Please, can someone explain in simple terms what is difference between primary and secondary frequency regulation?

If I understood correctly, primary regulation is a spontaneous reaction of turbine's control valves ( in case of steam turbine) to changes of frequency in the system.

I don't know what is secondary regulation.

Thanks

 

I have struggled greatly with this same quandary over the last couple of years. I believe it's related to a UCTE document about frequency regulation or -control for many of the countries in the EU (it can be found at www.ucte.org). Unfortunately, I can't understand or explain exactly what the difference is after reading bits and pieces of the document (which was an English translation of a document originally written in French, I believe).

I believe primary frequency regulation refers to Droop Speed Control and how a primer mover's governor responds to grid frequency disturbances when operating in "straight" or "plain" Droop governor mode. The "spontaneous" reaction of a turbine's control valves would actually be in response to some action determined necessary by the governor (control system), such as responding to changes in grid frequency/turbine-generator speed.

I believe secondary frequency regulation refers to remote control of units being operated in Droop Speed Control by entities such as system operators/regulators, etc. For, example, in an underfrequency condition, a supervisory/regulatory entity may have the ability to raise load by raising the turbine speed reference (up to the rated output capability of the prime mover, or, in some cases, through a special control scheme); or in an overfrequency condition, a supervisory/regulatory entity may have the ability to lower load by lowering the turbine speed reference.

This remote secondary frequency control might be accomplished by contact inputs to the RAISE and LOWER SPEED/LOAD setpoint functions, or some kind of analog speed/load reference. This kind of control would have to be enabled by the local operator to allow remote control. Some sites get paid additional monies for allowing this type of remote control capability, and even more when it's used.

But, that's my (limited) understanding of the two: primary- versus secondary frequency regulation. I would welcome anyone else's inputs, comments, or corrections. It's not something which seems to be understood or explained very well anywhere that I have searched. I've seen it referred to as special control schemes in the turbine control system, or as detailed operating procedures. It seems to be something of a nebulous term. Droop speed control isn't understood or explained very well anywhere, either, and that's generally what's referred to as primary frequency control/regulation in most documents I've read or discussions I've been party to.

 

CSA, thank you for your reply. I didn't read a document directy from UCTE, but I beleive it at least, containes parts of it.

In article which I read, it is stated something like this:
Primary regulation:
When ther is a deviation of grid's frequency from its nominal value, it will reflect to small change in turbine's speed. In such case, turbine's speed controllers changing output power in order to obtain balance between production and consumption. As a result of control action (almost always P-only control !?!) there is a new steady value of frequency which can differe from nominal value, but stability of power system is secured.

Secondary regulation:
Goal of secondary regulation is to restore grid's frequency to its nominal value, by acting on cotrollers of prime movers (turbines) but only to those power plants that are assigned to participate in secondary regulation. So PI controller is mainly used in power plants that are chosen for secondary regulation.

I know this is very weak and full of holes, but that is only I could find. I don't have access to full ucte article, do you?

Thanks

 

Try:

http://www.ucte.org/_library/ohb/policy1_v22.pdf

 

My simple explaination first. Then I will follow with more detail one.

The main function of primary response is to bring system frequency rate of change (dF/dT) to zero within time frame several seconds. It will be acheived by forcing the supply to be equal to the demand + losses. It MW response can be controlled via governor speed droop percentage set point. dF/dT can be zero at any stable frequency that the grid can exist. It does not necessarily occur at the scheduled frequency.

On the other hand, secondary response resets the system frequency back to the scheduled frequency. It does not produce any net generation to the system . Rather it add or subtract inertial energy of rotating mass of the system. Here is an illustration.

Generation = 6000MW

Demand + losses =6000MW

Balancing is done primary response

System frequency = 49.80Hz

Inertial energy of rotating mass = 59860MJ

Scheduled frequency =50.00Hz

Inertial energy of rotating mass @50Hz =60000MJ

It is clear from above that we have to add another 240MJ to the system if wish to reset the frequency to 50.0Hz.

Manual secondary response can be illustrated fairly simple. We just raise MW of any turbine generator by 2.4MW for 100 seconds. After 100seconds, we have added the desired 240MJ. Then we have to reduce it by 2.4MW too. Otherwise, system frequency will keep on increasing since it has additional energy to do so. Thus our system frequency should settle down at 50.00Hz. That is why secondary response must have feedback loop irrespectively we use AGC or operator's intervention.

More detail...

Many people get confusing the true meaning of infinite bus. I'm referring to the grid system operation. There is no such thing of infinite bus in the real world. Let me prove my point.

Suppose we are operating a grid system with installed capacity of 3 million MW. I suppose it is very close to infinite system, right? Our system's complex power is says 3.3million MVA. The system is rotating at 50Hz electrical frequency.

Assuming our system inertial energy constant of rotating mass(H)as 10MJ/MVA at 50 Hz rotating frequency. Thus the total inertial energy of rotating mass of our system at 50 Hz will be

Inertial energy constant =10 X 3,300,000 = 33,000,000MJ

When our system is operating at steady state frequency, says at 50.0000000Hz, then we can be sure that power produced is equal to demand + losses. We can conclude that there is no net torque to accelarate any rotor in the system. Assuming the power production and demand + losses are 2,700,000MW at t=0. Just after t=0, a 1MW demand is connected. That makes the new demand will be 2,700,001MW, right? All the turbine generators have no way to know that there is additional load being attached to the system if the system frequency does not change.

If we assume infinite bus means the bus frequency that cannot be accelarated & retarded, then we have fundamental problem with regard to the conservation of energy. If there is no additional load being supplied by any prime mover, then where does this 1MW come from? It seems like we just get 1MW out of thin air!

No. We can't get any energy out of nothing. Additional 1MW is derived from inertial energy of rotating mass. Our system has energy of rotating mass as we have shown above equals to 33,000,000MJ as long as the system frequency remains at 50Hz.

Assuming the level of generation does not chance from 2,700,000MW. Then 1MW additional demand causes the system frequency to go down and after T seconds all the rotors cease to rotate, where

T = 33,000,000MJ/(1MJ/s)= 33,000,000seconds (381days)

It may take long but surely all the rotors will stop to rotate since its inertial energy of rotating mass has been used up!

How the prime movers will respond? Primary reponse takes frequency deviation in order to increase or decrease generation. Not later than 0.1Hz, primary response will provide additional output so that the frequency will not go down anymore. Normally its response is important during the major load disturbances such as loss of bulk generating unit or transmission line. For a small load change, secondary response via operator's intervention is good enough to restore order.

What does the true meaning of infinite bus then? The bus will become infinite if we make it to become infinite by means of system set up, frequency control and regulation, spinning reserve management, etc. It is actually the operational objective rather than the physical law.

 

A reasonably good explanation, and a thought-provoking piece on the inertia of electrical grids and the concept of an "infinite" grid.

Not sure I agree with everything exactly as stated, as you are arguing against conventions, but there is food for thought here.

Thanks!