Surge Transfer Study for Power Transformer using …

1 Surge Transfer Study for Power Transformer using emtdc / pscad Veerabrahmam Bathini, Chandra Shekhar Reddy Atla, Dr. K. Balaraman and K. Parthasarathy 1 Abstract- The lightning and switching surges can be transferred from one voltage level to another through Transformer couplings. A distribution system, which may not be directly exposed to the overvoltages of atmospheric origin, but connected to a utility system through a Transformer of high turns ratio will be exposed to overvoltages on the secondary side due to overvoltages on the primary windings. The resulting stresses on the distribution system may exceed the BIL levels. This paper presents modeling of high frequency autotransformer and frequency dependent Surge arrester models and results of simulations for lightning and switching surges transferred through 502 MVA, 380/132 kV autotransformer using emtdc / pscad . Surge arresters are usually provided on the high voltage side and low voltage side of the autotransformers.

2 The purpose of the present paper is to analyze the surges transferred towards tertiary of autotransformer. If these surges are to be controlled to safe levels it may be necessary to provide the Surge arresters at tertiary side also. This aspect has been highlighted in present paper. I. INTRODUCTION The most common primary distribution voltage in industrial systems is kV. However, for large Power demands, the utility system voltage may be as high as 380/400 kV. The Surge Transfer through the transformers depends upon the voltage turn ratio, as well as electrostatic and electromagnetic couplings of the windings. The lightning and steep fronted waves are partially transferred through the electromagnetic coupling, which is the mechanism that governs the Transformer operation at Power frequencies and depends upon the turn s ratio. The magnitude of these surges transferred through electromagnetic coupling is far less than the magnitude of surges transferred through electrostatic coupling hence electrostatic effects dominate the coupling of transients from the primary to the secondary windings.

3 For slower switching surges, the electromagnetic coupling effect predominates [1]. The overvoltages caused by Transfer of lightning and steep fronted waves or switching surges are compared with BIL of the equipments on low voltage side. In case the magnitude of transferred overvoltages exceed the BIL levels, mitigation techniques like provision of properly rated Surge arresters (SA), Surge capacitors etc., have to be 1 Veerabrahmam Bathini, Sr. Power system Engineer, is with M/s Power research and development consultants Pvt. Ltd, Bangalore, India. Chandra Shekhar Reddy Atla, Power system Engineer, is with M/s Power research and development consultants Pvt. Ltd, Bangalore, India. Dr. K Balaraman, CGM, Power System Group, is with M/s PRDC Pvt. Ltd., Bangalore. (e;mail: Prof. K Parthasarathy, Retired Professor from IISc, Bangalore.)

4 Employed to control these overvoltages. This paper concentrates on mitigation technique provided by Surge arrester. The selection of an appropriate Surge arrester is an important consideration. System overvoltages under normal and faulted conditions, system grounding and ground fault clearance times should be considered in selecting a Surge arrester. The selection procedure is as follows [2] [7]. Arrester rated voltage (Vn): selected based on maximum temporary overvoltages (TOV) appearing in the Power network, considering earth fault factor. Maximum continuous operating voltage (MCOV): selected based on the maximum system steady state operating voltage. Energy Capability: selected based on switching and lightning overvoltage studies. This paper presents the modeling of high frequency autotransformer and frequency dependent Surge arrester to conduct Surge Transfer studies for 502 MVA, 380/132 kV autotransformer using emtdc / pscad .

5 Considering a worst case scenario for simulation, the lightning impulse or switching impulse injected currents at high voltage (HV) and low voltage (LV) terminals of the autotransformer are selected based on the V;I characteristics of corresponding Surge arresters. The modeling methodologies, data considered for case Study and simulation results are presented in following sections. II. MODELING This section presents the modeling details of 502 MVA, 380/132 kV autotransformer and Surge arresters. A. Autotransformer Model The parameter specifications of 502 MVA, 380/132 kV autotransformer provided by manufacturer are presented in Table 1. TABLE 1 AUTOTRANSFORMER PARAMETERS Parameter Value 1 Rated capacity 502 MVA 2 Rated voltages (High/medium/low) 380/132 kV 3 Lightning BIL (High/medium/low) 1300/650/95 kV 4 Switching BIL (High/medium/low) 1050/650/95 kV 5 frequency 60 Hz 6 Type of system grounding HV Solidly LV Solidly TV Effectively 7 Common neutral (autotransformers) Solidly 8 Short circuit Impedances : %Z (on 500 ZHL= 16th NATIONAL Power SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010548 Department of Electrical Engineering, Univ.)

6 College of Engg., Osmania University, Hyderabad, , INDIA. Parameter ValueMVA base) ZHT= Terminal to ground capacitances and terminal to terminal capacitances CHG CLG CTG CHL CHT CLT = In pscad / emtdc , the built in model for autotransformer with tertiary is not available. modeling implementation is described in this phase three winding transformers are used tothree phase autotransformer with tertiary as shown in FigThe leakage impedances for three single phase three winding transformers can be determined from the leakage impedances of the autotransformer by using the procedure Appendix [3]. The input data required for this model can be extracted from the data provided in Table 1. To represent high frequency model for the autotransformercapacitances mentioned in Table 1 are connected as shown in Fig. 2. Fig. 1: Autotransformer model in pscad / emtdc . B. Frequency Dependent Surge Arrester Modeling Surge arrester dynamic characteristics are significant for studies involving lightning and other fast transient surges.

7 The time to crest for surges used in lightning studies can range from s to several Hs. For a given current magnitude in an arrester, the voltage developed across the arrester can increase by approximately 6% as time to crest of current is decreased from 8 s to s. One approach for an arrester model for lightning studies would be to use a simple noncharacteristics based on s discharge voltage. This would give conservative results (higher voltages) for surges with slower time to crest. The frequency dependent model will give good results for current surges with times to crest from to 40 s [4]. The Surge arrester model proposed by Pinceti [derived from IEEE model [4] is used in the present paper for #2#3#1#2#3#1#2#3#1 HVaHVbHVcValue = = = = pF = pF = pF = 1889 pF = model for three phase is not available. Hence the modeling implementation is described in this section. Three used to represent as shown in The leakage impedances for three single phase three winding transformers can be determined from the leakage impedances procedure described in The input data required for this model can be the data provided in Table 1.]

8 To represent high autotransformer, terminal connected as shown in model in pscad / emtdc . Modeling Surge arrester dynamic characteristics are significant for studies involving lightning and other fast transient surges. The time to crest for surges used in lightning studies can range to several Hs. For a given current magnitude in an arrester, the voltage developed across the arrester can increase by approximately 6% as time to crest of current is decreased . One approach for an arrester model for would be to use a simple non;linear V#I discharge voltage. This would give conservative results (higher voltages) for surges with slower time to crest. The frequency dependent model will give o crest from s model proposed by Pinceti [5] [4] is used in the present paper for performing Surge Transfer Study . The Surge arrester model is presented in Fig. 3. Fig. 2: High frequency Autotransformer model Fig. 3: Frequency dependent Surge arrester model proposed by Pinceti This model is composed by two sections of nonresistance usually designated by separated by inductance L1 and L0.

9 TM') is added to avoid the numerical L0 are computed based on the procedure described in The computation procedure is described in flow chart shown in Fig. 4. Vn is arrester rated voltage (voltage (kV) for the discharge current of impulse, Vr1/T2 is the residual voltagecurrent 10 kA, 1/T2 Hs steep front can vary between 2 and 20 s. The nonlinear resistorsA1 can be modeled as a piecewise linear characteristic of A1 arrester is selected from manufacturer datasheet and V#I characteristic of A0 is selected based on curves proposed by IEEE [4] which are shown in Fig. 4: Flowchart to calculate elements [uF] [uF] [uF]1889e-6 [uF]K=Vr1/T2/Vr8/20 K< 1/ 28/2018/201/ 28/2008 TrnrrTrrVVLVVVVLVnV = =performing Surge Transfer Study . The Surge arrester model is equency Autotransformer model : Frequency dependent Surge arrester model proposed by Pinceti [5]. This model is composed by two sections of non;linear resistance usually designated by A0 and A1 which are.

10 The resistance R (about 1 added to avoid the numerical problems. The values L1 computed based on the procedure described in [5]. described in flow chart shown is arrester rated voltage (kV), Vr8/20 is the residual the discharge current of 10 kA, 8/20 s is the residual voltage (kV) for the discharge impulse. The fall time T2 he nonlinear resistors A0 and can be modeled as a piecewise linear V#I curves. V#I selected from manufacturer data is selected based on curves which are shown in Fig. 5. Fig. 4: Flowchart to calculate elements L0 and L1 [5]. [uF]1889e-6 [uF] [uF]Data Sheet Vr1/T2L1= Vn L0= Vn 16th NATIONAL Power SYSTEMS CONFERENCE, 15th-17th DECEMBER, 2010549 Department of Electrical Engineering, Univ. College of Engg., Osmania University, Hyderabad, , INDIA. The V#I characteristic of A0 and value of L1 in the model have to be properly adjusted to match the manufacturer s data with respect to switching and lighting characteristics.)