Diesel Genset Droop Speed Control And Isochronous Speed Control

Guys any help please ~~

Droop versus Isochronous.

First - More Fuel = more power = more watts out of generator and/or more speed.

To get the diesels to share load they need to be in droop, then more load = less speed. If in Isochronous then load will wildly go back and forth between diesels assuming isolated unless you have a fancy governor that can share load without dropping speed.

Regulator = Voltage = VARs NOT EQUAL to POWER has nothing to do with power. We have some operators that are confused by this also since the governor control says speed.

On a utility generator the governor has a speed component included so if the speed is too high the valves close, if too low, the valves open (unless they are wide open anyway). Since it can be considered to be connected to an infinite bus then more energy into turbine == more MW out. Normally lower frequency on the load means lower watts consumed and higher frequency means more watts consumed.

Hi There,

Thanks for your explanation !

So , more fuel = more mechanical power generated by prime mover = more real power generated by alternator?

But, I thought the mechanical power generated by prime mover is in the form of rotational speed of the flywheel ( since the cylinders in the engine is generating more energy and this is actually being converted to rotational movement of the flywheel) which in turns turn the shaft of the engine which is coupled to the alternator and in turn, increase the speed of the alternator?

I have read some somewhere that when the load demand increases, the voltage at the stator windings of the generator will decrease and therefore, the AVR has to supply more excitation current to the rotor of the generator to bring up the voltage to its rated voltage value.

Why will the voltage decrease when the load demand increases?

So sorry for the plentiful questions.

the mechanical power generated by prime mover is in the form of rotational speed of the flywheel

Not exactly - it is more torque gives more power. Think of an auto on a flat highway going 1500 rpm. You need more gas/torque to keep the same rpm going up a hill. Similar priinciple for a generator and load.

when the load demand increases, the voltage at the stator windings of the generator will decrease and therefore, the AVR has to supply more excitation current to the rotor of the generator to bring up the voltage to its rated voltage value.

Correct - If one looks at the current in a rotor for the same voltage & power factor as the load goes up the current goes up to combat the "back emf, Lenz's law" caused by more current flow in the stator. If you did not have an automatic voltage regulator then the voltage would sag.

Very useful piece of information indeed !

Thanks !

But I wish to know in detail some of the theories discussed ( like how does AVR affect the VAR of the generator , how does a governor increase the real power of the generator when it is in parallel with other generators etc... )

Here is a simple explanation, and I apologize if it's to simplistic. The field windings generate a magnetic field that is rotating. The armature is also an electromagnet. The armature shaft can either be connected to a mechanical load (and it behaves as a motor) or it can be driven by an engine (acting as a generator). As a motor, the greater the mechanical load, the more the armature field lags behind the rotating field. Likewise the more power delivered by the engine, the more the armature field leads the rotating field. When you parallel two generators, then the amount of power each delivers to the common load is determined by how much each armature field leads the rotating field, and this is determined by the throttle setting on the engine. In essense, the generator shafts are connected together with a flexible magnetic coupling.

The governor increases the engine power when the rotation rate is too slow, and decreases it when it is too fast. In isochronous, this control is very tight. Not much speed error is tolerated before the governor adjusts the engine. In contrast, in droop mode, more error is tolerated. If you parallel two engines in isochronous mode, the one that is set slightly higher will take all the load. It is difficult to get the throttles set close enough together for the load to be shared.

The first generator to be connected to the load should be set to isochronous and its throttle adjusted so that the power frequency is correct. Subsequent generators paralleled with the first should be set in droop mode and the throttle adjusted to accept their share of the power (Kilowatts, not amperes).

If the voltage regulators are not set properly, there will be cross currents between the paralleled generators. This extra current is not delivered to the load and only serves to heat up the generator windings. You will notice that the sum of the currents times the voltage is greater the the power reading. Cross current compensation in the voltage regulator detects this condition and adjusts the excitation to the armature to minimize it.

This particular question is more in the realm of Speed Regulation, if we design a governor for 10% droop or regulation;

Speed at 100% rated is equivalent to 0% load and essentially 0% valve/fuel opening.

Speed at 90% rated is equivalent to 100% load and 100% valve/fuel opening.

Speed at 95% rated is equivalent to 50% load and 50% valve/fuel opening.

Let me explain each of those conditions;

In the first instance, this describes any prime mover waiting to be synchronized. Whatever you are using for a speed sensing device is nulled or has found its place in space.

The second instance describes a unit that is isolated from a system and while running at full speed no load, a load of 100% is applied. The valves/fuel have to open to 100% to satisfy the load. The system speed dropped because the system was designed to droop 10% from no load to full load.

In the third instance and we are still speaking of running in an island mode, 50% load was applied and required 50% valve/fuel opening to satisfy the load. The speed changed 5% because it was designed for 10% speed regulation.

Taking that a step further let's say we have two units side by side both designed for 100 MW and 10% speed regulation. Both units are on an isolated system and a system load of 200 MW is placed on the bus. Neither unit has the capability to pick up the 200 MW so both units decay in speed to 90% and both units are running valves/fuel wide open. If this load is reduced to 100 MW, both units will increase in speed to 95% and the valves/fuel will be 50% open.

This is all assuming you are loading the governor up and not touching it from that point. In actuality you would be biasing the governor up to keep the system speed up. In fact if your governor has 10% regulation the amount of input to the governor to make up for the speed decay would be present in case of a breaker opening and the turbine would increase speed to 110% and require resetting to full speed no load.

One thing that I find interesting about the whole concept is that if you were at full speed, full load, valves wide open and you increased speed to 110% you would drive the valves shut.