Circuit-breakers in medium voltage (MV) installations cannot provide any protection against exceptionally high peak short-circuit currents, as they are too slow.
By detecting and limiting a short-circuit current at the first rise – that is in less than a millisecond – ABB’s Is-limiter, regarded as the ‘world’s fastest switching device’, ensures that the maximum instantaneous current occurring remains well below the level of the peak short-circuit current.
The Is-limiter has both technical and economic advantages when used in transformer or generator feeders and in switchgear sectionalizing, or connectedin parallel with reactors. It is ideal for solving switchgear short-circuit problems in power stations, heavy industry and utility applications.
THE IS-LIMITER PRINCIPLE
The Is-limiter consists of an extremely fast switch, able to carry a high-rated current but having a low switching capacity,and a high rupturing capacity fuse arranged in parallel. In order to achieve the desired short opening time, a small charge is used as the energy store for opening of the switch (main conductor). When the main conductor is opened, the current continues to flow through the parallel fuse, where it is limited within 0.5 milliseconds, and then finally interrupted at the next voltage zero passage.
The current flowing through the Is-limiter is monitored by an electronic measuring and tripping device. At the very first rise of a short-circuit current, this device decides whether tripping of the Is-limiter is necessary. The three phases are operated independently of one another.
Is-limiters are frequently used in interconnections between systems or in bus sections that would not be adequately short-circuit proof when connected by a circuit-breaker. They offer a number of advantages, including:
Reduction of the series network impedance. The voltage drops caused by load surges (for example during motor starting) can be significantly reduced.
Improvement of the current distribution at the feeder transformers.
The load-dependent losses of the feeder transformers are reduced.
Increased reliability of the power supply. On failure of one feeder transformer, the load is taken over by the other feeder transformers without current interruption. The cost of a new switchboard with higher short-circuit capacity that would otherwise be required is therefore saved.
A remarkable advantage of the use of an Is-limiter is that the voltage in the part of the system not affected by a short-circuit only drops for a fraction of a millisecond so that even sensitive loads (such as computers) remain protected from drops in the system voltage.
For this reason the IS-limiter is ideal for use as a link between an ‘unprotected’ and a ‘protected’ switchboard or section of a switchboard.
LINKING PUBLIC AND PRIVATE NETWORKS
Where a system with its own power generating facilities is interconnected with public supply networks, there is a risk of exceeding the permissible short-circuit current in the utility network. The most appropriate technical solution – and sometimes the only one – is to install an Is-limiter in the interconnection with the public utility network
If necessary, the Is-limiter can be provided with a directional tripping criterion, so it will trip only on short-circuits in the public supply network if a generator is in operation.
WORKING IN PARALLEL WITH A REACTOR
The Is-limiter can also be connected in parallel with a reactor. If a short-circuit occurs behind the reactor, the Is-limiter trips and the current commutates at the first current rise to the parallel reactor, which then limits the short-circuit current to the permissible level