CURRENT LIMITING REACTOR

CLR is a well-known fault current limiting technique. Compared with many other methods, it is more economical. In addition its effect on the reliability of substation is negligible. However, it occupies a relatively large area in the substation, due to safety considerations. More- over, it may degrade both voltage stability and transient stability of the system.

TYPE OF CLR

Dry type CLR and oil type CLR are the two well-known types of CLR. Dry type is an air-core reactor with copper or aluminum windings. Generally, iron cores are not used in CLRs, due to the possibility of saturation. Since this device is installed in series with the main circuit, possibility of iron core saturation, specially, during short circuit conditions, is high. Therefore, dry type air-core re- actor is the common type of CLR, used in power systems. One of the main problems, associated with this device, is the safety problem due to the magnetic flux distributed through the space around CLR. Therefore, air-core CLRs require proper fencing due to the personnel safety considerations.

Characteristics of oil type reactors are mainly similar to the dry type. However, the oil type is specifically designed for the heavily polluted environments. Moreover, oil type CLR has got the following advantages:

Dielectric constant of oil is greater than air. This will result in the smaller size of oil type CLR, compared with the dry type.
Heat transfer capability of oil is higher the air. This will result in some advantages and savings during the design stage.

TECHNICAL SPECIFICATIONS OF CLR

Important technical parameters of CLRs may be listed as follows:

Nominal voltage;
Nominal frequency;
Short circuit capacity of the system;
Basic insulation level;
Continuous operating current;
Rated inductance;
Type (dry or oil);
Class (indoor or outdoor).

PRACTICAL CONSIDERATIONS OF CLR

CLRs may significantly reduce short circuit level. However, some practical restrictions must be considered, before installing CLRs.

VOLTAGE DROP: CLRs may affect voltage profile of the system. Hence, when CLR is recommended for a system, voltage stability studies of the system should be repeated.

TRANSIENT STABILITY: in addition to voltage stability, CLR may also degrade transient stability of the system.

ENERGY CONSUMPTION: since the main current of power system is continuously passing through the CLR, energy consumption of the device might be significant. This issue must be considered in the CLR design stage.

DISTRIBUTED MAGNETIC FLUX: required safety clearances around the CLR should be double-checked, in order to consider the high magnetic flux, distributed through the space. This will necessitate careful fencing.

TRANSIENT RECOVERY VOLTAGE: when the circuit breakers interrupt short circuit or even normal load current, a transient voltage appears across the opened contacts. This voltage is known as Transient Recovery Voltage. TRV and its rate of rise, known as Rate of Rise of Recovery Voltage are considered as important parameters for the circuit breaker manufacturers. If either TRV or RRRV exceeds the circuit breaker capability, possibility of secondary arc will be increased. This will impose a significant stress on the circuit breaker and other equipment.

CLR affects both TRV and RRRV in the following manner:

It reduces the peak of TRV. This is an advantage of CLR.
It increases RRRV. This is a disadvantage of CLR. Unfortunately, RRRV is more critical than TRV. Therefore, prior to the installation of CLR, accurate transient studies are required.

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