Background:
350Kw co-gen in Indiana, mature radial network, local loads mostly residential.
Gen is 480Y, grounded at the GSU. GSU is 480Y to 12.47Y.
While running on load banks, resistive and reactive, the gen was fine, metering all looked very good. Voltages, currents, KVAR all good. The problems start when they parallel with the gird.
After running for a few minutes, they are getting phase current unbalance trips, so the utility brought out their metering dude and he took some data with the basic power analyzer in from the meter, see .jpg file.
A phase current lags by 47 degrees, B lags by 25 degrees, and C LEADS by 26 degrees. Yes, two phases lag and one leads.
The resultant VARS are 0, so our voltage regulator is happy trying to regulate PF=1. The currents are fairly well balanced on true RMS meters, but unstable as heck. PF meter was quickly jumping all over from lagging to leading, from .9 to .2.
The volts before sync were very close. (284, 283, 287 L-N, A, B, C respectively) so I don't suspect a voltage unbalance causing the problem.
The local utility load is highly residential, old, and only has over current protection, no VAR or PF metering. They have single phase L-N capacitor banks spread throughout the radial fed and they swear they are all OK. Radial feed at the substation was about 240A @ 12.47Kv, so our gen is a drop in the bucket.
They are running a 12.47KV Y solidly grounded system, most loads connected line to neutral. The xmfr feeding the gen is a Y-Y wound 12.47K to 480v. The gen is Y of course, solidly grounded. Yea, not a good situation, but the utility says they won’t change. I suggested to customer they float the neutral at the gen since they don’t really need it.
I am concerned about the harmonics, (Y-Y xmfrs have high third harmonics, as you know) so they are going to get a full analyzer out there soon. They had other problems (breaker tripping) so we could not load it up past about 60%. The unbalance seemed to level out (lower % unbalance) at the higher power, but it was still pretty unstable. I’ll bet we had some high reactive circulating currents in the gen.
I would greatly appreciate your feedback. Have you ever seen a system with 2 phases lagging and one leading? With the current phase angles that far off? Any thoughts on the PF stability?
I can understand how they have the different PF on the different phases: I suspect a cap bank got stuck with too much KVAR being added to the system. (Utility swears this is not the case.) But why the unstable readings and why the severe current unbalance? The genset CB tripped on % unbal showing 868A on B phase and 1680A on C. (it did not capture A phase). I suspect a bad trip unit (AC Pro retrofit kit) so they are replacing that today.
But I was standing right in front of the CB when it tripped, and there was NO WAY it opened under that kind of load. I have been doing this for 30 years and when a IEEE 1558 CB (an old GE AK-100) opens under that kind of load, you know it!
Below is some data from the meter captured well before the trip (at low load). The voltage angles are right on. CT's are 1500:5.
Meter Configuration
Meter Time:
2/19/2009 3:42:03 PM
3 Element 3 Phase 4-Wire WYE
Transformer Factor
1
Line Frequency
60
Hardware Options
NA
Toolbox Information
Measurements
Va Angle 0.000
Vb Angle 119.000
Vc Angle 240.300
Va 285.150 V
Vb 284.325 V
Vc 286.500 V
Ia Angle 47.800
Ib Angle 145.700
Ic Angle 214.900
Ia 0.240 A
Ib 0.372 A
Ic 0.268 A
kW 0.153
kVA 0.160
kVar 0.047
PF 0.956
Thanks, in advance, for your comments,
Steve
350Kw co-gen in Indiana, mature radial network, local loads mostly residential.
Gen is 480Y, grounded at the GSU. GSU is 480Y to 12.47Y.
While running on load banks, resistive and reactive, the gen was fine, metering all looked very good. Voltages, currents, KVAR all good. The problems start when they parallel with the gird.
After running for a few minutes, they are getting phase current unbalance trips, so the utility brought out their metering dude and he took some data with the basic power analyzer in from the meter, see .jpg file.
A phase current lags by 47 degrees, B lags by 25 degrees, and C LEADS by 26 degrees. Yes, two phases lag and one leads.
The resultant VARS are 0, so our voltage regulator is happy trying to regulate PF=1. The currents are fairly well balanced on true RMS meters, but unstable as heck. PF meter was quickly jumping all over from lagging to leading, from .9 to .2.
The volts before sync were very close. (284, 283, 287 L-N, A, B, C respectively) so I don't suspect a voltage unbalance causing the problem.
The local utility load is highly residential, old, and only has over current protection, no VAR or PF metering. They have single phase L-N capacitor banks spread throughout the radial fed and they swear they are all OK. Radial feed at the substation was about 240A @ 12.47Kv, so our gen is a drop in the bucket.
They are running a 12.47KV Y solidly grounded system, most loads connected line to neutral. The xmfr feeding the gen is a Y-Y wound 12.47K to 480v. The gen is Y of course, solidly grounded. Yea, not a good situation, but the utility says they won’t change. I suggested to customer they float the neutral at the gen since they don’t really need it.
I am concerned about the harmonics, (Y-Y xmfrs have high third harmonics, as you know) so they are going to get a full analyzer out there soon. They had other problems (breaker tripping) so we could not load it up past about 60%. The unbalance seemed to level out (lower % unbalance) at the higher power, but it was still pretty unstable. I’ll bet we had some high reactive circulating currents in the gen.
I would greatly appreciate your feedback. Have you ever seen a system with 2 phases lagging and one leading? With the current phase angles that far off? Any thoughts on the PF stability?
I can understand how they have the different PF on the different phases: I suspect a cap bank got stuck with too much KVAR being added to the system. (Utility swears this is not the case.) But why the unstable readings and why the severe current unbalance? The genset CB tripped on % unbal showing 868A on B phase and 1680A on C. (it did not capture A phase). I suspect a bad trip unit (AC Pro retrofit kit) so they are replacing that today.
But I was standing right in front of the CB when it tripped, and there was NO WAY it opened under that kind of load. I have been doing this for 30 years and when a IEEE 1558 CB (an old GE AK-100) opens under that kind of load, you know it!
Below is some data from the meter captured well before the trip (at low load). The voltage angles are right on. CT's are 1500:5.
Meter Configuration
Meter Time:
2/19/2009 3:42:03 PM
3 Element 3 Phase 4-Wire WYE
Transformer Factor
1
Line Frequency
60
Hardware Options
NA
Toolbox Information
Measurements
Va Angle 0.000
Vb Angle 119.000
Vc Angle 240.300
Va 285.150 V
Vb 284.325 V
Vc 286.500 V
Ia Angle 47.800
Ib Angle 145.700
Ic Angle 214.900
Ia 0.240 A
Ib 0.372 A
Ic 0.268 A
kW 0.153
kVA 0.160
kVar 0.047
PF 0.956
Thanks, in advance, for your comments,
Steve
