Generator capability whilst connected to national grid question
Generator capability whilst connected to national grid question
(OP)
Hi,
Please could you help my understanding on the generator performance and change in p.f when connected to a grid system in the following scenario;
A generator is connected to the national grid via a step-up transformer. The maximum achievable voltage at max power that can be achieved is 420kV, in the highest tap position, with a P.F of 0.98 at the grid connection point. The grid code states that the normal operating range is 400kV +/- 5% however during abnormal conditions voltages could be between +/- 10% and should last no longer than 15 minutes. My question is if we were connected to the grid and an abnormal condition arose and grid voltage was near 440kV, what would happen to the generator? My understanding is that the AVR would try to maintain grid voltage and that a trip of the protection system would be instigated on an upper excitation limit, however I am not certain on this and would appreciate some clarification.
Thanks,
Please could you help my understanding on the generator performance and change in p.f when connected to a grid system in the following scenario;
A generator is connected to the national grid via a step-up transformer. The maximum achievable voltage at max power that can be achieved is 420kV, in the highest tap position, with a P.F of 0.98 at the grid connection point. The grid code states that the normal operating range is 400kV +/- 5% however during abnormal conditions voltages could be between +/- 10% and should last no longer than 15 minutes. My question is if we were connected to the grid and an abnormal condition arose and grid voltage was near 440kV, what would happen to the generator? My understanding is that the AVR would try to maintain grid voltage and that a trip of the protection system would be instigated on an upper excitation limit, however I am not certain on this and would appreciate some clarification.
Thanks,






RE: Generator capability whilst connected to national grid question
If the AVR is in automatic voltage control, the GSU transformer tapchanger remains fixed, and the AVR is regulating the voltage on the main terminals then the AVR will reduce the field to try to reduce the machine terminal voltage as it compensates for the rise in terminal voltage brought about by the rise in grid voltage. You might conceivably hit the under-excitation limiter with the machine operating at a leading power factor as it absorbs reactive power from the grid. This is a bad area of the capability chart for a generator to be working in as the machine stability is compromised and a major grid disturbance could cause it to lose synchronism.
RE: Generator capability whilst connected to national grid question
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RE: Generator capability whilst connected to national grid question
"The grid code states that the normal operating range is 400kV +/- 5% however during abnormal conditions voltages could be between +/- 10% and should last no longer to 15 minutes."
You must adequate your AVR for that contingence.
RE: Generator capability whilst connected to national grid question
Of course there is an upper limit for generator field excitation--a system would need to include protection for over excitation.
I suggest contacting the manufacturer of the generator in question for assistance with your design.
RE: Generator capability whilst connected to national grid question
I'm guessing you're UK based? In the UK the large generators are given reactive instructions and they use the tapchanger on the step up transformer (bearing in mind the AVR acts to maintain the generator terminal volts) to adjust the reactive power at the HV side of the step up transformer. The generators should all have under excitation protection (I've seen a unit trip on this), posibly pole slipping protection as well as systems to prevent over excitation.
RE: Generator capability whilst connected to national grid question
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Generator capability whilst connected to national grid question
I guess you have limited experience in an operational power plant, because if you did then you would realise that the AVR is a key part of the generator's control. To say that an AVR is unsuitable for use with a grid-paralleled machine is quite simply untrue, as demonstrated by the numerous machines throughout the world equipped with an AVR and operating in a satisfactory manner.
RE: Generator capability whilst connected to national grid question
We experience high voltage ranges over long periods of time. We have had to start adding incoming transformers with On Line Tap Changes with (OLTC) 17 steps of 1.25%, giving plus / minus 10%. Now if we need to connect to the grid that is operating near or outside our limits of control, the voltage regulator on the transformer steps and allows the factory voltage to remain constant with in the new increased range. It allows for fast synchonization. Also if you are required to remain connected to the utility during the out of range periods, when it is high, you will electrically stress all your insualtion on the electrical equipment. This is seen as insulation failures. If the voltage is low, all the torque values will drop and the current will increase and possibly cause tripping if you are on your power limits with your design. The electricians turn up all the overloads to prevent "nuisance tripping' and now you have lost your protection.
RE: Generator capability whilst connected to national grid question
Do your AVRs sense the high side of the GSU transformer or the low side? If they sense the low side then most generators are capable of flexing their own terminal voltage even when connected to an infinite bus because of the GSU transformer impedance. The flow of reactive power can - and does - influence the local grid voltage and is one method of controlling it. I am puzzled by this misconception that a grid can't be influenced by a generator. A grid node close to a large generator or plant certainly will be affected by the AVRs. The influence on voltage is often relatively small, but the 'infinite bus' is a convenient first-order modelling tool rather than a statement of fact, even on a dense and heavily interconnected grid like in the UK. In a larger and less dense grid like in much of the US and I suspect in S.A. too the effect of a generator on the local grid will be more significant.
RE: Generator capability whilst connected to national grid question
The generator/AVR will be operated in PF mode.
That said, I have seen the exception.
A city was supplied by a diesel generator plant.
A large hydro generating installation was constructed on the other side of the country.
A transmission line was built and the city was fed from the hydro plant. The diesel plant was mothballed.
The city grew and the transmission line became overloaded and low voltages became an issue.
The diesel plant was re-commissioned and run in either fixed excitation or voltage control mode to control the voltage at the city.
Due to the present cost of fuel, very little real power was produced by the diesel plant.
Yes, a generator can and will influence the grid voltage if the grid connection is relatively soft, and as Scotty points out; The bus is far from infinite.
scott88; Assuming that the AVR is run in PF mode, once the voltage rises past the point that the AVR is at full output, the export of VARs will be reduced. The export of real power will continue as real power production depends on throttle or valve setting and not voltage.
With a large enough voltage rise it is possible that the power factor may drop to the point that the KVA load may overload the generator and/or the GSU.
This is assuming that there are no over-voltage failures and nothing saturates.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Generator capability whilst connected to national grid question
The differences in mode possibly stem from differing methods of despatching units. In the UK the merchant stations are largely despatched on MW output with fixed terminal voltage, and the reactive power is regulated on the GSU transformers. In the specific instance of the plant I worked at until recently we were contracted to provide reactive power support, but rarely did in reality. The grid at that location is fairly robust and generally sat slightly high at about 282kV or so, so there was little need to export reactive power under normal conditions. The station was largely Var-neutral at the grid connection, with the Var consumption of the transformers being met from the generators. In the event that a reactive despatch was received, it would be met in the first instance by tapping the GSU transformers against the fixed output voltage of the generators with a possible trim by flexing one or more generator terminal voltages slightly to meet the despatch more precisely.
It would make a certain amount of sense for a large baseload plant to operate in PF control, and allow the merchant stations to flex both power and reactive power to suit the grid needs.
RE: Generator capability whilst connected to national grid question
That being said, the voltage set point given to the AVR can be adjusted up or down to reach the desired power factor by an external control loop. If the voltage set point is adjusted fast enough, the AVR might technically be in voltage control mode, but would behave as if it were in PF mode.
Modern AVRs should provide under/over excitation limiting rather than having protective relays trip the unit at the excitation limits. Any unit trips should be based on over voltage relays protecting voltage sensitive elements or volts/hertz relays protecting power transformers.
Our somewhat remote hydro plants can swing the high side bus voltages a couple of percent if they were to swing from the under excitation limit to the over excitation limit.
RE: Generator capability whilst connected to national grid question
RE: Generator capability whilst connected to national grid question
You are correct, I am speaking from my experience in the area of cogeneration with reciprocating-engine/generators, which are typically very tiny compared with the grid. As has been said above, the type of voltage (and frequency) control depend largely upon the size of generator vs. stiffness of the grid.
RE: Generator capability whilst connected to national grid question
Waross is very knowledgable about these smaller reciprocating engine driven plants is islanded mode or attached to small grids, you two will probably swap some ideas. I apologise if my earlier reply seemed rude.
RE: Generator capability whilst connected to national grid question
RE: Generator capability whilst connected to national grid question
RE: Generator capability whilst connected to national grid question
if the machine is too small say 80MW interconnecting to a 400kV system, it is s slave unit and should follow the gird system and what will happen was already mentioned by a lot of experts here already.
I did not see any words mention that the unit is running in PF controll mode.
RE: Generator capability whilst connected to national grid question
In my province's system it is quite routine to use the major 'utility' [publicly owned] generating stations as reactive resources; none of their generating unit output transformers are equipped with underload tapchangers. Although some do have off load tapchangers, these are commonly selected to an optimal tap position either upon commissioning or very soon thereafter; it takes a major change in system characteristics for an alteration in tap position to even be considered.
The allusion to NERC standard VAR-002-2b by bacon4life is interesting; a number of what are coloquially referred to as 'non-utility' [private partnership] generators have connected to our grid in recent years, and although the NUGs comply with the requirement mentioned, in practice their reactive output is essentially ignored by the system operator when dispatching reactive resources...to the point where I have seen more than half of the VARs being added to the system when static capacitors are placed in service absorbed by the NUGs. As near as I can tell the only thing the NUGs respond to is their internal under- or over-excitation limits.
Ridiculous, in my view, but above my pay grade.
Smaller distribution-connected generators may more commonly, in my experience, be equipped with reactive output control as well as the classic AVR scheme. Distribution votages are much more commonly regulated with ULTCs, these being manually adjusted as needed when other reactive resources such as DOL static capacitors are being switched in or out of service; the smaller NUGs have no obligation to support voltage and prefer to operate their equipment at as low a current as possible, hence the choice to operate near zero reactive.
Hope this helps.