GOVERNING SYSTEM: OVERVIEW

Dr M.S.R.Murty

INTRODUCTION

Governing system is an important control system in the power plant as it regulates the turbine speed, power and participates in the grid frequency regulation. For starting, loading governing system is the main operator interface. Steady state and dynamic performance of the power system depends on the power plant response capabilities in which governing system plays a key role. With the development of electro- hydraulic governors, processing capabilities have been enhanced but several adjustable parameters have been provided. A thorough understanding of the governing process is necessary for such adjustment.

In this paper an overview of the steam turbine governing system is given. The role of governing system in frequency control is also discussed.

BASIC GOVERNING SCHEME

Need for governing systemThe load on a turbine generating unit does not remain constant and can vary as per consumer requirement. The mismatch between load and generation results in the speed (or frequency) variation. When the load varies, the generation also has to vary to match it to keep the speed constant. This job is done by the governing system. Speed which is an indicator of the generation – load mismatch is used to increase or decrease the generation.

Basic schemeGoverning system controls the steam flow to the turbine in response to the control signals like speed error, power error. It can also be configured to respond to pressure error. It is a closed loop control system in which control action goes on till the power mismatch is reduced to zero.

As shown in the basic scheme given in Fig. 1, the inlet steam flow is controlled by the control valve or the governor valve. It is a regulating valve. The stop valve shown in the figure ahead of control valve is used for protection. It is either closed or open. In emergencies steam flow is stopped by closing this valve by the protective devices.

The governing process can be functionally expressed in the form of signal flow block diagram shown in Fig.2. The electronic part output is a voltage or current signal and is converted into a hydraulic pressure or a piston position signal by the electro- hydraulic converter (EHC). Some designs use high pressure servo valves. The control valves are finally operated by hydraulic control valve servo motors.

1

The steam flow through the control valve is proportional to the valve opening in the operating range. So when valve position changes, turbine steam flow changes and turbine power output also changes proportionally. Thus governing system changes the turbine mechanical power output.

In no load unsynchronized condition, all the power is used to accelerate the rotor only (after meeting rotational losses) and hence the speed changes. The rate of speed change is governed by the inertia of the entire rotor system. In the grid connected condition, only power pumped into the system changes when governing system changes the valve opening.

Grid

Reference

ST : stream

G : generator

SV : stop valve

CV

SVSteam

Speed

Power

GOVERNINGSYSTEM

N

ST

CV : control

Fig. 1 STEAM TURBINE GOVERNING SCHEME

SPEED

+

Valve Position

SET POINT

-

Mechanical Power

+GOVERNOR TURBINEROTOR

INERTIA

Fig 2 GOVERNING SYSTEM FUNCTIONAL BLOCK DIAGRAM

2

When the turbine generator unit is being started, governing system controls the speed precisely by regulating the steam flow. Once the unit is synchronized to the power system grid, same control system is used to load the machine. As the connected system has very large inertia (‘infinite bus’), one machine cannot change the frequency of the grid. But it can participate in the power system frequency regulation as part of a group of generators that are used for automatic load frequency control. (ALFC).

As shown in the block diagram, the valve opening changes either by changing the reference setting or by the change in speed (or frequency). This is called primary regulation. The reference setting can also be changed remotely by power system load frequency control. This is called secondary regulation. Only some generating units in a power system may be used for secondary regulation.

.

ELECTRO HYDRAULIC GOVERNING SYSTEM

Basically the controls can be described as i) speed control when the machine is not connected to the grid or in isolation and ii) load control when the machine is connected to the grid. The governing system has three functional parts: i) sensing part ii) processing part and iii) amplification. These functions are realized using a set of electronic, hydraulic and mechanical elements, in the electro-hydraulic governor (EHG), as shown in Fig. 3.

Earlier, only mechanical-hydraulic elements were employed in mechanical-hydraulic governor (MHG). With the developments in electronics technology, the microprocessor- based and digital signal processor (DSP) based governors are being offered by various manufacturers.

Sensing: to sense speed and power (or MW). The well known fly ball governor is a mechanical speed sensor which converts speed signal in to a mechanical movement signal.

Steam

G

· Actuation of Valve (Servomotor)

· Hydraulic Amplification

Speed & MW

· Primary Amplification

· Sensing

· Processing

Electro-hydraulicConverter

ControlValve

E H

HYDRALICPART

ST

N

ELECTRONICPART

Fig 3 ELECTRO – HYDRAULIC GOVERNOR SCHEME

3

Nowadays electronic sensors using Hall Effect principle and/or hydraulic sensor (a special pump whose output pressure varies with pump speed linearly) is used for speed measurement.Processing: to evolve the desired valve opening command signal: proportional (P) or proportional integral (PI) or proportional integral derivative (PID) or a combination of these. In digital governors the processing is done using software blocks.Amplification is necessary to obtain sufficient power to operate the steam control valve (where forces due to steam pressure also act)

Speed controller and load controllerIn the era of mechanical- hydraulic governors (MHG), the control action is mainly proportional. That is valve opening command is just proportional to the speed error. In the isolated operation where speed control is active and in the inter connected operation where power output or MW only is controlled same control action is present. In the electronic governors it has become easier to realize complex control logic. Separate control actions are incorporated for speed control and load control, as shown in Fig. 4.

Speed control loop demands additional capability to dampen the speed oscillations. This is obtained using so called proportional derivative (PD) controller. In this the valve opening command is proportional to the rate of change (or derivative) of the error also. This can improve the dynamic response considerably.

Load control loop deals only with MW error, which is obtained using a MW- transducer and is mainly a proportional integral (PI) controller. This loop is active when the steam turbine generator is connected to the grid.There is a selection logic which decides which control loop should prevail.

Mechanical hydraulic governor as backupAs mentioned earlier mechanical hydraulic governor comprising hydraulic speed sensor, primary amplification devices (called follow up pistons) are provided as backup to the electro hydraulic governor (EHG) in BHEL – KWU sets.

VALVEOPENING

COMMAND

–

–

Load

Speed

+

+

LoadRef.

SpeedRef.

SPEEDCONTROLLER

(PDP)

LOADCONTROLLER

( CP I )

SELECTIONLOGIC

(MIN – MAX)

Fig 4 SPEED CONTROLLER AND LOAD CONTROLLER IN EHG

4

The EHG system and MHG system will be continuously generating command signals for the governor valve opening. Normally both will have the same value. There is a ‘minimum logic’ provided hydraulically (called hydraulic minimum). According to this whichever calls for lesser valve opening will prevail. In this way in case there is a failure in electronic part mechanical governing system will take over. The turbine can be run with MHG alone.

It may be noted that MHGs are functioning reliably in several power stations in India for more than thirty years. PERFORMANCE ASPECTS

Regulation or droop characteristicWhenever there is a mismatch in power, speed changes. As seen earlier, the governing system senses this speed change and adjusts valve opening which in turn changes power output. This action stops once the power mismatch is made zero. But the speed error remains. What should be the change in power output for a change in speed is decided by the ‘regulation’. If 4 % change in speed causes 100 % change in power output, then the regulation is said to be 4 % (or in per unit 0.04).

The regulation can be expressed in the form of power – frequency characteristic as shown in Fig. 6. At 100 % load the generation is also 100 %, frequency (or speed) is also 100%. When load reduces frequency increases, as generation remains the same. When load reduces by 50 %, frequency increases by 2 %, in the characteristic shown. When load reduces by 100 %, frequency increases by 4 %. In other words 4 % rise in frequency should reduce power generation by 100 %. This 4 % is called ‘droop’ of 4 %. The characteristic is of ‘drooping’ type. Droop or regulation is an important parameter in the frequency regulation. In thermal power plants droop value is generally 4 % or 5 %.

Hydraulic Minimum

Electro – Hydraulic Controller

Mechanical – Hydraulic Controller

VALVEOPENINGCOMMAND

E H C

M H C

MIN

Fig 5 MECHANICAL HYDRAULIC GOVERNOR AS BACKUP

5

In terms of control system steady state gain it is expressed as inverse of droop: gain of 25 in per unit corresponds to 4 % (or 0.04 p.u) droop.

Transient performanceThe governing system, as noted earlier is a closed loop control system. Stability is an important parameter in any feedback control system. Stability and speed of response depend on the signal modifications done by various blocks in the loop. The closed loop gain depends on the individual block gains and the adjustable gains provided in the speed controller and load controller. The gain at the steady state and during the transient is important in deciding the performance. If the gain is not proper there can be hunting in the system as shown in Fig. 7. Various parameters like speed, power, valve opening will be oscillating continuously and may ultimately result in the trip of the turbine.

Frequency(Hz)

50

52

4% Drop

Load0% 50% 100%

Fig 6 REGULATION OR DROOP CHARACTERSTIC

Time (Sec)

Speed(%)

Unstable

Oscillatory (Hunting)

Fig 7 TYPICAL SPEED HUNTING TRANSIENT

6

Lift- flow characteristicAn important characteristic that decides the loop gain is the valve lift versus flow characteristic. Due to the nature of design, this characteristic is nonlinear. Though linearization is done either in the forward path or reverse path using mechanical cam, the gain introduced is different at low openings. The effective closed loop gain is less resulting in less damping capability at low loads.

Transient speed riseGoverning system maintains the turbine speed as set by the reference. When there are disturbances, the response should be quick otherwise speed may continue to deviate. Transient speed rise (TSR) is one important criterion that is used to judge the response capability of the governing system. Load throw off or load rejection is a major disturbance. When the TG unit is running at full load, if the circuit breaker opens, load is cut off. The full load steam flow causes the rotor to accelerate. The steam inflow is to be cutoff as soon as possible. It cannot be done instantaneously as the hydro mechanical elements take certain time to respond. Speed shoots up and then falls gradually due to the closure of control valve, as shown in Fig. 8. The peak value of speed is called transient speed rise (TSR).

Even when the control valves are closed steam remaining in the steam volumes of reheater piping, turbine cylinders (‘entrained steam’) continue to do the work and increase the speed for few seconds. There is an emergency governor provided to stop the turbine if the speed crosses its setting, usually 112 %. The standards specify that the TSR value should be less than the emergency governor setting. That means when there is a full load throw-off, governing system should act fast so that turbine does not trip.

There are other devices provided in the governing system which help in minimizing transient speed rise like load shedding relay (LSR) which cause feed forward action to close governing valves before speed variation is sensed by the speed transducer.

I P turbine control loop

Load

100%

Time(sec)

t

100%0%

Speed (%)

TSR

(6 - 10%)

4% Droop

Fig 8 LOAD REJECTION RESPONSE SHOWING TRANSIENT SPEED RISE (TSR)

7

In large steam turbines steam after reheater enters intermediate pressure (IP) turbine through another set of stop valves (called interceptor valves) and control valves, as shown in Fig. 9.

Normally small load variations are met by regulating steam admission in HP turbine only. But HP turbine generates only about 30 % of the total power output (IP turbine about 45 % and LP turbine the rest). So when large changes in generation is to be brought, IP control valves are also regulated through separate sets of electro-hydraulic elements, as shown in Fig. 10 below. In fact IP control action significantly reduces the transient speed rise.

Governor insensitivity or dead band The governing system action depends on speed sensing. There is a minimum value of speed which cannot be picked by the sensing mechanism and hence may remain uncorrected. This minimum value is called governor insensitivity or dead band. The characteristic is shown in Fig. 11.

Reheater

IPCV

HPCVStream

Condenser

LPTHP

T IPT N

R H

Fig 9 STREAM TURBINE SCHEME WITH HP AND LP PARTS

ToHPCV

ToIPCV

Speed / LoadController

Output

E H C(IP)

E H C (IP)Func-tion

Fig.10 IP TURBINE CONTROLS

8

Sometimes due to wear and tear dead band increases over a period of time. This is detrimental to the frequency regulation. In control system analysis, it is well known that dead band or hysteresis in a closed loop causes instability or limit cycle oscillations. Governor hunting may occur. At the same time, governor should not react for very small changes in frequency, so dead band is introduced intentionally in the electronic governor which is an adjustable feature.

FREE GOVERNOR MODE OF OPERATION

When frequency changes in the grid every TG unit reacts and adjusts its generation as dictated by power frequency or droop characteristic. For instance when frequency falls by 0.1 %, generation has to be increased by 20 % with droop of 5 %. In Indian situation most of the generating units operate at their peak values and no additional generation is possible. With the result many units do not increase their generation and load shedding is resorted to. In some cases, due to various operational reasons generating companies do not like to their machines to respond, even though spare capacity is available. The governing is bypassed. If most of the generating stations in a grid do not respond naturally, there is a danger of grid becoming unstable also.

In the recently approved Grid code it has been made mandatory for each generator to be provided with capability to allow up to 105 % MCR( maximum continuous rating capacity) generation whenever situation demands. This is called Free Governor Mode of operation (FGMO). It has been reported that after introduction of FGMO, frequency profile has improved considerably.

ValveOpening

Dead band or insensitive zone

Speed frequency

Fig 11 DEAD BAND CHARACTERISTIC

9

INTERFACE WITH OTHER SYSTEMS

Turbine automation packageThe governing system is part of comprehensive turbine automation package called Electronic Automation system for steam turbines (EAST). The governing system gets commands from automatic turbine run up system (ATRS) and automatic synchronizer during the starting and synchronization phase. The rate of rise of speed (and load) is decided by turbine stress evaluator (TSE) package.

The process of regulation by governing system causes disturbances in steam pressure, drum level, steam temperature, furnace draft etc., So the boiler control systems are also involved in the governing process.

Unit Coordinated Control systemThe Unit coordinated control system (UCC) or unit demand control system (UDC) is used in the recent DDCs of power plants. When a command is given for increasing or decreasing power generation, UCC generates command signals for boiler as well as turbine based on various considerations like boiler storage time constant, boiler thermal stresses, availability of auxiliaries etc., The governing system thus interfaces with the UCC. It is also referred to as coordinated master control system (CMC) in some power plants. In the BHEL/ABB UDC, the command to change boiler generation and turbine power are simultaneously given. But the governing system actually receives the command after a time delay equivalent to the time taken by the boiler to increase its generation. This is shown by the steam flow coordinator block in Fig. 12.

Automatic Load frequency Control systemThe responsibility of maintaining grid frequency is given to Automatic Load Frequency Control (ALFC) system or automatic generation control (AGC) system. Whenever there is a mismatch between generation and load in a grid or an area of a power system (such as Regional electricity Board in India), the grid frequency varies and ALFC gives commands to adjust the generation through the governing systems, as shown in Fig. 13. Due to the absence of thermal rate limits hydro units are preferred. But the generations of many large thermal

Governing System

Boiler Load Set Point

TurbineLoadSet Point(TLSP)

BLSP to Boiler combustion Controller

Time delay based on boiler storage constant

UnitLoadSet Point(ULSP)

U D C

STEAMFLOW

CO-ORDINATOR

To

Fig 12 UNIT DEMAND CONTROL SYSTEM INTERFACE WITH GOVERNING SYSTEM

10

units are also increased/ decreased. The governing system must respond quickly for such requests. The load controller of electro hydraulic turbine control system has provision for such interfacing.

CONCLUSIONSThe governing system plays an important role in the start up, synchronization and loading of a steam turbine generating unit. It has to ensure stable and secure operation. With the developments in technology, digital governors are increasingly being used. In this paper a process overview of the governing system is given. Though BHEL/KWU design is mainly dealt with, the concepts are applicable to the governing systems of other manufacturers also.

Set point○

Generator Power

Frequency Total Generation

TotalLoad

Primary regulation

Other m/c

To OtherMachines

Set point AreaFreq-uency

Secondary regulation

-- +

++

+○

○

○

AUTOMATICLOAD REQUENCY

CONTROLLER

Governor Turbine

GRIDINERTIA

Fig 13 AUTOMATIC LOAD RFEQUENCY CONTROL SYSTEM

11