SPEED GOVERNOR SYSYTEM AND ITS DERIVATIONS

Real power and frequency control

1. UNIT II REAL POWER - FREQUENCY CONTROL SYLLABUS Basics of speed governing mechanism and modeling - speed-load characteristics – load sharing between two synchronous machines in parallel. Control area concept LFC control of a single area system . Static and dynamic analysis of uncontrolled and controlled cases. Integration of economic dispatch control with LFC. Two-area system – modeling - static analysis of uncontrolled case - tie line with frequency bias control of two-area system - state variable model.

2. Speed changer Lower Raiser X A X B X C X D X E Speed Governor Pilot valve High pressure oil To Turbine Steam Steam valve Main piston Hydraulic amplifier l 1 l 2 l 3 l 4 Fundamentals of Speed Governing System 2

3. The system consists of following components Fly ball governor Hydraulic amplifier Linkage mechanism Speed changer Fly ball speed governor: This is the heart of the system which senses the change in speed of the system. As the speed increases, the fly ball moves outwards and the point B on linkage mechanism moves downwards. The reverse happens when the speed decreases. Hydraulic amplifier It consists of pilot value and main piston. Low power level pilot valve movement is converted into high power level pilot valve movement. This is necessary in order to open or close the steam value against high pressure system. Linkage mechanism ABC is a rigid link pivoted at B and CDE is another rigid Link pivoted at D. This link mechanism provides a movement of control valve in proportion to the change in speed . 4

4. N & F L Raise command A point Move Downwards Xc point move upwards Xe point move downwards Valve move downwards Getting more inputs To restore the freq. we increase the generation

5. Speed Governor Model The governor compensates for changes in the shaft speed Changes in load will eventually lead to a change in shaft speed. Change in shaft speed tends to change in system frequency . Turbine model The prime mover driving a generator unit may be a steam turbine or a hydro turbine. The models for the prime mover must take account of the steam supply and boiler control system characteristics in the case of steam turbine on the penstock for a hydro turbine. The dynamic response of steam turbine in terms of changes in generator power output Δ P G to change in steam valve opening Δ X E . 6

6. Generator load or Power system model To develop the mathematical model of an isolated generator, which is only supplying local load and is not supplying power to another area, Suppose there is a real load change of Δ P D . Due to the action of the turbine controllers, the generator increases its output by an amount Δ P G . The net surplus power ( Δ P G - Δ P D ) will be absorbed by the system in two ways. By increasing the kinetic energy in the rotor. As frequency changes, the motor load changes being sensitive to speed . 7 Model of Load frequency control of single area

7. Speed-Load characteristics The parameter R is referred to as speed regulation or droop . It can be expressed in percent as

8. Requirements of parallel operation of Alternators Alternators to be operated in parallel should meet the following requirements. They must have the same output voltage rating. The rated speed of the machines should be such as to give the same frequency. The prime movers of the alternators should have same speed load drooping characteristics, so as to load the alternators in proportion to their output rating. The alternators should be of the same type so as to generate voltages of the same waveform. They may be differ in KVA rating. Load sharing between two synchronous machines in parallel If two or more generators with drooping governor characteristics are connected to a power system, there will be a unique frequency at which they will share a load change They are initially at nominal frequency f , with outputs P 1 and P 2 . When a load increases Δ P L causes the units to slow down, the governors increase output until they reach a new common operating frequency f’. The amount of load picked up by each unit depends on the droop characteristics:

9. 5.If the percentage of regulation of the units are nearly equal, the change in the outputs of each unit will be nearly in proportion to its rating. EX.01.Two identical 60 MW synchronous machines are operate in parall e l , the gov e rnor settings on the machines are such that they have 4 % and 3% droop s no load to full load % speed drops. Determine (a) the load taken by each machine for a total load of 100 MW . (b) The % no-load speed to be made by the speeder motor if the machines are to share the load equally. A B C D E F G 60MW 60MW 100MW x 100-x 104 103

10. EX.02.Two turbo-alternators are rated at 25 MW each. They are running in pa ralle l . The speed-load characteristics of the driving turbines are such that the frequency of a l ternator 1 drops uniformly from 50 Hz on no load to 48 Hz on full load, and that of alte rnator 2 from 50 Hz to 48.5 Hz : (a) how the two machines will share a load of 30 M W and find the bus-bar frequency at this load? (b) Compute the maximum load that th ese two units can deliver without o v erloading either of them .

11. System frequency=48.5

12. Steady state response of Uncontrolled case Consider speed changer has a fixed speed setting. ▲Pc=0 Reduced Block diagram is General block diagram for LFC

13. When several generators are with governor speed regulation R 1 , R 2 …. Rn

14. Block diagram of Load frequency control Block diagram reduces to

15. Kp =1/B=1/.01=100 Hz/ p.uMW

16. Dynamic Analysis of Uncontrolled Case (Single Area) A static response of ALFC loop will inform about frequency accuracy, whereas, the dynamic response of ALFC loop will inform about the stability of the loop . To obtain the dynamic response representing the change in frequency as a function of time for a step change in load. The block diagram reduces as shown in below. Taking Inverse Laplace transform for an expression ▲F(s) is tedious, because the denominator will be third order . We can simplify the anal y sis by making the following assumptions. 1. The action of speed governor and turbine is instantaneously compared with rest of the power system. 2. The time constant of the power system Tp = 20 sec Time constant of governor TG= 0 . 4 sec Time constant of turbine Tt =0.5 sec

17. Important Points for Uncontrolled Single Area By reducing value of 'R' , it is possible to increase AFRC (Area Frequency Response Characteristics). Hence static frequency error may be reduced. With smaller time constant T G and T t , the system response shows some oscillations before settling down with a drop-in frequency . But if these time constants are neglected, response is purely exponential . If the overall closed loop system time constant is calculated from the response curve, it is found to be much smaller than the open loop time constant Tp s of the power system . This has been possible due to the feedback loop as provided through the speed go v erning system . From the nature of the system response equation it may be observed that s y stem may be made still faster by reducing ' R ' . For the uncontrolled system there exists a steady state frequency error as a result of increase in load demand, however small it may be. When the load demand increases speed or frequency of the system drops though initially kinetic energy of rotating inertia may be used to meet up the demand. Eventually it will be balanced by an increase in system generation and decrease in load as associated with the dropping frequency.

18. Ex.01 : For a system of regulation = 4 Hz / p. u MW, kp = 150, Tp = 18 sec, ▲PD = 0.01 p.u.find the dynamic response for uncontrolled case.

19. Integral control of single area system. There is a considerable droop in speed on frequency of the turbine for a given speed changer setting . Su ch a large deviation ' (± 0.5 Hz) cannot be tolerated and we must develop some su i tab l e contro l strategy to achieve much better frequency constancy. For th i s p u rpose , a signal from ▲ f is fed through an integrator to the speed changer . T h e Integral controller actuates the l oa d reference point until the freq u e n cy deviation becomes zero. Integral controller gives zero steady state error . The negative polarity m e ans positi v e f requency error to give rise to a n e gative or decrease comm a nd .

20. Static Analysis or Steady State Response (Uncontrolled Case) Put Δ Pc=0, the block d i ag r a m re du c e s as sho w n i n below

21. Equating the coefficients ,

22. Equating the coefficients ,

23. MULTI AREA SYSTEM Introduction Interconnected power system Control Area Tie line Normal condition Operation Emergency condition Operation Net area interchange

24. AGC of Two area interconnected power system An ext e nded power system can be di v i ded into a number o f l o ad frequency c o n tro l areas interconnect e d by mean s of tie lin es . Without lo s s of ge nerating , we shall co n sider a two - area cas e connect e d b y a s in gle tie lin e has illustr a ted in below figure . It is conveniently assumed that each control area can be represented by an equivalent turbine, generator and governor system. Symbols used with suffix ‘1’ refers to area ‘1’ and those suffix ‘2’ refers to area ‘2’. In an isolated control area case, the incremental power ( Δ P G - Δ P D ) was accounted for by the rate of increase of stored kinetic energy and increase in area load caused by increase in frequency. Since a tie line transports power in or out of an area, this fact must be accounted for in the incremental power balance equation of each area. Power transport e d out o f a r ea 1 i s giv e n by

25. + - + - - - - - - + + Δ PG 1 (s) + Δ PG 2 (s)

26. Frequency Response of AGC without IC

27. Frequency Response of AGC with IC

28. AGC in two area system

29. AGC in two area system with IC

30. Power variation in Multi area system

31. Tie-line With Frequency Bias Control Of Two area systems. The persistent static frequency error is intolerable in the single control area case. A persistent static error in tie-line power flow called "inadvertent exchange" - would mean that one area would have to support the other on a steady state basis. The basic principle in good operation of pool must be that each area absorbs its own load in normal steady state. In two area- system, we could conceive of the arrangement that area '1 ‘ be responsible for frequency reset and area '2' take care of the tie line power. The ACE's would be fed via slow integrators or to the respective speed changers. But this arrangement is not so good. The block diagram of two area LFC with tie-line bias control is shown in below. The control strategy is termed as tie line bias control, and is based upon the principle that all operating pool members must contribute their share to frequency control, in addition to taking care of their own net interchange .

32. + - + - - - - - - + + Δ PG 1 (s) + Δ PG 2 (s) -b 1 + + -b2

33. Determination of Tie Line with Frequency Bias Control of Two Area System Principle : All operating pool members must contribute their share to frequency control, in addition to taking care of their own net interchange. ACE is the change in area frequency which, when used in integral control loop forced the steady state frequency error to zero. In order to make the steady state tie line power error to zero, another integral control loop (one for each area) must be introduced to integrate the incremental tie line power signal and feed it back to the speed changer.

34. Speed changer commands are, Steady State Response

35. The reset control is implemented by sampled data techniques. At sampling intervals of one second, all tie-line power data are fed into the central energy control area, where they are added and compared with predetermined power. Now this error is added with biased frequency error, to give ACE results. If optimum dispatch is employed, a tertiary slower loop is added. Under normal operating conditions, besides meeting respective area loads, scheduled interchange between areas can take place. Under abnormal conditions, such as loss of generation in area, power can flow from other areas, through the interconnection. Such pool operation where mutual assistance is possible which reduces the reserve capacity needed. For a large system with many areas, the kinetic energy of the rotatory inertia is high. A sudden load change may not cause any considerable transient frequency deviation.

36. + - + - - - - - - + + Δ PG 1 (s) + Δ PG 2 (s)