DYNAMIC DISTURBANCE OF WIND FARM ... - IEEE Xplore

Proceedings of the 1st International Nuclear and Renewable Energy Conference (INREC10), Amman, Jordan, March 21-24, 2010

DYNAMIC DISTURBANCE OF WIND FARM CONNECTED TO 14 BUS NETWORK A. M. Amin

Erhab B. Youssef

M.M.A. Mahfouz

Helwan University Helwan University Department of Electrical Power Department of Electrical Power P.O. Box 11792,Helwan,Egypt P.O. Box 11792,Helwan,Egypt [email protected]

[email protected] [email protected]

ABSTRACT Induction generators are the most popular in wind energy conversion system due to its simplicity and absence of synchronization problem. However the major drawback of this machine is its additional reactive burden on the electric network. This paper studies the voltage stability of wind farms (WFs) Connected to IEEE 14 bus network during system disturbances such as a load change, three phase fault or island condition. Those disturbances have a great impact on the system voltages and frequency. A static synchronous compensator (STATCOM) is proposed to maintain voltage and frequency within grid codes requirements. The proposed system shows an excellent performance it maintains the voltage and frequency constants by injection controllable reactive power during system disturbances. MATLAB/SIMULINK and the Power System Analysis Toolbox (PSAT) package are used for the simulation. 1.

Helwan University Department of Electrical Power P.O. Box 11792,Helwan,Egypt

INTRODUCTION

Wind turbine technology h a s u n d e r g o n e a revolution during the last century. The attention has continued to grow as the demands on reducing polluting emissions have increased. With the development of wind turbine technology, large scale wind farms at hundreds MW rated capacity are being developed in many countries. These modern wind farms are usually connected to the power grid. The wind power penetration levels in the networks could b e h i g h , f o r e x a m p l e , average w i n d power penetration levels of 20-30 % with peak penetration level up to 100%. Which will effectively reduce the requirement on the fossil fuel based conventional power generation; however, it also presents many challenges to modern power systems. The issues, such as power system operation and control, system stability and power quality, need to be addressed in order to realize power quality for the power systems integrating large scale wind power [3, 4, 7, 10].Technical constraints of power generation integration in a power system may in general be associated with the thermal limit, frequency and Voltage control and stability. Grid codes are set up to specify the relevant requirements [8]; these specifications have to be met in order to integrate wind turbines into the grid. The predominant generator employed in WF ’ s i s t h e asynchronous type, which requires reactive power for its excitation. Whilst asynchronous generators have particular advantages for wind

turbine generator (WTG) applications, t h e i r demand for reactive power can b e p r o b l e m a t i c . To achieve continuous voltage regulation under varying system conditions such as under fault conditions, this demand increases substantially, contributing to voltage instability and possible voltage Collapse [5],[6], by compensating capacitors located at the site of the WF's itself, however during fault conditions additional reactive power resources are needed. Flexible ac transmission systems (FACTS) devices improve the transmission o f electric power and include a family of shun and series devices. static synchronous compensators (STATCOMs) can provide t h e reactive power required for the regulation voltage .STATCOMs have not yet been widely used in distribution networks due to their cost, however when used in this context they can assist WFs to continue to provide active power during fault conditions for some time. STATCOMs are shunt devices that use voltage-sourced converter (VSC) technology, which allows them to regulate the voltage at the bus to which they’re connected by either generating or absorbing reactive power [6]. 2.

SYSTEM MODELING

Figure 1(a), shows the one-line diagram for the IEEE 14 bus[11] the interconnected system with one additional wind farm connected at bus 8 that is PV bus which consist of 50units of 2MW,690V ,60 Hz squirrel cage induction generator wind energy conversion system which serves local load and STATCOM connected in parallel to wind farm. Figure 1(b) ,shows the direct connection of squirrel cage induction generator to grid through transformer and transmission line, STATCOM connected in parallel to wind energy conversion systems at load bus voltage. The data used in simulation f o r wind farms and STATCOM described in table below, 100MVA is selected for base power and the frequency i s 60Hz for interconnected power system. Table 1.Simulation Data Turbine D a t a Shaft stiffness Rated wind speed Turbine rotor speed range Rotor diameter Gear ratio STATCOM Data

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2.5pu/rad 12m/s 9.52rpm 75m 1:86.5

Proceedings of the 1st International Nuclear annd Renewable Energy Conference (INREC10), Amman, Jordan, March M 21-24, 2010

Rated Power Rated voltage Rated frequency Maximum current Minimum current Gain of the voltage control Time constant of voltage control Generator Data Rated power Rated voltage Rated frequency Stator resistance Stator reactance Rotor resistance Rotor reactance Mutual reactance Generator rotor inertia Number of poles pairs 3.

500MVA 6690V 660Hz 1.1pu -1.0 pu 50 0.2s 22MW 6690 V 660Hz 0..048 pu 0..075 pu 0.018 pu 0.120 pu 3.8 pu 00.5 s 2

STATCOM MODEL

A simplified representation of the STATCOM, including a DC side capacitor, an inverter, and series inductancce X in the three lines connecting to the transmission line is shown in Figure 2. This inductance accounts for the leakage oof the actual power transformers. The circuit also includdes resistance RC in shunt with the capacitor to represent the sswitching losses in the inverter, and resistance R in series w with the AC lines to represent the inverter and transformer coonduction losses. The inverter block in the circuit is treated as an ideal, lossless power transformer .It operates in two modees inductive and capacitive mo d e will depend upon the volttage level of the power system. If the voltage is V1 higheer than V2, the STATCOM will absorb reactive power; if the ssystem voltage is V1 lower than V2, it will generate react ive power. For balanced three phase system STATCOM cann be described in steady-state, wh e r ea s the DC circuit is ddescribed by the following differential equation, in terms of thhe voltage V on the capacitor [12].

dV P Vdc R( P 2 + Q 2 ) = − − dt CVdc CRdc CVdcV 2

(1)

4 bus network. Figure 1.a. The IEEE 14

Figure 1.b. Wind farm conn nection to bus 8.

The active and reactive power injections into thhe ac system are:

P = V 2G − GVKVdc cos(θ − α ) − KVdccVB sin(θ − α ) (2)

Q = −V B + BVKVdc cos(θ − α ) − KVdccVG sin(θ − α ) 2

(3) √

Where K = and m is modulation index=√ and α is √2 / phase angle of inverter voltage. Of primary inteerest is controller for transmission line ASVCs. In this case, k is a constant factor, and the only available control input is the anglee α of the inverter voltage vector [1] .This type of controller is useed in paper where k=0.9 constant and α is varying within range thhe maximum and minimum value to get minimum and maximum m current in two modes.. The block diagram of control of voltagge for STATCOM is shown in Figure 3.

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S circuit Figure 2.Simpified circuit of STATCOM


Figure 4.a. Voltage terminal at wind farm bus

Figure 3. Block diagram of STATCOM controller. 4.

SIMULATION STUDIES

Simulation studies for the proposed system were carried out d u r i n g d i f f e r e n t system disturbances occurrences with and without STATCOM using MATLAB/SIMULIN and the Power System Analysis Toolbox (PSAT) package. 4.1. Sudden Load change The dynamic performance of self excited induction generator with STATCOM connected in parallel with load bus is examined under disturbance and results are depicts in figures.4-7. 4.1.l. load increase: sudden increase in load demand occurs at (t=3 s to t =4 s) by 40% as shown in figure 4(a)simulated generator terminal voltage magnitude drops from 1 pu to 0.98 pu due to reactive power increased to apply load demand but using fixed capacitor as source for reactive power voltage cannot remain constant. F i g u r e 4(b) reactive powers transferred to wind farm increased to support load demand, STATCOM used for this reason where in this case STATCOM generates reactive power required to maintain voltage stable as shown in figure 4(c). A transient drop in the kinetic energy, speed and active power of the IG in order to provide the sudden increase in generator output power are represented in figure 4(d) and (e). After This transient, the increase in load power was balanced by the grid, and generator speed returned to its original value. The active power exported to the grid was decreased, whereas the local bus load increased.

Figure 4.b. Reactive power at bus 8

Figure 4. c. Reactive power generated by STATCOM

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Figure 4. d. Stator frequency at wind farm bus

Figure 5. a. Voltage terminal at wind farm bus.

Figure 5.b. Reactive power generated by STATCOM

Figure 4.e. Active power at wind farm bus Figure 4. Grid performance for wind farm during increasing load condition. 4.2 Three Phase Short Circuit Faults Figure 1(b), Shows three-phase short-circuit faults that occurs at 3second and cleared after 100 ms. The voltage at the wind turbine drops during the fault period, which leads to the reduction in rotor acceleration and the active power transmitted to grid is zero .The results with and without the STATCOM in operation are presented in figure 5(a),(c) and(d) as the generator terminal voltage cannot recovered after the fault and the generator will be tripped by the overspeed protection .the STATCOM has effectively restored the generator terminal voltage and the system will restore normal operation. Figure 5(b), Shows the reactive power amount that was generated by STATCOM d u rin g fault and increasing time of voltage to recover its normal operation as fast as possible it can be seen that the STATCOM control is an effective.

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Figure 5.c. Stator frequency at wind farm bus

Proceedings of the 1st International Nuclear annd Renewable Energy Conference (INREC10), Amman, Jordan, March M 21-24, 2010

Figure 6.b. Reactive power geenerated by STATCOM

farm bus Figure 5.d. Active power at wind fa Figure 4. Grid performance for wind farm duriing three faults at bus 8 condition. 4.3 Island P e r f o r ma n c e During island condition, self-excitation will occur when the I G l o s e s i t s c o n n e c t i o n with t h e g r i d . Unstable voltage and frequency will be observed during self- exxcitation [2], [5], and [9]. In the simulated event, the wind d farm lost its connection to the grid at t = 2.5 s and remained in islanding mode thereafter .Figure 7(d) illusstrates that the active power of IG exported to grid dropp ped rapidly from 0.4 to 0 pu. After the grid-disconnection trannsient .As showed in figure 6 (a) generator voltage terminal coollapse due loss reactive power and this time the load demannd change due to voltage collapse. To continue wind farm serving load demand at island condition STATCOM uused as shown in figure (b), the load bus voltage magnitude wa s fixed at steady state. Rotor speed of IG accelerates in islannd mode due to reduction of active power exported to grrid that causes difference between frequency of IG and grrid. Due to stall turbine that lead to active stall control to keep speed constant and it will be take be covered in futture work in this case wind farm will trip and isolated from sysstem .

Figure 6.c. Stator frequen ncy at wind bus.

Figure 6.d. Active powerr at wind farm bus. 5.

CONCLUSIONS

Disturbances normally occur in powerr system networks such as load change, faults and island mod de in case of wind farm connection to the grid. These h a v e great impacts on wind farms performance during direct co onnection of wind energy conversion system with network.

Figure 6.a. Voltage terminal at windd farm bus

s that the proposed The previous results above show system ,using STATCOM can coverr the variations of voltage and frequency during network dissturbances and keep its

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values within the grid code. STATCOM work a s a reactive power controller has been successfully designed to regulate the load bus voltage during disturbances by injection reactive power demand to compensate for the voltage drop. STATCOM has a great effect on voltage stability during disturbance .Constant frequency will be studied in future work to serves load at island case with constant voltage and frequency in future work with artificial intelligent control techniques. 6.

REFERENCES

[1] C. D. Schauder and H. Mehta, "Vector analysis and control Of advanced static VAR compensators ", Proc. Inst. Elect Eng.–C, vol. 140, no. 4, pp. 299–306, Jul. 1993. [2] Z. Saad-Saoud, M. L. Lisboa, J. B. Ekanayake, N. Jenkins, and G. Strbac,"Application of STATCOMs to wind farms,"Proc. IEE–Gener. Transm . Distrib. vol. 145, no. pp. 511–516, Sep. 1998. [3] DEFU Committee reports 111-E (2nd edition): Connection of wind turbines to low and medium voltage networks1998. [4] IEC 61400-21: Power quality requirements For wind whines (2001). [5] W. L. Chen and Y. Y. Hsu, "A novel active voltage and frequency regulator to improve grid-disconnection transients of self-excited induction generator system ," in Proc.5th IEEE Int. Conf. Power Electron. Drive Syst.,2003, vol. 2, pp. 1586–159. [6] Genevieve Coath and Majid Al-Dabbagh" The Influence of STATCOMs on Islanded Weak Power Systems Embedded Wind Farms ". In Proc. IEEE Int. Conf. Power Electron. Drive system 2003. [7] English version of Technical Regulations TF 3.2.6, "Wind turbines connected to grids with voltage below100kV Technical regulations for the properties and the control wind turbines ", Eltra and Kraft systems, 2004. [8] Z. Chen, Senior Member, IEEE," Issues of Connecting Wind Farms into Power Systems," in 2005 IEEE/PES Transmission and Distribution Conference & Exhibition: Asia and Pacific Dalian, China. [9] Woei-Luen Chen, Member, IEEE, and Yuan-Yih Hsu, Senior Member,IEEE,"Controller Design for an Induction Generator Driven by a Variable- Speed Wind Turbine,"inIEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 21, NO. 3, SEPTEMBER 2006. [10] IEC 61400-12: Wind turbine generator systems. Power performance measurement techniques. [11] http://www.power.uwaterloo.ca/~fmilano/. [12] F. Milano, Documentation for PSAT, volume 1.3.3.

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