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Electricity

1. Control2. Switchgear3. Grid and off-grid

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1. Control

Controls for a 3 machine 2400kW plant – Sri Lanka

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Introduction

C t l ll th f d di t bl t tControl allows the safe and predictable start, run-up, synchronising and loading, unloading, disconnection and shut-down of turbine-generator installations.

Instrumentation monitors and displays the electrical, hydraulic and mechanical conditions, also raising alarms in the event of abnormal operation

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in the event of abnormal operation.

Protection monitors the electrical conditions to trip and shut down the plant in the event of a fault – either in the plant or the network.

Mechanical control

Mechanical control systems act on the water supplyMechanical control systems act on the water supply• initially and finally on the Main Inlet Valve Fully Open/Fully closed

to synchronise and unload at start/stop:• Jet deflectors on Pelton and Turgo-impulse turbines• Guide vanes on Francis turbinesOpen/Close – all continuously adjustable

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to control flow or load in operation • Spear valves on Peltons and Turgos• Guide vanes on Francis turbinesOpen/Close – all continuously adjustable

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Mechanical control

Runner

Main Inlet Valve Spear Valve(s)

Mechanical control

Governor

Main Inlet Valve

Runner

Jet Deflector(s)

Spear Valve(s)

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Mechanical control

M i I l t V lMain Inlet Valve

Guide Vanes

Mechanical control

Main Inlet Valve

Guide Vanes

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Mechanical control

Moveable Runner Blades

Guide Vanes

Electrical control:

Electrical control systems act on the electricity supply, • to synchronise and disconnect at the Main Circuit

Breaker Open/Close

• Continuously on flow control

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• Continuously on flow control –Spear valves on Pelton and TurgoGuide vanes on Francis turbinesIncrease/Decrease – continuously adjustable

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Governing

For speed / load control of the water turbine in isolated (island) mode

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Governing

Used for synchronising and Operating isolated from a t k ( id)network (grid)

If load (kW) is added to generator, shaft speed falls, and governing system opens guide vanes or spear valves to increase speed.

This increases power output to balance mechanical load on

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This increases power output to balance mechanical load on turbine and speed is restored.

A control loop dampens the response.

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Governor Operation

Water admission controlled by guide vanes or jet deflector

Speed is measured by shaft encoder

Speed sensor

Nmeas

NrefΣ Amp

Nref-Nmeas

Governor or ECS

Governor opens and lSpeed is measured by shaft encoder

and compared with an (adustable) reference speed setting in the governor or

electronic control system.Used for synchronising and

Operating isolated from a network (grid)

Rotor coupling to generator shaf

Water inlet Power in

Main Inlet Valve TurbinePower out

closes guide vanes

(grid)

Note: For small schemes, a head level controller and plc controls are now more commonly used

AVR Operation

Rotor excitation controlled by output of solid state AVR amplified and fed as dc to main rotor winding

Operating isolated from the network

fed as dc to main rotor winding

Generator stator voltage is measured by a VT and compared with an (adustable) reference

voltage setting in the AVR.

For synchronising p gIf electrical load is added to

generator, terminal voltage falls, and AVR increases excitation to increase voltage and minimise error in control loop.

y g

If measured speed V1 is lower than reference voltage Vref error signal (Vref – V1) is amplified and used to increase main excitation a little, but quite precisely

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Mechanical and electrical aspects of startup

Design is generally site specific, but general principles for start logic are:• Test all trips are reset, no alarms are raised, all control voltages are available• Test for full head behind MIV• Open MIV fully and prove• Part-open spear valve or guide vanes and prove• Turbine accelerates and passes half speed driving governor• Governing system takes control of speed (on guide vanes or jet deflector)

towards unloaded setting• Generator excitation builds up and generator excites• AVR takes control of speed towards no load voltage• Autosynchroniser takes control of reference speed and senses generated

frequency and voltage• Speed of rotation and generated frequency are controlled by the admission of

water to the runner – under speed governor action in closed loop towards a preset speed.

• Open circuit voltage is controlled by the excitation of the generator rotor under Automatic Voltage Regulator (AVR) action in closed loop, towards a preset voltage.

• All operations are checked within time-outs, and aborted to shut-down if they fail

SynchronisingCircuit breaker open.

Network ‘running’ at Vgrid and fgrid

Generator speed and voltage is

Vgrid f grid

Generator speed and voltage is

increased until Vgen = Vgrid

and fgen = fgrid.

Synchroscope

compares phase

of Vgen and Vgrid.

Synchroscope look for stationary at 12.00

f genVgen

Turbine

Circuit Breaker

Stator

Sh f

Generator

gen grid

CB is closed when scope is Vary speed set point until fgen = fgrid

Rotor

Shaft

AVR

Governing System

Power control

Vary voltage set point until Vgen = Vgrid

Nref

Vref

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Autosynchronising

Manual synchronising is normally only for backup,

Usual for start, run-up, and synchronising to be automated.

• Turbine controlled and protected by PLC or relay logic

• Set points on governing system and AVR are motorised

• Auto-synchroniser responds to freq, phase and voltage difference to match ‘running’ and generator supplies.

When conditions are correctWhen conditions are correct

• Check-synchroniser verifies this and allows through a ‘CLOSE’ instruction to main Circuit Breaker, and breaker closes.

• Any neutral earthing contactor on generator is opened at this point

• Once closed increase reference speed setting to pick up active power and load and initiate any other control such as head level or load-share.

Electrical aspects of operation

After connection to the network

Speed of rotation and generated frequency are dictated (precisely) by the synchronous connection and governing system is set to maximum power (or load limit) by increasing the reference speed Nref above synchronous.

Generator voltage is dictated (precisely) by the synchronousGenerator voltage is dictated (precisely) by the synchronous connection and the Automatic Voltage Regulator (AVR) can only control generated power factor.

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Head level control

Where storage is small relative to design flow, and can runoff into storage can vary quickly head level control can adjust discharge to keep free surface level nearly constant in headpond.

Radio transmitter

Ultrasonic level detector

DC supply

Reducing water level closes spear, increasing water level opens spear

Radio receiver

PID controller

Motorised spear valve

increasing water level opens spear.Needs aux supplies at intake

Means that turbine output follows inflow

Instrumentation (typical)

Electrical Mechanical Alarms

Bus voltage Inlet pressure DC failure

Bus frequency Shaft speed Overspeed

Gen voltage Bearing temp Overtemp

Gen frequency Stator temp Overtemp

Phase current Hours run Vibration

Power Head level Start fail

Power factor Valve status Stop fail

EnergyEnergy

Main CB status

NE breaker st.

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Mech & elec aspects of shutdownDesign is again generally site specific, but general principles for planned

stop logic are:

• Initiate stop, this drives down the reference speed setting on the governing systemgoverning system

• As this passes synchronous speed, governor senses speed high

• Error signal acts to correct this by inserting jet deflector, or closing guide vanes

• Water supply to turbine runner is reduced slowly to zero

• Depending on turbine type either generator starts to motor and Reverse Power protection trips main circuit breaker, or

• When spear valve/guide vanes are closed logic trips main circuit breaker

• Generator runs down, excitation collapses and generator de-excites

• Spear valve (turgo and Pelton) and main valve are closed in sequence

• All operations are checked within time-outs, and aborted to shut-down if they fail.

Emergency ShutdownDesign is again generally site specific, but general principles for

emergency stop logic following a protection trip are:• Protection trips main circuit breaker• Load has been removed and water supply is still on, so speed takes offpp y , p• As this passes reference speed setting, governing system senses speed

high• Error signal acts to correct this by inserting jet deflector quickly or closing

guide vanes slowly • Generator runs down, excitation collapses and generator de-excites• Spear valve (impulse turbines) and main valve are closed slowly in

sequence• Guide vanes (Reaction turbines) and main valve are closed slowly in

sequence• Water supply from pipeline is reduced slowly to zero to avoid pressure

rise – could take several minutes.• With Francis turbine this inevitably means a period of time when there is

no mechanical power take off, and the runner is still being driven. Flywheels are necessary to limit speed rise while avoiding pressure rise.

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2. Switchgear

They accommodate all current and voltage

Network circuit breakers are 3 phase switches, with mechanically operated contacts that open and close. The breakers can be fixed or withdrawable.

current and voltage transformers and protection relays

Operators and auto sync may open or close them locally or from remote positions.

Local or remoteLocal or remote protection equipment may open them.

They must open and close very quickly under normal and abnormal conditions.

Requirements and Obligations

It is a legal and professional requirement that all distributed generators are designed, manufactured and installed to operate in an inherently safeand installed to operate in an inherently safe manner, under normal, abnormal & fault conditions.

Statutory obligations– Health & Safety at Work Act 1974– Electricity Act 1989y– Electricity Supply Regulations 1988

Under these Regulations no person may operate generating plant in parallel with a Distribution Network Operator’s (DNO's) system without their written agreement and the plant must meet minimum standards of operation, protection and disconnection in the event of a distribution network or plant failure.

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Requirements and Obligations

Written agreement of DNO will require demonstrableWritten agreement of DNO will require demonstrable compliance with

• Local or International standards for equipment and installation practice

• Connection and operating standards, such as

Engineering Recommendation G59/1, G75 & ETR113• Prevents connection to faulty network

• Prevents displacement of V or frequency outside statutory limitsPrevents displacement of V or frequency outside statutory limits

• Prevents back-energising of network

• Neutral earthing compliance

• Electrical and mechanical protection

General Protection Considerations

Switchgear of new DG must have a prescribed minimumsystem of protection relays that will disconnect they p ygenerator from network if

a fault develops on either side of the generator circuit breaker;

the system voltage or frequency goes out of agreed limits within thestatutory maxima and minima;

the network becomes disconnected at a remote point.

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System voltage and frequency limits

General Protection Considerations

y g q y

• V: 10% of 400/230V (253-207) 6% of 11kV

• Frequencies: +1%/-6% of 50Hz (50.5-47)

• Time for operation: 0.5secs (> by agreement)transient network disturbances such as auto reclosure of– transient network disturbances such as auto-reclosure of network circuit breakers and motor starting

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11kV lines:3-phase & single phasesingle phase

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11kV lines:3-phase & single phase

- dropping to LV

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Switchrooms and Metering Pointsg

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Connection layouts

• Generator connected directly to heat load. Off-grid

B ttR tifi D C l d

a.c.

Batteryd.c.

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Rectifier

a.c.

D.C. loadd.c.

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Connection layouts

Batteryd.c.

Rectifier Inverterd.c.

A.C. load

a.c.a.c.

• Current rectified then reinverted

• Still off-grid

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Connection layouts

d.c.Rectifier Inverter

a.c.

HouseProtection

a.c. Generation meter

• Current rectified then reinverted

• Relays in case of fault

• Export or use onsite

• Rectified, inverter & protectionmay be supplied as one unit

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Grid

Import meter