We operate Brush Electric Machines Frame BDAX-365ER generators equipped with EX2000 PWM digital exciters that are brushless. These exciters were supplied with AVR, FVR, UEL, RCC, OEL, and V/hz limiter as standard features. Optional PSS and VAR/PF controllers were also supplied. The units have the ability to operate in AVR, VAR control, and PF control modes.

NERC requires our synchronous generators to be operated in Automatic Voltage Regulation unless permission is given to operate them otherwise. They also require an annual Reactive Capability test, wherein the units are required to produce the maximum lagging (positive) VARs for a period of 1 hour and, because we are black start capable, the minimum leading (negative) VARs for an instantaneous reading.

And now the issue:
We are limited by the OEL and UEL at +/- 5% of rated generator voltage(13.8 kV). However the manufacturer of the generator says that their machine is capable of +/- 10%.

Are UEL and OEL bypassed when regulator control is in Auto with generator mode in VAR control?

Are they bypassed when regulator is in Manual?

How much does a somewhat high grid voltage reduce the machines ability to produce lagging VARs?

 

Mark,

Brush Generators should be operated at no greater than + or - 1.05 PU nominal voltage when at BASE load. Failure to adhere to the manufacturers limits can cause over flux damage. This operational curve will be included in your BDAX manual in appendix A technical data.

The OEL limit has two components, one is the rotor field current and the other is the stator current limit.

The UEL is determined by the capability curve for the machine model at its rated name plate operating voltage. When commissioning the PSS or when re validating its operation both the OEL and the UEL will be tested to prove stability.

When in VAR control both the UEL and OEL protection will still be functional.

If you are operating you generator through an on line tap changer, you can adjust the tap so that you can achieve your one hour run on the OEL limiter without exceeding your 1.05 PU voltage, but care should be observed during this test to monitor the stator temperature closely so as not to risk damage. Prior to completing the test, you should verify that you OEL setting is appropriate to your machine.

The UEL test is always performed during commissioning of the excitation controller to prove stability at base load and again the capability curve as supplied by Brush should not be exceeded as at base load there is the increased risk of pole slipping if you exceed the manufacturers limits as shown on the capability curve

Regards
Graham Walker

 

I would listen to this, very spot on---

>Brush Generators should be operated at no greater than + or
>- 1.05 PU nominal voltage when at BASE load. Failure to
>adhere to the manufacturers limits can cause over flux
>damage. This operational curve will be included in your BDAX
>manual in appendix A technical data.

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