intermittency mitigation and power quality …intermittency mitigation and power quality improvement...
TRANSCRIPT
ISBN: 97-8-93-81195-82-6 Proceedings of NJCIET 2015
Canara Engineering College, Mangalore NJCIET-2015 433
Intermittency Mitigation and Power Quality Improvement of Grid Integrated with
Distributed Energy Resources
Prasanna
1, Shivalinga Swamy
2,
1P.G Scholar, Department of EEE, AMC Engineering College,Bangalore, India,
Email: [email protected] 2Assistant Professor, Department of EEE, AMC Engineering College,Bangalore, India,
Email: [email protected]
Abstract.Renewable energy generation is growing fast and ideas such as smart grid are trying to change the role of a consumer
from being a passive consumer to an active contributor who can supply stored excess power in various DERs, such as solar,
wind, hydro, Fuel cells and Storage technologies back to the distribution grid or the micro -grid. Increasing amounts of
distributed energy resources located on the customer side of the electric grid represent both a challenge and an opportunity for
grid operators. There is a corresponding increase in power quality problems and intermittencies on the dist ribution grid. In
order to reduce the intermittencies and improve the power quality of the distribution grid, an Ultracapacitor integrated powe r
conditioner is proposed in this paper. The simulation of Ultracapacitor supported Unified Power Quality Conditi oner system,
which consists of grid-tied inverter and DC/DC converter is performed using MATLAB/ Simulink.
Keywords: The Unified Power Quality Conditioner (UPQC) Active power filter (APF); DC/DC converter; d –q Control;
dynamic voltage restorer (DVR); Ultracapacitors(UCAP).
1 Introduction
The benefits associated with distributed energy resources (DERs) installations are often significant and numerous. DERs can be
defined as the concept of connecting generating units of small sizes, between several kW to a few MW. The primary source of
energy for these generators can be the renewable sources such as wind, solar, hydro, and biomass. These generators are
connected either to the medium voltage or low voltage sections of the electric grid [5]. Most often th ey are connected near the
load centers or the low voltage networks. They almost always provide tangible economic benefits, such as energy savings or
transmission and distribution upgrade deferrals, as well as intangible benefits, such as power quality impr ovements that lengthen
maintenance or repair intervals for power equipment.With the increase in the penetration of the distribution energy resources
(DERs), there is a corresponding increase in the power quality problems and intermittencies on the distribution grid in the
secondsto minutetime scale [1]. As more sensitive loads, such as computers, automation equipments, communication equipments
and manufacturing processes have come into wide use, the power quality is a big issue in both customer and utility company.
Since these equipments are very sensitive for the input voltage disturbances, the inadequate operation or the fault of these loads
brings about huge losses [3]. Also the insufficient power quality can be caused by failures and switching operations in the
network (voltage dips and transients) and by network disturbances from loads (flickers, harmonics and phase imbalance). The
elimination or mitigation of disturbances propagated from the source side and the other loads interconnected is critical for
improving the operational reliability of these critical loads. There have been increased focus in the recent years on the concept of
mitigating intermittent output of distributed energy resources (DERs) using Unified Power Quality Conditioner (UPQC) with
energy storage devices.The Unified Power Quality Conditioner (UPQC) is a distribution level controller which is to compensate
both source voltage sag and load current imperfections in power distribution system. Recently the Energy Storage System (ESS)
is widely applied in power electronics devices [11]. Despite the variety of the energy storage devices, the Ultracapacitor (UCAP)
or so-called electrochemical capacitor is one of the most popular choices for the energy storage system because of its distinct
characters. The reason behind overview of ultracapacitor energy storages system is that ultracapacitors charge very rapidly and it
can charge fully within few seconds. It hasextremely low internal resistance or effective series resistance and consequently high
efficiency (95% or more), with extremely low heating levels.It provides fast access to the stored energy and has very high rates
of charge and discharge.Ultracapacitors have wide operating temperature range (from -45°C to +55°C) and fast transient
response (~20 micro seconds).Ultracapacitors can be charged up to their full capacity and discharged all the down to a zero -
charge state.
ISBN: 97-8-93-81195-82-6 Proceedings of NJCIET 2015
Canara Engineering College, Mangalore NJCIET-2015 434
The research work presented here aims at analyzing the technical issues and solutions of connecting distributed energy reso urces
to a grid/distribution system. The UPQC can compensate the reactive power, harmonic current, voltage sag and swell, voltage
unbalance, and the voltage interruption. The elimination or mitigation of disturbances propagated from the source side and th e
other loads interconnected is critical for improving the operational reliability of these critical loads. The performance of
proposed system was verified through model built in MATLAB/Simulink.
2.System Description
The system has two inverter and an ultracapacitor integrated DC/DC converter connected to distribution grid integrated with
DERs. Two inverters acts as series dynamic voltage restorer (DVR) and shunt active power filter (APF) respectively. DVR
prevents sensitive loads from experiencing voltage sags/swells, and APF prevents the grid from supplying non sinusoidal
currents when the load is nonlinear [1]. The design goal of the traditional UPQC was limited to improve the power quality of the
distribution grid by being able to provide sag, swell, and harmonic current compensation. In this paper, energy storage
integration into the power conditioner topology is being proposed, which will allow the integrated system to provide addition al
functionality.
Most DERs are connected to the grid through a three-phase grid-tied inverter which can be controlled to meet various control
objectives such as supplying active power, reactive power, harmonic compensation, and frequency regulation etc. A three phase
shunt active power filter (APF) can be controlled i n such a way that it meets all the same control objectives. Furthermore, a
three-phase grid-tied inverter is similar in topology to a three-phase shunt APF which opens up the possibility of exploring the
vast amount of literature available for shunt APFs and modifying the control strategies to suit the present control objectives of
the grid-tied inverter.Integration of active power filter (APF) with UCAP energy storage which gives the integrated APF -UCAP
system active power capability which aids in providing renewable intermittency smoothing, active and reactive power support to
the distribution grid. This cannot be achieved by a conventional APF with dc-link capacitor which does not have active power
capability.
Fig.1.One line diagram of grid integrated DERs system.
2.1Three Phase Inverters
The one-line diagram of the system is shown in Fig. 1. The power stage consists of two back-to-back three-phase voltage source
inverters connected through a DC-link capacitor. UCAP energy storage is connected to the dc-link capacitor through a
bidirectional dc–dc converter. The series inverter is responsible for compensating the voltage sags and swells; and the shunt
inverter is responsible for active/reactive power support and renewable intermittency smoothing. The complete circuit diagram
of the series DVR, shunt APF, and the bidirectional dc–dc converter is shown in Fig. 2. Both the inverter systems consist of
ISBN: 97-8-93-81195-82-6 Proceedings of NJCIET 2015
Canara Engineering College, Mangalore NJCIET-2015 435
IGBT module, its gate-driver, LC filter, and an isolation transformer. The dc-link voltage Vdc is regulated at 260V for optimum
voltage and current compensation of the converter and the line–line voltage Vab is 208V. The goal of this project is to provide
the integrated power conditioner and UCAP system with active power capability 1) to compensate temporary voltage sag (0.1–
0.9p.u.) and swell (1.1–1.2p.u.), which last from 3s to 1min and to provide active/reactive support and renewable intermittency
smoothing, which is in the seconds to minutes time scale.
Fig. 2. Model of power conditioner with UCAP energy storage
Fig. 3. Proposed MATLAB Simulink model.
2.2 Bidirectional Dc–Dc Converter and Ultracapacitor
A bidirectional DC/DC converter is required as an interface between the UCAP and the dc-link, since the UCAP voltage varies with the amount of energy discharged, while the dc-link voltage has to be stiff. The dc–dc converter should operate in Discharge mode, while providing active/reactive power support and voltage sag compensation. The dc–dc converter should also be able to operate in bidirectional mode to be able to charge or absorb additional power from the grid during intermittency smoothing. In this
ISBN: 97-8-93-81195-82-6 Proceedings of NJCIET 2015
Canara Engineering College, Mangalore NJCIET-2015 436
–neu
=
1
0
√
√
of th
−
e s
)
olta
−
e
)
−
)
−
)
√
(1)
a
=
(
−
c
) in(
−
) )
+
) )
= 3 (
) (
)
= 3 (
) (
)
the s
ase d nce
3
nverte
bet
r c
we
(4)
=
1
0
√
ver
√
r co
cos −s
(5)
by eries i an be computed using (3) from the rms values of injected voltage Vinj2aand load current ILa and ϕ is the ph iffere en the two waveforms.
paper, the bidirectional dc–dc converter acts as a boost converter, while discharging power from the UCAP and acts as a buck converter while charging the UCAP from the grid.
Ultracapacitors can deliver very high power in a short time span; they have higher power density and lowe r energy density when
compared with Li-ion batteries [9]. The major advantage UCAPs have over batteries is their power density characteristics, high
number of charge–discharge cycles over their lifetime, and higher terminal voltage per module. These are id eal characteristics
for providing active/reactive power support and intermittency smoothing to the distribution grid on a short -term basis. In [6], it is
proposed that UCAPs are currently viable as short-term energy storage for bridging power in kilowatt range in the seconds to
few minutes timescale. The choice of the number of UCAPs necessary for providing grid support depends on the amount of
support needed, terminal voltage of the UCAP, dc-link voltage, and distribution grid voltages. For a 260V dc-link voltage, it is
practical and cost effective to use three modules in the UCAP bank.
3 Controller Implementation
The series inverter controller implementation is based on the in-phase compensation method that requires PLL for estimating θ,
and this has been implemented using the fictitious power method described in [4]. Based on the estimated θ and the line –line
source, voltages Vab, Vbc, Vca (which are available for this delta -sourced system) are transformed into the d–q domain and the
line tral components ource v g Vs , Vsb, and Vsc which are not available can then be estimated using equations
⎡ ⎤ cos( sin( ⎢ ⎥ ⎢ ⎥ −sin os( ⎣ ⎦ √
⎡ s . ⎤
m* ⎢(sin( − ⎥
. ⎢ ⎥
(2)
⎣(sin( − . ⎦
(3)
These voltages are normalized to unit sine waves using line–neutral system voltage of 120 Vrms as reference andcompared with unit sine waves in-phase with actual system voltages Vs from (2) to find the injected voltage references Vref necessary to
maintain a constant voltage at the load terminals, where m is the modulation index, which is 0.45 for this case. Therefore, whenever there is a voltage sag or swell on the source side, a corresponding voltage Vinj2 is injected in-phase by the DVR and
UCAP system to negate the effect and retain a constant voltage VLat the load end. The actual active and reactive power supplied
= −
= − 2
⎡ ⎤ ⎢ ⎥ sin
⎢ ⎥ in cos
⎣ ⎦
The shunt in te ntroller implementation is based on the ‘id−iq’ method, which is modified to provide active and reactive power compensation, such that id controls the reactive power and iq controls the active power. Therefore, b ased on the references for active and reactive powers Pref and Qref, the reference currents iqref and idref in d–q domain can be calculated using
(4), where vsq is the system voltage in q-domain and the reference currents are calculated using (5)
ISBN: 97-8-93-81195-82-6 Proceedings of NJCIET 2015
Canara Engineering College, Mangalore NJCIET-2015 437
4 Modes of Operation
Average current mode controlis used to regulate the output voltage of the bidirectional dc –dc converter in both Buck and Boost modes while charging and discharging the UCAP bank. This method tends to be more stable when compared with other methods like voltage mode control and peak current mode control. Average current mode control is used to regulate the output voltage of the bidirectional dc–dc converter in both Buckand Boost modes, while charging and discharging the UCAP bank.
While the UCAP-APF system is discharging power, the dc-link voltage Vout tends to be less than Vref , which causes the
reference current Iucref to be positive, thereby operating the dc–dc converter in Boost mode. Along similar lines, when the UCAP-
APF system is absorbing power from the grid, the dc-link voltage Vout tends to be greater than Vref , which causes the reference
current Iucref to be negative and thereby operating the dc–dc converter in Buck mode. Average current mode control technique
was found as the ideal method for UCAP-APF integration as it tends to be more stable when compared with other methods like voltage mode control and peak current mode control. This is a major advantage in the present topology, where the stability of the dc–dc converter has to be ensured over a wide operating range and in both Buck and Boost modes of operation.
The higher level integrated controller is designed to make system level decisions on the inverter and DC/DC converter controllers. Based on various system parameters like Pload, Qload, Pgrid, Qgrid, Vucap, Vdc, Idclnk, and Iucap, the higher level integrated
controller will decide on operating in one of the following modes: active power support mode, reactivepower support mode, renewable intermittency smoothingmode, sag/swell compensation mode, and UCAP chargemode.
In active power support mode and renewable intermittency smoothing mode, the UCAP-UPQC system must provide activepower to the grid. Therefore, the active power capabilityof the UCAP-UPQC system must be assessed by the higherlevel integrated controller. Based on the Pgridand Ploadvalues, the reference Prefis calculated in the higher levelintegrated controller, and it will
decide if the UCAP hasenough energy to respond to the Prefcommand based on theUCAP state of charge. If the UCAP has
enough capacity to respond to the request, then the dc–dc converter controller is operated in grid support mode; otherwise, it is operated in charge mode, where the UCAP is recharged and the power request is met at a later time. In grid support mode, the dc–dc converter will operate in a bidirectional fashion in both Buck and Boost modes to respond to the active power requests and regulate the dc-link voltage in a stable fashion, while the inverter controller should respond such that the commanded Prefis
supplied by the inverter through current control.
In reactive power support mode, the UCAP-UPQC system must provide reactive power to the grid. In this mode, the UCAP-
UPQC does not provide any active power to the grid and even the UPQC losses are supplied by the grid. Based on the Qgridand
Qloadvalues, the reference Qrefis calculated in the higher level integrated controller. In this mode, the dc–dc converter controller
can be programmed to operate in grid support mode directly because the active power requirement for operating in this mode is minimal. Therefore, the goal of the dc–dc converter controller is to regulate the dc-link voltage in a stable fashion, while the
inverter controller should respond such that the commanded Qrefis supplied by the inverter through current control.In sag/swell
compensation mode, the UCAP-PC system is programmed to prevent sensitive loads from disturbances on the supply-side like voltage sag or voltage swell. These disturbances require short-term energy storage, and in this mode, the dc–dc converter controller can be programmed to operate in grid support mode. Therefore, the goal of the dc–dc converter controller is to regulate the dc-link voltage in a stable fashion during both sag/swell events. It is also required that the dc–dc converter be able to discharge and meet the active power requirements during a voltage sag and to be able absorb active power in a stable fashion during a voltage swell event.
In charge mode, the UCAP is recharged by absorbing active power from the grid when the UCAP state of charge falls below
50%. The rate at which the UCAP can be charged is assessed by the higher level integrated controller based on the Pgridand
Ploadvalues and the reference Prefis calculated. Then the dc–dc converter controller is commanded to operate in charge mode,
wherein the dc–dc converter will operate in Buck Mode to absorb the power from the grid and the inverter controller must respond to supply commanded Pref..
ISBN: 97-8-93-81195-82-6 Proceedings of NJCIET 2015
Canara Engineering College, Mangalore NJCIET-2015 438
5 Simulation Result An extensive simulation study is carried out using MATLAB/Simulink in order to verify the proposed control strategy. The
simulation of the proposed UCAP integrated power conditioner system is carried out in Simulink for a 208V, 50Hz system,
where 208V is 1p.u. The system response for a three-phase voltage sag which lasts for 0.1s and has a depth of 0.64p.u is shown
Fig. 4 (a) Source and load rms voltages Vsrms and VLrms during sag.
(b) Source voltages Vsab (blue), Vsbc (red), and Vsca (green) during sag.
in Fig. 4(a-d). During voltage sag, the source voltage Vsrms is reduced to 0.36p.u, whiletheload voltage VLrms is maintained
constant at around 1.01p.u due to voltages injected in-phase by the series inverter. This can also be observed from the plots of
the line–line source voltages (Vsab, Vsbc, and Vsca) [Fig. 4(b)], the line–line load voltages (VLab, VLbc, and VLca) [Fig. 4(c)],
and the line–neutral injected voltages of the series inverter (Vinj2a, Vinj2b, and Vinj2c) [Fig. 4(d)].
Fig 4. (c) Injected voltages Vinj2a blue), Vinj2b (red), and Vinj2c (green) during sag.
(d) Load voltages VLab (blue), VLbc (red), and VLca (green) during sag.
ISBN: 97-8-93-81195-82-6 Proceedings of NJCIET 2015
Canara Engineering College, Mangalore NJCIET-2015 439
Fig. 5. (a) Currents and voltages of dc–dc converter.
In Fig 5(a), the plots of the bidirectional dc–dc converter are presented, and it can be observed that the dc-link voltage Vfdc is
regulated at 260V, the average dc link current Idclnkav and the average UCAP current Iucav increase to provide the active power
required by the load during the sag. This can also be observed from various active power plots shown in Fig 5(b), where the
power supplied to the load Pload remains constant even during the voltage sag when the grid power Pgrid is decreasing. The
active power deficit of the grid is met by the DVR power Pdvr, which is almost equal to the input power to the inverter Pdc in
available from the UCAP. Therefore, it can be concluded from the plots that the active power deficit between the grid and load
during the voltage sag event is being met by the UCAP-based energy storage system through bidirectional dc–dc converter and
the inverter. It can also be noticed that the grid reactive power Qgrid reduces during the voltage sag while Q dvr increases to
compensate for the reactive power loss in the system. Similar analysis can also be carried out for voltage sags that occur in one
of the phases (A, B, or C) or in two of the phases (AB, BC, or CA); however, three -phase voltage sag case requires the
maximum active power support and is presented here.
Fig.5 (b) Active and reactive power of grid, load, and inverter during voltage sag
ISBN: 97-8-93-81195-82-6 Proceedings of NJCIET 2015
Canara Engineering College, Mangalore NJCIET-2015 440
6Conclusion
In this paper, the concept of integrating UCAP-based rechargeable energy storage to a power conditioner system to improve the
power quality of the distribution grid is presented. With this integration, the DVR portion of the power conditioner will be able
to independently compensate voltage sags and swells and the APF portion of the power conditioner will be able to provide
active/reactive power support and renewable intermittency smoothing to the distribution grid. UCAP integration through a
bidirectional dc–dc converter at the dc-link of the power conditioner is proposed. The control strategy of the series inverter
(DVR) is based on in phase compensation and the control strategy of the shunt inverter (APF) is based on ‘id−iq’ method.
Designs of major components in the power stage of the bidirectional dc–dc converter are discussed. Average current mode
control is used to regulate the output voltage of the dc–dc converter due to its inherently stable characteristic. A higher level
integrated controller that takes decisions based on the system parameters provides inputs to the inverters and dc –dc converter
controllers to carry out their control actions. The simulation of the integrated UCAP-UPQC system which consists of the UCAP,
bidirectional dc–dc converter, and the series and shunt inverters is carried out using MATLAB/Simulink. Results from
simulation successfully mitigate intermittency due to DERs and UPQC compensate for both volt age sag/swell and reactive
power, thereby verifying the concepts introduced in this paper. Similarly UCAP based energy storages can be deployed in the
future in a microgrid or a low-voltage distribution grid to respond to dynamic changes in the voltage profiles and power profiles
on the distribution grid.
References
1. Deepak Somayajula and Mariesa L. Crow “An Ultracapacitor Integrated Power Conditioner for Reducing Intermittencies and Improv ing
Power Quality of Distribution Grid” IEEE Transactions on Sustainable Energy.
2. Srivastava, A. K., Kumar, A. A., & Schulz, N. N. (2012). Impact of distributed generations with energy storage devices on the electric grid.
Retrieved from http://krex.ksu.edu
3. B. Han, H. Lee and J. Lee, “Unified Power Quality Conditioner with Electric Double Layer Capacitor, Myongji University.
4. A. K., Kumar, A. A., & Schulz, N. N. (2012). Impact of distributed generations with energy storage devices on the electric gr id. Retrieved
from http://krex.ksu.edu. 5.Robert Lasseter, Abbas Akhil, Chris Marnay and John Stephens, “Integration of Distributed Energy Resources: The CERTS Micro Grid
Concept,” Energy Systems Integration Program Public Interest Energy Research California Energy Commission.
6. Kuldeep Sahay,Bharti Dwivedi “Supercapacitors Energy Storage System For Power Quality Improvement: An Overview” JES 2009 on -line
journal.esrgroups.org/jes 7. K. Purchala, R. Belmans, KULeuven and L. Exarchakos “Distributed generation and the grid integratio n issues”.
8. Paulo F. Ribeiro and Mariesa L. Crow “Energy Storage Systems for Advanced Power Applications” proceedings of the ieee, vol. 8 9, no. 12,
december 2001
9. Li, Xiaomeng, "Ultracapacitor character analysis and its application in unified power q uality conditioner as energy storage system" (2010). Doctoral Dissertations. Paper 1905.
10. W. P. Poore, T. K. Stovall and B. J. Kirby “Connecting Distributed Energy Resources to the Grid: Their Benefits to the DE R
Owner/Customer” OAK RIDGE NATIONAL LABORATORY Oak Ridge, Tennessee. http://www.osti.gov/bridge.
11. Marian Gaiceanu,”Power Conditioning System Topology For Grid Integration Of Wind And Fuel Cell Energy” The Annals Of Duna rea De
Jos University Of Galati. FASCICLE III, 2006 12. DNV GL Energy, Energy Advisory,” A Review of Distributed Energy Resources” www.dnvgl.com
13. International Energy Agency Demand Side Management Programme,”Integration of Demand Side Management, Distributed Generati on,
Renewable Energy Sources and Energy Storages”
14. Ken Darrow and Bruce Hedman,” The Role of Distributed Generation in Power Quality and Reliability” New York State Energy Research
and Development Authority,17 Columbia Circle,Albany, New York.
15. Ignacio.J.Perez1Arriaga,”Managing large scale penetration of intermittent renewables” 2011 MITEI SYMPOSIUM Framework paper
16. MATLAB Simulink SimPowerSystem, Available at: http://www.mathworks.com/products/simpower/