INTRODUCTION

Governing system is an important control system in the power plant as it regulates the turbinespeed, power and participates in the grid frequency regulation. For starting, loading governingsystem 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 akey role. With the development of electro- hydraulic governors, processing capabilities have been enhanced but several adjustable parameters have been provided. A thoroughunderstanding 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 system

The 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 speedconstant. This job is done by the governing system. Speed which is an indicator of thegeneration – load mismatch is used to increase or decrease the generation.

Basic scheme

Governing system controls the steam flow to the turbine in response to the control signals likespeed error, power error. It can also be configured to respond to pressure error. It is a closedloop control system in which control action goes on till the power mismatch is reduced tozero.As shown in the basic scheme given in Fig. 1, the inlet steam flow is controlled by the controlvalve or the governor valve. It is a regulating valve. The stop valve shown in the figure aheadof control valve is used for protection. It is either closed or open. In emergencies steam flowis stopped by closing this valve by the protective devices.The governing process can be functionally expressed in the form of signal flow block diagramshown in Fig.2. The electronic part output is a voltage or current signal and is converted into ahydraulic pressure or a piston position signal by the electro- hydraulic converter (EHC). Somedesigns use high pressure servo valves. The control valves are finally operated by hydrauliccontrol valve servo motors.

 

The steam flow through the control valve is proportional to the valve opening in the operatingrange. So when valve position changes, turbine steam flow changes and turbine power outputalso 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 pumpedinto the system changes when governing system changes the valve opening.

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 systemgrid, same control system is used to load the machine. As the connected system has very largeinertia (‘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 areused for automatic load frequency control. (ALFC).As shown in the block diagram, the valve opening changes either by changing the referencesetting or by the change in speed (or frequency). This is called

primary regulation

. Thereference setting can also be changed remotely by power system load frequency control. Thisis called

secondary regulation

. Only some generating units in a power system may be usedfor secondary regulation..

ELECTRO HYDRAULIC GOVERNING SYSTEM

Basically the controls can be described as i) speed control when the machine is not connectedto 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 mechanicalelements, in the electro-hydraulic governor (EHG), as shown in Fig. 3

 

 

 

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 systemgrid, same control system is used to load the machine. As the connected system has very largeinertia (‘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 areused for automatic load frequency control. (ALFC).As shown in the block diagram, the valve opening changes either by changing the referencesetting or by the change in speed (or frequency). This is called

primary regulation

. Thereference setting can also be changed remotely by power system load frequency control. Thisis called

secondary regulation

. Only some generating units in a power system may be usedfor secondary regulation..

ELECTRO HYDRAULIC GOVERNING SYSTEM

Basically the controls can be described as i) speed control when the machine is not connectedto 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 mechanicalelements, in the electro-hydraulic governor (EHG), as shown in Fig. 3.Earlier, only mechanical-hydraulic elements were employed in mechanical-hydraulicgovernor (MHG). With the developments in electronics technology, the microprocessor- basedand digital signal processor (DSP) based governors are being offered by variousmanufacturers.

Sensing:

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

 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 controller

In 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 theisolated 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 areincorporated for speed control and load control

Speed control loop demands additional capability to dampen the speed oscillations. This isobtained using so called proportional derivative (PD) controller. In this the valve openingcommand is proportional to the rate of change (or derivative) of the error also. This canimprove the dynamic response considerably.Load control loop deals only with MW error, which is obtained using a MW- transducer and ismainly a proportional integral (PI) controller. This loop is active when the steam turbinegenerator is connected to the grid.There is a selection logic which decides which control loop should prevail.

 Mechanical hydraulic governor as backup

As 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

The EHG system and MHG system will be continuously generating command signals for thegovernor 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 partmechanical governing system will take over. The turbine can be run with MHG alone.

PERFORMANCE ASPECTS Regulation or droop characteristic

Whenever there is a mismatch in power, speed changes. As seen earlier, the governing systemsenses this speed change and adjusts valve opening which in turn changes power output. Thisaction 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 inFig. 6. At 100 % load the generation is also 100 %, frequency (or speed) is also 100%. Whenload 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 %.

 

Automatic Load frequency Control system

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

GoverningSystemBoiler Load SetPointTurbineLoadSet Point (TLSP)

 

BLSP to Boiler combustionTime delay based on boiler storage constantUnitLoadSet Point(ULSP)

U D C

STEAMFLOWCO-ORDINATOR To