By Magnus Callavik, Anders Blomberg, Jürgen Häfner, Björn Jacobson
A hybrid high-voltage direct current (HVDC) breaker that delivers negligible conduction losses while preserving ultra-fast current interruption capability has been developed by ABB. Advancements in voltage source converter-based (VSC) HVDC transmission systems make it possible to build an HVDC grid with many terminals. Compared with high-voltage alternating current (HVAC) grids, active power conduction losses are relatively low and reactive power conduction losses are zero in an HVDC grid. While HVDC grids are more attractive, the relatively low impedance in HVDC grids is a challenge when a short circuit fault occurs because the fault penetration is faster and deeper. Consequently, fast and reliable HVDC breakers are needed to isolate faults and avoid a collapse of the common HVDC grid voltage, with the ability to clear faults within a few milliseconds.
Recently, plans for large scale use of embedded VSC-HVDC transmission in point-to-point overhead lines have been proposed in Germany, in the network development plan (NEP). New power generation sources from remote sites such as renewable sources drive the case for VSC-HVDC systems in the areas of active power transmission and reactive power compensation. A hybrid HVDC breaker-or ABB's new hybrid HVDC breaker-can provide the additional benefit of interrupting HVDC line faults.
Existing mechanical HVDC breakers are capable of interrupting HVDC currents within several tens of milliseconds but are too slow to fulfill the requirements of a reliable HVDC grid. In addition, building mechanical HVDC breakers requires the installation of additional passive components to create the resonance circuit and generate the current zero crossing so the breaker will succeed in breaking the current once it opens. Existing HVDC switches have been used for more than 30 years in the neutral switchyard of bipolar HVDC installations-with various functions such as rerouting HVDC current during reconfiguration of the main circuit or helping extinguish fault currents. The metallic return transfer breaker (MRTB), for example, is used to commutate the current from the ground path to a metal conductor when there are restrictions on how long an HVDC current can be routed through the ground. The two main differences between these transfer breakers and the hybrid HVDC breaker is that the transfer breakers operate slower than the hybrid breaker and that part of the current is transferred, not interrupted.
System Design Requirements
An HVDC grid is formed when more than two converter stations are interconnected on the HVDC side through HVDC cables or overhead lines. Each converter station of the multi-terminal HVDC grid couples the HVDC grid to an ac grid. To maintain the converter's active and reactive power control capability, the HVDC voltage should be above 80 percent of the nominal HVDC voltage. If the converters lose control capability because of low HVDC voltage, the consequences can be voltage collapse in the HVDC grid and high current or voltage stresses for the converter. This can affect the coupled ac grid voltage. An HVDC short-circuit fault can suddenly drop HVDC voltage from nominal level to near zero at the fault location. Voltage reduction in other places of the HVDC grid mainly depends on the electrical distance to the fault location and HVDC reactors installed near the converter stations. For an HVDC grid connected by HVDC cables, a short-circuit fault must be cleared within five milliseconds to not disturb converter stations that are connected to the same ac grid, since this would influence the combined grid performance.
In addition, HVDC switches are necessary to limit the short-circuit current rating capability of the HVDC breaker. Thus, grid performance and breaker design criteria make dc breaker a significantly different challenge compared to ac fault clearing times.
Hybrid HVDC Breaker
The hybrid HVDC breaker consists of an additional branch, a bypass formed by a semiconductor-based load commutation switch in series with a fast mechanical disconnector. See Figure 1. The main semiconductor-based HVDC breaker is separated into several sections with individual arrester banks dimensioned for full voltage and current breaking capability, whereas the load commutation switch matches lower voltage and energy capability. After fault clearance, a disconnecting circuit breaker interrupts the residual current and isolates the faulty line from the HVDC grid to protect the arrester banks of the hybrid HVDC breaker from thermal overload.
Prototype Design of the Hybrid HVDC Breaker
The hybrid HVDC breaker is designed to achieve a current breaking capability of 9.0 kA in an HVDC grid with rated voltage of 320 kV and rated HVDC transmission current of 2 kA. The maximum current breaking capability is independent of the current rating and depends on the design of the main HVDC breaker only. The fast disconnector and main HVDC breaker are for switching voltages exceeding 1.5 p.u. in consideration of fast voltage transients during current breaking.
The main breaker consists of several breaker cells with individual arrester banks limiting the maximum voltage across each cell to a specific level during current breaking. Each main breaker cell, shown in Figure 2, contains four breaker stacks, which break the current in either current direction.
Each stack is composed of up to 20 series connected insulated gate bipolar transistor (IGBT) HVDC breaker positions. Because of the large transient current (di/dt) stress during current breaking, a mechanical design with low stray inductance is required. Application of press pack IGBTs with 4.5-kV voltage rating enables a compact stack design and ensures a stable short circuit failure mode in case of individual component failure. Individual resideual current device snubbers across each IGBT position ensure equal voltage distribution during current breaking. Optically powered gate units enable operation of the IGBT HVDC breaker independent of current and voltage conditions in the HVDC grid. A cooling system is not required for the IGBT stacks.
For the load commutation switch design, one IGBT HVDC breaker position for each current direction fulfills the requirements of the voltage rating. Parallel connection of IGBT modules increases the rated current of the hybrid HVDC breaker. Series connected, redundant IGBT HVDC breaker positions improve the reliability of the load commutation switch. A matrix of IGBT positions for each current direction is chosen for the present design. Since the load commutation switch is continuously exposed to the line current, a cooling system is required.
Hybrid HVDC Breaker Test Results
During the design of the hybrid HVDC breaker prototype, tests were performed to verify expected performance. The first focused on validation of the system components, such as the semiconductor devices and their current breaking capability, and the ultra-fast disconnector operation dielectric properties. The voltage and current capability of one cell of the main breaker was verified.
The extension of this test focused on the overall performance of the hybrid HVDC breaker system as outlined in Figure 1. Successful verification testing has proven the performance of the hybrid HVDC components, and the breaker has been verified in a demonstration at ABB facilities. The maximum rated fault current of 9 kA is the limit for the existing generation of semiconductors. The next generation will allow breaking performance up to 16 kA. The tests verified switching performance of the power electronic parts and the opening speed of the mechanical ultra-fast disconnector. The test object consisted of one 80-kV unidirectional main breaker cell, along with the ultra-fast disconnector and load commutation switch. The higher voltage rating is accomplished by connecting in series several main breaker cells, but the switching stress per cell is the same as in the demonstrator. Tests have been performed for normal breaking events and situations with failed components in the breaker to verify reliable detection and safe operation in such cases.
Outlooks and Conclusions
Fast, reliable and nearly zero loss HVDC breakers and current limiters, based on the hybrid HVDC breaker concept, have been verified at ABB's high-power laboratories in Sweden and Switzerland for HVDC voltages up to 320 kV and rated currents of 2 kA.