reactive
Electrical
- Jul 9, 2002
- 54
Client has been having irregular 11kV VT failures on an submerged arc furnace installation. System details as follows:
- 2 x VTs per furnace 11/28/75kV connected phase to phase upstream of the furnace breaker.
- Supply transformer 88/11kV 75MVA.
- 4,8 MVAr plain capacitor bank switched with the furnace.
- 30 MVA Furnace fed via 3 single phase furnace transformers.
- Furnace breaker - air blast.
- Failure is external flashover on VT connection leads.
- There is a second furnace connected to the same supply transformer.
- surge arresters on one furnace only (the one with the failures, however connected downstream of the furnace breaker).
- No VT failures on the other furnace.
I am pretty sure that the failures are simply due to external flashover due to badly installed VTs. However, what interests me is a transient that was recorded.
High speed voltage transient measurements were connected to the open delta output of the VTs (i.e. upstream of the furnace breaker). During furnace overcurrent trips (not at light load) on the first day two events were recorded which showed medium frequency voltage oscillations (approx. 17kHz) exponentially damped (dying in approx. 0,4ms), magnitude about 2p.u. Funny thing is that these appeared on all three traces (measuring phase to phase) simultaneously.
The traces were not seen again during the two week measurement period. I do not know of any system changes during the period - strange ??
I assume that this is restrike with some kind of coupling through the two capacitor banks. Is there anyone who can explain this in more detail or comment.
- 2 x VTs per furnace 11/28/75kV connected phase to phase upstream of the furnace breaker.
- Supply transformer 88/11kV 75MVA.
- 4,8 MVAr plain capacitor bank switched with the furnace.
- 30 MVA Furnace fed via 3 single phase furnace transformers.
- Furnace breaker - air blast.
- Failure is external flashover on VT connection leads.
- There is a second furnace connected to the same supply transformer.
- surge arresters on one furnace only (the one with the failures, however connected downstream of the furnace breaker).
- No VT failures on the other furnace.
I am pretty sure that the failures are simply due to external flashover due to badly installed VTs. However, what interests me is a transient that was recorded.
High speed voltage transient measurements were connected to the open delta output of the VTs (i.e. upstream of the furnace breaker). During furnace overcurrent trips (not at light load) on the first day two events were recorded which showed medium frequency voltage oscillations (approx. 17kHz) exponentially damped (dying in approx. 0,4ms), magnitude about 2p.u. Funny thing is that these appeared on all three traces (measuring phase to phase) simultaneously.
The traces were not seen again during the two week measurement period. I do not know of any system changes during the period - strange ??
I assume that this is restrike with some kind of coupling through the two capacitor banks. Is there anyone who can explain this in more detail or comment.