Hi Community Members,
We have recently faced some outages of our 7.6MVA Steam Turbine Generator on actuation of Reverse Power Protection. I am writing here to know your expert opinions:
1. Protection Relay SIEMENS SIPROTEC 7UM62 is installed at our Generator Breaker, 02 stages of Reverse Power Protection are enabled in it.
Setpoint: -119.3 kW (-1.57% of 7.6MVA)
Time Delay (with Trip SOV operation): 50 ms
Time Delay (without Trip SOV operation): 5 sec
2. As Steam Turbine Control is entirely a local PLC based (ComAP/intellisys) which also provides some generator protections, so another Reverse power stage with following settings in enabled in that
Setpoint: -182.4kW
Time Delay: 02 sec
3. Hydraulic governor system is installed on this Steam Turbine which usually cause hunting issues leading to load fluctuation and ultimately tripping the turbine on Reverse Power Protection from ComAP controller as its time delay is lesser that Siprotec 7UM62 (without Trip SOV operation)
4. Moreover, even normal stop initiated by Operators lead to reverse power from Siprotec 7UM62with 50ms delay.

My question here is; whether its advisable to allow the machine to wait for reverse power to actually happen? Shouldn't we trip the machine before it actually experiences reverse power?


There is a function (Change in Active Power) which monitors the active power of last 3 cycles (60 msec for 50 Hz system) and allows the Siprotec to generate a trip contact in case Active Power fluctuates. Can you guide percentage of active power we may use as setpoint? So that normal load fluctuations (driven by operator) don't cause any unwanted trip.

 

To answer your question simply; yes you should wait for reverse power to trip the generator breaker for the steam turbine in any scenario.
In your description, you are mixing terms a bit since there are two 'trips' happening when a steam turbine is tripped; controls trip and generator breaker trip.
A controls trip is when the steam valves are shut to stop steam flow to the turbine (SOV).
A generator breaker trip should only be actuated by the relaying and is typically performed by the reverse power relay.

From reading this, it sounds like you are under the impression that reverse power is damaging to the machine(turbine or generator). It can be damaging if reverse power for a long time (more than a few minutes) but a steam turbine generator breaker should not trip unless reverse power is observed to verify that the steam has indeed been stopped from entering. This prevents overspeed of the machine. If there is any steam available to the turbine and you remove the generator load, there is nothing stopping the turbine from spinning other than friction, which is minimal.
If the machine is experiencing reverse power for long time, windage friction inside the machine will cause heat to build up and cause damage but it takes quite a while for that to occur.

Most generator breaker reverse power relays I have encountered are set for reverse power for greater than 3 seconds = trip generator breaker. IMO, 50ms is too short of a time delay since that is barely 3 cycles and could be a false indication of reverse power.