TG LOAD HUNTING AND REVERSE POWER

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Thread Starter

skg

We have a 18 MW generator running in parallel with the utility grid. The max generation is around 14 MW.After 4MW of internal consumption remaining 10 MW is exported to Grid.

Couple of days back it was noticed that there was a severe hunting in the TG load ie.. from 10 MW it went down to 2 MW and again picked up to 8MW and again 4 MW.(As seen in DCS trends). There was a variation in the grid frequency, also in the range of 50 to 51.5 Hz during that period. After some time the grid breaker tripped on over current and also an reverse active power protection alarm was noticed in the Generator relay panel. However after the tripping of the grid breaker the generator came on home load, but during the time the grid breaker tripped many (not all) motors has tripped in the plant.

The droop setting is 4 %. The DCS trends are not so fast to show the actual MW during the disturbance. The frequency had actually gone up to 52.5 Hz momentarily, as understood from the nearby grid substation. (Two local grid substation was disconnected from the main grid and our Power plant and Wind generation plant of 32 MW started feeding the islanded local grid). Our over frequency relay was set at 51.5 Hz but with a 2 sec delay.

The plant load is 5 MW only around 300 AMPS(11kv). So how the grid incommer breaker tripped at OVERCURRENT (recorded 2400 amps nearly balanced). Please note that the instance when the grid 11 kv breaker tripped recording a current of 2400 amps, the same time reverse power alarm was actuated in the generator protection relay. If the plant load is 300 amps how come the current has gone up to 2400 AMPS? Was the excess amps due to the reverse power in the generator.Is it possible to draw so high current during reverse power? No fault was found in the plant side and most of the motors were running after islanding. Some motors have tripped probably due to voltage drop due to such high current.

Request your opinion
 
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Namatimangan08

1) Your generator load swing because of system frequency swing. If the grid that you serve is smaller than 1000MW (My estimate), it is possible that your generator could trigger the load swing of the grid.

2) It was not the voltage drop due to high current. Opposite is true, i.e high current due to voltage drop. This can happen and almost always happen when the system undergoes load-frequency swing.

3) There are two important mechanisms that cause the load swing namely governor speed droop and AVR. Load swing causes by governor speed droops of all parallel generators. Whilst AVR causes voltage swing.

4) Why the load swing happens in the first place? One of the most probable reason is load -frequency response is Not Just In Time due to improper setting or some other dynamics problem.....
 
Skg... before this thread becomes another "endless" (excuse the pun) on grid-size, can you provide a additional information, such as whether this is a new phenomenon, type of prime-mover, and size of GSU (if one exists.)

Regards, Phil Corso

 
1) There was a transmission line fault and two 66 KV grid substation got isolated from the main grid

2) The total generation feeding to the isolated grid comprising of two 66 KV substation was 46 MW (32 MW FROM Wind powered generators and 14 MW from our STG).

3) The hunting started as soon as the two substations got isolated isolated from main grid

4) 2400 Amperes was recorded when the 11 KV grid breaker at our plant end tripped.The same time around 2100 AMPS current was recoded to be flowing towards the generator(motoring?),which is confirmed by the reverse power alarm of 1.5 times the Rated MW of the generator

5)GENERATOR STEP UP TRANSFORMER AT out plant end is 11kv/66kv, 20 MVA.
 
SKG... "One Picture is Worth a Thousand Words!" Can you provide a simple Single-Line-Diagram (SLD) or a hand-drawn sketch of system?

Regards, Phil Corso (cepsicon[at]AOL[dot]com)
 
SKG... thank you for the sketch. Several more questions:

1) Is wind-power plant (or Wind-farm or WPP) provided with typical induction-generators or self-excited induction-generators?

2) Is Rev-Pwr alarm actually 1.5 x Gen rating, or is it 1.5 x power required to motor the turbine? (or was it too, alarm only!)

3) Is STG a condensing or non-condensing unit?

4) Which protective-relay cleared the 66kV fault to isolate HV subs and the STG and WPP sources?

5) Is it acceptable to say the GSU xfmr impedance is about 10-12%?

6) For the ph-ph fault on the 66kV grid, the per-unit 11kV currents would have been 1.0 in one phase and 0.5 in each of the other two phases! Are you sure that ph-ph fault appeared as "2.4kA, nearly balanced" in 11kV generator?

Regards, Phil
 
Dear SKG,

I have few questions for your problem,

1) I hope the generator you are mentioning is STG set.

2) If it is STG set whether the machine is equipped with Woodward governor

3) If operating with woodward governor what mode operation you are adopting i.e. load mode or speed mode.

4) when your plant and other wind farm was feeding the isolated grid what was the load on your machine.
With above questions answers i will try for proper RCA ( Root cause analysis)

Regards,
Rangacharya
 
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