Governing - general.pdf

Dr. K.V. Vidyanandan AGM & Sr. Faculty Member (PMI) [email protected]

Introduction to Governing System Need of Load Frequency Controls Importance of Inertia Governor Droop TSR and Dead-band

Introduction to Governing System Need of Load Frequency Controls Importance of Inertia Governor Droop TSR and Dead-band

Frequency deviation from nominal value (50Hz) ( 50Hz) represents mismatch between generation (supply (supply)) and load (demand (demand). ). Dr. K.V. Vidyanandan

WHY SPEED DROPS AS LOAD LOAD INCREASES ? 

When there is an increase in system load, due to the inherent mechanical delay, the governor cannot act instantaneously.

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In order to meet the extra demand, kinetic energy of the rotating mass is released and there by machine speed reduces.

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The deviation in speed (∆ω) is used to activate the governor to open the CV further and the increased steam flow arrests the speed drop.

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The steady state speed after the load increase will be less than the nominal value.

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The magnitude of deceleration depends upon the quantity of the power mismatch and the inertia of the turbine-generator. turbine-generator. Dr. K.V. Vidyanandan

NEED OF CONSTANT FREQUENCY 

For satisfactory operation of the power system, the frequency should remain close to nominal value (50/60 Hz)



This will ensure constancy of speed of induction motors

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Variation in frequency will affect the performance of equipments and can interfere with system protection schemes

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Considerable drop in frequency will result in high magnetising currents in motors and transformers

Dr. K.V. Vidyanandan

IMPACTS OF FREQUENCY DEVIATION IN POWER PLANTS Most of the turbine generators must have frequency regulated to avoid mechanical resonances. If a rotating machine spins at or near one of its resonant modes, mechanical vibration damage can occur.

Manufacturers design their machine's resonant frequencies to be far away from the intended frequency of operation, so this is not typically a concern unless frequency deviates more than 5%.


IMPACTS OF FREQUENCY DEVIATION LOAD IMPACTS 

Poor power system frequency control can degrade power quality.

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Most industrial processes which require a high precision reduce their risk by using variable frequency drives (VFD).

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Most VFDs are insensitive to supply frequency, and they precisely regulates the output frequency. Thus systems using VFD are insensitive to small deviations in power system frequency.

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Converters used to rectify the AC source are not frequency sensitive in the range of ±5%. Dr. K.V. Vidyanandan

FREQUENCY AS A SYSTEM HEALTH INDICATOR 

Frequency provides an indication of the interconnected system’s generation-load balance.

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It is instantly available everywhere within the interconnection without the need for additional communications.

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This facilitates dispersed, autonomous response to system casualties by generators and loads.

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Assuming that all control systems such as AGC and speed governors are working correctly, a low system frequency is indicative of a low generation reserve. Dr. K.V. Vidyanandan

PROCESS OF ENERGY CONVERSION 

Primary source of electrical energy supplied by utilities are the Kinetic Energy of water and steam

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Prime movers convert the kinetic energy into shaft work

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This is in turn converted into electrical energy by Synchronous Generators

Prime mover governing systems provide a means of controlling power generated and frequency, a function generally called as Load Frequency Control (LFC) or Automatic Generation Control (AGC).


AUTOMATIC GENERATION CONTROL

Functions of AGC are: 

To maintain power balance in the system.

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To make sure that operating limits are not exceeded in the





Generators



Tie-lines

To maintain the system frequency constant under all conditions


COMPONENTS OF AGC 

Primary control – Immediate (automatic) action to sudden change of load,

for example, reaction to frequency change. 

Secondary control – To bring tie-line flows to scheduled. – Corrective actions are done by operators.



Economic dispatch – Make sure that the units are scheduled in the most

economical way.


FUNDAMENTAL QUESTIONS

Is it always necessary that load variation results in frequency variation..?

Under what situation load variation results in frequency variation..?

Why frequency variation occur with load change..?


BLOCK DIAGRAM OF GENERATION AND CONTROL SYSTEM


GOVERNING SYSTEM

The engineering of Governing System is a combination of 

Mechanical-Hydraulic System



Electrical Power System



Control System


GOVERNOR FUNCTIONS The main functions of governing system for steam turbines are: 

Speed (frequency) and load (power) control: →

mainly through HPCV

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Overspeed control:



Overspeed trip:



Start-up and shutdown control

→

→

mainly through the IV

through HPSV and IPSV


FEATURES OF SPEED GOVERNOR

All governors must have five fundamental features : 1. A way to set the desired speed 2. A way to sense actual speed 3. A way to compare the actual speed to the desired speed 4. A way for the governor to change steam flow to the turbine 5. A way to stabilize the rotor speed after a load change


GOVERNOR ACTION

Operator

Controller Desired Speed

Deviation

Negative (Balancing) Feedback

Governor Feedback Loop Turbine Actual Speed Sensor Dr. K.V. Vidyanandan

Steam Supply Actuator

GOVERNOR ACTION STEAM CONTROL VALVE

TURBINE

GEN

When turbine speed deviates from the set value, governor action modulates the control valve to regulate the steam flow. Dr. K.V. Vidyanandan

HYDRO POWER GENERATION


HYDRO TURBINE GOVERNOR

In hydro turbines, due to water inertia, a change in gate position produces an initial turbine power change which is opposite to that sought. This is because flow will not establish immediately. For stable control performance, a large transient (temporary droop) with a long resetting time is required.

A gain reduction compensation retards the gate movement until the water flow and power output have time to catch up. Dr. K.V. Vidyanandan

TYPES OF TURBINE GOVERNORS 

Mechanical

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Electro Mechanical

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Hydraulic

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Electro Hydraulic

MECHANICAL Speed transducer is mechanical centrifugal type speed governor, which directly actuates control valves through mechanical linkages.

ELECTRO MECHANICAL Mechanical centrifugal type speed governor is connected to hydraulic system either mechanically or hydraulically. Dr. K.V. Vidyanandan

TYPES OF TURBINE GOVERNORS HYDRAULIC Speed transducer is a centrifugal pump whose discharge pressure is a function of machine speed. This signal is sent to a hydraulic converter, to generate a high power hydraulic signal for the operation of different control valves or gate.

ELECTRO HYDRAULIC Electronic Transducer is used for measuring the machine speed. This signal is processed electronically and then sends to an Electro Hydraulic Converter for converting the electronic signals into proportional hydraulic signals forVidyanandan the operation of control valves. Dr. K.V.

METHODS OF GOVERNOR CONTROL 

Throttle Governing

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Nozzle Governing

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By-pass Governing

THROTTLE GOVERNING In this method of governing, steam is throttled to a suitable pressure using one or more sets of throttle valves (control valves). All these valves operate simultaneously and the throttle control is achieved usually by controlling the steam admission to the HP cylinder. Arc of steam admission in this governing method is 3600. Dr. K.V. Vidyanandan

METHODS OF GOVERNOR CONTROL NOZZLE GOVERNING Various groups of Nozzles with suitable isolating valves are used for regulating the steam flow to the turbine. These nozzles are grouped in 2, 3, 4 or more and their operation is done sequentially. These nozzle groups achieve control by regulating the steam flow to the first stage of HP cylinder. Arc of steam admission in this method is < 1800.


HYDRO TURBINE GOVERNOR FEATURES 

Hydraulic turbines can be modelled by a first-order model if water hammer (wave) and surge effects are neglected.

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Second-order models for hydraulic turbines with water hammer effect in the penstock are considered

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The speed controller of the hydro turbine governor has a permanent droop and a transient droop.

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Reversible hydraulic machines are used for pump-storage plants.

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Optimal pumping speed: 12-20% above optimal turbine speed.

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This needs variable-speed operation, thus power electronics. Dr. K.V. Vidyanandan

GOVERNOR COMPONENTS The main parts of a ball-head hydro-mechanical governor are: 

Speeder Spring

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Thrust Bearing

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Flyweights

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Pilot Valve

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Servo (Power) Piston

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Drive Shaft


SPEEDER SPRING 

Speeder spring is used to set desired speed.

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Applying more force down on the speeder spring causes the governor to increase steam.

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This initial force is usually set by the operator for the “reference” speed.

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It can be set by a screw adjustment, a knob, a lever, an electric motor, air pressure, or solenoids, depending on the specific governor.


ISOCHRONOUS GOVERNOR

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The ability to return to the original speed (constant speed) after a change in load is called isochronous speed control

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Governor components include comparator and integrator

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Works only when unit supplies to an isolated load or only one unit in a multi-machine system need to respond to load change


For a 5% droop, a 5% increase in frequency causes change in turbine output from 100% to 0%


The ratio of speed deviation (∆ω) or frequency deviation (∆f) to the change in valve

position

(∆Y) or

power

output (∆P) is equal to the droop parameter R.

Unit of R: Hz/MW Dr. K.V. Vidyanandan

Droop (or speed regulation) permits a machine to share load with other machines in an interconnected system. Typical values of droop for steam turbines vary from 2.5 to 8% and generally set at 5%.


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Governor Droop, Speed Regulation or Speed Error are the common terms used in describing a turbine’s response to changes in system frequency (or speed).

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New grid code requires all units above 10 MW capacity should have an operating governor with droop.

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Droop distributes frequency regulation to all generators in the interconnected network.

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Recommended droop settings for thermal units: 4 to 5% with a maximum dead band of ± 0.036 Hz.

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Minimum value should not be less than 2.5% to maintain stability in a speed-droop governor. Dr. K.V. Vidyanandan

WHY DROOP IS NECESSARY..? An increase in load will cause the turbine to slow down. The governor will respond by increasing the steam until the speed has returned to the original value. Due to the combined effects of inertia and power lag, the speed will continue to increase beyond the setting, causing a speed overshoot. The governor again will respond to decrease speed to correct for the overshoot. It will over-correct the speed in the reverse direction causing undershoot. This overcorrection of speed in both directions (instability) will amplify until the turbine trips out on over speed. With droop, an increase in load will result in reduction in speed reference and the problem of instability will not occur. Dr. K.V. Vidyanandan

LOAD SHARING BETWEEN UNITS


TRANSIENT SPEED RISE During total load rejection in a unit, speed of the turbinegenerator shoots up temporarily before settling down to steady state value.

This temporary speed rise is called transient speed rise and is expressed as percentage speed rise of rated speed on full load throw off. Typical Value of TSR : 5-7% Dr. K.V. Vidyanandan

TRANSIENT SPEED RISE


GOVERNOR DEAD BAND

Due to inherent inertia of the components of governing elements and friction present in governing system, certain motion of governing system is lost before corrective signal can actuate the control valves. The lost of the motion is called DEAD BAND of the system and is expressed as percentage of rated speed. Dr. K.V. Vidyanandan

SPEED GOVERNOR FUNCTIONING


HYDRAULIC SPEED GOVERNOR


SPEED MEASUREMENT (HYDRAULIC)


A TYPICAL ELECTRO-HYDRAULIC CONVERTER


A TYPICAL OVER SPEED TRIP


LOAD-FREQUENCY CONTROLS In order to ensure the power balance in the system, three types of load-frequency controls are commonly used. These are:

Primary Control, Secondary Control and Tertiary Control 

Primary control maintains power supply-demand balance by using proportional control action.

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Secondary control restores the frequency after every supplydemand mismatch using integral action.

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Tertiary control ensures economic allocation of secondary control reserve.


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Inherent control of the prime mover

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Fast in Response (< 1 min)

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With increase in speed, steam or water flow reduces

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With decrease in speed, steam or water flow increases

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Control parameter is Droop

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Droop vary between 2.5 - 8%

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Results in static frequency error after load change


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Supplementary Control

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Relatively Slow Response (~ few min.)

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Comes into service after the Primary Control

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Restores the frequency back to nominal value after a load change

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Reset control action is provided

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Error acts on Speeder Motor to shift the Droop line up or down

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Controllers commonly used are: Integral and PI


TIME SPAN OF PRIMARY, SECONDARY AND TERTIARY CONTROL


GOVERNING OIL SYSTEM (KWU)


Comments and suggestions are welcome

Dr. K.V. Vidyanandan AGM & Sr. Faculty Member (PMI) NTPC Ltd., Noida. [email protected]

Governing - general.pdf

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