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## Neutral point grounding resistor AACF Cabinets ## SIMPAX - AACF Cabinets / secondary forcing resistors

The KFW / ZKFW resistor series is used in MV compensated networks as secondary forcing resistors. They are used to force the flow of the active component of short-circuit current, by connecting the resistor to the secondary winding of the arc-suppression coils (secondary winding 500V). The mere compensation of earth fault current ensured by arc-suppression coils causes a relatively high unreliability of the protective automation.

The arc-suppression coil has a current transformer whose secondary side is connected to the protective devices. The disadvantage of such solution is the fact that if the short-circuit current is not properly compensated (above 50%), it is not certain whether the protective automation will activate. Therefore, it is advantageous to use a forcing resistor.

### Calculations of forced primary current.

The resistor operating current is selected in such a way as to force a specific primary current, on the primary side of the arc-suppression coil. The table below shows how to calculate the forced primary current depending on the resistor operating current. According to the parameters specified in the “Parameters of secondary forcing resistors”, the resistor can be made for any operating current, in the range of 500A – 1000A, which gives the range of primary currents 20A – 40A.

##### Guidance:
• UNETWORK - Network rated voltage
• UARC-SUPPRESSION COIL - Arc-suppression coil operation rated voltage UARC-SUPPRESSION COIL = UNETWORK / √3
• URESISTOR – Resistor rated operating voltage: voltage on Petersen coil secondary winding = 500V
• VD – Petersen coil transmission VD = UARC-SUPPRESSION COIL / URESISTOR
• IR – Resistor rated current: standard 500A
• IW – Rated primary forced current: IW = IR / VD
• Example of calculations for 15,75K

Example of calculations for 15,75K

UARC-SUPPRESSION COIL = 15750V / √3 = 9090V

URESISTOR = 500V

Calculations of Petersen coil transmission VD = UARC-SUPPRESSION COIL / URESISTOR = 9090V / 500V = 18,18

IR = 500A for 1,0Ω of the resistor resistance

Calculations of primary forced current: IW = IR / VD = 500A / 18,18 = 27,5A

### ZKFW secondary resistors parameters

 Rated operating voltage: 500 VAC Rated Insulation voltage: 3500VAC / 1 min. Rated short-circuit current: 20A – 300A (possibility to create resistor adjusted for different current values – tapping connections): Rated Resistance: 0,5 - 1,0  (or other depending on expected rated short-circuit current) Rated short-circuit current loading time / resistor’s heat resistance: 5s – 60s. Standard AACF cycle is ≤ 5s. Increasing the heat resistance causes more frequent resistor activations. Max. number of consecutive short-circuit cycles 1 or more when their total time does not exceed the heat resistance. Protection class IP23 Primary current forced on primary side 0A – 40A (see computational point of primary forced current on previous page) Resistor can be adjusted for any forced current or can have tapping connections enabling joining the required forced currents (e.g. 15A, 20A, 25A)

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Grabonóg 76