Switching devices for the ITER magnet power supply system

Switching devices for the ITER magnet power supply system

ELSEVIER Fusion Engineering and Design 24 (1994) 419-424 Fusion Engineering and Design Switching devices for the ITER magnet power supply system V...

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ELSEVIER

Fusion Engineering and Design 24 (1994) 419-424

Fusion Engineering and Design

Switching devices for the ITER magnet power supply system V. Kuchinski, B. Larionov, N. Mikhailov, S. Pavluk, A. Roshal, V. Risev, S. Uralov Efremov Research Institute, SINTEZ Scientific-Technical Centre, St. Petersburg, 189631, Russian Federation Received 23 February 1993 Handling Editor: Robert W. Conn

Abstract The task of creation of a new generation of switches was set forth in the ITER research and development programme. The set of switches consists of circuit breakers and making switches both for normal and protective mode of operation. In addition, there may arise a demand for a polarity reverser to change the output current polarity of the supply converter. All of these switches are to operate with steady state currents of about 40 kA and voltages of up to 20 kV. The required response time is not more than several milliseconds. In this paper the results of development and investigation of several multiaction mechanical switches for the ITER power supply and protection systems are described. An extensive test programme was carried out to check the design concepts of the switches and determine their real parameters. The results of the experimental study indicated that all of these switches can be used as the basis for the design of the ITER switches.

1. General considerations The conceptual design phase of the I T E R project specified the main requirements for various highpower switches to be used in the power supply and protection systems of the I T E R magnet superconducting coils. A set of switches consists of circuit breakers and making switches both for normal and protective operation. In addition, there m a y arise a demand for a polarity reverser to change the converter current polarity. All of these switches are to operate with steady state d.c. currents of about 40 kA and voltages of up to 20 kV (for the polarity reverser, up to 5 kV). The response time of the switches must be not more than several milliseconds. Elsevier Science S.A. SSDI 0920-3796(94)00078-L

The data on the existing high power switches used in the main t o k a m a k devices such as T F T R , J E T and JT-60 indicate that there are no devices which could operate with steady state currents of 40 kA. Therefore the task of the creation of a new generation of switches was set forth in the I T E R research and development programme. The investigations of non-standard high power switches for large-scale electrophysical installations were initiated at the Efremov Institute in the mid 1960s. Since then a lot of different switching devices have been designed. They were used, in particular, for the power supply and protective systems of the Russian tokamaks: T-10, T-15, TSP. These switches were rated at pulse currents of up to 300 kA and voltages of up to 40 kV.

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V. Kuchinski et al. / Fusion Engineering and Design 24 (1994) 419 424

In connection with the ITER R&D program the conceptual design of several switching networks has been completed at the Efremov Institute. For the circuit breaker, which has to operate each cycle, the arcless commutation mode with a current counterpulse pause was chosen. It is necessary that a counterpulse pause as short as possible be obtained to reduce the counterpulse capacitor energy. This task can be solved by using a fast mechanical switch as well as a thyristor switch. Taking into account the 40 kA level of the d.c. breaker current a pure thyristor switch must consist of several parallel branches, which leads to a complicated switch design and increases the switch cost, so a combination of a mechanical switch with only one thyristor set may have some advantages. The breaker for the protection system can be not only a multiaction switch but also a singleaction one. The main feature of this breaker must be an extremely high reliability under different operation conditions. In our opinion a pirobreaker with elements of a rheostat to reduce the possible overvoltage is very attractive for this purpose. Investigation of this pirobreaker has been started at the Efremov Institute. The making switch can be made as a two-stage device. One stage can be a mechanical switch and

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£d Fig. 1. Simplified scheme of the circuit breaker: FB, fast breaker; C~, C 2, L~, L 2, VSI, ¢ounterpulse network; C a, L a, VS2, electrodynamic drive network, R, switching or dump resistor: L~t, saturable inductor.

the other stage can be a vacuum gap to provide a fast response and conditions for arcless commutation of the mechanical stage. The reversing switch for the polarity reverser also can be a mechanical device because of currentless contact switching. In this paper some results of the design and investigation of several multiaction mechanical devices are described. All these switches have similar main components: the water cooling contact system (main executive unit), the electrodynamic drive, the control unit with current and voltage probes, and auxiliaries (capacitor chargers, water and pneumatic supply units). The breaker comprises additionally a counterpulse network and saturable inductor. The description of the switch main executive units is given below.

2. D e s i g n d e s c r i p t i o n

2. I. Circuit breaker

The simplified scheme of the circuit breaker is shown in Fig. 1. The principle of the device action is based on arcless interruption of the power circuit by the fast breaker during an artificial currentless pause, formed by the counterpulse network charged to a voltage which depends on the interruption current. The main advantage of this breaker over the commercial analogues is a very short time of contact switching, which results in an essential reduction of the energy of the counterpulse capacitors. The breaker high speed operation is achieved by using light membrane contacts with extremely limited movement in combination with a very fast electrodynamic drive. A cross-section of the fast breaker is shown in Fig. 2. The breaker consists of the following basic parts: membrane contact system; electrodynamic drive; pneumatic drive. The movable contacts are made in the form of circular copper membranes, which alternate with the solid fixed contacts. The thickness of the membranes is 0.5 mm and the average membrane diameter is 350 mm. The contacts are silver plated

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V. Kuchinski et al. / Fusion Engineering and Design 24 (1994) 419-424

cooling water

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Fig. 2. High-power fast breaker (in closed position): 1, membrane contacts; 2, fixed contacts; 3, solenoid inductor; 4, thin metal cylinder (corrugated); 5, liquid dielectric; 6, pneumatic valve; 7, plane inductor; 8, terminals.

to prevent contact oxidation. The electrodynamic pulse drive consists of a solenoid inductor and a corrugated metal cylinder, which envelops the contact system and is filled up with a liquid dielectric. The fixed contacts have a channel for cooling water. Pressure gas is given into the membrane contacts to keep the contact system in the closed position before interruption. The action of the breaker is based on producing high pressure in the liquid dielectric by using the electrodynamic drive. High pressure in the liquid dielectric compresses the membrane contacts and separates them from the fixed contacts creating four gaps, which are filled with the liquid dielectric. Though each gap does not exceed 2 mm, all of them taken together create an interval sufficient to provide the required dielectric strength. At the same time the pneumatic valve is opened by the plane inductor. Pressure gas located inside the membrange goes out of the breaker to keep the membrane contacts in the compressed position for a long time after commutation. The total switching process does not take more than 150 ms, including the period of the electric strength recovery. In order to increase the reliability of the breaker operation the command to start the breaker drive is sent by the control unit only after producing the current counterpulse through the contact set and with the current zero identified.

2.2. Making switch Two types of mechanical closers have been designed to be used as a main switch. One of them is based on using slight membrane contacts similar to those described above and is characterized by a high operation speed and longer lifetime. The second closer has a more massive coaxial contact set and allows some switching at the maximum operating voltage in the case of a vacuum gap fault, thus giving it some advantages when applied for the protection systems. A cross-section of the mechanical closer with the membrane contact is given in Fig. 3. The closer consists of the following basic parts: membrane contact; two fixed contacts; pneumatic valve; electrodynamic drive inductor; terminals. The movable membrane contact is placed between two massive fixed contacts. Initially the movable and fixed contacts are separated from each other by gaps and fixed in this state, because the gas pressure outside the membrane exceeds its inner pressure. The contacts are closed as a result of operation of the pneumatic valve, resulting in the pressure decreasing outside of the membrane. The fast pneumatic valve is actuated by the pulse electrodynamic drive. The commutation time is approximately 1 ms. A cross-section of the mechanical closer with coaxial contacts is shown in Fig. 4. The contact set of this device comprises two coaxial fixed contacts, the movable contact and the additional

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Fig. 3. Mechanical closer with membrane contacts (in closed position): 1, membrane contact; 2, fixed contacts; 3, pneumatic valve; 4, electrodynamic drive inductor; 5, terminals.

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V. Kuchinski et al. / Fusion Engineering and Design 24 (1994) 419- 424

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Fig. 4. Mechanical closer with coaxial contacts (in open position): 1, fixed contact; 2, movable contact; 3, elastic contact: 4, membrane; 5, electrodynamic inductor.

elastic contacts. The movable contact is located on the dielectric membrane under the inductor. The movable contact is connected with the metallic plate, which initially hermetically joins the insulating case of the drive inductor. So the movable contact position is stable in spite of the gas pressure over the membrane exceeding the pressure under it. The switching process is initiated by the electrodynamic drive. First, the inductor pushes the movable contact to the elastic contacts, which prevent the movable contact from impact, when it gets in touch with the fixed contacts at the final stage of the switching. Later the pressure over the membrane provides reliable contact between the movable and fixed contacts for a long period of time. 2.3. P o l a r i t y r e v e r s e r

The operating principle of the reverser with a cross-section of the reversing switch is shown in Fig. 5. There are two separate make-and-break contact sets, each consisting of two pairs of fixed contacts and one movable roller contact. Each roller contact consists of ten rollers. The movable contacts are prepressed by springs located inside the pusher. Four thin aluminium plates are located on the insulating pusher surface opposite the drive inductors. Within each group the movable contact is displaced and fixed at one of two

Fig. 5. Simplified electric scheme and cross-section of the reversing switch: 1, fixed contacts: 2, movable contact: 3. electrodynamic drive inductor; 4, contact set pusher. stable positions with the help of the pusher, which acts upon the suspension. The switching process is initiated by two inductors, which make the pusher turn around the axis, finally resulting in displacement of the movable contact and one contact pair is closed and the other is opened. It takes not more than 10ms for the movable contacts to pass from one stable position to the other and for electric strength recovery. The reversing switch consists of four equal modules connected in parallel. Each module can operate with a d.c. current up to 10 kA.

3. Technical data and experimental investigation results The basic technical data on the switching devices and their main units are summarized in Table 1. The experimental study and testing of full-scale models have been carried out in order to determine the real characteristics of the switching networks. The whole test program can be divided into four parts: mechanical testing of the contact systems and drives; high-voltage tests; thermal testing of the contact systems under steady-state current; commutation capability testing under real conditions. The main results of these tests are summarized in Table 2.

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V. Kuchinski et al./ Fusion Engineering and Design 24 (1994) 419-424

Table 1 Technical data on the switching devices Element

Parameter

Circuit breaker

Membrane Total system

Main executive unit

Drive power supply Counterpulse unit

Auxiliaries

Maximum current (kA) Maximum voltage (kV) Volume (m 3) Weight (t) Constant current (kA) Pulse current (kA) Closed contact resistance (la.O) Switching time (ms) Electric strength recovery time (ms) Capacitor energy (kJ) Capacitor voltage (kV) Discharging current (kA) Capacitor energy (kJ) Maximum voltage (kV) Currentless pause duration ( ~ ) Cooling water flow rate (1 min -l) Gas pressure (MPa)

40 20 7 4.5 40 150 5

Polarity reverser

Making switch Coaxial

40 20 2.5 1 40 150 5

40 5 5 1.5 10 40 30

2.5 I 400 5

1

1

7 2

2.7 3 20

1.4 3 17

16 3 4

15

5

5

12

1

2

1.8

0.15 0.05 16 6 60 40 20 150

-

-

0.5

Table 2 Results of the experimental study of the switches Parameter

Circuit breaker

Making switch Membrane

Response time (ms) Maximum voltage across the contact (HV test) (kV) Maximum temperature of the contacts at the rated current (°C) Voltage recovery rate (V s -l)

0.15 60 100 2.5 x 108

0.95 60 100

Polarity reverser Coaxial 0.75 40 55

7 20 120 5x 106

HV, high voltage. D u r i n g m e c h a n i c a l testing the effect o f the elect r o d y n a m i c a n d p n e u m a t i c drive p a r a m e t e r s (cap a c i t o r energy, gas pressure etc.) on the time o f the c o n t a c t o p e n i n g a n d closing was investigated. A s can be seen f r o m T a b l e 2, a response time o f less t h a n 1 m s was o b t a i n e d for the m a k i n g switches. T h e expected r a t e d values for the b r e a k e r a n d reverser were confirmed. Testing has shown n o c o n t a c t b o u n c e d u r i n g switching. The

j i t t e r o f the b r e a k e r o p e n i n g time was e x t r e m e l y small (less t h a n 10 ~tm). This is very i m p o r t a n t for the breakers, which o p e r a t e with the c o u n t e r p u l s e network. T h e high voltage tests were carried o u t with d.c. voltages, which are a p p r o x i m a t e l y 3 times m o r e t h a n the r a t e d values (see T a b l e 2). D u r i n g the c u r r e n t h e a t i n g test, c o o l i n g w a t e r was flowing t h r o u g h the c o n t a c t sets o f the testing

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V. Kuchinski et al. [ Fusion Engineering and Design 24 (1994) 419 424

switches and the dependence of the contact temperature on the current and water flow values were studied. Since the m a x i m u m steady state current available at the existing stand was 25 kA, only part of the contact system was used under thermal testing to reach the nominal current load of 40 kA through the whole contact system. The m a x i m u m contact temperatures for the devices are given in Table 2. The cooling water temperature was checked and found to be according to design values (less than 30°C). During the last part of the test p r o g r a m m e the real switching processes were checked. In order to determine the accuracy needed for forming the current zero pause, a study of the breaker commutation capability without a counterpulse current was carried out. The commutation test with currents of up to 500 A showed reliable breaking of the circuit with an inductance of the order of 1 ~tH. N o contact erosion was seen after testing.

For the making switches it was important to identify an admissible level of voltage on the contacts during the last phase of their closing. This voltage can create an arc between the contacts and damage the contact surfaces. The tests showed that there were no traces of arc on the membrane contacts at a voltage level of up to 100V. This voltage level corresponds to the voltage across the ignited vacuum gap, which shunts the mechanical closer. For the coaxial device this level was about 200V, and for a limited number of commutations the permissible voltage could be much higher. For instance, the switch can operate under a voltage of 20 kV two or three times. This is very important for a protective mode of operation. Now tests on the service life of the switches are under preparation. The results of the experimental study indicated that all the executive units can be used as the basis for the design of the switches for the I T E R goals.