IMPROVED EQUIVALENCING TECHNIQUE FOR SHORT CIRCUIT CONTRIBUTION OF WIND FARMS IN LARGE POWER SYSTEMS

The Eighth Asia-Pacific Conference on Wind Engineering, December 10 14, 2013, Chennai, India IMPROVED EQUIVALENCING TECHNIQUE FOR SHORT CIRCUIT CONTRIBUTION OF WIND FARMS IN LARGE POWER SYSTEMS Aditya Patil 1, Chandrasekhar Reddy Atla 2, Dr. Balaraman K 3 1 Research student at Power Research & Development Consultants Pvt Ltd., India. aditya@prdcinfotech.com 2 Research Student at Power Research & Development Consultants Pvt Ltd., India. sekhar.atla@gmail.com 3 CGM at Power Research & Development Consultants Pvt Ltd., India. balaraman@prdcinfotech.com ABSTRACT The utilization of wind power- a major renewable energy source is expected to constantly increase during the next few years. Interaction between the Wind Farms (WFs) and the grid needs to be investigated for different power system studies. A number of issues related to the influence of large wind power penetration into the grid are evacuation feasibility; transient stability and the ensuing short-circuit levels. This paper focuses on the short circuit contribution from wind farms to the grid when integrated to large power systems. To determine the short circuit level of a large wind farm, it is necessary to utilize the equivalent model to minimize the modeling complications. The simulations presented in the literature show that the fault current contribution from equivalent model has large variation as compared to individual turbine models. Hence this paper focuses on improvements of equivalent technique for short circuit studies. Case studies are carried out on different wind farms to determine the fault current contribution. Keywords: wind farm equivalencing, short circuit contribution of wind power plant, Fault Ride Through, wind power generation Introduction Electrical networks are normally designed such that maximum short-circuit current never exceed the rating of the switchgear and thermal, mechanical endurance of the equipment. WF s are generally concentrated in high wind potential locations which mostly fall in remote windy areas which are typically equipped with weak electrical networks. The short circuit levels at the point of common coupling (PCC) will change with the integration of wind farms into the grid. It should be noticed that wind farms mainly affect short circuit level of medium level voltages which are under the DIStribution COMpanies (DISCOMs). Although during high windy seasons, the short circuit level of the higher voltage level may also be affected. Effect of wind power integration was not considered earlier as the penetration level was less in comparison with the grid capacity. Earlier countries like Denmark, Spain, Netherlands, Ireland, Portugal, Germany etc., had penetration level lesser than 10%. With rapid increase in wind turbine technology and awareness about harnessing the renewable energy, wind power penetration has reached around 15-20% in all of the above nations according to World Wind Energy Report (WWEA) 2010. The wind penetration in India according to [India Wind Energy Outlook 2012] is around 12% and increasing. Such high penetrations cannot be neglected as it can have implications in the total system operation and stability. Proc. of the 8th Asia-Pacific Conference on Wind Engineering Nagesh R. Iyer, Prem Krishna, S. Selvi Rajan and P. Harikrishna (eds) Copyright c 2013 APCWE-VIII. All rights reserved. Published by Research Publishing, Singapore. ISBN: 978-981-07-8011-1 doi:10.3850/978-981-07-8012-8 285 1269

In India the wind turbines are equipped with Induction Generators (IGs). These induction generators do not have voltage control facility. Hence to provide voltage control facility in these machines during the system transients, arrangements such as mechanically switched capacitor banks area adopted. However, in order to improve dynamic performance the use of STATCOMs SVCs etc., are being suggested. These arrangements will also assist the wind turbines to keep connected to the grid during the faults and under voltage condition. This phenomenon is known as Fault Ride Through (FRT) of the wind turbines. The FRT requirement of the wind turbines depend on the fault current and the critical clearing time for the worst case fault scenario. The induction generators in wind farms are Squirrel Cage Induction Generator (SCIG), Wound Rotor Induction Generator (WRIG) and Doubly Fed Induction Generator (DFIG). During the fault for the first few cycles (approximately 2-3 cycles), DFIG s behave as IGs and consequently modeled as IG as described by Morren J & de Haan S W H. During this operation the power electronic converters (rotor and stator side) are blocked by the crow bar protection or current limitation function of rotor side converter and the transients of rotor side current starts decaying. The short-circuit behavior of IGs is strongly dependent on their characteristics. Some approximate equations to determine the maximum short-circuit current for induction generators has been shown by Boutsika T. et al. This paper presents the case studies on comparison of fault contribution in detailed model and equivalent model of wind farms. The paper focuses on improving the equivalencing technique such that the fault current contribution for equivalent technique is nearer to detailed model. Four different wind farms connected to the grid are considered for case studies. SCIGs, WRIGs and DFIG wind turbines are considered for short circuit studies. The type IV wind turbine with Full End Converter (FEC) is not analyzed as it is completely decoupled with the grid through the converters. Both the detailed and equivalent models are developed in MiPower TM simulation software with [MiPower user manual]. Three phase to ground fault is considered for the analysis as this fault yields the maximum fault current. Modeling of IGs for short circuit level calculations This type of wind turbines consist of IGs which are directly coupled to the electrical grid. The equivalent circuit of the squirrel cage induction generator has been modeled for the short circuit analysis (Divya K C. et al. in [June 2006]). For WRIG equivalent circuit an external resistance R ex is added in series with rotor resistance. DFIG can be modeled as IG during the fault application (Morren J & de Haan S W H). The SCIG operation can also simulated using the fourth order model expressed in a reference rotating at synchronous speed w s as explained by Ong C M in Prentice Hall publications in 1998 and Krause P C., McGraw Hill, 1986. Calculation of fault contribution from the wind farm The fault contribution from the wind farm is calculated similar to any power system fault contribution. Figure. 1 shows an example on equivalent representation of wind farm connected to a grid. 1270

Fig. 1 Equivalent representation of wind farm The equivalent wind generator model, pad mounted transformer model and equivalent collector system model are represented in Figure. 1 This entire wind farm model is connected to 34.5/230 kv nearest pooling substation. The existing wind farm equivalencing is carried out referring to Equivalencing collector system of a large wind power plant by Muljadi E. et. al. The grid equivalent is modeled at 230kV with 3phase to ground and single line to ground short circuit fault levels. Now, to determine the fault contribution from the wind farm, it is necessary to calculate the contribution from bus 2 to bus 3. This is done by conducting operations on Z bus matrix. Assume three phase to ground fault occurs at bus p. The performance equation during the fault is given as.1 The is the unknown voltage vector,.2 is the known voltage vector prior to the fault condition. current vector during the fault bus (say fault bus p) is the unknown bus 3 and is the three phase bus impedance matrix...4 1271

The three phase voltage vector at the faulted bus p, is given as,...5 Combining Eq 1 and Eq 5 we can write as..6 Solving the above equation 6 for yields, 7 Now to calculate fault current contribution from the wind farm, it is necessary to calculate contribution through that branch or element (referring fig 1, the element connected between 2 and 3 needs to be considered). The fault current through the branch/element is represented, in terms of voltage across the element as shown in Eq 8...8 Where is the fault current contribution from the i-j branch, is the admittance of the i-j branch obtained from admittance matrix, is the faulted bus voltage and is the bus voltage of the branch impedance. The fault contribution from the equivalent wind farm can be known by the Eq. 8 and the admittance of the branch is known as old. The expected fault contribution if the detailed wind farm is represented can be calculated internally by utilizing all the data entered for equivalencing. For this new fault current new will be determined. The difference between the new and old will give the correction factor z. This correction factor will aid in manipulating the old. Hence the correction factor will be included in the equivalent impedance of the pad mounted transformer. With inclusion of this correction factor z, there will be a appropriate variation in the Z bus elements in the short circuit path of the wind farm. Hence fault current nearer to the detailed model will be witnessed. Case studies In this section three phase to ground fault study is carried out on the LV bus of 34.5/230 kv pooling substation. Four different wind farms are considered for the analysis. Each wind farm is modeled as detailed representation and equivalent representation. The calculation of correction factor as explained in the earlier section is carried out and an appropriate change in the equivalent impedance is done. Simulations are carried out on detailed models, existing equivalent model and proposed equivalent model. The comparision of the results are shown in Table 4. All simulations are conducted in MiPower TM simulation software. Details of different cases considered for the studies are as follows: 1272

Case 1: Wind farm 1 consists of 20 wind turbines. The machine details are shown in Table 1 and network configuration is given in [Final project report on WECC Wind Generator Development ]. All the other wind farms are having similar machine details with different wind farm network layout. *All values are in pu on machine MVA rating. Table 1: Wind farm machine details Description Induction Generator details Nominal voltage [kv] 0.575 kv Rated power 2.5 MW Rated MVA 2.84 MVA Efficiency at nominal operation 97% Acceleration constant 1 sec Number of pole pairs 2 Stator resistance Rs [pu] 0.04066 Magnetic reactance Xm [pu] 2.2942 Stator reactance Xs [pu] 0.066733 Rotor resistance Rr [pu] 0.00433 Rotor reactance Xr [pu] 0.0418667 Power factor 0.88 Case 2: Wind farm 2 consists of 24 wind turbines and machine details are shown in Table 1 Case 3: Wind farm 3 consists of 28 wind turbines and machine details are shown in Table 1 Case 4: Wind farm 4 consists of 37 wind turbines and machine details are shown in Table 1 Table 2 represents the pad mounted transformer details and the short circuit level details of the grid connection. Table 2: Transformer and grid short circuit level details Transformer MVA 3 MVA Primary/Secondary voltage 0.575kV / 34.5 kv Impedance Z in pu 6.25 % Grid short circuit level at 220kV 3 phase fault level 1500 MVA Single line to ground fault level 940 MVA The wind farm internal network is at 34.5kV and the details of the line are shown in Table 3 Table 3: Wind farm internal network details Line details Voltage level R in ohms X in ohms B/2 in mho Dog Conductor 34.5 kv 0.162 0.383 1.51e-006 1273

A three phase to ground fault simulation study is analyzed upon assuming fault occurrence at the LV side of the main pooling substation transformer. The fault contribution for detailed wind farm model, existing equivalent wind farm model and proposed equivalent wind farm model are shown in Table 4. Table 4: Comparison of results of detailed wind farm model, existing equivalent wind farm model and proposed equivalent wind farm model No. of wind turbines and wind farm capacity Case No. 1 Wind farm- 1 (20 turbines= 50MW) 2 Wind farm- 2 (24 turbines=60 MW) 3 Wind farm- 3 (28 turbines=70) 4 Wind farm- 4 (37 turbines=92.5 MW) Fault current contribution in Amperes with detailed wind farm model Fault current contribution in Amperes with equivalent wind farm model Fault current contribution in Amperes equivalent wind farm model with proposed correction factor 4658.84 5217.31 4964.7 5241.04 4976.19 5097.2 6190.3 5856.3 6000.2 7373.9 6259.96 6706.41 The results show that the current contributions from the equivalent model vary when compared to the detailed model. These results may mislead the system operators during integration of a large wind farm into the grid. The above variations can be reduced by introducing the correction factor z in the equivalent wind farm model. This proposed technique will act in the Z bus matrix of the equivalent wind farm and reduce the variation of fault contribution. Conclusions Simulation studies have been carried out with the existing equivalencing technique and proposed equivalencing technique for short circuit study. The proposed equivalencing technique presented better results compared to existing equivalencing technique. The results in this paper are encouraging and show that the fault current contribution from proposed equivalent wind farm model are nearer to detailed wind farm model. References Boutsika T., Papathanassiou S., and Drossos N., Calculation of the fault level contribution of distributed generation according to IEC standard 60909, presented at the CIGRE DG Symp., Athens, Greece, Apr. 2005 Divya K C. and Nagendra Rao P S., Models for wind turbine generating systems and their application in load flow studies, ELSEIVER,Electric Power Systems Research, vol 76, Issues9-10, June 2006, Pages 844-856. Final project report on WECC Wind Generator Development Prepared for CIEE by: National Renewable Energy Laboratory, pp 53. 1274

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