increasing hosting capacity of pv solar systems using smart … · 2018. 12. 8. · review of...

978-1-5386-6159-8/18/$31.00 ©2018 IEEE

Increasing Hosting Capacity of PV Solar Systems

using Smart Inverter Volt-Var Control

Sibin Mohan

Dept. of Electrical and Computer

Engineering

The University of Western

Ontario

London, ON, Canada

[email protected]

Syeed Hasan


Engineering


Ontario

London, ON, Canada

[email protected]

Yafet Gebremariam


Engineering


Ontario

London, ON, Canada

[email protected]

Rajiv K Varma


Engineering


Ontario

London, ON, Canada

[email protected]

Abstract— The steady state voltage rise caused by reverse

power flows and intermittency in renewable power is the main

limiting factor for integration of distributed generators in

medium and low voltage distribution lines. With the

advancement in smart inverter technologies, the Volt-Var control

using the remaining capacity of the inverter can be used to

provide effective voltage control in distribution systems. In this

paper, the efficacy of Volt-Var control on two PV solar farms

connected to a realistic feeder in Ontario, Canada, is

demonstrated for increasing the hosting capacity up to the

thermal limit of the feeder. Implementation of such smart

inverter controls will greatly help in the integration of PV solar

systems in power grids.

Keywords— PV Solar Systems, Smart Inverters, Distributed

Generation, Reactive Power Control, Hosting Capacity.

I. INTRODUCTION

With increasing concern over global warming, the use of renewable energy is growing at a rapid rate around the world [1]. In 2013, over 140 countries agreed to UN’s framework convention on climate change to reduce carbon emission to combat global warming. This boosted the growth of renewable energy systems. Policies like priority dispatch from renewable energy sources, special feed in tariffs, quota obligations and energy tax exemptions has accelerated the growth of harvesting renewable energy sources. In different renewable sources, wind and PV showed highest growth. The installed capacity of solar PV systems has already reached 229GW [2], and that of wind power systems has increased to 486 GW [3].

A commitment was made by the Canadian Government under the Climate change plan for Canada to buy 20% of electricity from renewable energy sources[4]. This policy along with offset programs, procurement through requests for proposals, standard offer and feed-in tariff programs, and legislated renewable portfolio standards accelerated the growth of wind and PV installations in Canada. Over the last ten years, wind power has grown thirty-fold to 12239 MW in 2017, which is estimated to equal about 10 percent of total Canadian potential wind generation capacity. Similarly, solar photovoltaic (PV) has grown substantially, reaching 2206 MW in 2016 from only 10 MW in 2002. Majority of these installations are in Ontario[4].

The large-scale integration of DGs has changed the

structure of the traditional power system and led to many

operational challenges like steady state voltage rise and dips,

voltage flicker, harmonics and resonances, false tripping of

protection equipment, etc. [5], [6]. The intermittency of the

renewable sources causes steady state voltage limit violation.

This increases the number of operation of traditional voltage

control devices like On Load Tap Changers (OLTC) and

switched capacitor banks [7], [8], leading to a reduction in their

lifecycles. Thus, the main limiting factor for the integration of

the DG is the voltage limit violation caused by the reverse

power flow and intermittency in renewable power [9]. A

strategy to estimate the hosting capacity of solar PV in a radial

distribution network considering the voltage limit is proposed

in [10]. Due to the above-stated voltage limit violations, the

hosting capacity of DGs must be limited substantially below

the thermal limit of the distribution lines.

EPRI introduced the concept of smart inverters whereby

Distributed Energy Resources (DER) inverters such as PV

solar inverters can provide multiple functions of real and

reactive power control either autonomously or in response to

utility issued commands [11]. These smart inverter functions

include: volt/var control, volt/watt control, frequency/watt

control, primary frequency control, ramp rate control, etc. [11],

[12]. The grid interconnection standard IEEE 1547 is revised to

include the smart inverter features [13].

The application of volt/var control for providing voltage

support during varying DG power level is demonstrated in

[14]. A volt/var control strategy to minimize the voltage

deviation and overall power loss is demonstrated in [15]. A

review of different volt/var control techniques is presented in

[16]. From various literature [16,17], it is evident that volt/var

control can provide voltage support and can reduce the energy

losses.

With the large-scale integration of PV solar farms, there is

a possibility of having multiple PV solar farms connected to

the same feeder. Due to the steady state voltage limit, the

capacity of distribution systems to connect these solar farms,

known as hosting capacity, can be limited much below the

thermal limit of the feeder. In this paper, the effectiveness of

Proceedings of the National Power Systems Conference (NPSC) - 2018, December 14-16, NIT Tiruchirappalli, India

the application of volt/var control of two PV solar farms

connected to a realistic distribution feeder in Ontario, Canada

for increasing their hosting capacity is investigated.

The rest of the paper is organized as follows. The study

system is described in section II, and the controller is

presented in section III. The simulation results are

demonstrated in section IV and the conclusion of this work is

provided in section V.

II. STUDY SYSTEM

The single line diagram of the study system is shown in Fig. 1. A realistic 27.6 kV distribution feeder in Ontario is considered as the study system. The transformer is rated 115 kV/ 27.6 kV, 100 MVA and has 0.075 pu impedance. The PV Solar Farm 1 is connected at 30 km distance from the substation. The PV Solar Farm 2 is connected 5 km away from the PV solar farm 1. The power output of both the solar farms are varied in this study. The distribution lines are represented by their equivalent pi models. The resistances , , and inductances , , denote the resistance and inductance of distribution lines between Bus 2 and Bus 3, Bus 3 and Bus 4, and Bus 4 and Bus 5 respectively. The total load of 6 MVA at 0.9 power factor is modelled as a constant RL load and is connected at Bus 5, 40 km from the substation.

𝑻𝟏 𝑹𝑻𝑳𝟏 𝑳𝑻𝑳𝟏 𝑳𝑻𝑳𝟐 𝑳𝑻𝑳𝟑 𝑹𝑻𝑳𝟐 𝑹𝑻𝑳𝟑

Bus 1 Bus 2 Bus 3 Bus 4 Bus 5

Load

PV Solar Farm 1 PV Solar Farm 2

Distribution

Substation

Fig. 1. Single Line Diagram of the Study System

III. PV SOLAR FARM CONTROLLER

The single line diagram of a grid connected PV Solar farm

is shown in Fig. 2(a). The PV power is fed to the grid through

a Voltage Source Converter (VSC). The inductance L,

capacitance C and inductance of the ∆-Y transformer acts as

the filter for the VSC output. L and C are designed such that

the total harmonic distortion (THD) is less than 5%. The

resistance and represents the parasitic resistance of

the inductance L and capacitance C, respectively. The voltage

at the point of common coupling (PCC) is measured and

denoted as in Fig. 2. The inverter output current is

represented as .

The architecture of the PV solar farm controller is shown in

Fig. 2(b). The inverter current , and PCC voltage are

measured and fed to the controller. The controller is designed

on the d-q frame. The PCC voltage angle ϴ for the

transformation is calculated by a phase locked loop (PLL).

The , and , are transformed to the d-q frame using abc

to dq transformation [18].

The voltage vector is aligned to the d axis in this model

thus making = 0, to have decoupled control of the active

and reactive power [18]. The active power is proportional to

direct axis current and reactive power is proportional to

quadrature axis current . The DC link controller maintains

the DC link voltage at the MPPT voltage to extract the

maximum power from solar array. The DC link controller

determines the direct axis current reference ( ) to maintain

the DC link voltage at the reference. The quadrature axis

current reference ( ) is generated by the Volt -Var

controller.

A. Volt-Var Controller

The volt-var characteristic of the system shows the

relationship between the voltage and the amount of reactive

power required to maintain the voltage with in the utility

limits. A typical volt-var curve is shown in Fig. 3. The upper

part of the curve represents capacitive reactive power, and the

lower part represents inductive reactive power. The volt var

controller operates the PV solar farm at unity power factor

when the voltage is within the utility limit. The is then

generated from the as per (1). As per [13], the inverters

are oversized to provide 44% reactive power while operating

at rated capacity. The is limited such that the inverter

current limit according to [13] is not violated.

(1)

Sinusoidal Pulse Width Modulation at 10 kHz switching

frequency is used for generating the gating pulses of the VSC.

The modulation index for the switching is calculated by the

inner current control loop.

IV. SIMULATION STUDIES

Load flow studies are done initially to find out the impact

of PV integration on feeder voltage for various X/R ratios of

the distribution line. Volt-var curves are modelled according

to the results of the load flow studies with following

objectives: (i) to increase the hosting capacity while

maintaining the voltage within the steady state limit, and (ii)

inverter current limit not to be violated. The time domain

simulation studies with the developed volt-var controller is

done in MATLAB Simulink to test its effectiveness in

increasing the hosting capacity.

A. Impact of PV Generation on Feeder Voltage for various

X/R ratios of distribution feeder

The impact of PV generation on feeder voltage for various

X/R ratios are studied by keeping the X/R ratio of the

distribution lines at 2.47 and 1.


1) Ratio X/R = 2.47

The resistance , , and inductance , ,

in Fig. 1 are varied to make X/R =2.47. The studies are

performed for the following cases: (i) only PV Solar Farm 1

connected, (ii) only Solar Farm 2 is connected, and (iii) both

PV Solar Farms are connected.

Fig. 4 shows the variation of bus 3 voltage for various PV

power generation levels. The steady state voltage limits of

1.06 and 0.94 are shown by yellow and green line

respectively. Blue line shows the voltage with only PV Solar

Farm 1 connected. Orange line shows the voltage with Solar

Farm 2 connected and black one shows the voltage with both

Solar Farms connected. The PV power is increased till 45

MW.

1.06

0.94

Capacitive

Inductive

% Voltage

% A

vail

ab

le V

AR

0.5

1.5

Fig. 3. Volt - Var Curve [11]

With only PV solar farm 1 connected, the voltage at bus 3

started decreasing for power increase beyond 25 MW. The load

flow solution fails to converge for power greater than 40 MW.

With PV Solar Farm 2 connected, the voltage starts reducing

for 25 MW and load flow solution fails to converge for power

above 40 MW. With both the solar farms producing power, the

voltage at bus 3 starts reducing for both PV producing 20 MW

each and fails to converge beyond 40 MW. The voltage of the

feeder remains within the limit for a broad range of power and

the voltage falls below the lower limit of 0.94 pu for power

above 35 MW.

Fig. 4. Bus 3 Voltage for different PV power level for X/R = 2.47

2) X/R = 1

To study the impact of PV power injection on bus voltage

for a lower X/R ratio, the feeder X/R ratio is kept at 1 and

following studies are done. The studies performed are for the

𝐢𝐪𝐫𝐞𝐟

𝒊𝒒

𝑽𝒅𝒄𝟐

𝐢𝐝𝐫𝐞𝐟 𝑽𝒅𝒄_𝑴𝑷𝑷𝑻𝟐

𝐢𝐝

𝐯𝐝

𝛚𝟎𝐋

𝐕𝐝𝐜𝟐

𝑲𝑷𝟑 +𝑲𝑰𝟑𝑺

6PWM

𝛚𝟎𝐋

𝐢𝐝

𝐢𝐪 𝐯𝐝 𝐯𝐪

𝒊𝒊𝒏𝒗

𝝆

𝒗𝒑𝒄𝒄

PLL

𝐯𝐩𝐜𝐜

𝐯𝐪

min

𝟏.𝟏𝟐 − 𝒊𝒅𝟐

(b)

𝑮𝑨𝑻𝑰𝑵𝑮 𝑺𝑰𝑮𝑵𝑨𝑳𝑺

𝒊𝒊𝒏𝒗 𝒗𝒑𝒄𝒄 ∆ 𝐘

𝐋 𝐑𝐟𝟏

𝐑𝐟𝟐

𝐂

𝐏𝐂𝐂

𝐕𝐒𝐂

𝑽𝒅𝒄

𝑮𝑨𝑻𝑰𝑵𝑮 𝑺𝑰𝑮𝑵𝑨𝑳𝑺

6 𝐒𝐨𝐥𝐚𝐫 𝐏𝐚𝐧𝐞𝐥

(a)

𝑲𝑷𝟏 +𝑲𝑰𝟏𝑺

𝐊𝐏𝟒 +𝐊𝐈𝟒𝐒

abc

dq

Volt-Var

Controller

Fig. 2. Single Line Diagram of a grid connected PV solar farm with the volt var controller


following cases: (i) only PV Solar Farm 1 is connected, (ii)

only Solar Farm 2 is connected, and (iii) both are connected.

Fig. 5 shows the variation of bus 3 voltage for various PV

power generation levels. The steady state voltage limits of

1.06 and 0.94 are shown by yellow and green line

respectively. Blue line shows the voltage with only PV Solar

Farm 1 connected. Orange line shows the voltage with Solar

Farm 2 connected and black one shows the voltage with both

Solar Farms connected. The PV power is increased till 45

MW.

For all the three cases, the voltage exceeded steady state

limit of 1.06 for a very low PV power of 8 MW, which is

much lower than the thermal limit of the feeder.

Fig. 5. Bus 3 Voltage for different PV power level for X/R =1

Fig. 6. Line Current for different PV power for X/R =1

Fig. 6 shows the current flow in distribution line between

bus 2 and 3 for different power generation of PV solar farms.

Blue line shows the line current for only PV Solar Farm 1

connected. Orange line shows the line current for Solar Farm

2 connected and black one shows the line current for both

Solar Farms connected. The thermal limit of the line is 680A,

and is shown by the yellow line. The voltage limits are

violated for PV power generation of 10 MW for all the three

cases as shown in Fig. 5. The line current for the 10 MW

power is approximately 100 A as depicted in Fig. 6, which is

approximately 15% of the thermal limit of the line. This

shows that the DG integration is limited much below the

thermal limit due to the voltage limit violation.

B. Impact of PV Power on Feeder Voltage for various X/R

ratios with Volt-Var Control

The effectiveness of the volt-var control by utilizing the

remaining inverter capacity to increase the real power

generation of the PV solar farm to the thermal limit of the

feeder is presented in this section. From the studies conducted

for system with different X/R ratios, it is observed that the

voltage limits are violated for a lower PV power injection on

system with X/R =1. So the system with X/R = 1 is

considered for this study.

1) Determination of Volt-Var Curve

The volt-var curve characterisitics are determined by load

flow studies in Power world software. The load flow studies

are conducted for the PV power from zero to 45 MW. From

this study, the reactive power required to maintain the voltage

within the steady state limit is found out and the volt-var is

curve is plotted using this data. The volt-var curve for all the

three cases are shown in Fig. 7. From the volt-var curve, the

equations of are derived. For example, the equations

obtained for the case with only PV solar farm 2 is connected is

shown in (2).

(2)

Fig. 7. Volt -Var curve for X/R =1

From Fig. 5, it can be observed that the highest feeder

voltage is 1.32 pu, which occurs for 45 MW power generation


by PV Solar Farm 1. The reactive power required for this

condition is 10 MVA, which is within the limit of 44%

reactive power injection required by the grid code [13]. Thus

it shows that for this study, the volt - var control for the whole

range of PV power genration can be provided without

violating the inverter current limit.

Time domain simulation in MATLAB Simulink is done with

the obtained volt- var characteristics and the results are shown

in Fig. 8. The dotted curves show the voltage at Bus 3 without

voltage control and the thick lines show the voltage at Bus 3

with Volt – Var control. The steady state voltage limits of 1.06

and 0.94 are shown by yellow and green line respectively.

Blue line shows the voltage with only PV Solar Farm 1

connected. Orange line shows the voltage with Solar Farm 2

connected and black one shows the voltage with both Solar

Farms connected. The PV power is increased till 40 MW.

It is observed that, using the volt-var control, the PV

power generation can be increased till 40MW without

violating the voltage limit. Whereas without the volt-var

control, the DG power generation must be limited below 10

MW.

Fig. 8. Bus 3 Voltage for different PV power level with Volt Var Control for

X/R =1

The current through the line between bus 2 and bus 3 for

different PV power generation with volt-var control is shown

in Fig. 9. Blue line shows the line current for only PV Solar

Farm 1 connected. Orange line shows the line current for

Solar Farm 2 connected and black one shows the line current

for both Solar Farms connected. The thermal limit of the line

is 680A and it is shown by the yellow line.

From Fig. 9, it can be observed that the thermal limit of the

line is exceeded for PV power generation of 35 MW. The

volt-var control is able to maintain the voltage within the

steady state limit for this range as noted from Fig. 8.

This study shows that the volt-var control using the

remaining inverter capacity can maintain the voltage within

the steady state limit and thus help in increasing the DG

interconnection till the thermal limit of the feeder.

In this study, PV integration could be increased up to

100% of thermal limit using smart inverter volt-var control.

However, the percent increase of renewable integration on

different distribution systems will depend upon their specific

short circuit level, X/R ratio and thermal limits.

Fig. 9. Line Current for different PV power for X/R =1 with Volt-Var control

V. CONCLUSION

The integration of distributed generators in medium and

low voltage lines is primarily limited by steady state voltage

limit violations. Simulation studies are reported in this paper

for a realistic distribution feeder in Ontario with the smart

inverter volt-var control implemented on two PV solar farms.

In this study, the power generation of PV solar farms is

limited to 15% of the thermal capacity of the feeder due to

violation of steady state voltage limit. With the advancement

of smart inverters and the revised grid codes, the inverters are

oversized to provide 44% reactive power support even at the

rated capacity. By using the remaining inverter capacity, volt-

var control can be provided to increase the grid integration of

distributed generators. It is shown in this study, that with the

volt-var control using the remaining inverter capacity, the

renewable power generation can be increased from 17% to

100% of the feeder thermal limit without violating the steady

state voltage limits. This smart inverter control helps in

increasing the DG integration to the existing feeders without

the needing to install additional expensive voltage regulating

devices.

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