Synchronisation Problem

If the voltages of the alternators are not equal to the bus voltage, there will be reactive current between the alternators and the bus. In other words, the power factor seen by the alternators will be low.

They will explode and kill everyone within 5 miles.

RTFM!

I don't know, try it and impart your newly found knowledge amongst CR4 members.

my problem is: i want to synchronise a powersystem having 3 generators with a total power of 48MW on load with another powersystem of 120MW on load. i intend to raise simultaneously the voltage of the 3 generators to come close to that of those of the 120MW powersystem. i wish to know the effect of the reactive power injected into the 48MW powersystem while raising the voltage on load.

The reactive power of the 48 MW system is determined by the load of that system. It may change a little with voltage. Unless you have to raise the voltage of the 48 MW system to abnormal levels to match 120MW there should be no problem - it is probably more significant that voltage increase will increase kW demand - square law for resistive load, approximately 3% increase of voltage is 6% kW icrease.

Just try adjusting system to match 120MW maximum voltage expected at 48MW to 120MW connection breaker and see what happens. Also raise frequency to maximum required for interconnection, since this will also raise kW load.

Since the power users decide on the load at any moment, this is always a "try it and see" operation - if you have a choice you may decide to connect when load is small or most stable.

Dear nzenowo,

Your question reads like your worry is about the effect of adjusting voltage of 48MW system while it is isolated from 120MW system. Some replies assume you worry about reactive flows after two systems are connected.

You have not specified interconnection 48MW to 120MW, if that is your concern. There is a great difference in possible disturbance between bus duct and a breaker between two systems at same voltage and interconnection by a 5 MVA transformer.

More detail is needed if help is to be given.

67model

The problem is, the 3 generators of 48MW are connected on a 10KV busbar and the 6 generators of 120MW are also connected on its own 10KV bus bar. Both powersystems are on load. I wish to synchronise both bus bars by playing simultaneosly on the excitation system of the generators of the 48MW powersystem to bring the voltage difference to synchronisation acceptance criteria. I worry about the effect of the reactive power injected into the 48MW powersystem when playing on the excitation system of the generators to raise or lower the voltage during the synchronisation process since the 48MW powersystem is on load. If at all it has a negative impact, how can the effect be minimised?

You are correct to be concerned, especially since you seem to be obsessed with matching the voltage without any regard for frequency and phase angle. If this is your first attempt at synchronizing these two busses then you really need the onsite guidance of a skilled and experienced electrical power engineer.

Attempting to do this with the aid of anonymous strangers on the web is a formula for a very disastrous and costly outcome.

As I wrote in post #7, any change of reactive power to the load caused by voltage adjustment of the 48 MW system is not a worry, so long as the load voltage is not outside its limits.

I would suppose that the first step would be to adjust the 120MW system to 10 kV rated voltage. Adjusting the 48MW system to 10 kV would make it also at nominal voltage for its loads, so the reactive load power will be "as designed".

Take note of RAMCONSULT's warning, a direct 10 KV connection will give highest currents for any mismatch. Thus frequency, phase and voltage must be matched as closely as possible before closing the bus-to-bus breaker.

Generators are designed to withstand and factory tested by a short circuit at rated voltage. It follows that two identical generators connected together while 180 degrees out of phase will survive. However, one generator connected 180 degrees out to two paralleled identical generators will get currents exceeding those on short circuit and could be damaged. The same facts apply to 3 generators versus 6 of the same model.

The advice to get an experienced engineer to oversee what you do is good. However, he will expect you to have thought it out for yourself, as he will have to do.

My first thought is "Has this interconnection ever been done before? Or is this a new, untried event?".

My second is "Having connected the systems together, can the outgoing breakers, particularly the "48 MW" system, cope with a short circuit fed by 48 + 120 = 168 MW of generation?

The system design should have considered this, but one cannot be sure and if one is asked to do a task which might injure you or others or damage expensive plant all of which tends to finding "some one to blame" checking everything is required.

Dear 67model,

i wish to know why any change of reactive power to the load caused by voltage adjustment of the 48 MW system is not a worry, so long as the load voltage is not outside its limits. Where does the excess reactive power injected into the system goes to?

Frequency and phase angle have already been taken care of.

The interconnection has already been done using a manual approach.

The voltage of the 48 MW system is changing all the time due to variations in the load as consumers start their motors or turn off a floodlight or pump without thought about its effect on generators. I guess that your "48MW system" generators have an automatic system to keep the voltage steady, control the load on an incoming generator - loading-up from cold, so most of the time changes are not noticeable.

However, when large load changes happen, for example, if a feeder trips or is switched on, there may be significant step changes, 5% or even 10%.

Down at the house level, 230V equipment is designed for +/- 10% variation about the nominal voltage while still working well enough - so "steps" mentioned above are tolerable. The 10 kV to 400/230V transformers have a regulation of about 4% between no load and rated load - call that +/- 2%. This leaves +/- 8% variation acceptable at 10 kV, so why worry about voltage adjustment for synchronising?

It is most sensible to adjust the 120MW system to 10 kV before interconnecting to "48MW system". This requires the 48MW system to be adjusted to its design value of 10kV, so why worry about voltage in that system?

I am feeling that you are thinking of "rules" about generator voltage change effect which apply to connection to the theoretical "infinite Bus" rather than the reality of a whole finite 48 MW system.

If you raise the voltage (AVR setting) of all its generators by the same amount, then any increase in inductive current to the load will be shared well enough between them (I am assuming the load is resistive with a lesser inductive component which is usual - the "0.8 lagging power factor" load has resistive current of 80% of the total and lagging current of 60%).

The system can be simplified to two ideal generators, each connected by a series inductor to a bus loaded by a resistor and inductor in parallel. The effect of setting the voltage (excitation) of the two generators to different values in this model determines load voltage and the circulation of currents between the two machines. This a situation analysed in many textbooks. I would mention that the usual "voltage droop" characteristic of an AVR makes the generator behave, in normal operation, as if it were a voltage generator in series with an inductance which drops about 5% of generator rated voltage at rated current

Often, when synchronising systems, a more precise control of voltage, frequency and phase can be got by adjusting one generator in "manual" and using only its controls to move the system. Obviously, raising its AVR setting is going to increase its lagging power and reduce the lagging power of the other generators - however, so long as its current/power factor is within rating, this is OK.

I do not know what common automatic voltage/frequency regulation systems you have (if any) on your 48 MW and 120MW groups. Often, basic systems only have "screwdriver" adjustments for system voltage and frequency, not being intended for matching two systems. Such a system will also try to counteract control of voltage and frequency by a single generator.

In practice, it is necessary to put all generators of the incoming system in manual AVR/governor control during the synchronising and coupling breaker closing procedure. There would also be the question of what happens if two systems are connected which both have automatic voltage/frequency control systems, which may "fight each other" when both are "on".

If your real concern is what happens after the coupling breaker is closed, you must be clear about your meaning of "load". This term is applied to the consumer's load on the system or the "load" on a generator and should not be mixed-up with reactive current flows between the two systems. If you match the two systems well before connecting, such reactive flows will be negligible.

Finally, I note that while you write "frequency and phase angle have already been taken care of ", these are usually far more worrying than voltage matching.

Draw a diagram with two equal voltage vectors displaced by 6 degrees - you will find that the voltage difference between them (the short side of an equilateral triangle) is about 10% of that of the two equal voltages representing two generators. An angular error of ~6 degrees maximum is a common requirement for auto synchronisers.

Consider that it is usually easy to get the system voltages matched to 2%. Consider also the basic inductive reactance of a generator for sudden load changes has about 10% rated voltage drop at rated current: you should see that connecting to an infinite bus with 10% terminal voltage difference [due to phase] will cause initial currents of generator rated current size.

Dear 67 model,

My main problem is this: To raise the voltage of the 48MW power system, i increase excitation of the generators which injects excess reactive power to the load to which it is connected. what becomes of this reactive power and what impact does it have on the load?

Dear Nzenowo

The power system must have many loads fed by a number of outgoing breakers. Without knowing the kind of load, plus the measured values,it is not possible to have an opinion.

1. Where are you measuring the MW & kVAr? Is it lagging or leading load?
2. what is the normal system voltage before raising excitation?
3. what are the normal MW & kVAr with voltage as 2. ?
4. To what voltage do you raise the system for excessive reactive power?
5. What are MW and kVAr with voltage as 4. ?
6. Is the excessive reactive power on one load feeder or all?
7. What kind of load is getting excessive reactive power ?
8. Please confirm the 48MW system is not connected to 120MW system when this occurs.
   

67model

Let's simplify this, OP has two isolated systems, both with the same nominal voltage (10kV) but different operating voltages (which we don't know). From the discussion it appears that the smaller system is operating below the larger system's voltage, and OP is concerned that when he raises the voltage to synchronize, the VAR flow increases above what he is comfortable with. He also wants to know how the VAR/Watt flow is going to distribute after synchronization and what the new bus voltage will settle at.

The answer is that no one can tell without knowing the characteristics/impedances of the generators, transformers, loads, and how the governor, AVR, and their associated Droop and Cross-current compensation subsystems are set up. Assumptions can be made, but only an experienced power engineer can predict with reasonable accuracy exactly what will happen, anything else is a SWAG (Stupid Wild A$$ Guess).

Dear Mr. spades,

As I understand the O.P., the raising of voltage is being attempted before synchronization - and for Synchronization there are a set of conditions to be satisfied.

Normally it will be stated, that the Phase Sequence should be the same, Frequency should be the same, Voltages should be the same.

During my college days our Professor taught that apart from the above conditions, it is better to have same SUSCEPTANCE RATIO, ADMITTANCE RATIO, of the In-Coming Generator is same - it will be good but not compulsory. But the Droop Charater is to taken care.

DHAYANANDHAN.S

Whatever your problems are with synchronization, they will be greater with non-synchronization. What are you trying to do, anyway, slam 3 gensets onto the bus all at once, rather than one at a time?

How did you answer this interview question? If you have done your synchronization process correctly (perfectly matched voltages, perfectly matched frequency, correct phase sequence) there will be NO VAR flow until you increase the excitation, and no Watt flow until you increase the fuel supply. This assumes that you have set up your droop and cross-compensation circuits correctly, but be careful, isolated systems such as this behave differently than grid connected systems.

There are many previous threads here that discuss this.

As long as phases follow the same order, the voltage matches, the frequencies are all a match, and the phase angles of all test phases (say the A phase on each) are identical, then there is no problem, but the instant any one of these generators trips (assuming of course they all have their own trip relays), the system under load requires more outputs from the other units in your "micro-grid".