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Energy Procedia 00 (2011)12000–000 Energy Procedia (2011) 1032 – 1041

Energy Procedia www.elsevier.com/locate/procedia

ICSGCE 2011: 27–30 September 2011, Chengdu, China

Grid Support with Variable Speed Wind Energy System and Battery Storage for Power Quality Sharad W. Mohoda* , Sudesh M. Hatwara, Mohan V. Awareb b

a Prof. Ram Meghe Institute of Tech & Research, Badnera, Amravati, India Electrical Engineering Department, Visvesvaraya National Institute of Technology, Nagpur, India

Abstract The proposed wind energy conversion system with battery energy storage is used to exchange the controllable real and reactive power in the grid and to maintain the power quality norms as per International Electro-technical commission IEC-61400-21 at the point of common coupling. The generated wind power can be extracted under varying wind speed and can be stored in the batteries at low power demand hours. In this scheme, inverter control is executed with hysteresis current control mode to achieve the faster dynamic switchover for the support of grid. The combinations of battery storage with wind energy generation system, which will synthesizes the output waveform by injecting or absorbing reactive power and enable the real power flow required by the load. The system reduces the burden on the conventional source and utilizes WEGS and battery storage power under load constraints. The system provides rapid response to support the grid. The scheme can also be operated as a stand-alone system in case of grid failure like a uninterrupted power supply. The system is simulated in MATLAB and results are presented. © 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license.

Selection and/or peer-review under responsibility of University of Electronic Science and Technology of China (UESTC) Keywords: Battery Storage; Power Quality; Wind Generating System.

1. Introduction The value of wind power can be extensively increased if is capable of contributing to the grid support. The wind energy is experiencing extraordinary growth. The worldwide capacity reached to 159,213 MW out of which 38,312 MW were added during 2008-09, shows that wind capacity doubles every three years. This environmentally friendly power source will be significantly improved for its long term goal. The increasing number of renewable energy sources and distributed generator requires a new strategy for operation and management of electric grid system. Today, wind energy generating system is connected

* Corresponding author. Tel.: +917212579901. E-mail address: [email protected].

1876-6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of University of Electronic Science and Technology of China (UESTC). Open access under CC BY-NC-ND license. doi:10.1016/j.egypro.2011.10.135

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into the power system to meet the consumers demand and to support the grid. However, the output power of wind generator is fluctuating and will affect operation of interconnected grid. The utility system cannot accept the new generation without the strict condition of voltage regulation due to real power fluctuation and reactive power generation/absorption. Thus addition of wind power into the grid system, affects the power quality. However, in practice the wide use of a nonlinear load connected to power distribution system or inverter based application, causes significant power quality degradation in the grid connected system in the terms of current/voltage harmonic, power factor and resonance problem. As a result of a nonlinear nature of the load, the purity of waveform of supplies may lose. Thus the power quality issue is becoming an increasingly important to the electricity consumer at all level of usages [1]. The power quality studies are also importance to wind turbine as a individual units can be large up to 5 MW, feeding into distribution circuit with high source impedance and with customer connected in close proximity. The impact of the wind generation on the power system will no longer be negligible if high penetration levels are going to be reached. At high penetration level there is need for additional voltage management in the grid. The penetration of wind generation into the power system will increased due to the use of variable speed wind generation to accommodate the maximum power in the power system. Thus, it promotes wind generating system through battery energy storage. The battery storage provides a rapid response for either charging or discharging the battery thus it acts as a constant voltage source in the power system. The battery storage is effective when wind speed output fluctuations are high particularly at speed just below the normal operating speed [2]-[4]. In variable speed induction generator system with grid connected power converter are increasingly used. The use of power converter system is now becoming a power conditioning system, such as to adjustability of voltage and reactive power and the control of supply of active power. These technical capabilities of wind generating system can be used in recent development in power control technology [5]-[7]. Today the concept of grid regulation has been developed and realized in software simulation. Now it is a task of grid controller to match the operating behavior of decentralized generating plants with that of conventional power station like thermal plant and support the grid. The configuration for grid control is shown in Fig.1 The proposed system is efficient and economical to support the grid system. The proposed control system with energy storage has the following objectives: • Unity power factor at the point of common coupling bus (PCC). • Reactive power support from wind generator and batteries to the load. • Stand-alone operation in case of grid failure. The paper is organized as follows. Section 2 introduces the system description. Section 3 gives the system configuration for grid support. Section 4 presents the control scheme. Section 5 describes the system simulations and Section 6 draws the conclusion. 2. System Description The proposed system for power quality in grid connected wind energy generation is shown in the Fig. 1. It consists of the following main modules. 2.1. Wind energy generating system The wind generating system is connected with turbine, induction generator, and AC-DC-AC converter, interfacing transformer. The static characteristic of wind turbine can be described by the relationship between total power in the wind and mechanical power of wind turbine as in equation (1).

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3

1 P = ρΠ R 2V 3 wind 2 wind

(1)

where ρ- air density (1.225 Kg/ m3) , R is rotor radius in meters, Vwind is the wind speed in m/sec. ,CP power coefficient. This coefficient can be express as a function of tip speed ratio λ and pitch angle θ. The mechanical power can be written as (2). P = CpP mech wind

(2)

1 3 C Pmech = ρΠ R 2Vwind p 2

(3)

By using the turbine rotational speed, ω turbine mechanical-torque is shown in (4) T =P /ω mech mech turbine

(4)

The speed - power characteristic of variable speed wind turbine is given in Fig. 2. Wind turbine characteristics

Turbine

Variable speed wind Generator

Thermal Plant

12 m/s

T

1.2

D C AC

A C DC

1

T.f.

11 m/s

Plant control

P C C

Load

Plant control

Power (pu)

0.8

Battery

10 m/s 0.6 9 m/s 0.4 0.2

Grid controller

Fig. 1.Grid support system configuration.

0 500

8 m/s 6 m/s 5 m/s

7 m/s

1000 1500 Turbine speed referred to generator side (rpm)

2000

Fig. 2. Power-speed characteristic of turbine

2.2. DC link for battery storage and wind energy generator In the inverter, the capacitor is used as the intermediate elements, which decouples the wind generating system and battery storage as shown in Fig. 3 and modeled with (5). C

d V =I −I dc(rect ) dc(inv) dt dc

(5)

where C is circuit capacitance, Vdc is rectifier voltage, Idc(rect) is rectified dc-side current, Idc(inv) is inverter dc-side current. The battery storage is connected to dc link and is represented by a voltage source Eb connected in series with an internal resistance Rb. The internal voltage varies with the charge status of the battery. The terminal voltage Vdc is given in (6). V = E + I *R dc b b b

(6)

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4

where I b represent the battery current. It is necessary to keep adequate dc link level to meet the inverter voltage as in equation (7). 2 2 V ≥ V dc M inv a

(7)

where Vinv is line-to-neutral rms voltage of inverter (230Vrms), Switching frequency - 2Khz , Inverter output frequency-50Hz and Ma .is modulation index (0.9).Thus the dc link is design for 800Volt . 3. System Configuration for Grid Support The proposed grid support scheme is configured on voltage source inverter based on current control technique for switching inverter as shown in Fig. 4 V.S.Inverter SB

SC

SA' SB'

SC'

SA

〈

Transformer

Turbine

Vwind

Wind Power Generator

Idc(inv) Idc(rect) Vdc

Rb

wind generator

Ib

Rectifier & Battery storage

SA

SB

Via Ri,Li

SC

Pulse Generator

C

Eb

Vdc

V dc V

ref +

Fig. 3. DC Link for battery storage and wind generator.

-

⊗

Ref.compensator current

V'sa

Voltage source (Grid )

Load Voltage sensor & transfer switch

Standalone Ref. Generator

PI-controller

Fig.4 Grid support scheme with voltage source inverter

3.1. Voltage source inverter The modern voltage source inverters are controlled using pulse width modulation. This PWM technique allows the desirable output voltage, control of harmonic at the output of the inverter. Thus PWM is the convenient form of synthesizing the desired waveform. The power flow is controlled by on/off ratio of the respective switches and desired current waveform can be obtained. The process of modulation allows a switched representation of waveform. PWM strategy is considered that is obtained by control lawsS An =

1 [1 + F (m, ωt )] 2 1⎡ 2Π ⎤ 1 + F (m, ω t ) − 2 ⎢⎣ 3 ⎥⎦

(9)

1⎡ 4Π ⎤ 1 + F ( m, ω t ) − 2 ⎢⎣ 3 ⎥⎦

(10)

S Bn = SCn =

(8)

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5

where SAn, SBn ,SCn are the average value of the switching variables in the interval nth. F = (m,sin ωt ) , mVdc and VLL = 0.612maVdc . In the over 2 modulation the fundamental-frequency voltage magnitude does not increase proportionally with ma . In over modulation region, more side band harmonics appear centered around the frequencies of harmonic m f and its multiple. Therefore the power loss in the load due to the harmonic frequency may not be high in over modulation. In this way, when the PWM inverter is connected in the inductive load in ac side, the current wave forms become closer to sinusoidal wave form.

denotes modulation index . one of the inverter leg is Van = ma

3.2. Current control-technique The PWM technique performance depends on the feed forward control of output voltage of voltage source inverter. The output current which depends not only on voltage but it also on load to be controlled and hence feedback from current sensor must be provided. A control system compares the actual current Ia, Ib, Ic with reference current I*sa , I*sb ,I*sc and generate appropriate switching variable SA, SB, SC for individual phases of the inverter. As a result the switching variable and output voltage are pulse width modulated in such a way that the output current waveform follows the reference waveform. A successful current technique ensures • Good utilization of dc supply voltage • Low static and dynamic current control error • Sufficient time allowance for proper operation of inverter switches and control system. The current generated from current control voltage source inverter can be controlled independently from the ac voltage, the active and reactive power control is decoupled and hence unity power factor operation is possible. The use of inverter can be of distinct benefit to network operation, Voltage control, harmonic reductions are the power quality benefit that can be gain without excessive compromise [8][12]. 3.3. Grid control Power station turbine and generator are generally fitted with proportional speed regulator. The speed – power curve of individual generators can used to derive a steady state frequency power characteristic ω = f ( P ) .The grid characteristic for active power generation is mostly flatter than individual g generators. The steady state operating point in the grid is found at the intersection of load and generation characteristic at the frequency ω , where the generation and consumption are approximately same and as 0

shown in Fig.5.

sine wave Template

a) Load Characteristics

Vdc

b) Grid chracteristic c) Generation Chracteristic a

ω

b

-

VdcRef

PI

X

Ref. current generator

I*sa

X


I*sb

X


I*sc

Power

Fig. 5.Speed –power characteristic

b

Inverter

iib

_

+ Isc

iia

a

_

+ Isb

c

0

+ Isa

Controller

iic

c

_ Hysteresis controller

Fig. 6. Control circuit of CC-VSI

CCVoltage Source Inverter

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6

4. Control Scheme In the proposed grid connected system, the magnitude of a grid current is determined from reference generated from grid voltage. The battery energy storage system (BESS) is connected in parallel to the dc capacitor. The battery will naturally maintain the dc-capacitor voltage within a small operating range. The BESS is the best suited since it rapidly injects or absorbed real power to stabilize the grid system. It also controls the distribution and transmission system in a very fast rate. The control scheme approach is based on injecting the currents into the grid having ‘bang-bang controller’ using a hysteresis current controlled technique. A hysteresis current controller keeps the control systems variable between the boundaries of hysteresis area and gives rise to correct signal for inverter operation. The control system designed for generating the switching signals to the inverter is shown in Fig. 6. 4.1. Grid synchronization With a three-phase balance system, the RMS voltage source amplitude is calculated at the sampling frequency from the source phase voltage ( VSa , V , VSc ) and is expressed as sample template Vsm , Sb sampled peak voltage, as in (11). Vsm =

{(

2 2 2 Vsa + V 2 + Vsc sb 3

)}

1/ 2

(11)

The unit vectors are obtained from AC source voltage and the RMS value of unit vector usa,usb,usc are represented in (12). V V V usa = Sa , usb = Sb , usc = Sc Vsm Vsm Vsm

(12)

The in-phase generated reference currents are derived using in-phase unit voltage template as, in (13)

isa* = kusa , isb* = kusb , isc* = kusc

(13)

where, k –is proportional constant magnitude of filtered source voltage for respective phases. This ensures that the grid current is controlled to be sinusoidal. This method is simple, robust and favorable as compared with other methods. 4.2. Current controller The bang-bang current controller is to control the current in the inverter. The reference current is generated as in equation (13) and actual current are detected by current sensors and are subtracted for obtaining a current error for a hysteresis based bang-bang controller. Thus the ON/OFF switching signals for IGBT of inverter are derived from hysteresis controller The switching function S A for phase ‘a’ is expressed as (14). * isa < (isa − HB) → S A = 0 ,

* isa > (isa − HB ) → S A = 1

(14)

where - HB is a current band of the controller. Similarly the switching function SB , SC can be derived for phases ‘b’ and ‘c’ respectively [13]-[15].

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7

5. Simulation of System The proposed control scheme is simulated using MATLAB/SIMULINK in power system block set. The system parameter for given system is given in Table I. Table 1. System Parameters 3-phase, 415Volt, 50 Hz.

Source Voltage Source ,line Inductance Wind Generator Induction Generator DC Link Parameter

0.5mH 150 kW, 415V, 50Hz,P =4, Ave.wind velocity-5m/s DC Link-800V, C= 5μF.

Rectifier-Three Arm Bridge Type Inverter-IGBT device, Three Arm Bridge Type Battery Parameters

Snubber R=100Ω, Ron=0.01Ω, C = 100μF. Rated-1200V, For.Current-50A , Gate Voltage-+/-20V.T-On delay-70ns, T-Off delay400ns ,Power dissipation-300W DC-800Volt.

Interfacing Transformer

415V/800V, 50 Hz,

Load Parameter

3-phase,415V,Non-linearLoad R=10Ω, C = 1µF

5.1. Steady state performance of the system A non linear load is considered for the simulation of this system. These nonlinear loads in the system will affect and disturb the grid current waveform. To have the grid current in distortion free, the correct amount of current must be injected to cancel out this distortion effect. The performance of the system is observed by operating the controller for the power quality improvement for these loads. The inverter is switched ‘on’ at 0.2 sec .The grid/source current Is, inverter injected current Iinv , load current IL are measured with and without controller operation. The current supplied from the source is made sinusoidal, harmonics-free as soon as controller is in operation and is shown in Fig. 7 (a). The injected current supplied from the inverter is shown in Fig.7 (b). The load current in the system is shown in Fig.7 (c).During this interval the load current will be the addition of source current and inverter current. The PCC voltage is shown in Fig.7.(d) sourceV_I (a)

50 0 -50 -100 0.16

0.17

0.18

0.19

0.2

0.21

0.22

0.23

0.24

50

0.25

(c)

50

(b)

0

0 -50

controller-OFF -50 0.16

0.17

0.18

0.19

0.2 0.21 Time

0.22

0.23

0.24

-100 0.16

0.25

0.17

0.18

0.19

0.2 0.21 Time

0.22

sourceV_I

500

Voltage Voltage

current (A)

_

100

Power Quality Mode

c urrent(A )

current(A)

100

0

-500 0.16

0.17

0.18

0.19

0.2

Time(s)

0.21

UPS-ON 0.22

0.23

0.24

0.25

Fig. 7. (a) Grid current (b) Inverter injected current (c) Load current d) PCC Voltage

0.23

0.24

0.25


8

5.2. DC Link The wind energy generator is operated to generate the power and supplied to rectifier to interface in the dc link. The dc link voltage is shown in Fig. 8(a). To transfer the real power from wind generator into the load, the generated power is fed to rectifier for charging the batteries. The battery maintains a constant dc terminal voltage while supplying current on demand. It is propose to decouple the energy storage from the inverter by a separately dc-dc converter. The current through the dc link is shown in Fig.8. (b). V -dc (V )

850

(a)

800 750

0

0.1

0.2

0.3

0.4

0.5

0.6

Fig. 8. a) DC link voltage b) Current through dc link

5.3. Unity power factor.

V oltage(V ) & Current(A )

The source current is maintained in phase with the source voltage indicating the unity power factor at point of common coupling and satisfies power quality norm. The result of in-phase source current and source voltage are shown in Fig. 9.This is due to the reference derived from the grid voltage. 400

Voltage

200 0 current

-200 -400 0.16

controller-OFF 0.17

0.18

0.19

100

controller-ON 0.2 0.21 Time(s)

0.22

0.23

0.24

0.25

Fig. 9. Source current and Source voltage at PCC

5.4. Power quality at PCC The current waveform before and after the inverter operation is analyze for power quality measure. The Fourier analysis of the waveform is expressed without the inverter- in the system and the THD of the source current signal is shown in Fig.10 (a) and the measured THD and its harmonics order is shown in Fig.10(b). FFT window: 1 of 50 cycles of selected signal

sourcecurrent(A)

100

(a)

50 0 -50 -100 0.1

0.102

M ag(%of Fundam ental)

0.104

0.106

0.108

0.11 0.112 Time (s)

0.114

0.116

0.118

0.12

Fundamental (50Hz) = 64.96 , THD= 22.91%

100

(b)

80 60 40 20 0 -2

0

2

4 6 Harmonic order

8

10

Fig. 10. a) Source current b) FFT of source current (before)

12

1039

1040


The power quality improvement is observed at point of common coupling, when the inverter and controller is in ON condition. The inverter is placed in the operation and source current waveform gets modified which is shown in Fig.11 (a) with its FFT in Fig.11 (b). It is shown that the THD has been improved considerably and it is within the standard norms. FFT window: 1 of 23.86 cycles of selected signal

source current (A)

50

(a) 0

-50 0.4

0.402

0.404

0.406

0.408

0.41 0.412 Time (s)

0.414

0.416

0.418

0.42

Fundamental (50Hz) = 44.28 , THD= 1.29%

Mag (% of Fundamental)

100

(b)

80 60 40 20 0 -2

0

2

4 6 Harmonic order

8

10

12

Fig. 11. a) Source Current b) FFT of source current (after)

5.5. Performance under stand-alone mode It is observed that the system is operating in power quality mode up to 0.6 sec. The dynamic performance of the system is monitor by operating the circuit breaker at 0.6 sec. Under such condition system performs as a stand-alone mode. The voltage sensor senses the condition and transfers the switches to generate the reference current in stand-alone reference generator. During this mode the inverter will support the critical load in absence of grid failure. Due to the unavailability of source, the inverter will supply the full load current in this duration. The grid current, load current and inverter current in stand-alone mode is shown in Fig.12. sourceV_I

current (A)

50

(a) 0 Stand-alone -Mode -50 0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

current (A)

100 0 -100 0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

100 Current (A)

0.8 (b)

0.8 (c)

0 -100 0.4

0.45

0.5

0.55

0.6 Time

0.65

0.7

0.75

0.8

Fig. 12. Grid Current b) Load Current c) Inverter- injected current.

6. Conclusion The scheme of variable speed type wind energy conversion scheme with batteries energy storage in grid interfaced is presented. The system has an interface of inverter in current controlled mode for exchange of real and reactive power support to non-linear load. The scheme utilizes power electronic switching device approach. The hysteresis current controller is used to generate the switching signal for inverter in such a way that it will cancel the harmonic current in the system. The control scheme is

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developed in view of the power quality improvement for grid system. The scheme maintains unity power factor and also harmonic free grid current at the point of common connection in distributed network. The exchange of wind power is regulated across the dc bus having energy storage and is made available under the steady state condition. This also allows the real power flow during the instantaneous demand of the load. Thus the power quality can be significantly enhanced through the use of inverter interfaced in grid system. The suggested control system is suited for rapid injection or absorption of reactive/real power flow in power system. The battery energy storage provides rapid response and enhances the performance under the fluctuation of wind turbine output and improves the voltage stability of the system. This scheme is providing a choice to select the most economical real power for the load amongst the available wind– battery-conventional resources. The system always operates within power quality norms also provides stand-alone mode and support the grid. References [1] Dusan Graovac,Vladimir A.Katic,Alfred Rufer , “Power quality problems compensation with universal power quality conditioning system,” IEEE Trans. on Power Delivery, vol. 22, no. 2, pp. 968-97, April 2007. [2] S.W.Mohod, M.V.Aware,“ Power quality issues & its mitigation technique in wind generation, ”Proc. of IEEE Int. Conf.on Harmonics and Quality of Power (ICHQP) , pp. 1-6, Sept. 2008. [3] Z. Chen, E. Spooner ,“Grid power quality with variable speed wind turbines”, IEEE Trans. on Energy Conversion, vol.16, no. 2, pp. 148-154, 2008. [4] J. M. Carrasco, “Power electronic system for grid integration of renewable energy source: a survey,” IEEE Trans. on Industrial Electronics, vol. 53, no. 4, pp. 1002-1014, 2006. [5] J.W.Smith and D.L.Brooks, “Voltage impacts of distributed wind generation on rural distribution feeders.” in Proc. IEEE.PES Transmission Distribution Conf. Exhib.,vol.1, pp. 492-497,Oct.28-2001. [6] A.Kehrli and M.Ross, “Understanding grid integration issues at a wind farm and solutions using voltage source converter FACT technology.” Proc. IEEE PES Gen.Meeting, vol. 3, pp.1822-1827, July 13-2003. [7] Sercen Teleke, Mesut E.Baran, Alex Q. Huang , Subhashish Bhattacharya, Loren Anderson, “Control strategy for battery energy storage for wind farms dispatching” IEEE Trans. On Energy Conv., vol. 24 , no. 3, pp. 725-731 , Sept.2009, [8] Chong Han, Alex Q.Huang, Mesut Baran, Wayne Litzenberger, Loren Anderson, “STATCOM impact study on the integration of a large wind farm into a weak loop power system.”, IEEE Trans. On Energy Conversion, vol. 23, no. 1, pp. 226-232, March 2008. [9] S. W. Mohod and M. V. Aware, “A STATCOM control scheme for grid connected wind energy system for power quality improvement,” IEEE, System Journal,vol.2,issue 3,pp.346-352,Sept.2010. [10] Bhim Singh ,P.Jayprakash,D.P.Kothari, “A T-Connected transformer and three-leg VSC–based DSTATCOM for power Quality improvement”, IEEE Trans.on Power Electronic,vol.29, no. 6, pp.2710-2716, Nov.2008. [11] Z.Yang,C.Shen,L.Zhang,M.L.Crow, “Integration of STATCOM and battery energy storage.”, IEEE Trans.on Power System,vol.16, no. 2, pp. 254-262, May 2001. [12] Carl Ngai-Man Ho,Victor S.P.Cheung,Henry Shu-Hung, “Constant-frequency hysteresis current control of grid-connected VSI without bandwidth control”,IEEE Trans.on Power Electronics,vol.24, no.11, pp. 2484-2494, Nov.2009 [13] Zeng,J.,Yu. C,Qi,Q.,Yan,Z., “A novel hysteresis current control for active power filter with constant frequency” , Elect.Power Syst. Res., vol. 68, pp. 75-82,2004. [14] Fu.Sheng Pai, Shyh-Iier Hung, “Design and operation of power converter for microturbine powered distributed generator with capacity expansion capability.”IEEE Trans. on Energy Conversion, vol. 3, no.1, pp.110-116, March 2008. [15] S.W.Mohod, M.V.Aware, “Battery Energy Storage to Strengthen the Wind Generator in Integrated Power System” Journal of Electronic Science & Technology.—Vol.9,No1,March 2011.

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