Parallel Generator and Synchronization With Grid


Thread Starter



I have lingering questions running through my head. I hope somebody can answers my burning questions.

1. A generator parallel with the utility grid, if let's I want the generator to supply 70% of the total load, the rest 30% from the grid, how to achieve this? generator supplies 50% of its capacity contributes to 70% of the total load. My assumption this can be done through governor control. I believe there is an explanation behind this.

2. A generator parallel with the utility grid, sudden load change has occurred (From high load to low load). my understanding is the generator will export the power to the grid in short period of time, governor and alternator would adjust the speed and voltage according to the reference value. The power supply by the generator would be reduced and generator continues to synchronize with grid to supply the power to the load. no power export to the grid by generator at this point. - Please correct me if I'm wrong.

3. Situation is almost the same as question no 1. a generator supplies 100% of its power to the load. no power export from the grid. the grid is in hot standby mode. Can it be done? if yes, please explain.

Most of us have lingering questions running through our heads; don't feel alone in this.

1. So, I think what you're trying to describe is something like this: A generator-set (prime mover and generator; remember: the generator is just converting torque to amperes, just like the motors at the other ends of the wires connecting the motors to the generator are converting amperes into torque) is rated at 140 MW, and the load on the grid is 100 MW. When the generator-set is supplying 50% of it's rated capacity (70 MW) it would be supplying 70% of the grid load, and other generator-sets would be supplying the remaining 30% of the load. If you want the 140 MW-rated generator-set to supply 70% of the grid load, then you have to calculate what that percentage is and then adjust the generator-set governor to supply that percentage.

There can be many different ways to tell the generator-set's governor to hold a particular load setpoint, from automatic to manual methods. There may be a way for the operator to input a load setpoint (in this case, 70 MW) and let the governor adjust the energy flow-rate into the generator's prime mover to maintain the 70 MW setpoint.

There might be a separate control system monitoring the grid which can send signals to multiple generator-set governors to control the percentage of grid load each generator-set is to provide. The signals can be discrete, or analog or some other method/protocol.

2. Now you're really trying to confuse yourself. There are two basic modes of generator-set governor operation: Droop Speed Control mode and Isochronous Speed Control mode. These two topics have been covered ad nauseum (in other words--many times, over and over and over and over and over and .... again) on, so much so that sometimes I think the name of this forum should be Use the 'Search' feature cleverly hidden at the far right of the Menu bar at the top of every (speed) webpage to search for many, many posts about Droop- and Isochronous speed control.

3. Let's say there is a grid which has a maximum load (the total of all the motors and lights and televisions and computers and computer monitors) which is 100 MW. And, there are several generator sets synchronized to the grid (and synchronized to each other), and that one of the generator-set prime movers is capable of producing 140 MW, and the other generator-sets are all rated for much less than 100 MW. If the operator of the generator-set rated for 140 MW increases the load being supplied by the unit to 100 MW then the operators of the other generator-sets would have to reduce their outputs to 0 MW in order for the grid frequency to remain at rated (50 Hz or 60 Hz, depending on where this imaginary grid was located). So, it would indeed be possible to have a single generator-set supplying all of the loads connected to a grid, with other generator-sets also synchronized to the grid but supplying none of the load.

These generator-sets would be operating at very low efficiency because they would be required to use some energy just to maintain rated speed and zero load.

These are very basic answers to your questions--without using much maths. As for Question 2, you need to do some studying in order to form your question better. Some of the threads you should find on utilize a bicycle analogy to try to explain droop- and isochronous speed control. A bicycle is a device for moving people--and sometimes goods and packages--by applying a force to a crankset (pedals) to create torque to move the people, goods and packages at some rate of speed. If the rate of speed is to be constant then the force being applied (the torque being applied) has to be proportional to the weight of the people, goods and packages such that the rate of speed at which the bicycle is traveling will remain constant.

An AC grid is very similar--in that the speed (frequency) of the system is supposed to remain constant. If more motors and/or lights and/or televisions and/or computers and computer monitors are turned on the primary effect on the AC grid is to cause the frequency to decrease. But, somewhere on the grid conscious operators and their automated equipment are increasing the energy flow-rate into one or more generator-set prime movers to keep the grid frequency constant. In this way, the total generation increases to match the load increase. When motors and/or lights and/or televisions and/or computers and computer monitors are turned off the effect on the grid is to increase the grid frequency--but, again, someone somewhere will reduce the energy flow-rate into one or more prime movers to keep the grid frequency at rated.

Back to the bicycle analogy, if there are multiple bicycles and riders all trying to move people and goods and packages at a constant speed if the number of people or goods or packages changes then the speed of the bicycle "train" will tend to change unless. One or more of the riders has to change the force being applied to the bicycle cranksets in order to keep the bicycle "train" moving at the desired speed.

Electricity, especially AC electricity, is very similar. Electricity is just a way of transmitting torque from generator-set prime movers (turbines; reciprocating engines; etc.) to motors and lights and televisions and computers and computer monitors. Generators are just devices for converting torque into amperes, in the same way that motors are just devices for converting amperes into torque. Generator-sets can be synchronized together to supply loads that are MUCH larger than any single generator-set could ever hope to provide.

If we think of bicycles, multiple bicycles, being used to move people and goods and packages (using something like a bicycle "train")--and they could be--and if we want to people, goods and packages to move at a constant speed, then the analogy to electrical generators works. If we want the bicycle "train" of people, goods and packages being pulled by multiple bicycles to move at a constant rate of speed then as the number of people, goods and packages changes then amount of force being applied to one or more bicycle cranksets has to change to keep the speed of the entire bicycle "train" constant. It's just that simple. Even think of just a tandem bicycle with two riders--if the number of goods and packages being moved on this single bicycle with two riders changes then one or both of the riders will have to change the amount of force they are applying to their crankset in order to maintain a desired speed. And, if there's no coordination between the two riders then the bicycle speed is not going to be very constant.

Moving people and goods and packages requires force, torque. Pumping water, moving air, compressing refrigeration gas, producing light and powering televisions and computers and computer monitors all require force, torque. On an AC grid, that torque (amperes) is supposed to flow at a constant rate (frequency). Changes in load (the number of motors and lights and televisions and computers and computer monitors) have the immediate effect of changing grid frequency--but a well-regulated grid will respond very quickly to change the energy flow-rates into the prime movers driving the generators to keep the grid frequency constant.

The split of how much of the total grid load each generator-set supplies is controlled by operators sending commands to the prime mover governors. Generators are really very dumb--they just convert torque into amperes. More torque; more amperes. Less torque; less amperes. It's the generator prime movers that produce the torque that's being applied to the generators. So, increasing the energy flow-rate into a prime mover increases the torque being produced by the prime mover, and that torque, when applied to a generator will cause the amperes flowing in the generator stator to increase.

If you don't like the bicycle analogy, think of multiple train engines connected together to pull lots of train cars and trying to do so at a constant rate of speed, while the number of people and train cars and packages and goods being carried by the train cars is constantly changing. It's exactly the same thing. Exactly.

Hope this helps!

Papoo Sankaran

Explaining complex technical subjects in the simplest form using 'easy to understand analogies is an art and you seem to have mastered it! And repeating it in many ways to make sure that it is understood is the quality of a good teacher. Look forward to read more such write ups.