David Hughes, of ABB UK, explains why Is-limiters now form a vital element in containerized MV switchgear solutions

There is a growing trend for process industry customers who are looking to upgrade their MV (medium voltage) distribution networks to request a fast-track solution in the form of containerized switchgear. These fully pre-engineered units are delivered to site ready to ‘plug and play'. This approach can result in a more cost-effective overall project that is around 20% faster to complete than the conventional ‘build on site' route. It also provides a smooth changeover with minimal disruption. Furthermore, with floor space at a premium, the containerized solution can release valuable space for more productive use.

However, the replacement of existing switchgear and cable connections with new equipment with higher short-circuit ratings can be technically challenging. Hence the increasing focus on the use of Is-limiters as part of the containerized solution, where they can reduce the short-circuit current in both extensions of existing systems and in new systems. With the capability to detect and limit a short-circuit current in less than a millisecond the Is-limiter has earned a reputation as the ‘world's fastest switching device'.

The function of the Is-limiter
Circuit-breakers cannot provide any protection against unduly high peak short-circuit currents, as they are too slow. By detecting and limiting a short-circuit current at the first rise, i.e. in less than  a millisecond, the Is-limiter ensures that the maximum instantaneous current occurring remains well below the level of the peak short-circuit current.
In comparison with complex conventional solutions, the Is-limiter has both technical and economic advantages when used in transformer or generator feeders, in switchgear sectionalizing and connected in parallel with reactors. So, in every regard, the Is-limiter is the ideal switching device to solve switchgear short-circuit problems in power stations, in heavy industry and in utility applications.

The Is-limiter principle
The Is-limiter consists in principle 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. In order to reach this decision, the instantaneous current and rate of current rise at the Is-limiter are constantly measured and evaluated. When the setpoints are simultaneously reached or exceeded, the Is-limiter trips.
The three phases are operated independently of one another. The loss-free conduction of a high operating current on the one hand and the limitation of the short-circuit current at the first current rise on the other hand are made possible by distributing these two functions of the Is-limiter between two conductors. In comparison with reactors, the Is-limiter avoids voltage drops and does not contribute to the peak short-circuit current.

A DC voltage of 150V generated in the power unit is used as the charging voltage for the tripping capacitors and at the same time as the supply voltage for the electronics. As far as necessary, the supply voltage is divided and stabilized within the individual assemblies. A watchdog module in the power unit constantly monitors the most important functions of the three tripping units.

Tripping unit
The current supplied by the tripping transformers for the corresponding phases is monitored in the tripping units. The three tripping units work independently of each other.
Both the rate of current rise and the instantaneous current value are used as criteria for tripping. Both variables are converted into proportional voltages and supplied via logical gates to an electronic measuring element. The latter provides an output signal when the rate of current rise and the instantaneous current value have both simultaneously reached the response value of the measuring element.

The output signal from the measuring element then activates a thyristor, which discharges a capacitor via the pulse transformer in the Is-limiter insert holder to the charge. At the same time, this discharge excites the corresponding flag indicator relay ‘Is-limiter tripped' in the indication unit..

Is-limiters in system interconnections
Is-limiters are frequently used in interconnections between systems or in bus sectionswhich would not be adequately short-circuit proof when connected by a circuit-breaker.
Each partial system should have at least one incoming feeder, so that power supply to each partial system can be maintained on tripping of the Is-limiter
There are a large number of advantages for the operation under normal conditions of bus sections connected by Is-limiters:
- Reduction of the series network impedance. The voltage drops caused by load surges (eg. 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

If a short-circuit occurs within a system or in an outgoing feeder, the Is-limiter trips at the first rise of the short-circuit current and divides the busbar system into two sections before the instantaneous current reaches an inadmissible high level. After tripping of the Is-limiter, the short-circuit is only fed by the transformer in the part of the system affected by the short-circuit. The short-circuit current is now selectively interrupted by the circuit-breaker.
A remarkable advantage of the use of an Is-limiter is that the voltage in the part of the system not affected by the 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.

Is-limiters used as a link between public and private networks
There are many examples of systems with their own power generating facilities being interconnected with public supply networks. The additional short-circuit current from these generators can result in the permissible short-circuit current in the utility network being exceeded. 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. This requires three additional current transformers in the neutral connections of the generators. The Is-limiter then only trips on short-circuits in the public supply network if a generator is in operation.

Is-limiter 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.

For normal operation, the Is-limiter bridges the reactor coil.
This avoids:
- Current dependent copper losses and the associated operating costs of the reactor.
- Current dependent voltage drop at the reactor, which frequently causes major
difficulties on start-up of large motors.
- Control problems with the generator.

Is-limiters as part of a totally integrated solution
Is-limiters are a vital element in ABB's fully equipped, totally integrated, containerized switchgear solution. Typically, this will comprise:
- 33kV gas insulated switchgear or 11kV vacuum switchgear
- Is-limiter
- Control equipment
- Batteries and chargers to provide backup power
- Heating and lighting
The equipment is housed in a robust container constructed in either GRP or steel according to site requirements. The actual size varies depending on the switchgear application,  but typically the container will be around 10 metres long, 5 metres wide and 4 metres high. This compact size enables the container to be easily transported on a standard low-loader.
Civil works are minimal since all that is required is basic foundations for the container to rest on - this results in a significant reduction in project costs . Once the unit is in place it is simply a question of connecting it up to the appropriate cables, so the need for outages is also minimized.

Minimal disruption
One of the most popular applications for containerized switchgear is at process sites where customers need to replace or upgrade an existing switchgear installation that is nearing the end of its working life. Because all the main construction work is carried out offline there is minimal disruption to the normal site operation. Furthermore, the transfer of circuits from old to new switchgear can be scheduled for the most convenient time that has least impact on production.

Fast-track
The combination of a pre-engineered solution and minimal civil works makes containerized switchgear the ideal approach when a fast-track solution is required. In most cases,  there will be a 20 per cent reduction in the time from order to energisation compared with the more traditional route.

Added safety
Since the majority of the construction work is carried out off-site, the containerized switchgear helps to reduced health and safety concerns, especially when worked in the limited space found in the typical industrial process environment.