OCCURRENCE OF DELAYED CURRENT ZERO CROSSING DUE TO LINE REACTIVE OVERCOMPENSATION LT CAMPINAS FERNAO DIAS 500 kv.

Transcription

1 OCCURRENCE OF DELAYED CURRENT ZERO CROSSING DUE TO LINE REACTIVE OVERCOMPENSATION LT CAMPINAS FERNAO DIAS 500 kv. Ricardo André Gonçalves 1 / 17

2 1 - INTRODUCRION Studies conducted on several 500 kv compensated lines indicate that it may have a risk of delayed current zeros during some switching scenarios. Transient studies indicated that under some switching cases, the line and in some instances reactor breakers (if it is switchable), may not see current zero crossings for several cycles. If the line breaker tries to open immediately after line energization, while the line currents are still in a transient state, the breaker currents will be offset with slow decaying DC. It will fail to open if there is no current zero within the interrupting time. The magnitude of the DC offset is governed by the point on the voltage wave where the breaker contacts close, line resistance, line inductance, and reactive shunt compensation. 2 / 17

3 1 - INTRODUCRION The Brazilian Energy Research Company (EPE) defines the expansion of the transmission system of Basic Network. One feature increasingly used in the expansion is dividing an existing line into two to insert a new substation. Such a procedure can eventually cause problems if the original line is compensated. Sectioning alters the degree of compensation of the new sections of the original line affecting the transient requests imposed on the terminal equipment during the transient switching. The new substation Fernao Dias 500 kv will split the previous Campinas Cachoeira Paulista 500 kv transmission line, which originally was 223 km extension, in two new transmission lines. The original line has at the Campinas terminal a switchable reactor of 112 Mvar. With the sectioning the new line Campinas Fernao Dias will have an extension of only 66 km. 3 / 17

4 1 - INTRODUCRION Therefore, when the reactor at Campinas terminal is inserted, the line become overcompensated since the shunt capacitance of the line only generates, in 500 kv, something around 75 Mvar, which in turn makes the line to be 150% compensated. An overcompensated line may eventually provide conditions to enhance delayed current zeros for the currents passing through the circuit breakers at its ends, during the energizing or automatic three phase auto-reclosing switching, as a consequence of the strong DC component. The interruption of these transient currents can cause significant demands on line circuit breakers, which are not covered by the Technical Specification Standards and Tests of high voltage circuit breakers. 4 / 17

5 2 - Description of the Phenomenon This type of problem has already been investigated in some papers. One good example of such an investigation was presented by Eletrobras/Chesf in XXIII SNPTEE, from where the following basic theory is reproduced. FIGURE 1 Equivalent Simplified Circuit Capacitance C represents the total positive sequence capacitance of the transmission line; Inductance L represents the positive sequence equivalent inductance of the line and reactors; Resistance R represents, in simplified form, the combined effect of the equivalent resistance of the total losses in the line and in the reactor and the equivalent resistance of the system X/R ratio at the point of connection. 5 / 17

6 2 - Description of the Phenomenon By manipulating the variables and making the correct equation of the circuit, one arrives in a simplified way (considering typical values for the line parameters and shunt compensation) to the graph shown in Figure 2. Para k sh menor que 0,5, a relação é menor que 1, ou seja, a componente contínua é inferior ao valor de crista da componente alternada. (a) (b) FIGURE 2 Conditions for the Occurrence of Delayed Current Zero Crossing (Zero-missing) T d -> tempo de retardo 6 / 17

7 2 - Description of the Phenomenon Depending on the K sh and Q, if the breaker opens immediately after the energization or three phase auto-reclosing of a compensated transmission line, the breaker poles, in one or more phases, can try to interrupt the current with a high DC component and, depending on the value of the current at the time of separation of the contacts and zero delay time, severe interference can occur at the circuit breaker. Modern SF6 circuit-breakers, suitable for reactor switching with minimum current cut-off voltage, may not be able to interrupt these transient currents, resulting in an extension of arc time with excessive degradation of parts of the fire extinguishing chambers or until the breaker explodes. The arc extension can also cause a reduction in dielectric strength between contacts, causing multiple re-ignitions and arc restarts until a possible burst of the chambers occurs. 7 / 17

8 3 - The 500 kv Transmission Line Campinas - Fernao Dias Study In order to analyse the Phenomena for this line, the following equivalent system was used in ATP 8 / 17

9 3 - The 500 kv Transmission Line Campinas - Fernao Dias Study - ATP Digital Simulations In order to evaluate the risks of occurrence of delayed zeros currents for the circuit breakers, digital simulations were performed using ATP for energization and three phase auto-reclosing, with and without the application of single-phase short circuits, with detailed representation of the transmission system in the area of interest through the previous described modeling suitable for studies of switching surges. For each switching condition, statistical closing times were simulated considering 4 ms of maximum pole spread, with monitoring of the currents in the switched circuit breaker and its integrals, in order to allow the selection of the closing moments of the poles that generate the highest DC component values. 9 / 17

10 3 - The 500 kv Transmission Line Campinas - Fernao Dias Study Ksh =1,5 ENERGIZATION BY CAMPINAS Oscillogram 1 shows the plots of the currents at terminals of the Campinas breakers, without short circuit and with the presence of the pre-insertion resistor 400 Ω. This case corresponds to the largest continuous current component through the circuit breaker, with a degree of compensation of 150% (Campinas reactor connected) and a ratio X0/X1 equal to 1.0 in the reactor. There is a high continuous component in the phase C (blue) current superimposed on a 60 Hz component, so that the resulting current does not reach zero during the entire simulation time, ie the delay time is greater than 30 cycles. Since the breakers have pre-insertion resistors, the effect of the dynamic resistance of the arc between contacts of the circuit breaker was not represented, but in practice this effect should not contribute to increase the damping and reduce the delay time. 10 / 17

11 3 - The 500 kv Transmission Line Campinas - Fernao Dias Study ENERGIZATION BY FERNÃO DIAS Oscillogram 2 shows the plots of the currents at terminals of the Fernão Dias breakers, without short circuit and with the presence of the pre-insertion resistor 400 Ω. There is a hugh DC component in the phase B (green) current superimposed on a 60 Hz component, so that the resulting current does not reach zero during the entire simulation time, ie the delay time is greater than 20 cycles. 11 / 17

12 3 - The 500 kv Transmission Line Campinas - Fernao Dias Study THREE PHASE SUCCESSFULL AUTO-RECLOSING BY CAMPINAS (after a single phase fault) Oscillogram 3 shows the plots of the currents at terminals of the Campinas breakers, with the presence of the 400 Ω pre-insertion resistor. There is a high DC component in the phase A (red) current superimposed on a 60 Hz component, so that the resulting current does not reach zero during the entire simulation time, ie the delay time is greater than 40 cycles. 12 / 17

13 3 - The 500 kv Transmission Line Campinas - Fernao Dias Study THREE PHASE SUCCESSFULL AUTO-RECLOSING BY F. DIAS (after a single phase fault) Oscillogram 4 shows the plots of the currents at terminals of the F. Dias breakers, with the presence of the 400 Ω pre-insertion resistor. There is a high DC component in the phase A (red) current superimposed on a 60 Hz component, so that the resulting current does not reach zero during the entire simulation time, ie the delay time is greater than 40 cycles. 13 / 17

14 4 - Conclusions - Delayed current zero phenomenon associated with the switching of shunt compensated lines has been demonstrated to be a slow decaying transient. This has a number of implications on line energization and three phase auto-reclosing. - Energization of shunt compensated line with over 60% compensation may result in large DC offsets in line breaker currents. Worse conditions are found when the composition is higher than 80%. - The delay in the appearance of current zeros can be as late as a second due to high X/R of shunt reactors. Opening of these breakers immediately following energization of such shunt compensated line will result in line breaker failure due to delayed current zeros. This effect is dependent on the degree of shunt compensation and also on the point on wave switching. 14 / 17

15 4 - Conclusions - The phenomenon depends on the characteristics of the line and its shunt compensation. It is not related to the performance of the circuit breaker and is not covered by the Specification Technical Standards and Tests of high voltage circuit breakers. Attempting to interrupt currents with delayed zeros with modern SF6 circuit breakers can cause excessive degradation of main chambers components, multiple arc re-ignitions, and even breaker explosion, representing a risk not anticipated by transmission agents. - For the line considered in this work, some actions were established in order to avoid the problem. Based on the results found, it was recommended that efforts to obtain authorization by ONS and ANEEL (Brazilian National Electric Agency) in order to relocate the switchable line reactor in Campinas to the same substation busbar. This measure will provide better reliability to the equipment (circuit breakers, reactors, etc.) and to the system, avoiding exposing the installation to failures due to the phenomenon of delayed zeros currents. In addition, it will provide greater flexibility to the system, allowing a better control of voltage in the Campinas 500 kv busbar. 15 / 17

16 4 - Conclusions - While this relocation does not take place, it has been recommended that line be only energized with the line reactor in Campinas out of operation. Moreover, whenever there is a signal of protection being sent to open the circuit breakers of the Campinas - Fernao Dias line, it should also be sent to the line reactor circuit breakers. Thus, in the event of a fault followed by automatic three phase auto-reclosing (500 ms dead time), at the moment of the reclosing, the line reactor would be disconnected, avoiding in such a way possible delayed zero currents conditions to be interrupted. - In the planning studies, whenever there is sectioning of a compensated transmission line to interconnect a new substation, the possible reallocation of terminal reactors to the busbar should be considered as part of the whole project. This situation must be regarded by ANEEL during the Study of Technical-Economic and Socio-Environmental Feasibility (Report R1). 16 / 17

17 I WOULD LIKE TO THANK THE SUPPORT GIVEN BY CIGRÉ THAT HELPED ME TO BE HERE MAKING THIS PRESENTATION OBRIGADO! THANK YOU! 17 / 17