# IS Limiter

## Introduction

Figure 1. Typical IS limiter design (image courtesy of [1])

IS limiters are fault current limiting devices that use chemical charges and current-limiting fuses to interrupt the fault current within the first quarter to half cycle (i.e. before the first peak).

In a typical IS limiter design, the device is composed of two current paths connected together in parallel – one path is an element rated for the full load current (which can have high continuous current ratings, e.g. 3000A), and the other path provides the current limiting function via a current-limiting fuse (which typically has a continuous current rating of <300A at 15kV).

In the event of a fault, the device operates by physically destroying the continuous current path using one or more chemical charges triggered by an electronic sensing and control unit. The fault current is thus forced through the current-limiting fuse.

Figure 2. IS limiter block diagram (courtesy of [1])

## Application

IS limiters are used for fault reduction in a system, typically to limit the fault current to within the rated capacity of downstream equipment. This is usually necessary due to system over-expansion or undersizing of equipment fault ratings. By using IS limiters, expensive upgrades or replacement of equipment can potentially be avoided.

A natural question to ask is why current-limiting fuses can't just be used in lieu of IS limiters? The answer is that they can - but within limits. Current-limiting fuses typically have continuous current ratings <300A [1], so if the application calls for currents lower than that, then current-limiting fuses by themselves are fine. However in many applications, the fault current needs to be limited on a busbar with higher continuous load currents. This is where IS limiters or other types of fault current limiting devices need to be used.

IS limiters are often installed at bus-ties / couplers to limit the fault contribution from one or more bus sections. This can allow the interconnection of systems that would normally have prospective short circuit currents exceeding fault ratings. IS limiters can also be installed at bus incomers to limit the upstream fault contribution.

## Parameter Settings

IS limiters typically operate using a combination of two parameters:

• Trigger level
• di/dt

### Trigger Level

The trigger level is the current magnitude at which the fault current limiter will initiate the transfer to the current limiting path (i.e. by activating the chemical charge). The trigger level is typically defined as the "the maximum short-circuit contribution permitted by a major power source, so that the total overall fault current superimposed from all short-circuit contributions will not exceed the momentary and interrupting ratings of the switchgear" [2].

The trigger level is set using trial and error, typically based on an initial trial value of the difference between the switchgear instantaneous rating and the peak instantaneous fault current. When selecting the trigger level, other considerations such as transformer inrush currents, overcurrent protection coordination and breaker operating margins should also be examined.

### di/dt

The di/dt parameter is a value describing the minimum fault current rate of change (in kA/s) at which the transfer will be initiated. This is used to avoid nuisance tripping for asymmetrical faults that may have high peak values (above the trigger level), but rms values within the ratings of the switchgear. The transfer to the current limiting path will only be initiated if the fault current exceeds both the trigger level and di/dt limit.

The di/dt limit can be approximated as follows [2]:

$\frac{di_{peak}}{dt} = \omega I_{M} \,$

Where $\omega \,$ is the system angular frequency (radians/s)

$I_{M} \,$ is the peak symmetrical short circuit current (A)

## References

[1] Chao, T., "Electronically Controlled Current Limiting Fuses", 1995, Proceedings of 1995 IEEE Annual Pulp and Paper Industry Technology Conference, Vancouver, Canada

[2] Wu, Al., and Yin,Y., "Fault-Current Limiter Applications in Medium- And High-Voltage Power Distribution Systems", 1998, IEEE Transactions on Industry Applications, Vol. 34, No. 1