#rd Harmonics from Steam Turbine Generator 1

[sjvanhecke]

I have a problem i am trying to resolve. I have acustomer that has a 3MW 6900V steam turbine generator that had the stator rewound about 4 months ago. Since the gen. was reinstalled they are getting nuetral overcurrent trips. In monitoring the nuetral current, I have discovered that not only is there substantially more nuetral current then before, but it is mostly third harmonic current. There is some 9th harmonic, but very little. We have eliminated the Drives for 3 fans by monitoring the current with the drives off and back on. There are two Gas turbine generators also, that do not have this problem nor do they contribute to it. We know this by attaching a power recorder to the system and monitoring the currents at all conditions, and when the steam turbine is not running the neutral current goes away. So what I am wanting to know is, what can cause third harmonic current in a generator? I understand by doing some research that if third harmonic current is generated on the phases it will add on the neutral, but where does it come from???

 

[bunnypig]

We have the same problem with an 11kv 1.5MW Diesel driven generator, it was originally installed as a standby generator at a large residential complex for essential supplies only.
We installed the required synchronising equipment to generate back into the utility company to reduce peak billing.
It synchronised OK but when load was increased the generator neutral earth fault relay operated.
The neutral current was measured at up to 10Amps through the neutral earthing resistor (68ohms).
Harmonic analysing equipment showed that this 10Amps was 95% 3rd harmonic with a little bit of 9th.
The neutral earth fault relay was an Alstom MCGG which does not have harmonic restraint, we replaced it with an Alstom KCGG which does. Problem solved !!! Not quite, the generator doesn't trip any more but the 3rd harmonic current is still a problem, the neutral earthing resistor is getting hot and smoking.
It is believed that the generator windings are wound as suggested in a previous posting, and is just inherrant to the generator design (Being made in France doesn't help).

 

[busbar]

Based on ANSI/IEEE Std 32-1972 …Neutral Grounding Devices the resistor has assigned a time rating. It is possible that it is intended for short-time duty and not continuous or extended duty. That characteristic should be reflected on the equipment nameplate.

An additional note in the standard is: “Where there is a third harmonic component of current, it shall not exceed 15 percent of the rated continuous duty current.”

 

[ScottyUK]

bunnypig,

One possible answer is to use a Y/D transformer to trap the triplen harmonics in the delta winding. This would be a typical configuration for a large generators with a step-up generator transformer to the grid, connecting the generator to the LV delta.

Is your generator actually generating 11kV, or is it stepped up for connection? 1.5MW seems a small rating for an 11kV set. If you have a transformer, what is its configuration?

 

[bunnypig]

ScottyUK,

Yes the the generator is 11kv, not stepped up. The powers that be in a multilevelled government organisation have decided to put their money into a reactor on the generator neutral to filter out the 3rd harmonic.

 

[jbartos]

Suggestion: Visit

http://www.iaei.org/magazine/03_a/lowenstein/magazine_03_lowenstein.htm

for more info on the third harmonic filtering

 

[jbartos]

Suggestion: The resistor power rating is supposed to have a large design margin in the resistor power rating since the resistor increases its resistance with increasing heat which leads to the higher voltage drop across the transformer primary, which serves as the current transformer primary rather than the voltage transformer primary.