Transients on Integrated Power System

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1 Chapter 3 Transients on Integrated Power System 3.1 Line Dropping and Load Rejection Line Dropping In three phase circuit capacitance switching, the determination of the voltage trapped after switching off is based on the ratio of the positive sequence capacitance C 1 (total capacitance per phase) to the zero sequence capacitance C 0 (capacitance from line-to-ground). The ratio is very large when the capacitor bank has ungrounded neutral. In this case, the voltage across the first phase approaches three times the line-to-ground voltage. For transmission lines and cables, the ratio is typically ranges from 1.6 to 2.0. This means that the voltage across fist pole to clear is in the range of 2.2 to 2.4 times of line-to-neutral voltage. It reduces the chances of a restrike and also results in a significant reduction in the transient voltage causes a restrike. During the restrike, the voltage across the switch will be divided into source side and to the line based on their respective surge impedances. The wave travels up and down the line corresponding to the oscillatory swing of the bank in the case of lumped capacitance. Resistance switching method is used in this case whereby a resistor with a series switch is included in parallel with the breaker. The resistor and the switch included in the circuit while switching and the resistance value chosen to effectively damp the transient oscillations due to transient. Later the resistor removed after switching. While switching with the resistance switching method, the load is a combination of R and C. Hence the system voltage is no longer at a peak at current zero when the interruption occur. Therefore, less voltage trapped on the line and reduces the chance of a restrike. The line voltage of an energized line will not be uniform throughout the line. When the line is removed from the source, a non-uniform charge will be trapped on the line. It will result waves with amplitudes depending upon the degree of nonuniformity. The waves travel up and down until losses damp them out. Sometimes a line is switched with transformer at line end. In this case, the trapped charge dissipate through the magnetizing impedance of the transformer. Until stauration of transformer core, transformer offers high impedance and the discharge is very slow. But, the impedance drops abruptly when the core runs into saturation. This results sharp increase in current and a rapid discharge of the line.anyhow, the restriking of the circuit breaker connected with a transformer is less than for unloaded lines. 9

2 10 Figure 3.1: Equivalent circuit during load rejection The secondary side switching of transformers connected to high voltage lines can itself create overvoltages, even without restriking. This comes about again as a result of the nonlinear properties of the transformer core. It is well known that magnetizing current of a transformer contains 3 rd hormanics. There is a possibility for considerable harmonic voltage generation due to higher impedance of both line and transformer Load Rejection Where the line is connected to a major load on a generating station, sudden load rejection will result in over speeding of the machines and a rise in voltage until such times as this can be checked by the governers and exciters. The amplitude of the over voltage can be evaluated approximately, as illustrated by Fig.3.1, by, X c V = E X c X s where E is the voltage behind the transient ractance, which is assumed to be constant over the subtransient period and its value before the incident, X s the transient ractance of the generator in series with the thransformer reactance, and X c the equivalent capacitive input reactance of the system. 3.2 Voltage Transients on Closing and Reclosing Lines The transient phenomena will vary according to the system configuration, the source type, line length, and terminations. For a line open at the far end, three types of transients on switching are shown in Fig.3.2. Figure 3.2(a) shows a transient which can be expected when the source is mainly inductive, for example, a single line connected to a transformer. It is single-frequency transient, and if switching takes place at maximum voltage in a phase, the transient oscillates to almost twice the value of the system voltage across the entire line length. Figure 3.2(b) shows a high-frequency transient which can be expected with infinite source impedance. This means that the system from which the line originates has a number of cables or lines connected to it, and the line being switched is not longer than the incoming lines. Many line terminations and connections at the point of origination of the line being switched means that these terminations present an overall low characteristic impedance compared to the line being switched. As a result the transient occurs at its natural frequency.

3 11 Figure 3.2: Profiles of switching surges on closing a line, dependent upon power system configuration Figure 3.3: (a).switching a open ended line (b) and (c) Voltage profiles on bus and line side, respectively Figure 3.2(c) shows the pattern of switching transient with complex source impedance, consisting of inductance of transformers and the surge impedance of other lines and cables feeding the system. The transient overvoltage occurs at a number of frequencies. Highest overvoltages occur when unloaded high-voltage transmission lines are energized and reenergized and this imposes voltage stresses on circuit breakers. Figure 3.3(a) shows closing of a line of surge impedance Z 0 and length l, open at the far end. Before the breaker is closed, the voltage on the supply side of the breaker terminal is equal to the power system voltage, while the line voltage is zero. At the moment of closing, the voltage at the sending end must rise from zero to the power frequency voltage. This takes place in the form of a traveling wave on the line with its peak at u m interacting with supply system parameters. As an unloaded line has capacitive impedance, the steady-state voltage at the supply end is higher than the system voltage, and due to Ferranti effect, the receiving-end voltage is higher than the sending end. Overvoltage factor can be defined as follows: OV total = u m u n where, u m is the highest peak voltage at a given point and u n is the power frequency voltage on the supply side of the breaker before switching. The increase in power-frequency overvoltage depends considerably on the line length. The transient voltage is not so simple to determine and depends upon the phase angle at the closing instant (Fig. 3.3). At instant t = t 1, maximum superposition of transient and power-frequency

4 12 voltage occurs. 3.3 Overvoltages by faults Overvoltage may be produced by certain types of asymmetrical fault such as a ground fault on one of the phase conductors of a three-phase transmission line. The situation is somewhat analogous to the switching transients except that here instead ofinjecting a current, a voltage equal and opposite to the prefault voltage at the fault point is applied. The losses in the system will dampen the transient by some percentage. Figure 3.4: Faulted Network Figure 3.4 shows the faulted system in which a ground fault has occured on the a phase at point F. Figure 3.5 shows the prefault steady-state voltage V fsa at the fault point. If it is assumed that a fault occurs at the peak of the prefault voltage and t = 0 at this instant, then the voltage injected at the fault point is as shown in Figure 3.5 (b). Figure 3.6 shows the de-energised network to which is applied the voltage V ffa. This is also known as the superimposed voltage. We can study the problem of overvoltage by asking how the de-energised network behaves in response to the application of this voltage. Complex analysis based on travelling wave theory (such as Bewley lattice diagrams) can be used to depict the level of overvoltage likely to be induced, particularly on the unfaulted phases due to the injection of the suddenly applied voltage at the fault point. A line-ground fault of the type considered here can produce an overvoltage on an unfaulted phase as high as approximately twice the normal line-to-neutral voltage. Figure 3.7 gives an example of such an overvoltage, in particular when the fault occurs at the midpoint of the line; then maximum voltage is at the midpoint ofthe unfaulted conductor.

5 13 Figure 3.5: Faulted Network Voltage Figure 3.6: Superimposed Network

6 14 Figure 3.7: Overvoltage phenomena on a faulted three phase system 3.4 Switching Surges on Integrated System The disturbance produced by the switching operation as modified by the interconnected system spreads through the system, setting up waves that travel along the lines and reflect to and fro at open ends. Figure 3.8(a) shows two buses with voltages of 138kV and 345kV interconnected by autotransformer. The 138kV bus is fed through the generator transformer. There are lines connected to both the buses. Figure 3.8 (b) shows a single phase representation. A fault at short distance down the line of 354kV bus is considered for the analysis. The switching operation due to this fault will evoke a response from both the line and the system. Figure 3.8 (c) shows the equivalent circuit of the system with a common voltage base. The lines are represented by the resistances R 1 and R 2 under transient conditions until it is modified by reflection from points down the line. The surge impedance of the circuit is given by, L1 C 1 The source circuit can be replaced by a parallel RL circuit in which L = L 1 + L 2 + L 3 and R = R 1. The response of a parallel RL circuit to a ramp of current I 0 t is given by, v(s) = RI 0 s[s + (R/L)] in operational form or V (t) = LI 0 (1 e Rt L )

7 15 Figure 3.8: Various representation of a generating and swtiching station. as a time function. The fault current I 0 is given by, I 0 = V p L where, V p is the peak system voltage. Substituting the value of I 0, V (t) = V p (1 e Rt L ) = Vp (1 e αt ) This shows that the fault will rise exponentially with a time constant. The approximate response of the equivalent circuit to a ramp of current is shown in Figure 3.9. Furthermore, a voltage of this form will travel down each of the connected lines. But, while considering the capacitance, it introduce a delay τ as indicated in Fig.3.9(b) The effect of the many waves on the bus voltage of a station will depend on the impedance Figure 3.9: Approximate repsonse of Fig.3.8(c) to a ramp of current. (a) Capacitance neglected. (b) Capacitance included.

8 16 Figure 3.10: First-generation encounter between wave and station of that station and connected lines, and also other stations that the wave has encountered. As a first approximation, each station can be represented by a parallel RL circuit. The inducteance will depend upon the source impedance of the station and the resistance upon the transmission lines connected to its bus. Figure 3.10 shows the first generation of encounters to be evaluated. This requires the application of the reflection and refraction coefficients. The reflected wave will be, ( ) Z2 Z 1 V r = av i = V i Z 2 + Z 1 where, V i is the incident wave, Z 2 is the impedance of the station being encountered, and Z 1 is the impedance of the line along which the incident wave is advancing. The transmitted wave or refracted wave is given by, ( ) 2Z2 V t = bv i = V i Z 2 + Z 1 It describes the voltage waves that travel down the several lines that R represents. After encounters of this type, the reflected and refracted waves return from where they came or proceed to more distant stations; normally termed as second generation encounters. The reflected wave and the transmitted wave are modified by the reflection and refraction coefficients appropriate to the next station or discontinuity. The coefficents will have the same general form but differ numerically because of different station and line characteristics. 3.5 EMTP (Electromagnetic Transients Program) for transient application: The EMTP is a computer program designed to solve electrical transient problems in lumped circuits, distributed circuits, or both. It was introduced by H.W.Dommel in the early 1960s. Transient analyses can be carried out in circuits with any arbitrary configutation of lumped parameters (R,L and C). Transmission lines, with distributed parameters, transposed or untransposed, can be included in the network. Losses in lines are approximated by lumped resistance. Nonlinear resistaors (for surge arresters) and nonlinear inductors (for saturable devices) can be represented. It is also possible to open or close switches to simulate breaker operations, flashovers etc. Both voltage and current sources are available with sine, ramp, or step functions. Trapped charges can be recognized. The EMTP is based on the application of the trapezoidal rule to convert the differential equations of the network components to algebraic equations. This approach is demonstrated in the following text for the inductance, capacitance, and lossless line. For the inductance L of a branch between the nodes k and m, it holds

9 17 Figure 3.11: (a) Inductance (b) Equivalent Impedance Network thus, v k v m = L i k,m (t) = i k,m (t t) + 1 L ( dik,m dt t ) t t (v k v m )dt Integration by means of the trapezoidal rule gives the following equations, i k,m (t) = t ( ) v k (t) v m (t) + I k,m (t t) 2L I k,m (t t) = i k,m (t t) + t ( ) v k (t t) v m (t t) 2L For the capacitance C of a branch between the nodes k and m, it holds Figure 3.12: (a) Capacitance (b) Equivalent Impedance Network v k (t) v m (t) = v k (t t) v m (t t) + 1 C t t t i k,m (t)dt Integration by means of the trapezoidal rule gives the following equations i k,m (t) = 2C ( ) v k (t) v m (t) + I k,m (t t) t

10 18 I k,m (t t) = i k,m (t t) 2C ( ) v k (t t) v m (t t) t For a single-phase lossless line between the terminals k and m, the following equation must be true. u m (t τ) + Zi m,k (t τ) = u k (t) Zi m,k (t) where, τ is the travel time(s). Resistance is simply represented by Ohm s law, Figure 3.13: Representation of resistance i k,m (t) = 1 R ( ) v k (t) v m (t) Considering one each of the components just described joining at node 1 to adjacent nodes 2,3,4 and 5 as shown in Fig Figure 3.14: Assembly of components connected to node 1 of a network The model equation for current at node 1 can be written as, i 12 (t) + i 13 (t) + i 14 (t) + i 15 (t) = i 1 (t) Substituting the values of currents from the above analysis, [ 1 + t Z 0 2L +2C t + 1 ] v 1 (t) t R 2L v 3(t) = 2C t v 4(t) 1 R v 5(t) = i 1 (t) I 12 (t τ) I 13 (t τ) I 14 (t τ) I 15 (t τ) This equation is a linear, algebric equation in unknown voltages, with the right hand side

11 19 known from the values of preceding steps. Each of the n nodes of the network can be trated in the same way to give similar equations. For easy calculation, these are assembled in matrix form: [G][v(t)] = [i(t)] [I] where, [G] - Nodal conductance matrix, [v(t)]-vector of n node voltages, [i(t)]-vector of current sources, and [I] - Vector of past history terms. It is usual for known voltages to be driving some of the nodes. These are handled by partitioning the nodes in above equation into the A nodes with unknown voltages and the B nodes with known voltages to give, [ ] [ ] [GAA ] [G AB ] [va (t)] [G BA ] [G BB ] [v B (t)] = [ ] [ia (t)] [i B (t)] from which the unknown voltage [v A (t)] is found by solving, where, [G AA ][v A (t)] = [I total ] [G AB ][v B (t)] [I total ] = [i A (t)][i A ] [ ] [IA ] [I B ] This equation helps to obtain the solution of linear equations in each time step with a constant coefficient matrix [G AA ], provided t is not changed. But, the right side of the equation must be recalculated in each time step. The equation [G AA ][v A (t)] = [I total ] [G AB ][v B (t)] is best solved by traingular factorization of the augmented matrix [G AA ], [G AB ] once for all, before entering the time loop. The same process is then extended to the vector [I total ] in each time step in the so-called forward solution, following by a back substitution to obtain [v A (t)] as indicated in Fig

12 Figure 3.15: Flowchart of transients program 20

13 Short Line or Kilometric Fault Faults occuring between a few kilometers to some hundreds of kilometers from the breaker are termed short-line faults (SLF) or kilometric faults. For such faults, the line impedance limits the current and, consequently, supports some of the system voltage. The generated voltage is divided on either side of the breaker in proportion to the impedance of the line and the source. Figure 3.16: Distribution of voltage on a faulted system Fig.3.16 shows the distribution of voltage. When the circuit breaker interrupts at current zero the generated voltage will be near the peak value. Let L be the source inductance, L 1 be the line inductance to the fault and C 1 be the line capacitance to the fault. E is the peak value of the e.m.f driving the fault. Peak voltage at the breaking point is given by, V = L 1 L + L 1 E Opening of the breaker is simulated by injecting a cancellation current, E ω(l + L 1 ) ωt = E (L + L 1 ) t This current wave, travelling on the line, produces a voltage wave, where Z is the surge impedance of the line. E (L + L 1 ) t.z v = 1 L1 C 1 and Z = L1 C 1 Time taken by the wave to reach faulted end, T is given by, L 1 C 1 When the wave reaches the faulted end of the line in time T, the voltage due to the wave at

14 22 the line side circuit terminal is, E (L + L 1 ) L1 E L1 C 1 = C 1 (L + L 1 ) L 1 = V The addition of initial voltage and the voltage due to transient wave at the circuit breaker terminal is zero. The voltage wave is reflected with opposite sign at the short circuit point and with the same sign at the circuit breaker terminal. Figure 3.17: Voltage at line terminal of circuit breaker during kilometric fault The voltage at the circuit line terminal is given by Fig It can be seen that the line side terminal of the circuit breaker can make a considerable swing in potential, +V to V, in a comparatively short time 2T. In this time, the other terminal of the circuit breaker would be still having a potential approximately equal to V. It is this fast rate of rise of the transient recovery voltage that may prejudice the successful operation of the circuit breaker. If the source can be represented by a parallel L and C circuit, the source side response will be (1-cosine) recovery wave with a period 2π LC and an amplitute of E E (L + L 1 ) L E 1 = (L + L 1 ) L Therefore the potential if the circuit breaker terminal on the source side is given by, ( ) E (L + L 1 ) L t 1 cos LC The breaker sees the difference of potential on the two terminals as shown in Fig Figure 3.18: Circuit breaker recovery voltage following a short-line fault

15 Contents 1 Standing Waves and Natural Frequencies Case:1 (r=g=0; Losses Neglected) Location of Poles Residues General Case (Considering r,l,g,c) Refelction and Refraction Coefficient Transients on Integrated Power System Line Dropping and Load Rejection Line Dropping Load Rejection Voltage Transients on Closing and Reclosing Lines Overvoltages by faults Switching Surges on Integrated System EMTP (Electromagnetic Transients Program) for transient application: Short Line or Kilometric Fault

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