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Fast Discharge Units (FDU) forming part of the ITER coil power supply systems are intended for protection of superconductive coils by extracting the energy stored in the magnet system in case of a quench or other abnormal events. It will be provided by opening the coil current circuits by normally closed circuit breakers, thus diverting the current into discharge resistors. The interruption of DC currents in the inductive circuits is provided with the help of counter pulse circuits (CPC), the discharge of which in the direction opposite to the coil current direction ensures the arc extinguishing in the circuit breakers. The paper describes the design of two CPC modifications and presents the results of the type tests carried out on CPC full-scale prototypes. The successful results of the tests demonstrated the working efficiency of CPCs, confirmed their compliance with the design requirements and gave green light to the manufacturing of these devices to be delivered to the ITER Site.

When the capacitor cell is discharged in the short-circuit mode, the current pulse amplitude and duration are maximal. Therefore, this mode is the most severe for discharge switches of capacitive energy storage. The characteristics of the transient process of the discharge capacitive energy storage and the current loads acting in the facility discharge circuit have been defined for this mode. The test bench for definition of the LTT (Light Triggered Thyristors) loading capacity is described. The limiting characteristics have been experimentally obtained for LTT, at which there emerges the thermo-generation peak. The process of OFF-ON switching of LTTs has been investigated, the necessity is shown to use the speed-up R-C circuits to ensure fast and stable transition of LTT into the conducting state. The design of the switch unit for the capacitive energy storage comprising LTTs and crowbar diodes is described, and the transient processes of current switching in crowbar diodes are considered. The tests carried out during switching of pu1se current up to 100 kA at a voltage of 6 kV have confirmed the workability of the switch unit. Keywords: Energy storage, Power semiconductor switches, Photothyristors, Pulse power systems

This article presents the results of testing heavy pulse current switches based on photothyristors. Transient processes are considered that are observed in the semiconductor switch when the capacitor is discharged upon inductance and with a signal-shaping network with an inductor and crowbar diodes. The limit currents are found for the semiconductor structure, at which thermal generation peaks are observed in oscillograph records of direct voltage drop. The energizing of the photothyristor is analyzed. The need for forcing RC circuits to ensure fast and steady transition of the semiconductor structure to the conductive state is shown. The transient process observed at reverse recovery of photothyristors in the discharge circuit with inductance is considered. The need for snubber circuits to suppress pulse overvoltage is established. The particulars of the transient switching of current to the crowbar diodes and the generation of pulse overvoltages at reverse recovery of photothyristors are considered for a discharge circuit with crowbar diodes. The parameters of snubber circuits to suppress these overvoltages are determined. The validity of the accepted engineering solutions is confirmed by the results of tests at switching of a pulse current of up to 100 kA.

Failures in capacitive energy storages result in destruction of faulty elements and other expensive components. Recovery of such failures requires long-term and expensive works to replace and to restore elements of the capacitive energy storage. Diagnostic hardware of the capacitive energy storages makes it possible to limit the consequences of such failures, to reduce the frequency of their occurrence, as well as to monitor the equipment and transmit information on functioning of the pulse current sources to the facility. The operation of data acquisition system in high voltage facilities is accompanied by the electromagnetic interference, and it is therefore necessary to provide the optical fiber transmission lines, the isolation of the diagnostics hardware components and galvanic isolation of their power supply.

The operation of counterpulse capacitor banks has specific features which stand in the way of ensuring reliable functioning of the semiconductor switches. These features are as follows. Its operation is in the surge current mode, in which the semiconductor switches may become overheated, with the ensuing thermal breakdown of the semiconductor. The discharge process ends with blocking of the switches and current interruption in the circuit containing inductance. The resulting high-voltage spike may bring about electric breakdown and failure of the semiconductor switch which had been overheated by the high surge current. This paper studies these features and examines the ways capable of providing reliable operation of pulsed-power thyristors and light-triggered thyristors as discharge switches. The ultimate current loads were derived by analyzing thermal generation peaks in the curves of direct-voltage drop across the thyristors. The methods and equipment employed to suppress switching overvoltages are discussed. Specific snubber circuits which provide suppression of overvoltages and protect the thyristor against pulses with an inadmissibly high current rise rate have been developed and tested. The possibility of reliable operation of semiconductor switches in a counterpulse capacitor bank has been demonstrated.

A capacitive energy storage is intended for generating high-power current pulses. The setup consists of two capacitive energy storage modules, a control console, and a cable collector for connecting a load to the setup. Each module is a capacitive energy storage with a 0.5-MJ stored energy and 18-kV voltage, which is based on eight capacitor cells with reverse switch-on dynistors as switches. The module volume is 1.3 m3. The semiconductor switches in the capacitor cells are activated by light pulses, which are transmitted from the control console through fiber-optic cables. The unit is designed for operating in the programmable discharge mode, at which the semiconductor switches in the capacitor cells are switched on nonsimultaneously but in accordance with a specified program. When the discharge of all the cells is switched on simultaneously and the load is short-circuited, the maximal amplitude of the output current pulse is 800 kA. The rise time of the discharge current pulse of the cell is 150 μs.

The capacitor banks are intended for generation of the counter current pulse in vacuum circuit breakers of a two-step powerful contactor (switchgear). This switchgear is part of the power supply system of ITER superconductive coils and provides fast discharge of coil magnetic field energy by breaking the coil current supply loop and connecting in the loop the high energy resistors. The superconductive coils of the ITER facility store considerable energy (several gigajoules) and the counter pulse capacitor bank should generate the pulse current not less than 70 kA during about 600 μs at an initial voltage up to 10 kV. After generation of the current pulse the discharge switch of the capacitor bank is closed and a high voltage exceeding the initial one (10 kV) is applied to the discharge switch. The paper presents the features of the transients during the commutation cycle, design of the capacitor bank and its main components (semiconductor discharge switch, inductor, charger, etc.), as well as the results of the capacitor bank testing.

Operation of Counter-Pulse Capacitor Banks (CPCB) is characterized by a number of features, which complicate the safe operation of semiconductor switches. These features are as follows. The necessity to operate in the surge-current mode at high currents up to 100kA within about 1 ms results in overheating and possible thermal damage of the semiconductor structure. The discharge process ends with closing of the switches and emergence of reverse voltage, which can be higher than the initial voltage of the capacitors. One of the key elements of the discharge circuit is the inductor, which limits the discharge current. Reverse recovery process in the circuit with this inductor causes generation of high-voltage spikes, which might result in the damage of a semiconductor device overheated by surge-current by this moment. The paper studies the above-listed features and analyzes the safe operation of Pulsed Power Thyristors (PPT) and Light Triggered Thyristors (LTT) under these conditions. The method for tracking of thermal generation peaks on the thyristor voltage drop curve was used for evaluation of allowable currents. Suppression of switching overvoltages is commonly provided by snubber R-C circuits. To eliminate spikes a snubber R-C circuit should contain, as a rule, large capacitors and resistors with a low resistance. But, in this case the rate of anode current pulse rise is very high, which might cause the thyristor to fail during switch-on. This made it necessary to develop special snubber circuits, which are free from disadvantages of usual circuits. The paper presents the results of experimental research of the thyristor switch-on and switch-off in CPCB prototypes. The developed method (suppression of impulse switching overvoltages) is demonstrated. The possibility of safe operation of the semiconductor switches in CPCB has been verified.

Superconductive magnetic systems capable of storing energy up to tens of gigajoules are used in modern electrophysical facilities. Protection of such systems is provided by Fast Discharge Units (FDU). In case of failure, in particular quench in a superconducting coil, rather fast and safe energy discharge from the magnetic system is provided by FDU, which breaks the coil supply loop and provides energy dissipation in high energy resistors. The ITER FDU should interrupt currents up to 70 kA with a voltage up to 10 kV. The FDU includes a two-stage switch consisting of a ByPass Switch (BPS, contactor with a pneumatic drive) and a fast vacuum circuit breaker (VCB). The other important part of FDU is a capacitor bank for generating the counter current pulse. The closed BPS carries a steady-state current up to 70 kA. Once the BPS opens, current passes into closed contacts of VCB. Shortly after, the VCB opens and the capacitor bank generates a counter current pulse, which compensates the current in VCB. In such a way the arcless current commutation is provided in the FDU. This paper studies the transients during FDU electrical switch-off. Peculiarities of the capacitor bank operation are considered. Capacitive storage rated for 5 kV, 70 kA is described. The tests results of capacitor bank are presented.

The pulsed current source is intended for research of high-current electrical discharges in dense media in laboratory conditions. The facility includes two modules of capacitive energy storage, control panel unit and cable collector which connects facility with the load‥ Each module is a functionally completed remote-operated capacitive energy storage 0.5 MJ, 18 kV, built on eight capacitor cells with reverse-switching dynistor (RSD) switches. The module has volume 1.3 m3 and comprises a charger with a high-frequency inverter, a protection contactor with normally closed high-voltage contacts to neutralize the cell charge, control units for triggering of RSD switches, control and diagnostics apparatus with a programmable logic controller. The semiconductor switches in the capacitor cells are triggered by light pulses transmitted from a control panel unit via fiber-optical cables. Information exchange between the modules control and diagnostics system and the control panel unit is also realized through fiber-optical cables. The facility is designed for operation in the programmable discharge mode, when semiconductor switches in the capacitor cells are switched on in the specified time sequence according to the preset program. A maximal current pulse amplitude of 800 kA at the module output is attained during simultaneous discharge of all cells in the short-circuit mode. The wave-front of the current pulse is 150 µs.

The module of the capacitive energy storage is intended to investigate high-current electrical discharges in dense media. The module is a functionally completed remote-operated capacitive energy storage built on eight capacitor cells with reverse-switching dynistor (RSD) switches. The module comprises a charger with a high-frequency inverter, a protection contactor with normally closed high-voltage contacts to neutralize cell charge, control units of RSD switches, control and diagnostics apparatus with a programmable logic controller. The semiconductor switches in the capacitor cells are triggered by light pulses transmitted from a remote control board via eight fiber-optical cables. Information exchange between the module control and diagnostics system and the remote terminal is also realized via fiber-optical cables. The storage is designed for operation in the programmable discharge mode, when semiconductor switches in the capacitor cells are switched on in the specified time sequence according to the preset program. A maximal current pulse amplitude of 400 kA at the module output is attained during simultaneous discharge of all eight cells in the short-circuit mode. The wave-front of the current pulse is 150 µs. The volume of the capacitor module is 1.3 m3.