Technical Technical Journal
Se tember 2011 2011
Capacitor Ca pacitor Switching – Technical Technical cons idera ideration tion
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 normally with 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 curr current ent of the circuit circuit break breakers ers should should be greate greaterr than than 1.5 times the rated rated curr current ent of capa capaci cito tors rs.. Due Due to fact fact that that swit switch chin ing g capa capacit citor or is very very simi similar lar to clos closin ing g of a circu circuit it break breaker er on short short circui circuit, t, It shall shall have have suffic sufficien ientt makin making g capac capacity ity and overload overload settin setting g should be set at 1.5 times rated current of capacitors. 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 630A 8 x Ie max/AC-3 630A 6 x Ie max/AC-3* * With minimum value of 5040A
Duration of Test 10s 10s
The power factor correction capacitor 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 t imes in case automatic.
Capacitor Switching – Technical cons ideration
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 be 1. 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 of amplification of predominant harmonics are avo ided. 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.
Capacitor Switching – Technical cons ideration
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/reactor th th th combination to slightly below the predominant harmonics like 5 , 7 11 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 info rmation, please contact ABB Limited Design Institute 88/3, 88/6, Basavanahalli Village 562123, Bangalore North, Karnataka, INDIA email:
[email protected] 002/2011
