GOVERNOR

BY

Er. TANUJ SHARMA

STUDENT OF PGDC HYDRO POWER PLANT(2010-11), NPTI

NANGAL

INTRODUCTION Governor is basically a speed control device. Speed control of turbines is required to ensure

proper operation of electrical and mechanical machinery. When a single AC turbine generator supplies electric supply to a small AC net work, a governor is necessary to maintain the speed of turbine at a fixed value irrespective of the load demand. Governors arranged in such a way that there is no change in the speed with the change of load are called astatic type of governors.

PRINCIPLE THE POWER PRODUCED BY GENERATOR SHOULD BE AT

FIXED FREQUENCY & FOR THAT, THE SPEED OF HYDRO TURBINE SHOULD REMAIN CONSTANT.SPEED TENDS TO CHANGE WITH VARIATION OF SYSTEM DEMAND. A GOVERNOR IS INCORPORATED IN THE TURBINE-GENERATOR TO ENSURE CONSTANT SPEED. IT SENSES CHANGE IN SPEED & THEN CONTROLS WICKET GATES OPENING AUTOMATICALLY THROUGH SERVOMOTORS SO AS TO OPPOSE CHANGES IN TURBINE SPEED. IN THIS WAY ,ITS KEEPS CONSTANT. A GOVERNOR ENABLES THE MACHINE TO START,STOP, SYNCHRONIZE & PROTECTS IT IN THE EVENT OF SHUT DOWN UNDER FAULT CONDITIONS.

WHAT IS GOVERNOR?

GOVERNOR MAINTAINS THE SPEED OR LOAD BY SENSING THE ACTUAL SPEED OR LOAD OF TURBINE AND COMAPARING IT WITH DESIRED SPEED OR LOAD SETTING. USING THIS COMPARISON THE GOVORNOR DETERMINES THE AMOUNT OF ENERGY SOURCE NECESSARY TO CONTROL DESIRED SPEED OR LOAD.

Water turbine governor, which is a speed sensitive governor, is designed to respond to small variation in speed and to correct in a stable manner. The governor is also called upon to respond in various ways to other control signals & to regulate the water supply to the turbine accordingly. Governors of both types i.e. mechanical and electro hydraulic or digital type are being designed and manufactured for their successful operation.

Two types of governors for speed and power control of hydraulic turbines are in use.

a) Mechanical governorb) Electro- hydraulic governor

ELECTRO- HYDRAULIC GOVERNOR

INTRODUCTION

In electronic governors hydraulic actuators speed sensing is by speed signal generator (SSG).The signal is converted to proportional voltage signal and amplified in a magnetic amplifier. Electronic hydraulic transducer converts proportional signal to release proportional pressure oil for servomotor operation. Feed back control is again by mechanical linkages. Control linkages i.e. feed back are mostly mechanical.

WORKING

SSG UNIVERSALAMPLIFIER

CONTROLAMPLIFIER

BOOSTERAMPLIFIER

E HT

SERVOMOTOR

FEED

BACKDROOP

UNITNO LOAD

GATE OPENING

65S

15F

PERMANENTDROOP

The governor is basically speed sensing type to speed deviation from a set value of speed. Temporary droop stabilizing technique has been used. The speed signal is produced with turbine shaft mounted toothed and magnetic pick up. The following fundamental relationship between speed and frequency of output signal implies:-

f= n × p / 60 where f= Frequency of the sensor’s output signal in

Hz

n= Speed of the toothed wheel in RPM p= Number of teethed on the toothed wheel Number of teethed are so chosen that rated speed,

the SSG produces 100Hz signal. The SSG’s output signal representing the machine speed is processed in frequency to voltage convertor which gives an output of 0 to 10VDC for input frequency signal of 0 to 200Hz. This signal is compared with the speed setting signal in an amplifier of gain “8”, so that at nominal speed and speed setting, the resultant signal is zero volt and varies by +0.8v/hz depending upon whether the speed is above or below normal.

The above signal is fed to control amplifier which comprises a phase advance amplifier and a multi input summing amplifier. The phase advance amplifier has a resistive- capacitive feedback and its output varies with the rate of change of input signal. Thus, it provides “speed error signal and its of change” for faster governor action.

In the summing amplifier the modified speed error signal is summed up with permanent droop and temporary droop signals. The signal levels and input resistors are so coordinated that output of summing amplifier under steady state is zero volts.

The droop signals are derived by comparing the gate reference signal with the gate feedback signal(+1 to -13vdc). The resultant signal directly applied to summing amplifier through potentiometer is called permanent droop signal, whereas differentiation of the resultant signal with variable time constant RC network provides the temporary droop signal. The temporary droop signal also known as damping signal transiently opposes the gate movement and thus it provides main stabilization to the speed control loop, which is otherwise highly unstable due to large interia of hydraulic system. Both time constant and percentage of the temporary droop signal are adjustable.

Permanent droop signal contributes gate v/s speed characteristic to the system. By virtue of this the machine accepts load when the frequency decreases and rejects load when the frequency increases, amount the permanent droop which is adjustable from 0 to 100% decides the slope of characteristics line in relation to the gate position. In other words permanent droop means speed regulation. A 10% droop a machine operating at 50hz will accept full load either if the frequency decreases by 10% or if the speed reference is raised to 55hz (50 Hz +10%) so as to open the gate fully. However, if the grid frequency is 48 hz, speed reference will have to be raised to 53(48+5) only.

The summing amplifier output is amplified is a booster amplifier and then applied to an electro-magnetic transducer(EMT) located in the hydro-mechanical cabinet. It is linking device between electrical and hydro-mechanical sections. The EMT actuating the hydraulic amplifier converts the electrical signal into the proportional mechanical movement of main distributing valve which in turn imparts oil to the servomotors for the controlling the gate movement.

Under steady state conditions governor provides balance current 0mA to the EMT. In the event of frequency rising or a lower command, it provides a proportional negative current for closing the gate and in the event of frequency falling or rise command it provides a proportional positive current for opening the gate. Once the gate acquires the new position the output current resets to 0mA under action of the permanent droop signal. Under dynamics conditions output current vary in the range of -60 to +60 mA. The relay control circuit enables the turbine to be started, stopped and protected.

DIFFERENT PARTS OF

THE GOVERNOR

Speed sensor- Speed sensor is installed on the extension of main generator shaft. Earlier permanent magnet generator(PMG) were used as sufficient power was required for moving actuator motor & set of fly balls. With the introduction of electro hydraulic governor with electronic circuits, the power requirement has considerably reduced & now a days, toothed wheel type sensors are installed. Both are explained as below:-

a) Permanent magnet generator- It is fixed on the extension of main generator shaft at the top. The frequency & voltage of its output signal is proportional to unit speed & the same is used as initial sensing speed. At normal speed, it produces 3-phase supply at 50Hz. It poles are of permanent magnet on which additional winding is provided for initial magnetizing & subsequent re- magnetizing. It is used on both type of governors.

b) Toothed wheel generator- In this case, a toothed wheel is mounted on an extension shaft. The number of teeth & diameter of this wheel depends upon speed of generator. A magnetic pick up mounted opposite to this tooth wheel to detect speed of machine. The output of this magnetic pick up is square wave whose frequency is proportional to machine speed & is fed to governor regulating circuit via a voltage convertor.

Servomotor- Two double acting servomotor are installed in turbine pit. These operate with oil pressure received from distribution valve & are of sufficient power to open/close all guide vanes against high hydraulic & friction loads. The cylindrical body is made of cast steel or fabricated from steel plates. The inner surface(cylinder) is accurately machined to enable piston to operate smoother & without any jerk. The piston & piston rings are made of cast iron. An interlock is provided to prevent operation of governor when the servomotor lock is engaged. These operate with oil at pressure of 20-40 kg/cm². a mechanical lock is provided on servomotor to lock the guide vanes in closed position. This lock should be applied before doing maintenance work on turbine or when turbine is shut down for longer period.

Oil pressure unit- A high pressure oil system is needed for developing and storing sufficient power the routine and emergency operation of servomotors. The oil pressure tank is made of boilerplate and is half filled with air and half with oil of 20- 40 kg/cm². this pressure is maintained by pumping oil if air volume is more and air volume is less. However since there are very little changes of air leakage, normally oil is pumped.

Oil pumps- These are two in number:- One is stand by with automatic change over provision on sensing decrease in oil pressure. These are positive displacement screw pumps, which are more efficient and quieter than conventional pumps. The may be mounted in the governor cabinet or outside it.

Electro hydraulic governor generally consists of two cabinets. One is hydro mechanical cabinet consisting of EHT, hydraulic amplifier, pilot valve, main slide valve & feedback circuit. Second is electronic cabinet consisting of various relays; setting for speed & frequency. These are explained below:-

1. Regulated stabilized power supply unit: This provides a suitable regulated power supply to various printed circuit boards for their functioning. It is normally mounted in draw out rack.

2. Speed measuring unit: It compares the frequency & or its rate of change signal from PMG with a reference & gives an output proportional to error difference between the two.

3. Phase advance unit: It provides an output consisting of speed error plus rate of change of speed error. The rate of change of speed error signal can be used to stabilize the speed loop & to improve the governor response.

4. Power amplifier unit: It amplifies signal received from speed reference changes, speed error signal & auxiliary signal into useful electrical current.

5. Transducer or electro hydraulic actuator with moving coil: It is an electro-mechanical device with produces a mechanical output proportional to item iv above.

6. Hydraulic amplifier: It is combination of valves to amplify mechanical output of item v above & to distribute pressure oil to servomotor. It is almost similar in electrical & mechanical type.

7. Guide vane transducer: It is an electro mechanical device for converting the position of guide vane servomotors into an electrical signal. In mechanical types solid mechanical feedback signal is sent to governor.

8. Speed setting: Resistor network to provide a speed regulation with respect to guide vane position.

9. Gate limit: Manual setting for the maximum gate opening.

10. Speed relays: The function of various speed relays is as follows:-

Governor control relay: Brings in governor control at about 50% speed.

Over speed protection relay: If turbine speed exceeds permissible limits & hydro mechanical device fails, this relay energizes a solenoid operated emergency close valve, which stops the machine.

Programmed control relay: This relay pick ups speed rise above 10- 30% & directly closes the turbine.

Control relay: These are used for various sequence controls e.g. auto start, manual start, load throw off, zero speed detector for automatic closing operation of brakes, limit adjustment rack etc.