6 Exercises The following five computer exercises accompany the course. Alternative Transients Program (ATP-EMTP) will be used to compute electrical transients. First electrical network should be created using built-in objects (visible by right mouse click on the drawing are as context menu!) that correspond to ATP components of the graphical preprocessor ATPDraw. Before executing ATP the ATPDraw file should to be saved (*.ADP). Make File command of ATPDraw generates ATP input data file *.ATP. Run ATP initiates ATP simulation and input data file will be passed to ATP in the command line. After the first execution of any data case, it is recommended to view ATP output file (*.LIS) immediately to check whether or not any error occurred or a warning message exists. ATP creates in case of error free execution a binary plot file *.PL4 that contains selected plot variables. The plot variables can be viewed and printed using graphical program PlotXY. Power System Transients (Kizilcay) 6-1
EXERCISE 1 Simple Series RLC Resonant Circuit to get familiar with ATPDraw, ATP and PlotXY; comparison of results with analytical expression; influence of time step on simulation results. Following resonant circuit will be constructed using ATPDraw. Voltage source: e(t) = 50 V for t $ 0 R = 2 S, L = 1 mh, C = 100 :F Use type-11 step function as voltage source. R, L and C can be represented in ATPDraw either by a RLC component or separately by connecting a resistor, inductor and capacitor in series. 1. Determine an appropriate time step )t for the ATP simulation taking into consideration the natural frequency of the above circuit. 2. Test the influence of the time step )t on simulation results (current flowing in the circuit) by setting )t = 0,5 ms, 0,1 ms, 10 :s und 1:s. 3. Replace the voltage source with step function by a cosine function (AC type-14 source). Amplitude of source voltage: 50 V with f = 50 Hz. 4. Insert a time controlled switch between source and resistance. Open the switch after t = 7 ms. Power System Transients (Kizilcay) 6-2
EXERCISE 2 Interruption of Small Inductive Currents to get familiar with numerical oscillations due to interruption of inductive currents; overvoltages due to inductive current chopping; simplified modelling of 30-MVA shunt reactors in the 500-kV system. Following circuit will be built using ATPDraw. The R and L values representing the reactor on the right side are to be calculated from the data: S r = 30 MVA, U r = 525 kv and X/R = 50. The source network on the left side is represented by R s = 2.5 S, X s = 25 S (L s = 66.3 mh) and C s = 1.0 :F. Specify the AC voltage source by type-14 source in ATPDraw: v(t) = V m cos (Tt + 2) V m = 2 500 kv 3 (peak value), f = 50 Hz 1. Observe voltage at node N3, when switch opens after t = 5 ms with a current margin of 0 A. 2. Add a stray capacitance of C = 1nF to the terminal N3 of the reactor. Now the current will be chopped with 2 A during opening action. Observe voltage waveforms at nodes N2 and N3. Is there any other method to damp the oscillations? Apply and test it. 3. Rerun the case with current chopped at 4 A and observe voltage waveforms at nodes N2 and N3. Power System Transients (Kizilcay) 6-3
EXERCISE 3 Three-Phase Source Network Representation to get familiar with multi-phase coupled RL elements; how to represent a source network with different positive- and zero-sequence short-circuit impedances; steady-state (phasor) solution of ATP-EMTP; transient simulation line-to-ground fault; numerical oscillations due to capacitor short-circuiting. The three-phase circuit given below will be modelled using ATPDraw. The three-phase coupled short-circuit impedance will be represented using type-51,52,53 mutually coupled RL elements (Section IV.C, ATP Rule Book), which corresponds in ATPDraw to the element Line lumped < RL Sym. 51 < 3 ph. Data of the 400-kV source network: V nq = 400 kv (nominal voltage) S kq = 15 GVA (short-circuit power) R 1Q / X 1Q = 0.1 (ratio of s.c. resistance to s.c. reactance for positive-seq. system) X 0Q / X 1Q = 3.3 (ratio of zero-sequence to positive-sequence s.c. reactance) R 0Q / R 1Q = 3.0 (ration of zero-sequence to positive-sequence s.c. resistance) 11. UnQ 1. Using the formula Z given in IEC 909 calculate short-circuit impedance Z 1Q of 1Q = " S kq 2 the positive sequence system. From the given impedance ratios short circuit resistances and Power System Transients (Kizilcay) 6-4
reactances for positive and zero sequence system can be calculated. These values can be entered directly into the ATPDraw object Line lumped < RL Sym. 51 < 3 ph. Create the three-phase circuit using ATPDraw. 2. The three-phase voltage source is represented in ATPDraw using Sources < AC 3-ph type 14. The peak value V m of the cosinusoidal voltage and phase angle are to be specified. Calculate V m from V nq. Enter 0/ for the angle of phase A voltage. 2. Run ATP and perform steady-state computation for the line-to-ground short-circuit current and check its value. T start of the AC type-14 source must be set equal to a negative value to obtain a phasor solution. 3. Initiate short-circuit by closing the switch after t = 6.7 ms in phase A. Examine voltage rise in the sound phases B and C, when phase A is short-circuited. Study the waveform of fault current. 4. Add line-to-ground capacitances of 1 :F at each phase as shown above. Rerun the case and observe waveforms of node voltages in phases B and C. Plot the fault current. Comment on current waveform. Power System Transients (Kizilcay) 6-5
EXERCISE 4 Travelling Waves on a Single-Phase Line use of LINE CONSTANTS and JMARTI SETUP routines via ATPDraw; comparison of line models CPDL, JMARTI and B-circuit representation; understanding of travelling wave phenomenon on lines. In this exercise a single-conductor overhead line is represented. First, line geometrical data, length, DC resistance per length, earth resistivity are required to create line model data using LINE CONSTANTS supporting routine via ATPDraw. The ATPDraw object supporting this feature is Line/Cable < 1 phase. Both switches are Measuring switches, i.e. they are close all the time and measure line current. Data of the overhead line (single-phase): Conductor data: 240/40 mm 2 ACSR, d in = 1.35 cm, d out = 2.19 cm, DC resistance, R _ = 0.116 S/km, height = 16.4 m, l = 50 km each section. Earth resistivity, D E = 100 S@m Source: type-11 step function with amplitude of 100 kv, T stop = 50 :s producing a rectangular pulse of width of 50 :s. 1. The line will be represented using line model Constant-parameter distributed line (called Bergeron in ATPDraw), JMARTI SETUP and 10 B-circuits each for 10 km line section. JMarti model takes frequency dependence of line parameters into account. Therefore a frequency range must be specified: Freq init [Hz] = 0.01, Decades = 8 (corresponds to f max = 1 MHz), Points/Decade = 10. Additionally, following data must be entered for JMarti model: Freq. matrix [Hz] = 750, Freq. SS [Hz] = 50. Power System Transients (Kizilcay) 6-6
2. Select an appropriate time step )t taking into account travel time of waves. 3. Examine voltage and current waveforms for the case with open-circuited receiving end at END2 and compare three line model responses. 4. Examine voltage and current waveforms for the case with short-circuited receiving end at END2 and compare three line model responses. 5. Examine voltage and current waveforms for the case when the receiving end at END2 is terminated with a resistance of 577 S and compare three line model responses. Power System Transients (Kizilcay) 6-7
EXERCISE 5 Line-to-Ground Fault on a 110-kV Overhead Line use of Bergeron model for a balanced three-phase line; understanding of travelling wave propagation on multi-phase lines. A line-to-ground fault of phase A voltage of phase A jumps suddenly from normal level to zero gives rise to travelling waves on the three phases. The overhead line is assumed to be balanced, hence it is sufficient to represent it using positive and zero sequence system data. The neutral point of the 110-kV source network is solidly grounded through the voltage sources. System data: Source network: V = 110 Q 3 kv (r.m.s. value), R Q = 0.331 S, X Q = 3.311 S Overhead line: Z' 1 = 0,12 + j0,3871 S/km C' 1 = 9 nf/km (positive sequence system) Z' 0 = 0,27 + j1,3927 S/km C' 0 = 4,912 nf/km (zero sequence system) l = 150 km 1. Model the three-phase voltage source using Sources < AC 3-ph type 14 in ATPDraw. 2. The three-phase short circuit impedance can be represented by Branch linear < RLC 3-ph in Power System Transients (Kizilcay) 6-8
ATPDraw. Set correctly Xopt under ATP Settings. 3. The three switches at the sending end of the line is always closed to measure line current. They can be represented by Probes & 3-phase < Probe Curr using ATPDraw. 4. Calculate characteristic impedance, Z c, and travel time, J, for the positive and zero sequence system. For simplification, omit the resistances in the calculations. The balanced transmission line will be represented in ATPDraw using Line distrib. < Transp. lines (Clark) < 3-phase. This corresponds to Bergeron method called Constant-Parameter Distributed Line in ATP Rule Book. Enter the required data as follows: positive-sequence zero-sequence R/l+ : R' 1 R/l0 : R' 0 (S/km) A+ : Z 1c A0 : Z 0c (S) B+ : J 1 B0 : J 0 (s) l = 150 km, ILINE = 2 Simulation of the simplified network: At first step, the short-circuit source impedance and line resistances will be omitted. 5. Initiate the line-to-ground fault by means of a time-controlled switch (Switches < Switch time-contr.) at the moment of negative peak value of voltage in phase A at the line end. 6. Compare voltage waveforms at the receiving end (END) of phases A, B and C just after the fault initiation. How can the induced voltages in sound phases B and C at the line end be estimated at the instant of fault initiation? Determine the travel time of voltage waves propagating in phase B. Can they be estimated with respect to travel times of positive and zero sequence system? 7. Plot the switch current of phase A at the sending of the line. Determine travel time of the current wave observed at node BEGA. How can be the amplitude of the current wave be estimated? 8. Include the short-circuit impedance of the source (R Q, X Q ) in the system model and add positive and zero sequence resistances to the line model. Compare voltage waveforms at the line end with those of the simplified network. How are the overvoltages at line end effected? Power System Transients (Kizilcay) 6-9