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from the Synchronizing Class department...
Cause of reverse power
Power generation equipment control. topic
Posted by blackstoneboy on 15 January, 2009 - 8:33 am
Can anyone kindly tell me what are the causes of reverse power in synchronization? Thanks... It'll be a great help.


1 out of 1 members thought this post was helpful...
Posted by CSA on 16 January, 2009 - 2:40 pm
Reverse power in synchronization?

I don't really understand the question involving reverse power and synchronization because synchronization is usually used to describe the procedure for connecting a generator to a grid with other generators and under such conditions there should not generally be reverse power flowing.

Reverse power generally describes a condition where the prime mover of a generator is not supplying sufficient torque to keep the generator rotor spinning at the same frequency as the grid to which the generator is connected. In other words, the generator has actually become a motor and is drawing current from the grid to which is connected and is supplying torque to the prime mover which is supposed to be supplying torque to the generator.

During synchronization, it might be possible to have the synchroscope rotating in the Slow direction (anti-clockwise) and then close the breaker. Under this condition, the generator would then be drawing a small amount of current from the grid instead of supplying a small amount of current to the grid (which occurs when the breaker is closed with the synchroscope rotates in the Fast (clockwise) direction).

I have seen some prime mover control systems (and exciter regulator control systems and synchronizers) that try to increase load slightly as soon as the generator breaker is closed. On a couple of sites, after a maintenance outage, I have seen the output of the control mis-connected to the control valve and cause the load to actually decrease instead of increase and cause the generator to trip on reverse power. One case was a paper mill that had a DC motor on the steam control valve actuator and the motor leads were reversed.


Posted by jojo on 17 January, 2009 - 12:38 pm
blackstoneboy,
it seems that you are trying to synchronise a generator, and as soon as its circuit breaker closes, the generator operates in reverse power. If this is the case, then there could be two causes of the problem:

1. If you are doing a manual synchronisation, ensure that you set the generator to run slightly faster than the grid frequency (shown by a synchroscope rotating clockwise). If you are doing this and still ending up in reverse power, check the connections of your synchroscope. You might have the generator and grid connections reversed)

2. If you are doing an automatic synchronisation, through a synchronising relay, again check that the generator and grid voltage connections are not inverted. In addition ensure that that the relay is set to close the circuit breaker only when the speed of the generator is equal to or higher than that of the grid (most probably indicated by positive slip in the relay setting). If the relay does not use slip for synchronisation, but synchronises only when the generator speed is the same as that of the grid (only a deviation in phase angle is accepted), then devise something such that as soon as the circuit breaker is closed, a raise command is given to the generator prime mover governor.


Posted by mehmet bozkurt on 10 June, 2011 - 5:48 am
Hi..!

Can you help me with a problem which a have in a hydroelectric power plant.

the problem is: reverse power

We start the generator and all values are in a normal range. Then we synchronize the machine to the national grid of turkey, but after that we see the generator protection relay trips and gives W< reverse power fault. We tried that to increase the excitation before the synchronization and then we push the button of synh. and the generator works in normal operation with synch. the relay is Microelettrica Scientifica.

Thanks,


Posted by 309EGuy on 10 June, 2011 - 9:43 am
Increasing the excitation will only affect the MVars generated and have no effect on MWs. Water flow and head determine the megawatts generated.

What type of turbine is this? Pelton or Francis/Kaplan? Provided water is flowing and the gates are open it is actually quite difficult to reverse power a hydro turbine. I've come across some original installations which had no reverse power trip protection. Time delayed minimum position limit switches on the wicket gates/needle valves would provide the protection.

Increasing water flow will increase the megawatts generated. What's happening to your wicket gate position immediately after breaker closure? It should increase a couple percent to increase power, and for a Pelton turbine the deflector should open out of the stream.

Are your reverse power relays settings realistic for a hydro turbine with a suitable delay? For the larger units it is normal practise to synchronize at a lower speed than the grid. This means immediately after breaker closure reverse power will be present until the wicket gates have slightly opened. Has the CT and PT wiring been checked?
Can this unit run as a synchronous condenser?

Please tell us more about your unit.


Posted by CSA on 10 June, 2011 - 11:19 am
Hello!

When did this problem start? After some outage? Is this a new unit?

What have you done to try to troubleshoot the problem? And what were the results of your efforts?

Reverse power is generally sensed by the "direction" of current flowing at or near the generator terminals. The CTs (current transformers) used for the sensing input to the reverse power relay are polarity-sensitive, and so if they were not verified during installation correctly, or if the wiring may have been disturbed during some outage and not verified prior to re-start, then the signal being applied to the reverse power relay may be "reversed".


Posted by safeek on 23 June, 2013 - 10:42 am
Your answer is very clear and correct the same problem was happened in my site also thank you for your clear answer.


Posted by riko on 23 June, 2013 - 12:51 pm
I agree. The wrong ct installation happened on my new gt fr6 power plant. The polarity of the ct was connected reversely. Soon after the generator Synchronized to the grid, we experienced reverse power and set the breaker to trip (only breaker not turbine-generator). No matter we thrust the turbine up by increasing fuel after sync, the reverse power always cut out.

Sooner we identified the misconnection of the ct, because this is the new power plant and the things like this happen in this situation.

But if reverse power occurs suddenly in well running generator, it is likely as the result of the failure of control system to establish proper amount of fuel after sync. You can start from inspecting the related device.

Good luck


Posted by Namatimangan08 on 10 June, 2011 - 1:55 pm
A hydro turbine especially the one that has a long penstock has one noticeable characteristic. When you attempt to increase the load by increasing the opening of flow control mechanism, the actual load will go down first probably for around 3-5 seconds. When you try to reduce the load by reducing the opening of flow control mechanism the actual load will go up first for about 3-5 seconds before it eventually goes down.

I will tell you why these are true.

What load you set prior to synchronizing?

There are other possibilities I wish to share later. First thing first. Please provide the answer to my question above.


Posted by G.Rajesh on 24 June, 2013 - 1:58 am
"When you try to reduce the load by reducing the opening of flow control mechanism the actual load will go up first for about 3-5 seconds before it eventually goes down.

I will tell you why these are true. "

We too waiting for your reply.

Take care
G.Rajesh


Posted by Namatimangan08 on 24 June, 2013 - 11:31 am
When you open the wicket gate, the water water passes across it will increase almost instantaneously due to the present of very high pressure difference over short distance. The same may not true for the flow from the wicket gate to the intake. It takes quite sometime for this massive water to accelerate owing to huge inertia it has.

Due to this the area just before the wicket gate has to go through flow "starvation" i.e. water leaving that section is higher that replenishment rate. As a result, high pressure drop will occur.

Since power is as a function of mass flow rate and pressure, temporary pressure drop will reduce power output if such drop in pressure cannot be compensated by additional mass flow. In this case, since flow starvation just occurs before the wicket gate, there is no way pressure drop can be compensated by additional mass flow. Therefore the output will go down for a while..

After a few seconds new steady state will achieve. Pressure rises back and mass flow rate reaches new steady state. Only then the output will increase and remain at the new steady state condition.


Posted by jaypat on 11 February, 2014 - 3:59 am
Reverse power in synchronization?

i just want to ask how this reverse power trip be tested. our generators circuit breaker is mitsubishi AE1000-SW

thank you for your soonest reply!


1 out of 2 members thought this post was helpful...
Posted by CSA on 14 February, 2014 - 12:20 am
Hi,

Reverse power occurs when the energy flow-rate into the prime mover is not sufficient to maintain rated speed while the generator breaker remains closed. In this case the generator actually becomes a motor, drawing amperes from the grid in order to maintain rate speed. In effect this results in the prime mover being rotated by the motorized generator instead of the prime mover rotating the generator.

To test the reverse power relay (this isn't a test of the breaker) just reduce the energy flow-rate into the prime mover until the reverse power relay operated--opening the generator breaker.

Hope this helps!


Posted by migaguiar on 16 February, 2014 - 6:21 am
Or to test, also just invert the angle 180 degrees into the relay configuration, and instead reducing the power you can "simulate" the reverse power actually going forward. i.e.: reverse power at -5MW => inverse the angle in the relay and do a +5MW (the 52G will trip per reverse power).

don't forget to put it back again to normal operation.


1 out of 2 members thought this post was helpful...
Posted by Phil Corso on 16 February, 2014 - 10:38 am
Jaypot... testing requires knowledge of the relay used.

Is it analog, solid-state or microprrocessor based. Also, a single-phase unit is normally used, but sometimes it can be three-phase.

Can you provide detail as to the manufacturer?

Regards,
Phil Corso

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