distribution grid capacity for reactive power support1177827/fulltext01.pdf · reactive power...

IN DEGREE PROJECT ELECTRICAL ENGINEERING,SECOND CYCLE, 30 CREDITS

, STOCKHOLM SWEDEN 2017

Distribution grid capacity for reactive power support

EYSTEINN EIRÍKSSON

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF ELECTRICAL ENGINEERING

Distribution grid capability for reactivepower support

Eysteinn Eiríksson

Master Thesis, 2017KTH - Royal Institute of Technology

Abstract

The modern power system is changing at a rate faster than would have been expected20 years ago. More and more conventional power plants will be shut down in favour ofdistribution generation (DG). This is happening now with the trend of introducing renew-able energy sources (RES) to the power system.

The grids were designed to transfer power from generating units connected to the highvoltage grids towards the end consumers connected to the low voltage grids. With changedpower mix, power flows in the system will change resulting in possible grid problems. Oneof the main problems is keeping the voltage within operational limits of the system. Whenthe generation exceeds the consumption in a distribution network, the power will flow fromthe low voltage network towards the high voltage network (reverse power flow) which willcause the voltage to rise in the low voltage network. Reactive power support from DG canbe a valuable resource to mitigate the problem. Reactive power is necessary to operatethe power system. The main source of reactive power is synchronous generators. If thissource is shut down, the reactive power must come from another source.

This thesis investigates if DG could be used to support reactive power to the highvoltage transmission network to control the voltage. For this purpose, a distributionsystem located close to Worms, Germany will be studied. This distribution system consistsof two MV feeders with high penetration of DG, mostly photovoltaic (PV) but also windturbines (WT). Consumption and generation measurement data was provided by the localdistribution system operator (DSO). A few reactive power control methods are introducedand tested on this system. From the results, it is concluded that it is possible to providereactive power support from distribution networks and a voltage dependent reactive powercontrol can be used to this purpose.

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1 Sammanfattning

Det moderna kraftsystemet förandras snabbara än vad som hade förväntats för 20 årsedan. Fler och fler konventionella kraftverk kommer att stängas till fördel för distribu-tionsgenering. Detta händer nu med trenden att introducera förnybara energikällor tillkraftsystemet.

Nätverket utformades för att överföra kraft från generatorer som är anslutna till högspän-ningsnätet mot konsumenter anslutna till lågspänningsnätet. Med ändrad kraftblandningkommer strömflödena i systemet att förändras vilket resulterar i eventuella nätproblem.Ett av huvudproblemen är att hålla spänningen inom operativa gränser för systemet.När generationen överstiger förbrukningen i ett distributionsnät, kommer strömmen attströmma från lågspänningsnätet till högspänningsnätet vilket kommer att leda till attspänningen stiger i lågspänningsnätet. Reaktivt kraftstöd från distributionsgenering kanvara en värdefull resurs för att mildra problemet. Reaktiv effekt är nödvändig för att drivaelsystemet. Huvudkällan för reaktiv kraft är synkrona generatorer. Om den här källanstängs av måste den reaktiva effekten komma från en annan källa.

Denna avhandling undersöker om distributionsgenering skulle kunna användas för attstödja reaktiv kraft till högspänningsöverföringsnätet för att styra spänningen. För dettaändamål studeras ett distributionssystem som ligger nära Worms, Tyskland. Detta distri-butionssystem består av två MV-matare med med mycket distributionsgenerering, främstsolceller men även vindturbiner. Förbruknings- och generationsmätningsdata tillhan-dahölls av den lokala distributionssystemoperatören. Några reaktiva effektstyrningsme-toder introduceras och testas på detta system. Av resultaten dras slutsatsen att detär möjligt att tillhandahålla reaktivt kraftstöd från distributionsnät och en spännings-beroende reaktiv effektstyrning kan användas för detta ändamål.

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2 Acknowledgement

I would like to thank my supervisors, Stefan Stankovic and Poria Hasanpor Divshali,at the Department of Electric Power and Energy Systems for excellent supervision duringthis project. I would also like to thank Lennart Söder at the Department of Electric Powerand Energy Systems for useful advice and for being my exminer. Lastly, I would like tothank my friends and family for endless support throughout the progression of this project.

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Contents

1 Sammanfattning 2

2 Acknowledgement 3

3 Introduction 73.1 Overview of the report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4 Background and literature review 84.1 Reactive power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.1.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.1.2 Importance of reactive power . . . . . . . . . . . . . . . . . . . . . 9

4.2 DG effect on distribution grid . . . . . . . . . . . . . . . . . . . . . . . . . 104.3 Regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.3.1 Voltage regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.3.2 Reactive power regulations . . . . . . . . . . . . . . . . . . . . . . . 114.3.3 Three phase connection . . . . . . . . . . . . . . . . . . . . . . . . . 11

4.4 Voltage control in distributed networks . . . . . . . . . . . . . . . . . . . . 124.4.1 Voltage Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.4.2 Active power control . . . . . . . . . . . . . . . . . . . . . . . . . . 124.4.3 Reactive power control . . . . . . . . . . . . . . . . . . . . . . . . . 13

5 Model Description 165.1 General introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165.2 German electricity market . . . . . . . . . . . . . . . . . . . . . . . . . . . 165.3 Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175.4 Feeder 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

5.4.1 General information . . . . . . . . . . . . . . . . . . . . . . . . . . 175.4.2 Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

5.5 Feeder 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185.5.1 General information . . . . . . . . . . . . . . . . . . . . . . . . . . 185.5.2 Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5.6 Generation Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.6.1 Active Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.6.2 Reactive Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5.7 Load Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215.7.1 Active Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215.7.2 Reactive Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

5.8 Reactive power capability . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

6 DigSilent Implementation 246.1 Power factors for small scale PV’s and loads . . . . . . . . . . . . . . . . . 24

7 Simulation Results 287.1 Active power flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287.2 Base Case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

7.2.1 Power flow at the primary substation . . . . . . . . . . . . . . . . . 297.2.2 Voltage profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

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7.3 No reactive power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337.4 Constant power factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

7.4.1 0.95 capacitive p.f. . . . . . . . . . . . . . . . . . . . . . . . . . . . 367.4.2 0.95 inductive p.f. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

7.5 Maximizing reactive power support . . . . . . . . . . . . . . . . . . . . . . 447.6 Sensitivity Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 457.7 Maximizing reactive power provision . . . . . . . . . . . . . . . . . . . . . 487.8 Maximizing reactive power consumption . . . . . . . . . . . . . . . . . . . 54

8 Comparison 58

9 Conclusion 59

10 Future Work 60

Appendices 61

A Detailed diagram of feeder 1 61

B Detailed diagram of feeder 2 62

C List of Figures and Tables 63

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Abbreviations and Symbols

DG Distributed GenerationHV High VoltageMV Medium VoltageLV Low VoltagePV PhotovoltaicQ Reactive powerSUS Secondary Unit Substation (MV to LV)WT Wind Turbinep.f. Power factorOLTC On-Load Tap Changerp.u. Per unitNLTC No-Load Tap ChangerDSO Distribution System Operatorcap. Capacitive power factor (generating Q)ind. Inductive power factor (consuming Q)RES Renewable Energy Sources

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3 Introduction

Distribution generation sources (DG) are becoming an increasingly important factor inthe active power production in power systems. One of the main sources of reactive powerare synchronous generators. With increasing renewable energy sources (RES) connectedto the distribution grid, these synchronous generators will be shut down one by one. Withgrowing demand of stable electric power, reactive power importance is increasing. If oneof the main source of reactive power is going away, there is a need for another source ofreactive power. In this thesis, DG will be proposed as a new source of reactive power. It willbe investigated if and to what extent the distribution grid is capable of providing reactivepower support. One of the problems with a large penetration of DG in low voltage networksis operating within voltage limits. DGs can cause reverse power flows and over-voltage inthe low voltage grid. Reactive power support can be used to mitigate this problem. Themain objective of this thesis is to investigate if DG is capable to provide reactive powersupport for voltage control in the overlaying grids, i.e. high voltage transmission grid,while still operating within voltage limits.

A real German distribution grid with a high penetration of DG (mainly photovoltaic(PV) but also wind turbines (WT)) is used to analyse this problem. This distribution gridwas modelled in DigSilent PowerFactory where all simulations were done. A few reactivepower control methods will be proposed and tested on this system.

3.1 Overview of the report

First, a literature review of reactive power, DG and voltage control will be given inChapter 4. Then a description of the system will be given in Chapter 5. In Chapter 6.1, theimplementation in DigSilent will be explained and the base case investigated. Differentmethods of reactive power control and the results are presented in Chapter 7. Finally,conclusion and future work is discussed in Chapter 9 and 10.

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4 Background and literature review

4.1 Reactive power

4.1.1 Basics

AC power systems produce and consume two types of electrical power, active and re-active power. Active power is the true power given to any load. Active power is measuredin units of watts (W). Reactive power moves back and forth in the power system. It isproduced by inductive and capacitive loads. It only exists when there is a phase displace-ment between voltage and current. It is measured in units of volt-ampere reactive (VAr).The apparent power is the total power (combination of active and reactive power). It ismeasured in units of volt-ampere (VA). [1]Equation 4.1 shows the relationship between active, reactive and apparent power.

S =√P 2 +Q2 (4.1)

where S is the apparent power, P is the active power and Q is the reactive power.Another way to see the relationship is to look at the power triangle shown in Figure 1

Figure 1: Power Triangle

The angle difference between the voltage and current is denoted with φ. The currentcan both lead and lag the voltage causing leading and lagging power factors. In this thesis,we will talk about capacitive power factor when the DGs/loads are injecting reactive powerand inductive power factor when they are consuming reactive power.

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4.1.2 Importance of reactive power

Importance of reactive power in power systems is increasing with growing demand forelectric power. Electric power must be generated in a stable, reliable and cost effectiveway. Reactive power is an important factor to be able to do that. The main reasons whyreactive power is so important are:

4.1.2.1 Voltage Control

Reactive power is an important factor in controlling the voltage. Over voltage candamage the insulation in equipments and low voltage can cause poor performance andalso overheating of the equipments because of possible bigger currents.How the voltage can be controlled by reactive power will be explained in more detail inSection 4.2.

4.1.2.2 Electrical Blackouts

If we look at the following equation:

P = U · I (4.2)

Power equals voltage times current. If the voltage is poorly controlled, it can cause highcurrent which can over load the lines and cause blackouts. In order to control the voltagecorrectly, reactive power is important like discussed in the previous chapter.

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4.2 DG effect on distribution grid

High penetration of distributed generation (DG) such as photovoltaic (PV) and windturbines (WT), have caused new challenges such as voltage rise and reverse power flows.This constantly growing use of DG in medium-voltage distribution networks will highlyimpact the development of future electrical systems.

Distribution networks are normally built up in the following way: The HV transmissionnetwork is connected to the MV distribution network via a primary substation. A numberof feeders are connected to this substation. These feeders are in general radially connected.Some feeders are connected in a ring but then one line is usually disconnected with a switchso that no loop flow can occur. Underground cables are mostly used in these feeders andthey have capacitive characteristics so they produce reactive power. Loads connected tothese feeders are mostly resistive, i.e. have a power factor very close to 1. Some big loadsare connected to the MV network and they are required to have a power factor within arange of 0.9 inductive and 0.9 capacitive [2]. The voltage drop along the feeder can beapproximated by:

∆U ≈ R ∗ PLoad +X ∗QLoadUN

(4.3)

Where,∆U Voltage change across the linePLoad Active power consumption by the load (negative)R Resistance of the lineQLoad Reactive power consumption by the load (negative)X Reactance of the lineUN Nominal voltagethe voltage in distribution networks with no DG decreases therefore from the primarysubstation to the end of the feeder.

When DGs are connected, the power flow can be reverse. So the voltage can be higherat the end of the feeder than at the primary substation.

∆U =R ∗ (PLoad + PDG) +X ∗ (QLoad +QDG)

UN(4.4)

If the generation is a lot higher than the demand, the voltage rise can exceed the limits.To avoid this, the reactive power in the generation units can be utilized. Normally, loadshave a power factor close to unity. PVs and WTs are usually connected through an inverterthat can adjust the active and reactive power almost freely but limited by the currentlimit of the inverter. When a DG unit is operated inductively, it consumes reactive powerso QDG comes negative which lowers the voltage. Capacitive operation however injectsreactive power which increases the voltage. The impact that reactive power adjustmentshave depends highly on the R/X ratio of the lines. In MV grids, the R/X ratio is usuallyaround 1, so active and reactive power have equal impact on the voltage rise.

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4.3 Regulations

The VDE 4105 code has regulations that must be followed for DG installation in Ger-many. [3]

4.3.1 Voltage regulations

Generation units connected to the MV grid in Germany are required to be able:

• to stay connected during fault

• to support the voltage by providing reactive power during the fault

• to consume the same or less reactive power after the fault clearance

4.3.2 Reactive power regulations

It is required that the capability of the inverters to feed in with a displacement up to0.95 leading or lagging and if the plant power exceeds 13.8 kVA, a displacement up to 0.90must be supported.

4.3.3 Three phase connection

Connections larger than 4.6 kVA must be three phase connected. Smaller connectionsare allowed to be one phase connected.

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4.4 Voltage control in distributed networks

In this chapter, a few control methods used in distribution grids will be discussed andexplained. As can be seen in equation 4.1, the voltage depends on the active and reactivepower, so by controling them, the voltage can be controlled. Active power controls andreactive power controls will therefore be explained in this chapter. There are also moredirect methods to control the voltage that will be explained.

4.4.1 Voltage Control

4.4.1.1 OLTC for transformers

On-load tap changers (OLTC) are used to control the voltage directly, i.e. withoutusing active or reactive power control. Normally, the HV/MV transformers that connectsthe distribution network to the transmission grid has an OLTC to control the voltage atthe MV busbar. However, this control could be more advanced. The OLTC controls thevoltage on the MV busbar. One or more feeders are connected to this busbar. If the DG isuneven, e.g. on one feeder there is a lot of generation but on another feeder there is not asmuch because of lack of DG capacity or clouds, then the voltage profile for these feederswould be very different and this could cause under or over-voltage. An advanced approachcould be to monitor the voltage at various points in the distribution grid. The downside ofthis approach however is additional cost. The MV/LV transformers normally have no-loadtap changers (NLTC). They have a fixed operation point and can only be changed on-sitewhen the transformer is disconnected. OLTC have a high advantage over NLTC. By usingONTC on the MV/LV transformers, the LV network could operate almost independent ofthe MV network. Then the voltage on the LV side could be set to below nominal voltageduring high in-feed and above nominal voltage during low in-feed. [4]

4.4.2 Active power control

One way to control the voltage is by controlling the active power. There are a few waysthat this can be done, for example:

4.4.2.1 Batteries

Batteries are built to store energy. When the DG is high and the voltage rises, batteriescan be used to store the extra energy and lower the voltage. Also, when the DG is lowand the voltage is low, batteries can use this extra energy to inject active power in to thegrid and prevent the voltage to lower further.

4.4.2.2 Active power curtailment

Active power curtailment can be used to avoid the disconnection of inverters due toover-voltage tripping. Active power can be limited to a fixed percentage of the nominalpower or voltage-dependent. Active power curtailment can also be used to increase hostingcapacity [5]. An example of a voltage dependent curtailment can be seen in Figure 2 wherethe curtailment starts at U=1.07 p.u. and the generation is completely shut down at U=1.1p.u.

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Figure 2: Example of active power curtailment

4.4.3 Reactive power control

In this section, the reactive power control of DGs will be explained in more detail. Ashas already been discussed in Chapter 4.2, the reactive power can be used to control thevoltage level. The voltage rise caused by the DG unit can be expressed as follows:

∆UDG ≈R · PDG +X ·QDG

UN(4.5)

Generation units must be able to operate between 0.95 inductive and 0.95 capacitive. Themost common ways to control the reactive power are: constant Q, constant PF, cosφ(P ),cosφ(U) and Q(V). These methods are explained in the following sections.

4.4.3.1 Constant Q

Constant amount of reactive power is consumed or provided by the DG unit indepen-dent of the voltage at the bus or the active power generated. This method is rather easy toimplement but the disadvantage is that this will unnecessary produce or consume reactivepower even when it is not needed.

4.4.3.2 Constant PF

With constant power factor, the DG unit is not consuming or providing reactive powerwhen there is no active power. However, it can still consume or provide reactive powerwhen it is not needed.

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4.4.3.3 cosφ(P )

With this method, the power factor changes according to the active power generation.This method improves the constant PF method as it can lower the reactive power con-sumption or provision when the infeed decreases. There are still some cases when thevoltage is not at its upper limit but the units are consuming a lot of reactive power, e.g.when clouds cover only part of the PV plants, causing the voltage to decrease but thereare some PVs generating maximum active power. An example of this controller can beseen in Figure 3.

Figure 3: Example of cosφ(P ) control

4.4.3.4 cosφ(U)

This method is very similar as cosφ(P ) as the voltage is very dependent on the activepower. This will however partially fix the cloud situation mentioned in Section 4.4.3.3 asthe power factor is only controlled by the voltage. An example of cosφ(U) control can beseen in Figure 4.

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Figure 4: Example of cosφ(U) control

4.4.3.5 Q(U)

This methods consumes/provides reactive power as a function of the voltage. Herethe reactive power is not a percentage of the active power, as in the cosφ methods. Nowit is possible to consume or generate reactive power even when there is no active powerproduction, e.g. at night if the voltage falls down below certain threshold it is possible toinject reactive power to make the voltage rise. This characteristic is shown in Figure 5.

Figure 5: Example of Q(U) control

The red dotted lines represent a certain threshold. So that when the voltage falls belowthe lower threshold, reactive power is injected to fix that. Also, when the voltage risesabove the upper threshold, reactive power is consumed to lower the voltage. In betweenthe two thresholds there is a dead-band, where no reactive power is consumed or provided.Of course, this is only an example of a Q(U) control. The dead-band could be wider ornothing at all. Also, there could be some constant reactive power consumption or provisionwithin the dead-band.

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5 Model Description

5.1 General introduction

In this thesis, a model was developed to study steady-state problems described in Chap-ter 3. To simulate this model, the power system toolbox PowerFactory 15.2 by DIgSILENThas been used. First version of this model was created by S. Geidel from Energynautics.The model was improved and expanded by L. Hulsmann in his master thesis from KTH[6]. Now this model has been further studied by me. Only the final version of this modelwill be described in this thesis.

This model consists of two MV feeders connected to the same primary substation.These feeders are a part of a German distribution grid. This Chapter will describes themodel in detail. First a summary of the German electricity market and then the locationand layout of the system.

5.2 German electricity market

Germany is the largest electricity market in Europe, it was opened for competitionin 1998. There is not a single system operator like in many other countries. Table 1summaries the electricity market in Germany.

Table 1: German electricity market

Electricity MarketDistribution Distribution Voltage Transmission Transmission VoltageGrid (km) Level (kV) Grid (km) Voltage Level # DSOs1780856

5.3 Location

The distribution system is located 50 km south of Frankfurt, Germany as can be seenin Figure 6.

Figure 6: Location of the system

There is one primary substation that connects the 110 kV grid to the 20 kV distributiongrid via two 45 MVA transformers. The two distribution feeders are shown in AppendicesA and B. They will be explained in more detail in the following sections.

5.4 Feeder 1

5.4.1 General information

There are 47 MV/LV secondary unit substations (SUS) at feeder 1. The furthest islocated at a distance of 22 km from the primary substation. A total number of 3700customers are connected to the LV grids of this feeder and one 7.3 MW PV farm. The PVfarm is connected to the MV side and is located in fre08, which is 18 km from the primarysubstation. A diagram of feeder 1 can be seen in Appendix A.

5.4.2 Parameters

The feeder consists mainly of underground cables apart from a few overhead linesinterconnecting villages. Information about this feeder is shown in Table 2. R/X ratiosalong the feeder are given in Table 4.

The MV/LV transformers at the SUS’s are equipped with off-load tap changers. All tapchangers have three possible positions with one of the following settings: 20.8/20.0/19.2kV to 400 V or 20.8/20.4/20.0 kV to 400V

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Table 2: Information about feeder 1

Number of substations 47Average number of customers per substation 79

Total number of customers 3712Number of small scale PV plants 278Capacity of small scale PV plants 4.4 MWCapacity of large scale PV plant 7.3 MW

Maximum load in 2015 3.7 MW

5.5 Feeder 2

5.5.1 General information

There are 39 MV/LV SUS’s at feeder 2. The furthest is located at a distance of 19.5 kmfrom the primary substation. A total number of 3020 customers are connected to the LVgrids of this feeder. There is one 9.6 MW wind farm connected to the MV grid in wah06(18 km from the primary substation). A diagram of feeder 2 can be seen in Appendix B.

5.5.2 Parameters

The entire MV network consists of underground cables. The LV grids also consistsof underground cables with the exception of ofs07 where the LV grid mainly consists ofoverhead lines. The R/X ratio along the feeder is given in Table 4. More informationabout this feeder can be seen in Table 3. The transformers at the SUS’s are equipped withoff-load tap changers with the same possible tap positions as described in 5.4.2.

Table 3: Information about feeder 2

Number of substations 40Average number of customers per substation 76

Total number of customers 3020Number of small scale PV plants 196Capacity of small scale PV plants 3.3 MW

Capacity of wind farm 9.6 MWMaximum load in 2015 5.7 MW

Table 4: R/X ratios along the feeders

Feeder 1 & 2R/X ratio in MV grid very close to the primary substation 0.9(0-1 km for feeder 1, 0-8 km for feeder 2)

R/X ratio in MV grid further away from the primary substation 1.7(1-22 km for feeder 1, 8-19.5 km for feeder 2)R/X ratio in LV distribution grid 2.6 (mostly)

R/X ratio in LV grid - customer connections 8.2

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The secondary substations were modelled in the following way: The LV networks werenot modelled in detail. The total load and generation from each substation were aggregatedinto a single load and a single generating unit as can be seen in Figure 7:

20 kV 400 V

�� 20 kV ∑ PV

∑Load

Figure 7: Aggregation of loads and PV’s

More information about how this aggregation was done is found in Section 5.6.1 forPV’s and Section 5.7.1 for loads.

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5.6 Generation Modelling

5.6.1 Active Power

There is only one large scale PV plant (the 7.3 MW PV plant in fre08). 15 minutesaverage active power values are available from the DSO. These values are available to di-rectly put in Power Factory. The small scale PV plants do not have smart meters, so thesame values are not available for them. The only data available for these plants are: thePV capacity of every installation and the SUS to which it is connected. For the modelling,the capacity of all PV’s connected to the same SUS is summed and one PV generator usedto represent all the small scale PV’s. The output of the large scale PV plant is used asa reference, e.g. if the average power output of the large scale PV plant is 30 % of thecapacity, the output of the small scale PV plants would also be 30 % of their capacity.This is a reasonable approximation for PV plants close to fre08. The village Freimersheimis only around 1 km from fre08. The PV’s in feeder 2 are between 10 to 16 km from fre08but due to lack of measurements, the large scale PV plant was also used as a reference infeeder 2. The active power for the wind farm in wah06 is directly taken from measurements.

5.6.2 Reactive Power

The small scale PV’s should operate at a unity power factor according to the DSO.However, because some of the LV networks are not included in the model, a unity powerfactor was not suitable for the small scale PV’s. In order to approximate the final reactivepower at the feeding point, a value of 0.9985 inductive (VAr consuming) was chosen forthe small scale PV’s in feeder 2. In feeder 1, a power factor value of 0.9992 inductive wasfound to be a good approximation for all small scale PVs. This is further explained inChapter 5.The large scale PV plant in fre08 has a cosφ(U) controller and 15 minute average mea-surements values for the reactive power are available. The cosφ(U) controller has the samecharacteristics as in Figure 4.The wind farm in wah06 operates at a constant power factor of 0.95 inductive (VAr con-suming).

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5.7 Load Modelling

5.7.1 Active Power

Three things are known about each customer from the DSO: The total yearly electricityconsumption, the SUS to which they are connected and the categorization of the customer.Each customer is categorized either as a RLM (German: Registrierende Leistungsmessung,English: Recorded power) or as a SLP (Standard Load Profile).

RLM customers are customers that consume more than 100 MWh per year. Theyare obligated to measure and record their power consumption. For these customers, a 15minute average value for the active power is available and can be used directly as an inputin Power Factory.

SLP customers are further categorized into household and industry customers. Stan-dard load profile is used to model these customers. This is a 15 minute average values overa large sample of customers of a common type. A typical household and industrial SLPfor one week is shown in Figure 8. The yearly electrical consumption for each customerconnected to the same SUS was summed up and then the SLP used to generate a 15minute average load profile.

Example: There are 57 household customers connected to substation fre03, their totalyearly consumption is 207777 kWh. The standard load profile for household has a yearlyconsumption of 3700 kWh. Then each 15 minute value is multiplied by 207777kWh

3700kWh. The

same thing was done for all industrial SLP customers. [6]

Figure 8: Standard Load Profile, residential vs. industrial

In feeder 1, there are 12 RLM customers and 3700 SLP customers.In feeder 2, there are 20 RLM customers and 3000 SLP customers.

21

5.7.2 Reactive Power

The loads usually have a unity power factor but they can also be slightly inductive orcapacitive.For the reactive power modelling a power factor of 0.94 inductive (VAr consuming) wasfound fitting for all loads on feeder 2 and a power factor of 0.975 inductive was foundfitting for all loads on feeder 1.

5.8 Reactive power capability

As mentioned in Section 4.3, all generation units must be able to operate with a powerfactor between 0.95 inductive and 0.95 capacitive. If the generation unit is generatingmaximum active and reactive power, then the apparent power will be:

PmaxSmax

= 0.95

Smax = 1.0526Pmax

So the PV inverter or the inverter in the wind turbine must be able to handle the currentwhen the apparent power is roughly 5% more than the maximum active power.The maximum reactive power at every time is therefore:

Qmax =√S2max − P 2 =

√1.0526P 2max − P 2

Here, Qmax is dependent on the instantaneous active power. This is shown in Figure 9where Smax is shown with bold blue half-circle and Pmax is shown with a red line.

Figure 9: Reactive power capability

22

It is however complicated to implement this controller because the active power gener-ation is not known beforehand and therefore Qmax is not known and is always changing.For a more simpler approach, Qmax could be constant. For example when the generationunit is generating maximum active power, the maximum reactive power is:

Qmax =√S2max − P 2max =

√(1.0526Pmax)2 − P 2max =

√0.108Pmax = 0.3287Pmax

Now Qmax is constant, 32.87 % of Pmax. This maximum limit of the reactive power isshown in Figure 9 with the black lines. In this project, this limit (32.87 % Pmax) was usedwhen reactive power controllers were tested.

23

6 DigSilent Implementation

As mentioned in Section 5.6.2, a fitting power factor for small scale PV’s and loads hadto be approximated. In this chapter this will be explained further and a base case will bepresented.

6.1 Power factors for small scale PV’s and loads

The system was modelled in DigSilent Power Factory and all load flow calculationsdone there. Reactive power measurements from the local DSO are available at the primarysubstation. These measurements were used to find a fitting power factor for the loads andgeneration units by comparing the measured and simulated reactive power flow at theprimary substation.

First, all loads and small scale PV’s are modelled with a unity power factor. Thelarge PV farm has the cosφ controller and the wind farm has a power factor of 0.95 VArconsuming. The reactive power from each feeder at the primary substation is shown inFigures 10 and 11 compared to the measured reactive power.

Figure 10: Reactive power from feeder 1, simulated and measured

24

Figure 11: Reactive power from feeder 2, simulated and measured

The root mean square error (RMSE) between the measured and simulated values are:

Feeder RMSE1 0.42 MVAr2 0.97 MVAr

From these figures it can be seen that a unity power factor for both loads and gener-ation is not a good approximation so there is need to find a fitting power factor to get amore accurate model.

There is too much reactive power generated for both feeders when the power factor forboth loads and DG is unity. Reactive power consumption is therefore needed in order tomatch the simulated values with the measurement data.

This was done with a trial and error method. Power factors were tried out and thenthe reactive power flow at the primary substation compared to the measurement data.The best outcome was obtained with the following values:

Feeder p.f. for loads p.f. for PVs1 0.975 ind. 0.9992 ind.2 0.94 ind. 0.9985 ind.

The comparison with these values can be seen in Figures 12 and 13.

25

Figure 12: Reactive power from feeder 1 whith corrected pf, simulated and measured

Figure 13: Reactive power from feeder 2 with corrected pf, simulated and measured

The root mean square error (RMSE) between the measured and simulated values are:

Feeder RMSE1 0.23 MVAr2 0.18 MVAr

26

The LV networks that are not modelled clearly have an effect on the reactive powerflow in the system. In order to model this, a new generator was put on all substationsthat have LV network not modelled (all SUS except the PV farm and wind farm (fre08,wah06)). This generator models the LV network. Active power is taken from the realgenerator and the power factor used to calculate Q that is consumed by this generator.The new generator is added to the substation like shown in Figure 14.

20 kV∑

PV

∑Load

P = 0, Q 6= 0

Figure 14: Modelling of loads and PV units at SUS

This generator is added so that it is easier to try Q controllers. Then this new Qgenerator does not change and different Q controllers can be implemented on the realgenerator.

27

7 Simulation Results

In this chapter, a few different control methods will be tested on the system in orderto conclude how much reactive power support can be delivered from the two feeders.

− No reactive power support

− Constant power factor

These control methods have been tested and then a voltage dependent reactive powermethod, Q(U), is used to maximize reactive power support.

7.1 Active power flow

The difference between load and generation from both feeders can be seen in Figures15 and 16. Positive values mean that the generation exceeds the consumption. Two pointsare marked in these Figures. The black circle marks the time when the generation minusconsumption is at minimum and the red circle marks the time when the generation minusconsumption is at maximum. These will be the two extreme scenarios used to check thevoltage in the system because this is when the voltages are most likely at maximum andminimum in the network. More information about these scenarios can be seen in Table 5.

Figure 15: Difference between generation and load, Feeder 1

Figure 16: Difference between generation and load, Feeder 2

28

Table 5: The scenarios for both feeders

Scenario Feeder 1 Feeder 2Time Gen [kW] Cons. [kW] Time Gen [kW] Cons. [kW]Max Gen-Cons. Day 6, 14:15 9620 2248 Day 1, 05:15 9600 2306Min Gen-Cons. Day 5, 20:45 0 2026 Day 5, 18:45 133 3765

7.2 Base Case

7.2.1 Power flow at the primary substation

The system was simulated without making any changes to it, i.e. the controller at thePV farm was not changed and the constant power factor at the wind farm was not changed.The data was from 6 days in 2015, from 18.06.2015 to 23.06.2015. The power flow at theprimary substation was recorded for these 6 days for both feeders and is shown in Figures17 - 18, (positive means power going from the feeder towards the primary substation andthe HV grid).

Figure 17: Power flow from the primary substation, Feeder 1, Base Case

29

Figure 18: Power flow from the primary substation, Feeder 2, Base Case

For feeder 1, when the large PV farm in fre08 starts to generate active power, it iscontrolled to consume reactive power. During daytime the feeder is taking reactive powerfrom the HV grid.

For feeder 2, the wind farm is consuming reactive power, so most of the time the feederis taking reactive power from the HV grid.

The average active power losses were calculated for both feeders in the following way:

PLosses =1

576

576∑i=1

(P iG−L + PiPS)

where P iG−L is the difference between generation and load at time i, taken from Figures15 and 16. P iPS is the active power flow from the primary substation at time i, taken fromFigures 17 and 18. There are 576 quarters for these 6 days. The average active powerlosses in this base case were calculated to be 27 kW for feeder 1 and 113 kW for feeder 2.

7.2.2 Voltage profiles

The voltage at each substation was also recorded for the 6 days. The two scenariosin Table 5 were used for the voltage profiles. The voltage profile for these two scenarioscan be seen in Figures 19 - 21. In Figure 19 substation 1 is the primary substation andsubstation 29 is wal01 which is the substation at the end of the feeder (see Appendix A).

30

Figure 19: Voltage profile for two scenarios at feeder 1, Base Case

Two voltage profiles were made for feeder 2. First one that is from the primary sub-station to the wind farm bus (wah06) and can be seen in Figure 20. The other goes tothe end of the feeder (ofs02), it can be seen in Figure 20. Buses one to five are the samefor both voltage profiles, bus one is the primary substation and bus five is mon01. SeeAppendix B where voltage profile A is marked in red and voltage profile B is marked inblue, the common part and other branches are coloured black.

Figure 20: Voltage profile for two scenarios at feeder 2 A, Base Case

In Figure 21 the voltage reaches maximum at mon01 (bus 5), that is because most ofthe generation is at wah06, and wah06 is at the end of a branch that goes from mon01.The voltage continues to rise in Figure 20 all the way to the wind farm bus wah06.

31

Figure 21: Voltage profile for two scenarios at feeder 2 B, Base Case

A voltage profile with respect to time was made for the most vulnerable bus (the busthat came closest to the voltage limits). In this case it was bus wah06 as can be seen inFigure 21. The voltage profile can be seen in Figure 22.The maximum voltage at bus wah06 was 1.072 p.u.

Figure 22: Voltage profile with respect to time for wah06, Base Case

32

7.3 No reactive power

The system was simulated with no source of reactive power, i.e. all the DGs are notinjecting or consuming any reactive power. The only change here from the base case isthat the controllers on the PV farm bus fre08 and the wind farm bus wah06 were takenaway. The generators added in Section 6.1 are however still consuming reactive power.The power flow for both feeders for the six days can be seen in Figures 23 - 24.

Figure 23: Power flow from the primary substation, Feeder 1, zero Q

Figure 24: Power flow from the primary substation, Feeder 2, zero Q

33

Even when there is no reactive power in the DG’s, there is still reactive power flowingfrom the feeder to the primary substation, for feeder 1 the average reactive power provisionis 700 kVAr. For feeder 2 the average reactive power provision is 167 kVAr. This can beexplained by that the distribution network is mostly supplied with underground cables andthey work like capacitors, i.e. they provide reactive power. The average reactive powerat the primary substation, standard deviation and network losses can be seen in Table 6and in Table 7, the voltage intervals (Umax and Umin) can be seen for both feeders. Thevoltage profile was made for the two scenarios in Figures 25 - 27. The red dashed line arescenario 1 from the Base Case and the blue dashed line are scenario 2 from the Base Case.

Figure 25: Voltage profile for two cases at feeder 1, zero Q

Figure 26: Voltage profile for two cases at feeder 2 A, zero Q

34

Figure 27: Voltage profile for two cases at feeder 2 B, zero Q

In scenario 2, the voltage profile doesn’t change from the base case. There is very littlegeneration in scenario 2 and therefore there is no reactive power consumption in the basecase and the voltage stays the same. In scenario 1 the voltage is higher when there is noconsumption of reactive power. Why this happens has been explained in Section 4.2.

A voltage profile with respect to time was made for the most vulnerable bus (the busthat came closest to the voltage limits). In this case it was the wind farm bus wah06 ascan be seen in Figure 26. The voltage profile can be seen in Figure 28. The maximumvoltage at bus wah06 was 1.088 p.u.

Figure 28: Voltage profile with respect to time for the wind farm bus wah06, zero Q

35

Table 6: Comparison between the base case and no reactive power method

MethodFeeder 1 [11.7 MW] Feeder 2 [12.9 MW]

Average Q St. Deviation of Q Average Losses Average Q St. Deviation of Q Average Losses

[kVAr] [kVAr] [kW] (% of BC) [kVAr] [kVAr] [kW] (% of BC)

Base case -388 581 27 (0%) 887 1044 113 (0%)

No reactive power -700 131 26 (-3.7%) -167 184 100 (-12%)

Table 7: Comparison between voltage intervals, base case and no reactive power method

MethodFeeder 1 Feeder 2

UMax UMin UMax UMin[p.u.] [p.u.] [p.u.] [p.u.]

Base Case 1.07 1.02 1.07 1.01No reactive power 1.08 1.02 1.09 1.01

7.4 Constant power factor

7.4.1 0.95 capacitive p.f.

The system was simulated when all DG’s had a constant power factor of 0.95 capacitive(providing reactive power). The power flow from the primary substation can be seen inFigures 29 - 30.

Figure 29: Power flow from the primary substation, Feeder 1, constant p.f. 0.95 cap.

36

Figure 30: Power flow from the primary substation, Feeder 2, constant p.f. 0.95 cap.

The mean values, standard deviations and average losses can be seen in Table 8 andthe voltage intervals can be seen in Table 9

Table 8: Comparison between Base Case, No Q and Constant p.f. 0.95 cap.




Base case -388 581 27 (0%) 887 1044 113 (0%)


Constant p.f. 0.95 cap. -1248 597 33 (+22%) -1350 832 113 (0%)

Table 9: Comparison between Base Case, No Q and Constant p.f. 0.95 cap., Voltage interval




Constant p.f. 0.95 cap. 1.09 1.02 1.11 1.02

37

The voltage profiles for the two scenarios in Table 5 can be seen in Figures 31 - 33.The dashed lines in the Figures are from the base case. There is no generation in scenario2 for feeder 1 and therefore the voltage profile doesn’t change at all because the reactivepower provision is zero on all DG’s as it was in the base case. On feeder 2, the generationin scenario 2 is very small so the voltage profile changes a little bit.

Figure 31: Voltage profile for two scenarios at feeder 1, constant p.f. 0.95 cap.

Figure 32: Voltage profile for two scenarios at feeder 2 A, constant p.f. 0.95 cap.

38

Figure 33: Voltage profile for two scenarios at feeder 2 B, constant p.f. 0.95 cap.

In this case, the voltage limit is violated on feeder 2 as can be seen in Figure 32.A voltage profile with respect to time was made for the most vulnerable bus (the bus

that came closest to the voltage limits). In this case it was the wind farm bus (wah06) thatexceeded the voltage limits the most as can be seen in Figure 32 (bus 16). The voltageprofile can be seen in Figure 34. The maximum voltage at wah06 was 1.106 p.u.

Figure 34: Voltage profile with respect to time for wah06, constant p.f. 0.95 cap.

39

7.4.2 0.95 inductive p.f.

The system was simulated when all DG’s had a constant inductive power factor of 0.95.The power flow can be seen in Figures 35 and 36. The mean values, standard deviationand losses can be seen in Table 10 and the voltage intervals can be seen in Table 19.

Figure 35: Power flow from the primary substation, Feeder 1, constant p.f. 0.95 ind.

Figure 36: Power flow from the primary substation, Feeder 2, constant p.f. 0.95 ind.

40

Table 10: Comparison between Base case, No Q and Constant p.f. methods




Base case -388 581 27 (0%) 887 1044 113 (0%)


Constant p.f. 0.95 cap. -1248 597 33 (+22%) -1350 832 113 (0%)

Constant p.f. 0.95 ind. -145 798 28 (+3.7%) 1039 1070 115 (+1.8%)

Table 11: Comparison between Base Case, No Q and Constant p.f. methods,Voltage intervals




Constant p.f. 0.95 cap. 1.09 1.02 1.11 1.02Constant p.f. 0.95 ind. 1.07 1.02 1.07 1.01

Voltage profile for both cases and both feeders can be seen in Figures 37 - 39.

Figure 37: Voltage profile for two cases at feeder 1, constant p.f. 0.95 ind.

41

Figure 38: Voltage profile for two cases at feeder 2 A, constant p.f. 0.95 ind.

Figure 39: Voltage profile for two cases at feeder 2 B, constant p.f. 0.95 ind.

The voltage profiles are almost identical to the base case. The reactive power flow onfeeder 2 is almost the same for this case and the base case because in both scenario 1 and 2there is almost no PV generation and therefore almost no reactive power consumption bythe PVs, the wind farm has a power factor of 0.95 inductive in the base case and thereforethe reactive power flow is almost the same in feeder 2 for this case and the base case. Onfeeder 1 in the base case, the PV farm has a cosφ(U) controller like shown in Figure 4 andin scenario 1, the voltage on the PV farm bus is 1.07 p.u. which means the power factorfrom the PV farm is around 0.92 inductive. The PV farm is consuming more reactivepower in the base case than in this case but the rest of the PVs are consuming with apower factor of 0.95 inductive in this case and nothing in the base case and therefore thereactive power flow on feeder 1 is similar in this case and the base case. For scenario 2 onfeeder 1, there is no generation and therefore this case is identical to the base case on thattime.

42

A voltage profile with respect to time was made for the most vulnerable bus.Again, it was the wind farm bus wah06 as can be seen in Figure 37. The voltage profilecan be seen in Figure 40. The maximum voltage was 1.072 p.u.

Figure 40: Voltage profile with respect to time for wah06, constant p.f. 0.95 ind.

43

7.5 Maximizing reactive power support

In this section, a Q(U) control will be used to maximize the reactive power support.In Figure 41, two examples of Q(U) control and the limits of the control can be seen.

Figure 41: Two examples of Q(U) control

The reactive power capability is from -32.87 %Pmax to 32.87 %Pmax represented by thered lines. The blue line in Figure 41 is an example of a Q(U) control. If this control isimplemented in a DG unit, it will provide reactive power ( 30% of active power capacity)until the voltage at the bus where the unit is connected reaches 0.96 p.u., then the reactivepower provision starts to decrease and when the voltage reaches 1.01 p.u. the reactiveprovision is 20% of active power capacity. The pink line is another example of a Q(U)control. This control is providing reactive power (20 % of active power capacity) until thevoltage reaches 0.91, then the reactive power provision starts to decrease and decreaseslinearly and reaches 0 when the voltage reaches 1.1 p.u. This control method will first beused to maximize the reactive power provision from both feeders and then to maximizethe reactive power consumption to both feeders.A sensitivity analysis was made in order to decide from which DG unit the reactive powersupport should come from. This analysis is explained in section 7.6.

44

7.6 Sensitivity Analysis

By controlling the reactive power in the DGs, the voltage profile changes. In order tobe able select which DG should contribute to the reactive power support the most, we needto find out which DG has the least effect on the voltage. Therefore, a sensitivity analysiswas made in order to decide where the reactive power should come from, i.e. which DGis the most sensitive to a change in reactive power consumption/provision. The analysisgoes like this:

− Set all DG power factors to 1, so there is no reactive power provision or consumption.

− Run a power flow and record the voltage at each substation.

− Put a constant 10 kVAr generation of reactive power at the first substation andrepeat step 2.

− Repeat step 3 for all the substation, one at a time.

The voltage change can be seen in Table 12 for feeder 1 and Table 13 for feeder 2.From these results, the obvious conclusion is that the distance from the primary substationhas the most affect on the sensitivity of the substations.

45

Table 12: Sensitivity, analysis Feeder 1

Name Distance from SS [m] ∆V [p.u.]∗10−5wal01 22047 3.10wal03 21664 3.04wal02 21175 2.97fre07 20290 2.85fre02 20032 2.81fre03 19475 2.73fre06 19124 2.68fre08 18422 2.58fre05 17650 2.47ket04 16734 2.35ket02 16468 2.32ket01 16011 2.25ket06 15849 2.23ket03 15622 2.19ket05 15068 2.11esl01 14632 2.05esl03 14300 2.00epp07 13078 1.91epp06 13029 1.91epp02 12714 1.86epp03 12713 1.86epp01 12372 1.81din01 12562 1.76flo02 11461 1.60flo01 10601 1.55flo05 11033 1.54flo04 11783 1.48ofl01 9086 1.33ofl02 8759 1.29ofl07 6884 1.02ofl06 6307 0.93ofl04 5985 0.89ofl05 5371 0.80ofl03 5067 0.76ghm02 2802 0.45ghm09 2420 0.39ghm13 2462 0.38ghm03 1412 0.31ghm07 1671 0.26ghm06 1643 0.25ghm04 1360 0.21ghm05 1200 0.19ghm01 931 0.15ghm11 380 0.06

46

Table 13: Sensitivity analysis, Feeder 2

Name Distance from SS [m] ∆V [p.u.]∗10−5ofs04 18939 2.90ofs08 18656 2.86ofs01 18608 2.85wah06 18193 2.76wah04 18110 2.75ofs11 17788 2.73ofs10 17625 2.71ofs05 17451 2.68ofs03 17218 2.65wah03 17360 2.65ofs09 16966 2.61wah05 17031 2.60ofs07 18118 2.57wah02 16463 2.52hos04 15739 2.43hos02 15536 2.40wah01 15559 2.39hos03 15122 2.34hos01 14655 2.27mon14 13404 2.09mon05 13196 2.05mon02 12609 1.97mon13 12322 1.93mon08 11852 1.86mon07 11531 1.81mon12 11462 1.81mon06 11252 1.77mon21 11252 1.77mon09 11899 1.76mon03 11110 1.75mon01 10601 1.68

These results will now be used to maximize the reactive power support with a Q(U)controller.

47

7.7 Maximizing reactive power provision

In this section, a Q(U) control will be used to maximize the reactive power provision.The sensitivity analysis performed in section 7.6 is used to determine the Q parameter forU = 1.1 p.u.. This was done to determine how much reactive power can be providedfor the worst case scenario (when the voltages are at maximum). The procedure goes likethis: First the reactive power provision was raised on the strongest bus (ghm11 on feeder1 and mon01 on feeder 2) until either the reactive power limits of the unit or the voltagelimits were met. Then the same was done for the next strongest bus (ghm01 on feeder 1and mon03 on feeder 2) until either limits were met. This was done for all buses with DGcapacity. The results from this can be seen in Table 14.

Table 14: Q parameters for U=1.1 p.u.

Substation Q [% Pmax] for U=1.1 p.u. FeederAll PV’s on F1 32.87 1

Wind farm bus wah06 12 2ofs01, ofs08, ofs01 0 2Rest of SUS on F2 32.87 2

On feeder 1, all PV units can provide maximum reactive power when the voltage isat maximum. On feeder 2, the PV units connected to the strongest 27 buses can providemaximum reactive power. Then the wind farm bus reached the voltage limits when thewind farm was providing reactive power of 12 % of its active power capacity. Then thethree remaining buses (ofs01, ofs08 and ofs04) could not provide any reactive power withoutviolating the voltage limits. Now we have Q parameters for one point in the Q(U) control(when U=1.1 p.u.). Because all PV units can provide maximum reactive power when thevoltage is at maximum, it can be concluded that these units can provide maximum reactivepower for all voltages. So all PV units on feeder 1 can have a constant Q control and allunits can provide maximum reactive power all the time without violating the voltagelimits. On feeder 2 this is not the case. In order to find how the control for the units onfeeder 2 changes for different voltage values, Q parameters were defined for U = 0.9 p.u..This was done with the same method, only now scenario 2 was used to determine theparameters because in scenario 2 the voltage is at minimum. In Table 15 the Q parameterfor U = 0.9 p.u. can be seen for feeder 2.

Table 15: Q parameters for U=0.9 p.u. on feeder 2

Substation Q [% Pmax] for U=0.9 p.u.Wind farm bus wah06 32.87ofs01, ofs08, ofs01 32.87Rest of SUS on F2 32.87

For U = 0.9 p.u., all DG units on feeder 2 can provide maximum reactive power. Nowit can be estimated that the DG units connected to the first 47 buses can have a constantQ control providing maximum reactive power at all times. The control for the wind farmbus wah06 and the other three buses ofs01, ofs08 and ofs04 need to decrease the reactivepower provision at some voltage value so that the voltage limits are not violated.

48

Now the Q parameters were found for these controllers at other voltage values (firstfor U=1.0 p.u., then for U=1.01 p.u. and so on). The resulting controllers can be seen inFigure 42 and in Tables 16 and 17 the resulting controllers can be seen.

Figure 42: Controllers for maximizing reactive power provision

The controller for the wind farm on wah06 is in red. The wind farm is providing 32.87% of active power capacity until the voltage reaches 1.09 p.u. and then it decreases linearlyand reaches 12 % of active power capacity at U = 1.1 p.u..

Table 16: Controller for the wind farm bus wah06

Q @ U=0.9 - 1.09 p.u. Q @ U=1.1 p.u.32.87 12

The controller for the PV units connected to ofs01, ofs08 and ofs04 is shown in greenin Figure 42. The units are providing 32.87 % of active power capacity until the voltagereaches 1.03 then the Q provision decreases linearly and reaches 0 when U = 1.1 p.u..

Table 17: Controller for ofs01, ofs08 and ofs04

Q @ U=0.9 - 1.03 p.u. Q @ U=1.1 p.u.32.87 0

The blue line in Figure 42 represents the controller for all PV units in feeder 1 and thePV units connected to the strongest 47 buses in feeder 2. These units provide 32.87 % ofactive power capacity all the time regardless of the voltage. The power flow and voltageprofiles can be seen in Figures 43 - 54.

49

Figure 43: Power flow from the primary substation, Feeder 1, maximizing reactive powerprovision

Figure 44: Power flow from the primary substation, Feeder 2, maximizing reactive powerprovision

The average Q values, standard deviation and active power losses can be seen in Table18.

50

Voltage profiles were made for the same two scenarios for both feeders. On feeder 1,the voltage reaches 1.099 p.u. for scenario 1 on the PV farm bus fre08 (bus 22 in Figure45). On feeder 2, the voltage reaches 1.099 on the wind farm bus wah06 (bus 16 in Figure46). The voltage intervals for both feeders can be seen in Table 19.

Figure 45: Voltage profile feeder 1, Maximizing reactive power provision

Figure 46: Voltage profile feeder 2 A, Maximizing reactive power provision

51

Figure 47: Voltage profile feeder 2 B, Maximizing reactive power provision

A voltage profile with respect to time was made for the bus that came closest to thevoltage limits. In this case it was bus wal01 on feeder 1. The maximum voltage was 1.099p.u. However, the voltage didn’t go over 1.09 p.u. often (less than 2% of the time).

Figure 48: Voltage profile with respect to time for wal01, Maximizing reactive powerprovision

52

Table 18: Comparison between maximizing Q provision to other control methods




Base case -388 581 27 (0%) 887 1044 113 (0%)


Constant p.f. 0.95 cap. -1248 597 33 (+22%) -1350 832 113 (0%)

Constant p.f. 0.95 ind. -145 798 28 (+3.7%) 1039 1070 115 (+1.8%)

Max Q prov. -4512 128 129 (+378%) -4471 526 210 (+86%)

Table 19: Comparison between maximizing Q provision to other control methods, Voltage interval




Constant p.f. 0.95 cap. 1.09 1.02 1.11 1.02Max Q prov. 1.099 1.03 1.099 1.03

53

7.8 Maximizing reactive power consumption

In this section, a Q(U) controller will be used to maximize the reactive power con-sumption to both feeders. First, the reactive power consumption from each DG unit wasfound for when U=0.9 p.u. and then the possible reactive power consumption was foundfor when U=1.1 p.u. for each DG unit. All the DG units can consume maximum reac-tive power (32.87 % of active power capacity) so they all have a constant Q controllerconsuming maximum Q all the time, regardless of the voltage.

Figure 49: Power flow from the primary substation, Feeder 1, maximizing reactive powerconsumption

Figure 50: Power flow from the primary substation, Feeder 2, maximizing reactive powerconsumption

The average Q values, standard deviation and active power losses can be seen in Table20.

54

Voltage profiles were made for the same two scenarios for both feeders. On both feeders,the voltage decreases compared to the base case. The voltage intervals for both feederscan be seen in Table 21.

Figure 51: Voltage profile feeder 1, Maximizing reactive power consumption

Figure 52: Voltage profile feeder 2 A, Maximizing reactive power consumption

55

Figure 53: Voltage profile feeder 2 B, Maximizing reactive power consumption

A voltage profile with respect to time was made for the bus that came closest to thevoltage limits. In this case it was bus wal01 on feeder 1. The maximum voltage was 1.07p.u.

Figure 54: Voltage profile with respect to time for wal01, Maximizing reactive powerconsumption

56





Base case -388 581 27 (0%) 887 1044 113 (0%)


Constant p.f. 0.95 cap. -1248 597 33 (+22%) -1350 832 113 (0%)

Constant p.f. 0.95 ind. -145 798 28 (+3.7%) 1039 1070 115 (+1.8%)

Max Q prov. -4512 128 129 (+378%) -4471 526 210 (+86%)

Max Q cons. 3216 133 88 (+226%) 4493 190 219 (+94%)





Constant p.f. 0.95 cap. 1.09 1.02 1.11 1.02Max Q prov. 1.099 1.03 1.099 1.03Max Q cons. 1.07 1.00 1.07 0.99

57

8 Comparison

In this section, the different methods will be compared. In Table 22 is the averageQ, standard deviation of Q and average losses for each method. In Table 23, the voltageintervals for each method can be seen.





Base case -388 581 27 (0%) 887 1044 113 (0%)


Constant p.f. 0.95 cap. -1248 597 33 (+22%) -1350 832 113 (0%)

Constant p.f. 0.95 ind. -145 798 28 (+3.7%) 1039 1070 115 (+1.8%)

Max Q prov. -4512 128 129 (+378%) -4471 526 210 (+86%)

Max Q cons. 3216 133 88 (+226%) 4493 190 219 (+94%)





Constant p.f. 0.95 cap. 1.09 1.02 1.11 1.02Max Q prov. 1.099 1.03 1.099 1.03Max Q cons. 1.07 1.00 1.07 0.99

In the no reactive power method, all the DG units have a unity power factor. Thereis however a small reactive power provision from the distribution grid (700 kVAr fromfeeder 1 and 167 kVAr from feeder 2). This is because of the cables in the network thathave a capacitive effect and are providing reactive power. This can also explain why themaximum consumption is lower than the maximum provision on feeder 1 although all DGunits are providing/consuming maximum Q in both cases. When all the DG units areconsuming reactive power, they first need to consume the reactive power provided by thenetwork and then they can consume reactive power from the HV transmission grid.

58

9 Conclusion

The increase of RES connected to the distribution network results in possible gridproblems. The main problem is the reactive power support which is the subject of thisthesis. The main source of reactive power support comes from conventional power plantsand with an increase share of RES, they are shutting down one by one so there is a needto find another source that can provide reactive power support for voltage control.

In this project, a German distribution grid with a high penetration of DG has beeninvestigated, this distribution grid consists of two MV feeders. The system was simulated inDigSilent PowerFactory and data from the local DSO used for modelling of consumptionand generation. The DG capacity is 11.7 MW and 12.9 MW in feeder 1 and feeder 2respectively. The objective of this project was to analyse if, and to what extent, thedistribution grid is capable of providing reactive power support for voltage control in theoverlaying grids, i.e. high voltage transmission grid. The maximization of reactive powersupport was done with a Q(U) controllers and sensitivity analysis was used to decidefrom which DG unit most of the reactive power support should come from. However,this sensitivity analysis was not needed when maximizing the reactive power consumptionbecause all the units could consume maximum reactive power without violating voltagelimits. The sensitivity analysis was neither needed when maximizing the reactive powerprovision from feeder 1 for the same reason. The main conclusion is that it is possibleto provide reactive power support from distribution grids with high penetration of DGto the overlaying grids for voltage control. The most effective way is to combine bothQ(U) controllers and constant Q controllers to the generation units like was done whenmaximizing reactive power provision on feeder 2, where the wind farm wah06 and PVs onthree other substations (ofs01, ofs04 and ofs08) had Q(U) controllers and the rest of thePVs had a constant Q controller (see Section 7.7). The active power losses increase whenmaximizing the reactive power support. When maximizing the reactive power provision,the active power losses increased 140% when both feeders are considered as the samenetwork and 120 % when maximizing the reactive power consumption. Although thisseems to be very high increase, the average losses are really small in the base case (1.9%losses on feeder 1 and 6.3 % losses on feeder 2)

59

10 Future Work

There are a few things that could be investigated more and better. First of all, a fewimprovements of the model could be made:

− Model all LV networks in full with individual SLP for each customer.

− Use more reference points for the PV modelling.

These changes would make the results more accurate and meaningful. To make these im-provements, more data from the local DSO are needed.In this project, the position of the on load tap changer (OLTC) in the primary substationwas not considered a variable. If it is, it can be used to lower the losses and also increasethe reactive power support. This report only investigates the impact of distributed gener-ation on the power system. There are other things that will increase in the power systemin the future, things like electric vehicles (EVs) and battery banks. If batteries were in-cluded in the system, it could be possible to provide the loads with batteries when thereis no generation. EV’s would also change the load profile significantly. It would be inter-esting to add these things to the model and investigate the affects it has on the power flow.

60

AppendicesA Detailed diagram of feeder 1

Primary Substationvghm11v v v v v

ghm05 ghm03 ghm09 ghm022802 mghm01

vghm04 vghm06

1643 m

vghm07vghm13

vofl06vofl04vofl05vofl03vofl08

vflo05vflo04 v

flo01

vflo03

vepp04

vepp01

vepp02

vepp06

13029 m

vepp03 vepp07vofl01vofl02vofl07

vfre05vesl01vesl03 v

ket05

vket03

vket01

vket02

16468 m

vket08 vket04vdin01vflo02

vfre08

18422 m vfre06

vfre03

19475 m vfre02

vfre07

vwal02

vwal03

vwal01

22047 m

61

B Detailed diagram of feeder 2Primary Substationxfld05xfld07xmon11xmon01 ��

xmon21

xmon09

11899 m

xmon17

12968 m

@@@@

xmon03

xmon07

xmon08

xmon13 xmon15 xmon14 xmon1813666 mx

mon02

xmon05

xwah01

xwah02

xwah05

xwah03

xwah04

xwah06

18193 m

xmon06

xhos02xhos03 x

hos04

xhos01xmon04 x

mon10

xmon12

xofs0219459 m

xofs04xofs11xofs05xofs09 x

ofs03

xofs10

xofs07

18118 m

xofs08

xofs01

18608 m

62

C List of Figures and Tables

List of Figures

1 Power Triangle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Example of active power curtailment . . . . . . . . . . . . . . . . . . . . . 133 Example of cosφ(P ) control . . . . . . . . . . . . . . . . . . . . . . . . . . 144 Example of cosφ(U) control . . . . . . . . . . . . . . . . . . . . . . . . . . 155 Example of Q(U) control . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 Location of the system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 Aggregation of loads and PV’s . . . . . . . . . . . . . . . . . . . . . . . . . 198 Standard Load Profile, residential vs. industrial . . . . . . . . . . . . . . . 219 Reactive power capability . . . . . . . . . . . . . . . . . . . . . . . . . . . 2210 Reactive power from feeder 1, simulated and measured . . . . . . . . . . . 2411 Reactive power from feeder 2, simulated and measured . . . . . . . . . . . 2512 Reactive power from feeder 1 whith corrected pf, simulated and measured . 2613 Reactive power from feeder 2 with corrected pf, simulated and measured . 2614 Modelling of loads and PV units at SUS . . . . . . . . . . . . . . . . . . . 2715 Difference between generation and load, Feeder 1 . . . . . . . . . . . . . . 2816 Difference between generation and load, Feeder 2 . . . . . . . . . . . . . . 2817 Power flow from the primary substation, Feeder 1, Base Case . . . . . . . . 2918 Power flow from the primary substation, Feeder 2, Base Case . . . . . . . . 3019 Voltage profile for two scenarios at feeder 1, Base Case . . . . . . . . . . . 3120 Voltage profile for two scenarios at feeder 2 A, Base Case . . . . . . . . . . 3121 Voltage profile for two scenarios at feeder 2 B, Base Case . . . . . . . . . . 3222 Voltage profile with respect to time for wah06, Base Case . . . . . . . . . . 3223 Power flow from the primary substation, Feeder 1, zero Q . . . . . . . . . . 3324 Power flow from the primary substation, Feeder 2, zero Q . . . . . . . . . . 3325 Voltage profile for two cases at feeder 1, zero Q . . . . . . . . . . . . . . . 3426 Voltage profile for two cases at feeder 2 A, zero Q . . . . . . . . . . . . . . 3427 Voltage profile for two cases at feeder 2 B, zero Q . . . . . . . . . . . . . . 3528 Voltage profile with respect to time for the wind farm bus wah06, zero Q . 3529 Power flow from the primary substation, Feeder 1, constant p.f. 0.95 cap. . 3630 Power flow from the primary substation, Feeder 2, constant p.f. 0.95 cap. . 3731 Voltage profile for two scenarios at feeder 1, constant p.f. 0.95 cap. . . . . 3832 Voltage profile for two scenarios at feeder 2 A, constant p.f. 0.95 cap. . . . 3833 Voltage profile for two scenarios at feeder 2 B, constant p.f. 0.95 cap. . . . 3934 Voltage profile with respect to time for wah06, constant p.f. 0.95 cap. . . . 3935 Power flow from the primary substation, Feeder 1, constant p.f. 0.95 ind. . 4036 Power flow from the primary substation, Feeder 2, constant p.f. 0.95 ind. . 4037 Voltage profile for two cases at feeder 1, constant p.f. 0.95 ind. . . . . . . . 4138 Voltage profile for two cases at feeder 2 A, constant p.f. 0.95 ind. . . . . . 4239 Voltage profile for two cases at feeder 2 B, constant p.f. 0.95 ind. . . . . . 4240 Voltage profile with respect to time for wah06, constant p.f. 0.95 ind. . . . 4341 Two examples of Q(U) control . . . . . . . . . . . . . . . . . . . . . . . . . 4442 Controllers for maximizing reactive power provision . . . . . . . . . . . . . 4943 Power flow from the primary substation, Feeder 1, maximizing reactive

power provision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

63

44 Power flow from the primary substation, Feeder 2, maximizing reactivepower provision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

45 Voltage profile feeder 1, Maximizing reactive power provision . . . . . . . . 5146 Voltage profile feeder 2 A, Maximizing reactive power provision . . . . . . 5147 Voltage profile feeder 2 B, Maximizing reactive power provision . . . . . . 5248 Voltage profile with respect to time for wal01, Maximizing reactive power

provision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5249 Power flow from the primary substation, Feeder 1, maximizing reactive

power consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5450 Power flow from the primary substation, Feeder 2, maximizing reactive

power consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5451 Voltage profile feeder 1, Maximizing reactive power consumption . . . . . . 5552 Voltage profile feeder 2 A, Maximizing reactive power consumption . . . . 5553 Voltage profile feeder 2 B, Maximizing reactive power consumption . . . . 5654 Voltage profile with respect to time for wal01, Maximizing reactive power

consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

64

List of Tables

1 German electricity market . . . . . . . . . . . . . . . . . . . . . . . . . . . 162 Information about feeder 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 Information about feeder 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 R/X ratios along the feeders . . . . . . . . . . . . . . . . . . . . . . . . . . 185 The scenarios for both feeders . . . . . . . . . . . . . . . . . . . . . . . . . 296 Comparison between the base case and no reactive power method . . . . . 367 Comparison between voltage intervals, base case and no reactive power method 368 Comparison between Base Case, No Q and Constant p.f. 0.95 cap. . . . . . 379 Comparison between Base Case, No Q and Constant p.f. 0.95 cap., Voltage interval 3710 Comparison between Base case, No Q and Constant p.f. methods . . . . . 4111 Comparison between Base Case, No Q and Constant p.f. methods,

Voltage intervals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4112 Sensitivity, analysis Feeder 1 . . . . . . . . . . . . . . . . . . . . . . . . . . 4613 Sensitivity analysis, Feeder 2 . . . . . . . . . . . . . . . . . . . . . . . . . . 4714 Q parameters for U=1.1 p.u. . . . . . . . . . . . . . . . . . . . . . . . . . . 4815 Q parameters for U=0.9 p.u. on feeder 2 . . . . . . . . . . . . . . . . . . . 4816 Controller for the wind farm bus wah06 . . . . . . . . . . . . . . . . . . . . 4917 Controller for ofs01, ofs08 and ofs04 . . . . . . . . . . . . . . . . . . . . . . 4918 Comparison between maximizing Q provision to other control methods . . 5319 Comparison between maximizing Q provision to other control methods, Voltage

interval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5320 Comparison between maximizing Q provision to other control methods . . 5721 Comparison between maximizing Q provision to other control methods, Voltage

interval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5722 Comparison between maximizing Q provision to other control methods . . 5823 Comparison between maximizing Q provision to other control methods, Voltage

interval . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

65

References

[1] Hadi Saadat, Power System Analysis WCB McGraw-Hill 1999

[2] TAB Mittelspannung 2008 Technical conditions for connection to the medium-voltagenetwork 2008

[3] VDE VDE-AR-N 4105 Generators connected to the low-voltage distribution network- technical requirements for the connection to and parallel operation with low-voltagedistribution networks 2011

[4] Damir Jakus, Josip Vasilj and Petar Sarajčev, Voltage Control in MV DistributionNetworks Through Coordinated Control of Tap Changers and Renewable Energy Sources2015

[5] Benoît Bletterie, Serdar Kadam and Julien Le Baut Increased hosting capacity by meansof active power curtailment 2016

[6] Leonard Hulsmann, Evaluation of two distribution grids in terms of PV penetrationlimits and effectiveness of reactive power controls. KTH Master Thesis, 2016.

[7] BNetzA, 2016. Monitoring Report 2016. Bundesnetzagentur fur Elektrizitat, Gas,Telekommunikation, Post und Eisenbahen, Bundeskartellamt.

[8] BMU, 2012. Distributed Generation in Germany: From policy planning to implemen-tation to performance, presented at the Great Wall Renewable Energy Forum 2012,Sino-German International Symposium on Renewable Energy and Distributed Gener-ation, Beijing, December 10th 2012.

[9] Afshin Samadi, Ebrahim Shayesteh, Robert Eriksson, Barry Rawn, Lennart Söder,Multi-objective coordinated droop-based voltage regulation in distribution grids with PVsystems. 2014

[10] Luis F. Ochoa, Andrew Keane, Gareth P. Harrison Minimizing the Reactive Sup-port for Distributed Generation: Enhanced Passive Operation and Smart DistributionNetworks, 2011

[11] Sarina Adhikari, Fangxing Li and Huijuan Li P-Q and P-V Control of PhotovoltaicGenerators in Distribution Systems 2015

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