REVRSE POWER RELAY

1. Please don't panic.

2. Please turn off all-caps (capital letters).

3. Please don't repeat the same post or idea in too many places.

I am not a power plant expert, even though I have successfully run a few small paralleling and split switchboards. But until better experts come aboard, I will try to take a crack at this.

If I understand theory somewhat correctly, the interactions between your equipment and the grid will force your equipment into synchronous rpm (frequency) with the grid. (Induction motors have slip, but synchronous motors and generators don't, unless something is really haywire.) [Finite bus equipment governors may have droop, but I hope to sidestep explaining that, and assume isochronous...I think, but whack me if I'm wrong.]

Under those assumptions, if your plant is going to put power into the grid (rather than absorb power out), your voltage must be a little bit higher. If not, the grid will feed power back into your equipment. This sounds like the "generator back power" message you got. From all this I would suspect that the voltage regulation on one or more of your equipment items is set too low, or a regulator is malfunctioning.

Or if a governor is malfunctioning, it could "try" to slow the generator down, which won't happen, because grid voltage flows backward and makes the generator keep up. Here is where my experience is not enough; I'm not sure how to distinguish these two conditions, nor do I know how your instruments can detect which is which. Therefore these comments are only some ideas. I hope they help a little.

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"How reverse power or back power relay work in gas or steam turbine. What are the basic setting? Pls send me detail"

Reverse power or back power it happened when the primary supply is removed while generator is connected to the grid (power system) and the field excited, the power system will drive the generator as a synchronous motor. For steam or gas turbine it causes overheating and potential damage to the turbine and turbine blades. This also occur by closing the steam or fuel gas valves too fast during a load reduction or by tripping the turbine while not corresponding tripping the generator breaker.

The relay type is directional relay, connected to operate when power flows into generator. It was to sense approximately 0.5% or higher of generator rating with time delay around 3 seconds or more (depend on generators vendor recommendation).

The following information should not be followed without generator vendor's recommendation:

As an example steam turbine generator with rating 12.5 MW at PF = 0.8, Voltage phase to phase = 13.8 kV. Recommended power reverse is 125 kW. Minimum pick-up relay = 0.025 A (CT secondary side) at 120 V.

CT ratio = 1000/5 = 200 and PT ratio = 14400/120 = 120.

Calculation:

Current occur when turbine steam stop flowing: Im = 125 kW/( 1.732 X 13.8kV) = 5.24 A at primary CT side.

Current at secondary CT: im =5.24/200 = 0.0262 A

Secondary voltage of machine nominal rating = (13.6/14.4) x 120 = 115 V

So, minimum pick-up relay at 115 V = I pick-up = (120/115) x 0.025 = 0.026.

It shown that im>I pick-up.

To avoid trip during paralleling time delay 5 or 3 seconds is selected.

If im < I pick-up we have to increase recommended power reverse (of course the vendor approval is required).