Capacitor Switching - Techncial Consideration

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Capacitor Switching – Technical cons ideration

In low voltage electrical network, Capacitors are used for reactive power compensation.

When these capacitors are switched, inrush peak current occur with high amplitude and

high frequencies during transient period which is lasts for short time (1ms to 2ms).

In accordance with IEC 60831-1 and IEC60931-1, Capacitor should function normallywith 130% of it rated current due to voltage harmonics. A tolerance of 15% is allowed.

Effectively the Capacitor current is

Ic max = 1.15 x 1.3 x Ir = 1.5 x Ir ( Ir is rated current of capacitor)

Following are the factor influencing these inrush currents

1. Inductances in the network (All elements like Transformer/Cable/Circuit

Breakers etc…)

2. Transformer power and short circuit voltage

3. Configuration of Power factor correction.

Circuit Breaker Selection

The circuit breakers should withstand inrush currents while connecting and

disconnecting the capacitor banks and Instantaneous protection should not trip due to

this inrush current. Circuit breakers should be withstanding periodic or permanent over

currents due to voltage harmonics and 15% tolerance of capacitor values. The rated

current of the circuit breakers should be greater than 1.5 times the rated current of

capacitors. Due to fact that switching capacitor is very similar to closing of a circuit

breaker on short circuit, It shall have sufficient making capacity and overload setting

should be set at 1.5 times rated current of capacitors.

Contactor Selection

The capacitor switching devices are classified under IEC 60947-4-1 with utilization

category of AC6b. Overload withstand current of the contactors are clearly defined in

IEC 60947-4-1 which is as below with basis of AC3 or AC4 duty.

Rated operational current Test current Duration of Test

630A 8 x Ie max/AC-3 10s

630A 6 x Ie max/AC-3* 10s

* With minimum value of5040A

The power factor correction capacitor is designed with either fixed design(single step) of

automatic (multiple steps). Inrush current can reach up to 30 times rated current in

single step and 100 times in case automatic.

Technical Journal Se tember 2011




Capacitor selection based on non linear loads.

The selection of filtering solutions depends on % of non linear loads connected to the

Transformer. The solution can be1. Standard capacitors solution

2. Detuned Capacitors

3. Active Harmonic filters

In an LV network wherein the non linear load which generates harmonics is around 20%

of the rating of the transformers, Standard capacitors can be used. In the event of non

linear load is in the range of 20% to 40%, capacitors with detuned reactors are the right

solution. If non linear loads are more than 50%, It is recommended that active harmonic

filtering solution is considered.

Selection of Series Reactors

A. Inrush Current Suppression:

In some cases due to parallel switching and/or due to high system fault level, the inrush

current could be higher than the permissible limit. Further, even if the inrush current is

within the limits, the associated equipments like the circuit breaker might not be able to

handle high inrush current.

Hence, the need for inrush current suppression reactors.

B. Harmonic current limiting: (Detuned Capacitor Banks)

In some cases there is a possibility of harmonics being imported into system where

capacitors are to be connected. This is due to harmonic generating loads on some other

systems. If the capacitors are directly connected in the grid without reactor, there is a

possibility of amplification of harmonics by these capacitors. This would not only result

in failure of capacitors but also affect the other equipment on the grid. Hence, the need

for harmonic current limiting reactor.

A harmonic suppression is reactor is rated, so as to limit the harmonic current flowing

into the capacitor. This also tunes the capacitor in such a way that the possibilities ofamplification of predominant harmonics are avoided.

However in the absence of any harmonic/system data, harmonic overloading is not

considered while designing the capacitors/reactors. This is required to be taken into

based percentage of non linear loads connected to the Transformer.




C. Partially Detuned Harmonic Filters:

The capacitor/reactor combination act as a low impedance alternative path for

harmonics. Depending on the system and compensation requirement more than one

harmonic filter could be designed for a system.

A harmonic filter reactor is usually rated so as to tune the capacitor/reactorcombination to slightly below the predominant harmonics like 5

th, 7

th 11

th etc.

Active Harmonics Filter Solutions

Active harmonic filters have to be designed based on the data which is arrived out of

actual harmonics analysis at site after complete facility is commissioned. Designing

active filters without harmonics analysis may not be an accurate solution.

ABB Circuit Breakers for Capacitor selection

ABB offer Emax and Tmax Circuit Breakers which allow the protection of the supply lines

of the power factor correction banks and the coordination with the contactors for

capacitor connection.

ABB Solution for Capacitor Switching

Safe, Reliable Type 2 coordinated solution for capacitor switching from ABB.

In case of further information, please contact

ABB LimitedDesign Institute88/3, 88/6, Basavanahalli Village562123, Bangalore North, Karnataka, INDIAemail: [email protected]

002/2011

Capacitor Switching - Techncial Consideration

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