A circuit breaker is a switching device that interrupts the abnormal or fault current and it is designed for closing or opening of an electrical circuit to protect the electrical system from damage. The circuit breaker is required to perform following three major duties under short circuit conditions.
• It is capable of breaking the faulty section of the system.
• It is capable of making the circuit in the greatest asymmetrical current in the current wave.
• It is capable of carrying fault safely for a short time while the other breaker is clearing the fault.
In addition to the above rating, the circuit breakers should be specified in terms of
• Number of poles
• Rated voltage
• Rated current
• Rated frequency
• Operating voltage
The classification of the circuit breaker is shown in the Figure 1.
HVDC Circuit Breaker
Over current protection of DC system is more challenging than AC system due to absence of natural zero crossing in a DC circuit. The design and operation DC system is different from AC circuit.
The basic requirements of a HVDC circuit breaker are:
• Creation of artificial current zero
• Dissipate the energy stored in the system inductances
• Withstand the voltage stress
• Prevention of restrike arc
The breaker which is used for the interruption of the high voltage direct current is known as the HVDC circuit breaker. The voltage breaking capacity of the HVDC circuit breaker is nearly 33KV, and for the current, it is 2KA. The fault current in the HVDC circuit breaker should be reduced to zero by using some external methods. The arc quenching medium of the air break circuit breaker is either oil or air blast.
The mechanical and solid-state breakers have both merits and demerits. Solid state breakers have ultra-speed, high switching losses and high cost. Mechanical breakers have low losses, low cost and very slow in operations. Integrating both with their merits and eliminating the demerits is called hybrid switching technique.
Hybrid HVDC Circuit Breaker
The hybrid DC breaker consists of three parallel branches to handle different tasks of the breaker. The first branch contains a mechanical switch that will carry the nominal current with metallic contacts resulting in conduction losses similar to conventional, mechanical circuit breakers. The second branch consists of semiconductors with a high switching performance. The third branch is metal oxide varistors (MOV) to limit the transient voltages and absorb the magnetic energy stored in the system.
Figures 3 and 4 show the current through the hybrid breaker and the voltage across the breaker when interrupting a rising fault current. The interruption of the current can be divided into five steps: fault detection, commutation, semiconductor conduction, semiconductor turn-off, and current limitation.
The arrangement of a very effective hybrid HVDC circuit breaker shown in figure below. Hybrid HVDC current consists of fast-mechanical disconnector (FMD) and a load commutation switch (LCS). The main branch consists of semiconductor interrupted and arrester bank. During normal operation current flows through the auxiliary branch, the current is transferred to main branch in case of fault. This happens through LCS, which is quickly opened and transferred the current to the arresters. To protect the load commutation switch from the voltage, build up across the breaker, the first mechanical disconnector opens as soon as the auxiliary branch does not carry current. A residual current breaker interrupts the residual arrester current to protect the arrester from thermal overload and isolate the fault line from the HVDC grid.
The HVDC circuit breaker operates in current limit mode. The maximum duration of current limit mode depends on the energy dissipation capability of the arrester breaker. To improve the reliability and rating a matrix LCS can be recommended. The main branch can be connected to a single arrester bank or parallel arrester bank parallel with several semiconductor cells.
Highest available component ratings, For the IGBT, two different values are given since the highest voltage rating is only available for relatively low current levels. Refer Table 1.
On-state forward voltage drop for ABB high power semiconductor components. The voltage drops are given for a junction temperature of 250C and 1250 respectively. Values in parentheses are interpolated values as they are exceed the component ratings. Refer Table 2.
Challenges of Hybrid HVDC Circuit Breaker
Nevertheless, the use of the conventional AC mechanical breaker in combination with a solid-state device is challenging due to:
• Fault detection and interruption times are required for the two components
• Different current rating capabilities, i.e. the conventional AC mechanical breaker can interrupt a fault current of some tens of kA but on the other hand controllable solid-state devices, such as IGBTs, can interrupt currents of only some kA.
• Arc voltage: The current from the mechanical breaker to the solid- state device an arc voltage which is double as high as the solid-state device voltage drop is required.
• Commutation time: High commutation time results increase in magnitude of the fault current and therefore the solid-state device is forced to interrupt very high currents.
• High-conduction time is required in order to completely commutate the current from the mechanical breaker to the solid-state device.
Features of Hybrid HVDC Circuit Breaker
The configurations of the hybrid HVDC circuit breaker should be designed to meet the following features:
• Arc-less interruption
• Lower turn-off current.
• Current limiting ability
• Lower varistor rating is required
• Overall turn-off process completes earlier
• Comparably lower commutation time possible
• Can be used in both AC and DC current interruptions
• Possible reduction of the conduction time of the solid-state breaker
• The solid-state device must handle (dissipate) comparably lower energy
• Lower temperature rise in the solid-state device due to lower peak current.
• Turn-on at lower fault current compared with the conventional hybrid breaker
• Required fault current handling capability of the mechanical contacts can be reduced.
The design of a hybrid DC-breaker is not straight forward. All different aspects have to be considered either by several iterations or by the means of an optimization. Even when optimal designs are found for the technical performance, external parameters as cost and reliability has to be considered for the final design and those parameters are not always easy to quantify.