Increasing Power in Synchronous Generator


Thread Starter


I am an electrical engineer and I am still fresh. I am studying the Steam turbine Generator and I have an STG connected to a grid (60 Hz,34.5 KV). the output STG power is 26 MW I need to increase it to 31.2 MW. I need to focus into generator side not prime mover. The question:

1- What is the primary step I should do?

2- What graph should I study (Capability curve, v/f .. etc)?

3- How can I know what is the maximum Active power?

4- What is the logic and control that prevent the generator to feed more?

I appreciate your help..


1) First, you need to understand that generators are electrical machines that convert torque from the prime mover (steam turbine in your example) into amperes, just like (electric) motors are electrical machines that convert amperes into torque to pump water or lift people/goods in a lift (elevator), or move air using a fan. There's no knob or switch directly connected to the motor that can change the motor's output (torque)--that has to be done by changing the load on the motor (increasing the amount of water being moved by the pump being driven by the motor; increasing the number of people in the car of a lift (elevator) being moved by the electric motor; increasing the amount of air being moved by the fan driven by the motor; etc.). Electric generators drive electric motors by the amperes flowing in the wires that connect the generators to the motors--but the actual work that's done by the motor is really done by the prime mover driving the generator. The torque that's being produced by the electric motor actually comes from the torque that's input to the generator. Both the electric motor and the electric generator are really "dumb" devices--they don't have "governors."

So, you can't divorce (separate) the prime mover from the generator when considering how to increase the amount of power produced by the generator. The formula for electric power is:
<pre>P = Vt * Ia * 3^0.5 * PF

where, P = Power (Watts), for a three-phase synchronous generator
Vt = Terminal Voltage
Ia = Armature Amperes
3^0.5 = Square root of 3
PF = Power Factor</pre>
Synchronous generators run at a relatively constant voltage (usually no more or less than 5% of rated terminal voltage). The square root of 3 is a constant (1.732, or something like that). So, these two values can be considered as fixed and not changing. The Power factor is variable, but we're going to consider it to be fixed at 1.0 (unity) for this discussion. That means if you want to changed the power output of a generator, you have to do something to change the amperes. And, there's no "AMPERE" knob on the generator--just a RAISE SPEED/LOAD or LOWER SPEED/LOAD switch or push-button or target on a computer display to change the amount of energy flowing into the prime mover (steam into the steam turbine in your case).

Changing the energy flow-rate into the prime mover driving the generator is the ONLY way to change the output of the generator--because the generator is an electric machine for converting torque into amperes. Again, if you can find a knob or switch or push-button connected to a synchronous generator to change the amount of amperes flowing out of the generator (which is how the power of a generator is changed--see the formula above) then I--and LOTS of other people will be very interested, and chagrined.

2) The "graph" you should study is a straight line--with the x-axis representing energy flow-rate into the prime mover(steam in your case) and the y-axis representing watts. As you increase the energy flow-rate into the prime mover the watts produced by the synchronous generator driven by the prime mover will increase linearly. It's y=mx+b, or, f(x)=mx+b.

3) The maximum active power produced by a generator-set (the generator and it's prime mover) is defined by the maximum power (torque) which can be produced by the prime mover (in a properly chosen generator-set). The generator can produce just about any amount of power--for an "instant"--it's just that the generator nameplate represents how much power can be produced at a continuous, steady operating condition without damaging the generator due to the heat that's produced when current is flowing in the generator's windings (that's what the capability curve represents). Most generators are rated for slightly more than the prime mover driving it can produce, so that the generator doesn't limit the prime mover's ability to produce power (on a continuous, steady-state basis, that is). There is also the coupling used to connect the prime mover and generator, and they usually have a maximum torque they can safely transmit without damaging the coupling. The maximum torque capability of the load coupling is sometimes programmed into the prime mover's governor (control system) to protect the load coupling (if there's a chance the prime mover output could possibly exceed the load coupling's rating).

> 4- What is the logic and control that prevent the generator to feed more?

The prime mover's governor--the control system that sends the signal(s) to the valve(s) that control the amount of energy flowing into the prime mover (steam into the steam turbine in your case) is what controls the generator output. Again, this is because a generator is a dumb device--it just converts torque into amperes, which are transmitted by wires to electric motors which then convert the amperes back into torque to do useful work (pumping water; moving people/goods in a lift; moving air; compressing refrigerant in an air conditioner or refrigerator; etc.).

Electricity is just a means of transmitting torque from one location to many others using wires. It's just like a hydraulic system--where a hydraulic pump develops pressure and flow, which is then connected to hydraulic motors and rams to produce torque or force. The hydraulic fluid is returned to the reservoir and then the hydraulic pump raises its pressure and flow to be reused over and over again.

Amperes are the hydraulic flow; volts are the hydraulic pressure. Electric generators are the devices that produce the pressure (voltage) and flow (amperes), and electric motors are the devices that convert the flow to torque to do work.

It's just not possible to consider the generator without the prime mover when trying to understand how to change the power produced by the generator. You don't change the terminal voltage of the generator to change it's power output. You can't change the square of three. And, while you can change the power factor, that's not how one changes the real power output of a synchronous AC generator (it's the result of changing the reactive current in a synchronous AC generator). To change the power output of a synchronous generator, one has to change the amount of torque being provided to the generator--by the prime mover driving the generator.

And, this is all done at relatively constant speeds (frequency) on most AC power systems (grids) around the world. Without electric power systems, one would have to have a LOT of small engines producing torque all over the country side and in cities, and if they used fossil fuel there would have to be LOTS of pipes and tanks and tanker trucks and tank farms to supply the engines. Electricity changes the scale of all that--by producing large amounts of torque in smaller areas and then transmitting that torque via wires to motors in lots of remote locations where torque is required.

Hope this helps!
Even though the post i very old, I am commenting now. The reply to the question is very well explained even understandable to a lay man. I am following posts of CSA and always notices very good replies to all the queries.

In continuation to the above question, I have one more query in similar line

If there is sufficient margin available in prime mover and steam generating unit, then what limits the overloading of generator, say up to 10% or 20% of the rated capacity? I hope again i will get very good answer from you.

>If there is sufficient margin available in prime mover and
>steam generating unit, then what limits the overloading of
>generator, say up to 10% or 20% of the rated capacity?

The prime mover governor is configured to protect the generator, and possibly the load coupling, against overloading.

Look, it's possible to operate a generator above it's rated capacity--but not for extended periods of time. 'Above rated capacity' means current flowing in the generator stator windings above rated capacity. As we all know when current flows through a conductor it generates heat. There are a LOT of conductors in a generator stator--and that means a lot of heat.

A generator's rating is determined by the ability of the generator cooling method to keep the heat produced by the current flowing in the stator windings (and the rotor windings!) to the level that it won't exceed the rating of the conductor insulation. If the heat gets excessive, then the insulation will deteriorate and there will be arcs and sparks--which ain't good inside the generator.

As was said previously, in general most generators are rated slightly higher than the prime movers driving them--so that the output of the prime mover doesn't have to be limited and it can produce as much power as possible. So, said in another way, by using a prime mover that is rated lower than the generator is rated the prime mover can be allowed to run at its full output (when desired) which means optimal efficiency, and it won't have to be throttled back to protect a generator which is rated lower than the prime mover.

Finally, there is always going to be the odd case where for some reason the prime mover was de-rated (usually for some permitting reasons) which could lead to the situation where the equipment was not being operated at its most optimal conditions. BUT, one should understand what those original conditions were and why that was done before undertaking raising the output of the equipment.

Hope this helps!!!