21, rue d'artois, F-75008 Paris http://www.cigre.org A3-308 Session 2004 CIGRÉ HIGH SPEED GROUNDING SWITCH FOR EXTRA-HIGH VOLTAGE LINES G.E. Agafonov, I.V. Babkin, B.E. Berlin Y. F. Kaminsky, S. V. Tretiakov, Y. I. Vishnevsky NIIVA Russia J.H. Yoon, J.H. Kang, B.H. Choi HHI Korea Summary At a single-phase short-circuit in extra-high voltage lines after breaking of faulty phase from both sides by the circuit breakers the secondary arc can exist for a long time in a place of a short-circuit, roused by electrostatic induction from the "sound" phases. One of the possible ways of a secondary arc elimination is employment of grounding switches, which are capable during a dead time at autoreclosing of circuit breakers at first to short-circuit purposely a faulty phase from both sides, thus having shunted and eliminated an arc in a fault location, and then to open it, by breaking of the induced currents, and to make a phase ready for making of the circuit breakers, i.e. renewal of power supply. The basic requirements to a switching performance of a grounding switch result from the transient analysis in power lines and include the requirements to switching of capacitive and inductive currents, including currents, which up to 100 ms have no zero value. The design of a high speed grounding switch (HSGS), meeting the given requirements was realized within the framework of GIS 800 kv project for KEPCO - Korean electric power company creating nowadays a system with such rated voltage. HSGS represents the single-break, puffer type device. For arcing capacity ensuring at large times the dead zone is provided for in compressed volume, sufficient for an intensive blow after the stopping of a moving contact. The distinctive feature of the given device is the usage of the stationary nozzle on the side of the moving contact. The given solution allows to lower the flow rate of SF6 on an initial stage of an arc blowout and to increase a gas pressure at bigger arc durations. For checking of the switching performance for large arcing times the new synthetic circuit was built, where the high-power rectifying installation was used as the current source permitting to obtain the unipolar currents up to 14 ка by amplitude and up to 150 ms by duration. Keywords: High speed grounding switch Extra high voltage Switching performance test. I.Babkin, niiva@mail.wplus.net
1. INTRODUCTION At a single-phase short circuit in extra-high voltage lines the circuit-breakers usually break a short circuit current on both sides of faulted phase. After breaking the short circuit current a secondary arc flows in a place of a short-circuit, roused by electrostatic induction from the "sound" phases. In this case if the interaction between phases is great, the secondary arc will not be quenched during a dead time before the circuit-breakers reclosing. One of the ways of secondary arc elimination is the use of high speed grounding switches (HSGS), which arrange the condition for extinguishing of a secondary arc by grounding of a faulted phase from both sides after short circuit arc quenching. After quenching of a secondary arc HSGS disconnect the induced currents: the first in time HSGS disconnects an electromagnetic induced current, and the second one - the electrostatic current. After the breaking operation by the HSGS the conditions are arranged for the circuit-breakers making on the ends of the faulted phase. HSGS should perform the making and breaking operations during a dead time. The principle of operation is illustrated by figure 1. The number of the particular requirements keeps aloof the high speed grounding switches with respect to other switchgear. In particular, among such requirements are: - relatively small interrupted currents at very large TRV values (8 ка and 700 kv for HSGS considered); - generally, for SF6 HSGS the design should be single-break, i.e. to provide an arc quenching and full insulation to earth of a system by one break; - the conditions can be arranged in a line, when the induced electromagnetic current switched by HSGS, will have no zero value within up to 80 ms, i.e. the maximum arcing time, given by environment, should make 80 ms. This HSGS is a part of SF6 GIS 800kV developed for Korean power company KEPCO (see report THE GAS INSULATED SWITCHGEAR RATED FOR 800 kv. 27 INT - B3-3). The main technical requirements to the equipment are listed in table I. Table I Technical requirements to a high speed grounding switch Name of parameter Value Switching of capacitive current: - current value, А 750 - recovery voltage value, kv 700 Switching of an induced current: - current value, А - recovery voltage value, kv 8000 700 CB is closed CB is closed Contact position HSGS is open CB is open HSGS is closed HSGS is open 0 0.1 0.5 1.0 Time, s Figure 1: C-O coordination of HSGS and CB 2
2. DESIGN On the basis of these requirements the HSGS design (figure 2) was developed and manufactured. The basic elements of HSGS design are an arcing device 1, reducer 2, drive 4 and metal construction 3. In such design, on the one hand, it is necessary, to provide the high velocity of the moving contact for breaking of the induced capacitive currents without restrikes, and on the other hand, the blasting duration providing the interruption of electromagnetic currents should be not less than 80 ms. (At the uniform stroke characteristic of a drive 80 ms after the contacts opening corresponds to a stationary position of moving contact). The design of an arcing device (figure 3) is based on puffer principles, the main distinctive feature of this device is the stationary fluoroplastic nozzle located on the side of a moving contact. For ensuring of the high speed of the contacts the powerful hydraulic drive is used in a design. To maintain the blasting required for an arc quenching in 80 ms after the contacts separation, the large rod end space is applied in HSGS design generating a long-term gas flow through a nozzle of small diameter after the stopping of the piston. The necessary rod end space and maximum mass of the moving parts were defined by computation in accordance with the given nozzle section, contacts velocity at opening and power characteristics of a drive. 1 - arcing device; 2 reducer; 4 drive; 3 - metal construction Figure 2: Design of HSGS 1 conductor; 2 tank; 3 - fixed contact; 4, 5 screens; 6 nozzle; 7 - moving contact; 8 - piston; 9 "dead zone" space; 10 - head end space; 11 - annular channel; 12 - lateral channels Figure 3: Design of arcing device 3
The arcing device includes the metal tank 2, filled by SF6, inside of which the currentcarrying conductor 1 with a fixed contact 3 and screen 4 is located. The metal screen 4 has the special shape, and its electrical contact with a stationary contact 3 is made on the side of an outer diameter of a screen, these measures practically exclude the possibility of an arc transition at opening to an enclosure of an arcing device. The moving contact 7 through the piston 8 is connected with a drive of a puffer device. The metal screen 5 and fluoroplastic nozzle 6 are located on an end wall of the puffer device enclosure. The number of holes connecting the inner space of the tank 2 with a head end space 10 during the whole contact travel is made on a lateral surface of a moving contact 7. The annular channel 11, connecting a rod end space 9 with the lateral channels 12 is made in the puffer device enclosure. The volume of a rod end space 9 of puffer device, corresponding to a final position of a moving contact ("dead zone"), is chosen sufficient for an arc blow during all arcing time up to 80 ms. HSGS works as follows: At making a drive rod 4 (figure 2) travels in a horizontal plane. The reducer 2 converts the horizontal travel in a vertical one and increases the rod travel transferring the motion to a moving contact. For avoidance of gas compression at traveling of a moving contact 7 (figure 3) the piston 8 in a head end space 10 has a check valve, and the head end space 10 is connected by channels with a tank space. At breaking a moving contact 7 and piston 8 are in a backward motion. In this case the holes on the piston 8 are overlapped, and the overpressure is arisen in a rod end space, which is applied through the annular channel 11 into the zone of nozzle 6. In traveling of a moving contact 7 the underpressure is generated in the head end area 10, due to which the arcing products are sucked into the area 10 through an inner space and the holes in the moving contact 7. Thus, in an initial section of the moving contact travel the arc quenching is made only by the suction, and then by combined influence upon arc by suction and blowing. 3. SWITCHING TESTS The number of the new synthetic schemes with voltage injection was used at experimental researches of the switching performance. The feature of HSGS, as it was mentioned, is the very large arcing time. It means, that the test circuit should provide the current flow through HSGS during 80 ms either by application of seven circuits for arc reignition, usually used at synthetic tests, or ensuring of the current waveform not having a zero value within the indicated time. As the availability of seven schemes is a problem not only for our test center, the second variant was chosen for the basis. Two circuits were selected for the analysis. - The first one presents an event similar to those, which should hypothetically take place in a system during HSGS operation, i.e. an alternating current has a very large dc component and does not pass the zero value during time up to 80 ms (figure 4). - The second circuit uses as a current source a powerful dc rectifier, which allows to obtain a unipolar current wave up to 150 ms by duration and the peak value up to 10 ka (figure 6). 4
CS1 CS2 AB PG HSGS 800 HVG PG power generator; CS1, CS2 closing switches; HVG high voltage generator; AB auxiliary breaker Figure 4: Test circuit for electromagnetic induced current switching with large DC component Prior to the test the closing switches CS1 and CS2 in the circuit (figure 4) are open and the auxiliary circuit-breaker AB and the test HSGS 800 are closed. At first CS1 is closed, then CS2 is closed. The maximum current flowing through HSGS can be achieved, if CS1 makes at the moment of zero voltage of the power generator PG, and CS2 - at the moment of peak current. After current in HSGS reaches zero value and arc quenching, the high voltage is applied to HSGS from HV power source. In figure 5 the simulation of current flowing through the closed HSGS and AB is shown. However, the experimental check of the circuit operation had shown, that the arc voltage of HSGS 800 is so great, that the maximum arc duration has made only 45 ms. Therefore it was necessary to refuse from this circuit at HSGS testing. Figure 5: Current curve simulation corresponding to the scheme in fig. 4 5
And so the full-scale switching tests of HSGS were carried out on a basis of test circuit with a rectifier installation (figure 6). CS PG RF HVG PG power generator; RF rectifier; HVG high voltage generator Figure 6: Test circuit for electromagnetic induced current switching with rectifier The current source in the circuit - the rectifier installation RF (assembled according to the six-phase rectification circuit) is powered from PG through limiting reactors L1, closing switch CS and step-up transformers Т. RF allows to obtain a rectified current of 14 ка with duration about 0,1 s. For surge control of a rectified current and for installation protection against the overvoltages the reactor L2 and R1-С1 chain was mounted at its output. The voltage source in the circuit - high-voltage generator (HVG) allows to obtain the required TRV peak value 700 kv. The TRV parameters control circuit consists of the reactor L 0 and connected in series resistors R and capacitors С. The arc duration was determined by a break time of the auxiliary circuit-breaker AB and arc voltage values of HSGS and AB. Current, stroke and transient recovery voltage curves are represented on the figures 7a, 7b. Figure 7a: Typical oscillograms of interrupted current and stroke 6
Figure 7b: Typical oscillograms of transient recovery voltage The HSGS - 800 prototype has passed a complete test cycle, including high-voltage tests with applying of a lightning impulse voltage of 2250 kv, breaking of currents 8 ка with duration up to 80 ms, breaking of capacitive currents, making of short circuit currents. The tests were carried out in the test centers of JSC "NIIVA" and KERI (Republic Korea). All tests have passed successfully. 7