Scibreak’s concept, VSC Assisted Resonant Current (VARC) circuit-breaker (CB), uses a mechanical vacuum interrupter (VI) together with auxiliary circuitry that creates a current zero-crossing in the internal arc during contact separation. This is a known principle with the following characteristics:
1. A mechanical interrupter is used to conduct the load current in normal closed conditions, making losses almost negligible.
2. An auxiliary circuit superposes a current pulse through the arc in the switch, causing the current through the switch to cross zero.
3. The mechanical interrupter itself executes the interruption when the current through it approaches zero.
The specific feature of the VARC CB is how the current pulse is excited and controlled. In most known concepts the current pulse is achieved either by releasing the charge in a pre-charged resonant circuit capacitor by a switch or by slow excitation due to negative resistance in the arc of certain breaker types. The current pulse in the VARC concept is excited, while the contacts are separating, by repetitive interactions of a voltage-source converter (VSC). There is no high voltage nor high current in the auxiliary circuit before the switching operation takes place. Instead energy is charged in the DC-link of a low-voltage VSC. This voltage is at least one order of magnitude lower than the transient inception voltage (TIV), which appears across the circuit breaker contacts after contact opening. How this can be possible will be explained below.
Tomas Modeer, our CEO and Simon Nee, chief engineer, inspecting prototypes developed in Scibreak’s head office and workshop in Stockholm.
Figure A. Two principles to achieve current pulses with identical amplitude.
Excitation of the resonant circuit aims to create a current pulse with higher amplitude than the line current to be interrupted. Such a pulse can be created by applying a voltage step across a resonant circuit. Exactly how this voltage step is created is irrelevant. One way obviously is to switch in a capacitor, pre-charged to sufficient voltage, like in some known DC CB concepts. This approach is sketched in the left figure in Figure A.
Another way to create a voltage step with the same amplitude is to execute a voltage reversals in a single-phase VSC bridge with a DC-link voltage that is half the voltage of the desired step.
If when the resonant circuit swings back, a second reversal is performed by the VSC and applied to the resonant circuit, a new current pulse is created and added to the first one — thereby doubling the resulting amplitude of the oscillating current. The desired amplitude of the final current pulse using two reversals can be achieved by the VSC using half the DC-link voltage compared to the first one.
The excitation using two half-cycles is illustrated in the left diagram in figure B below. This principle of course may be extended to use several reversals. Each added reversal is reflected by a corresponding reduction of the required DC-link voltage in the VSC. Excitation using seven reversals is shown as an example in Figure B below to the right.
Figure B. Illustration of reduction of needed DC link voltage by use of many reversals.
Obviously, some time will be required to perform many reversals. Is that a problem? To answer that question let us consider what is meant by a long time and a short time. While a mechanical actuator is “fast-acting” if it executes an opening command in a millisecond, power electronic devices naturally deal with switching times in the microsecond range. As a result, a VSC can easily execute many switching operations during the time interval needed for an ultra-fast switch to separate the contacts in a VI.
The child’s swing is an adequate illustration of this excitation method — even a small child brings about a high swing amplitude by repeated small injections of power.
A little girl on a swing makes small injections of power to reach high swing amplitude.
The VSC typically is a full-bridge single-phase converter containing four arms with one semiconductor switch in each arm. The rating of the semiconductor devices in these switches is directly proportional to the DC-link voltage in the VSC, so the rating of the required amount of semiconductor devices can be reduced in proportion to the number of reversals that can be utilized to reach the desired current pulse amplitude. The cost of semiconductors often is the dominating item in the bill of material for circuit breakers of this kind and therefore the principle described here is of paramount importance to achieve cost effectiveness for circuit breakers of this kind.
With 10 kHz resonance frequency five full cycles, each including two reversals, can be executed in 0.5 ms. The accumulated voltage step would ideally be 5×2×2×udc=20×udc. This example explains why, by repetitive action, a few semiconductors can achieve the same effect as many semiconductors acting simultaneously only once.
It is worth noting that current interruption is performed by the mechanical VI and not by the semiconductors. The latter are only used to excite the oscillating current. In order to charge the oscillating circuit with maximum speed the voltage of the VSC shall be in-phase with the oscillating current, so that commutation shall occur close to the oscillating current zero-crossings. Accordingly, the required semiconductor turn-off capability is limited but high current conduction capability is desired.
The use of power electronics makes it possible to perform accurate control of the oscillating current. Therefore, it is controlled such that a current zero-crossing occurs as soon as possible after sufficient gap between the contacts has been established. In this way arcing time in the switch is minimized.
Successful current interruption requires coordination between the motion of the moving contact in the mechanical interrupter and control of the excitation system. The voltage withstand capability of the mechanical switch increases with the expanding contact gap at an opening operation as indicated by the dashed line. The transient inception voltage at current interruption, determined by the MOV protective voltage, must be lower than the voltage withstand capability. The control system for the power electronic excitation system safe-guards the cooperation between the mechanical and the electrical system so that the conditions necessary for successful current interruption always will be satisfied.
The use of oscillating current to achieve current zero-crossing in the arc in the mechanical switch implies that zero cross-over will occur independently of the line current direction. The VARC circuit breaker therefore is bidirectional by nature.
Using an oscillating current with growing amplitude to create the current zero-crossing in the arc in the mechanical switch brings about an automatic adaptation of the amplitude of the pulse amplitude to the actual line current. Accordingly, low line currents are interrupted using a pulse with low amplitude, while high line currents are interrupted using a pulse with high amplitude. Such conditions are beneficial for the interruption of low line currents.
The use of power electronic converter for excitation of the oscillating current allows fast control of the oscillating current, which facilitates implementation of intelligent behavior of the circuit breaker response to various unusual events like e.g. re-ignition in the interrupting switch.
The VARC circuit breaker utilizes a vacuum interrupter as the main current interrupter. A section of a typical vacuum interrupter is shown below to the left. The voltage withstand capability of vacuum is exceptionally high and reaches values of tens of kilovolts per millimeter at low pressure as shown in the Paschen curve to the right in Figure C. Therefore, only a short stroke of the moving contact is needed to withstand high voltage. Vacuum interrupters nowadays dominate the world market of medium-voltage switchgear with rated voltage up to 40 kV. Those standard devices are mass-produced, making them available in many forms, accessible from many manufacturers at a reasonable price level.
Traditionally, conventional AC circuit breakers come with an actuator using a mechanical spring. Permanent magnets are sometimes also used as they provide latching and actuating. For these two types of device, the typical operating time is a few tens of milliseconds. For the applications, that the VARC concept is intended for, this is far too slow. As mentioned above, the operating time for circuit breakers is of paramount importance in high-voltage DC networks and for current-limiting AC circuit breakers. It must not exceed a few milliseconds.
Such very fast operation can be achieved in actuators using a Thomson coil. This principle has been described in several papers, which showed that this concept is suitable for the present application and can meet the high requirement on short operation time. The principle is illustrated in the figure below. A conductive disk is placed in the vicinity of a flat, spiral coil, which is energized by a high-current pulse obtained by discharging a capacitor bank. The magnetic field induces a mirror current with opposite direction in the armature metal disk. The two currents create an axial repulsive force, which can be applied on the moving contact of a vacuum interrupter, through a pulling rod. The force arises almost instantaneously when current starts flowing through the coil, which can be done in less than one millisecond. Typically, the current peak amplitude reaches several kiloamperes and the duration of the current pulse is a few milliseconds. The force falls off quickly when the axial distance between the coil and the disk increases. This force has the character of an impulse causing the disk to accelerate very fast, several hundred to up to thousand times the acceleration of gravity.
A prototype of a circuit breaker using a Thomson coil actuator to maneuver the moving contact has been designed, implemented and tested at SCiBreak. The vacuum interrupter, in this case rated for 24 kV/1600 A, 20 kA short-circuit current, is located on the top. The moving contact is pulled via an over-travel mechanism by a composite shaft connected with the armature disk in the Thomson coil arrangement. The purpose of the over-travel mechanism, which basically is a spring arrangement, is to safe-guard that the contact pressure in closed state is within certain specified limits in spite of small variations in the mechanical structure as well varying contact erosion. In this laboratory prototype the pulling rod is just a short composite shaft providing a low insulation between the actuator and the moving contact in the vacuum interrupter.
Two Thomson coils are installed in opposite directions. The first one for the OPEN operation and the second one is for the CLOSE operation. In both open and closed positions, the moving contact is maintained in place by a bi-stable spring mechanism. This mechanism provides a contact force in the closed position to reduce the contact resistance in the vacuum interrupter. More details are pictured in Figure D.
Vacuum interrupters can handle voltages up to approximately 100 kV. For HVDC transmission systems, typically using rated voltages from ±200 kV and above, several modules connected in series can be used to achieve the required voltage withstand capability. Each module is provided with its own voltage-limiting MOV protecting the module against over-voltage. The modular approach allows for the use of redundant modules, which significantly can increase the reliability of the circuit breaker as a whole.