Control Protection Micro Hydro Plant

Control Protection of SHP and MHP plant

CONTENTS

1 HYDRO SYSTEM CONTROL 1

1.1 MECHANICAL CONTROL 1 1.1.1 ELECTRICAL ACTUATORS 1

1.1.2 HYDRAULIC ACTUATORS 1

1.2 PLC CONTROL 2

1.3 SYNCHRONISATION 2

2 CONTROL PANEL SPECIFICATION 2

2.1 CONTROL PANEL COMPONENTS 3

2.2 CONTROL PANEL DESIGN AND CONNECTIONS 3

2.3 METERING 3

3 SYNCHRONOUS GENERATORS OPERATION 4

3.1 VOLTAGE CONTROL 4

3.2 AUTOMATIC VOLTAGE REGULATORS - AVR’S 4

3.3 POWER FACTOR AND REACTIVE POWER 5

3.4 POWER FACTOR FOR VOLTAGE CONTROL 5

3.5 POWER FACTOR CONTROL 5 3.5.1 SYNCHRONOUS MACHINES POWER FACTOR CONTROL 5

3.5.2 INDUCTION MACHINE POWER FACTOR CONTROL 6

3.6 ELECTRICAL GENERATOR PROTECTION 6

3.7 GENERATOR INSULATION PROTECTION 6

3.8 GENERATOR MECHANICAL PROTECTION 7 3.8.1 BEARING PROTECTION USING TEMPERATURE SENSORS. 7

3.8.2 GENERATOR OVER SPEED 7

4 SYSTEM EARTHING 7

4.1 LV CONNECTED 7

4.2 HV CONNECTED 7

5 ELECTRONIC LOAD CONTROL 8

5.1 THYRISTOR SYSTEMS - ADVANTAGES AND DISADVANTAGES 8

5.2 FAST SWITCHING IGBT 8

5.3 BASIC ELC ARRANGEMENT 8

5.4 ELC BURST FIRING OPERATION 10

5.5 PULSE WIDTH MODULATION 11

5.6 FREQUENCY PROTECTION 12

5.7 STAND ALONE SYSTEM - SYNCHRONOUS GENERATOR WITH ELC 12

5.8 ELC BALLAST LOADS 12

DR-S5-RevA.doc Section 5 Page 1

6 PERMANENT MAGNET GENERATORS 12

1 Hydro System Control A typical small hydro or micro hydro scheme will comprise a variety of electro-mechancial

systems. All of these systems will need monitoring and control - ideally automatically if not

manually. Specific items that need control and monitoring are:

1. Intake – e.g. screen cleaning and level monitoring

2. Turbine operation:

3. Turbine spear valve or vane control

4. Main Inlet Valve

5. Bearings

6. Generator

7. Excitation

8. Synchronisation

9. Power Factor

10. Bearings and windings

Control can be manual or automatic or a mixture of the two. The choice will depend on the

particular situation, scheme size, location, labour costs and availability, connected loads etc.

1.1 Mechanical Control

Automatic control of mechanical items is usually arranged by using electric or hydraulic

actuators. The key features of these two types of actuators are given below.

1.1.1 Electrical actuators

1. Can provide linear or rotary actuation

2. Can be pulsed or continuous operation

3. Can provide very slow and small movements reliably

4. Can provide accurate position and torque feedback signals and indications

5. Relatively expensive for good quality units

1.1.2 Hydraulic actuators

1. Usually provide linear actuation

2. Can be pulsed or continuous operation

3. Speed of operation dependant upon oil temperature (can use expensive temperature

compensated flow control valves)

4. Difficult to achieve very small movements

5. Require good quality maintenance, oil changing and good filtering. A corollary of this is

tha a certain level of understandings of hydraulics will be needed in operation staff

6. The potential for oil pollution from leakages needs to taken into account by the

specification of bunds or other protection systems.

7. Are relatively cheap


1.2 PLC control

PLC systems are commonly available from a wide variety of manufacturers. They can form

the basis of dedicated hydro control and protection systems. Many systems are available in

modular format, with a wide variety of analogue and digital inputs and outputs. Systems can

be tailored to suit a particular application.

PLC systems can be connected to SCADA systems for data logging and remote access and

control. The use of standard PLC modules means there is a requirement to specify and

program them to suit the particular application. PLC software programming requires skill and

experience. The software designer needs to understand the mechanical, electrical and

hydraulic characteristics of the equipment that will be controlled – for example opening a

main pipeline valve needs to be done slowly to avoid dangerous surge pressures. Software

needs rigorous checking and debugging.

Note also that specialist PLC based systems specifically designed for hydro applications are

available. Such systems are pre-configured and programmed to operate a turbine and

generator. For this reason, their use can save on system design and implementation time.

However, such systems may not suit the requirements of your particular system and might be

difficult to adapt for particular system requirements that are not included in the standard

package.

1.3 Synchronisation

This subject will is covered in Section 7 of this manual. Suffice it to say that synchronisation

can be achieved by:

1. Controlling turbine speed on no load until it can be connected with the grid – requires

accurate and reliable turbine speed control based on water flow control or deflector

position

2. Controlling the load on the generator with fixed hydraulic power into the turbine until the

unit is in synch with the grid – requires Electronic Load Control in some form and a

synchronous generator. ELC system operation are covered below.

2 Control panel specification

Hydro system control panels need careful design and specification. The control panel will

usually include a selection of standard electrical protection and control modules and

protection relays as well as a PLC system, metering, contactors and relays, a synchroscope,

possibly data logging and remote monitoring facilities.

Note that the term “relay” is used in two senses:

for control and protection modules such as a “synch check relay”

for simple control relays – electrical switches

Care must be taken to ensure that the specified components are compatible with each other.


2.1 Control panel components

Hydro systems can have a very long operating life and the electrical components should be

suitable specified to match this. Components should be manufactured to suitable international

standards. Components must be compatible with each other, for example EMC interference

issues can arise. Backup power supplies are usually required to maintain cabinet power during

grid outages – can use batteries or UPS systems.

2.2 Control panel design and connections

Clear design and “as built” drawings must be created and maintained – these are vital for

successful build and operation. There will be a large number of electrical terminals for control

and monitoring cabling. These terminals need to be clearly labelled and logically laid out to

facilitate installation and connection and to make subsequent checking and fault finding

possible.

Consideration must be made at an early stage as to cable entry and cable routing within the

power house

Power Cabinets

A power cabinet usually refers to the section of control panel containing the main power

connections for the generator and load / grid. There will be three main functions:

1. Providing generator protection functions – e.g. overcurrent and earth fault. These

functions are usually provided using a suitable circuit breaker and / or protection relays

2. Providing a connection between the generator and the load / grid – usually using a

contactor or motorised circuit breaker

3. Providing terminals / connections for the power cabling

4.

Contactors compared with motorised breakers for connection

The choice mainly depends on system size:

1. The smallest motorised circuit breakers are around 400 kVA, and basic units can be de-

rated to about 60%. These will be Air Circuit Breakers (ACBs) usually referred to as

Moulded Case Circuit Breakers (MCCB)

2. Note that the maximum reasonable contactor size is 500 kVA

3. A breaker of some sort will usually be required to provide over current protection

2.3 Metering

A hydro control system requires electrical metering to monitor operation and identify

problems. Traditional analogue panel meters provide easy to read displays, but have limited

functionality. Often many separate meters are required – adding to cost and panel space

requirements. Multi function digital panel meters can provide many items of information from

a single meter. Many digital meters can be connected directly to the PLC systems via Modbus

or similar communication systems. This enables the PLC to monitor electrical parameters and

for them to be available to SCADA systems for logging and remote monitoring. The typical

parameters that can be monitored by a digital panel meter include:

1. Three phase volts and current

2. Power, power factor and cummulative power – often by individual phases

3. Frequency

4. Peak values of current and voltage

5. Harmonic distortion information


3 Synchronous Generators Operation Synchronous generators have a wound set of windings on the rotor. The current through the

rotor windings can be controlled and hence the voltage induced and the current capability of

the generator can be controlled. The control is done by the AVR – Automatic Voltage

Regulator. The figure below shows the basic control process of synchronous generator

operation.

Fig.3.1 – control of a synchronous generator

3.1 Voltage Control

Referring to Fig.3.1 above, the rotor current determines the magnetic field produced by the

rotor – this determines the induced voltage produced by the stator and the ability of the stator

to export power. If the generator is in stand alone mode the output voltage of the generator

will increase as the rotor current is increased. If the generator is grid connected the grid

system will determine generator voltage – changing the excitation will change the output

power factor.

3.2 Automatic Voltage Regulators - AVR’s

Modern AVR’s are solid state devices usually supplied by the generator manufacturer. AVR’s

monitor the main stator output voltage and produce a suitable D.C. voltage output to the

exciter. They can (and do) fail in service. The most common cause of failure is voltage spikes

produced by lightning. Other causes of failure are dampness and vibration. For these reasons,

it is advisable to:

1. Use surge arrestors on the line to a power house.

2. Consider mounting the AVR away from the generator in a suitable environment such as

in the control cabinet.

Manufacturers replacements are relatively expensive – typically £150 for a 10 kW machine

and £500 – 700 for a 500 kW machine (prices Sterling).

Stator Winding

Main rotor windingExciter Rotor Winding Rotating Diodes

Exciter Stator

Winding

AVR

Poweroutput

Voltage signal and

supply to AVR

Excitation from AVR

DC voltage

Magnetic flux

linkageMagnetic flux

linkage

A.C.

current

D.C

Current


3.3 Power Factor and Reactive Power The Power Factor is a measure of the relative amounts of Real and Reactive power

components. Reactive Power is the component of power where the current and voltage

waveforms are 90 degrees out of phase. Reactive power can do no useful work. However,

despite this, reactive power for loads has to be produced somewhere. Reactive power

produced by capacitors or generators has to go somewhere hence the need for a “reactive

power balance”.

Note carefully that the of the use of terms “leading” and “lagging” power factor – can mean

different things to different engineers. It is preferable to use of “export” or “import” reactive

power.

3.4 Power Factor for voltage control

A key effect of reactive power flows on power lines is that they cause a volt drop in the same

way that “real” power flows do. The voltage at a generator may be lowered by importing

reactive power which may effect its performance somewhat. Systems are in use that control

voltage at the generator by controlling the amount of reactive power imported or exported.

Note that any reactive power imported will have to be paid for. Conversely, exported reactive

power can be sold however, note that usually suppliers will want a firm capacity that can be

controlled.

3.5 Power Factor Control

Power Factor Control only applies to generators which are connected to the grid – or run in

parallel.

In a stand alone system, the reactive power supplied by the generator must match the reactive

power demand of the loads.

3.5.1 Synchronous Machines Power Factor Control

A Power Factor Control unit (PFC) will be connected to the AVR of the synchronous

generator. The PFC will take over control of the AVR and hence the excitation output when

power factor control is enabled (when the system is on the grid). When the generator is “over

excited” the system will export reactive power – it will be Capacitive. When the generator is

“under excited” the system will import reactive power – it will be Inductive. Power Factor

Control units can usually be set to either:

Maintain a constant power factor

Maintain a constant amount of reactive power

Usually a PFC unit is enabled by an auxiliary contact on the main contactor / breaker. PFC

units will not work at low generator powers – around 15 – 25% of rated generator power.

This figure varies with manufacturer, and can be significant factor in run of river systems

where the operator wants to run at low power sometimes.

When two or more generators are connected in parallel – either stand alone or grid connected

– care must be taken to avoid circulating reactive currents. It is not possible for two AVR’s to

operate at exactly the same voltage, and any difference will manifest as circulating current.


Quadrature Droop circuits are used to reduce excitation when reactive power flow increases

and will naturally balance reactive power supply between two or more generators. Other Load

Balancing circuits / relays are also available when running generators in parallel.

3.5.2 Induction Machine Power Factor Control

An Induction generator will inherently import reactive power, to provide it’s excitation. This

is usually provided by capacitor banks mounted near to the generator. Banks of capacitors are

used to provide power factor correction – the capacitors are switched in / out by a control unit

that is monitoring the system power factor. The switching of capacitors is controlled to

maintain a relatively constant power factor at the station terminals. In practice, this kind of

arrangement wWorks well and is commonly used. Note that the cost of the equipment is

considerable and that there will be the service related costs for replacing failed exciter

capacitors.

3.6 Electrical Generator Protection

Electrical protection is achieved using:

1. Overcurrent protection on the generator breaker

2. Earth fault protection on the generator breaker (and on generator earth during run up)

3. Core (winding) temperature monitoring

4. Limitation on the maximum excitation current

5. Stability limitations

6. Pole slipping protection, for larger generators

3.7 Generator Insulation Protection

Generator core insulation is required to ensure electrical separation of windings and

separation of the windings and the core material. Insulation resistance is reduced by

dampness. Insulation resistance will be reduced on a new machine, the varnish will not have

been completely cured. It is vital that the insulation resistance of the generator is sufficiently

high prior to the generator producing any voltage. Different grades of winding insulation are

available from manufacturers it is advisable to get the best grade available. The best grades

are sometimes referred to as “Marine Grade” or “Double Dipped Double Baked”, the latter

referring to the application and curing of the varnish.

For the reasons given above, the insulation resistance should be measured prior to generator

use and after shutdowns. All control and protection equipment must be disconnected prior to

insulation testing. Measure using a “megger” resistance tester which can produce voltages

higher than the operating voltages of the generator, usually 2 times normal operating voltage.

The required insulation resistance will be specified by the manufacturer, usually it should be

> 1 Meg Ohm, often 10-20 Meg Ohms.

If the insulation resistance testing indicates a low resistance the windings must be dried.

Drying can be achieved in several ways:

1. A good method is to short the generator main windings and inject a controllable d.c.

current into the exciter winding whilst rotating the machine at rated speed. Up to full

rated current may be induced and this will heat the core and dry the windings.

2. An alternative is to use air heaters blowing into the generator whilst it is rotated slowly


3. Anti condensation heaters can be installed in the generator to keep it warm when it is not

running – but this requires an external power source.

3.8 Generator Mechanical Protection

3.8.1 Bearing protection using temperature sensors.

Different sensor types are available but the most commonly used are the so called PT100

(platinum resistance sensor) resistance type which are robust and sufficiently accurate. The

sensor is made of platinum which has a particularly linear resistance vs temperature

characteristic. Thermocouples or thermistors are alternatives.

Look for an unusually rapid rise in temperature to indicate the imminent failure of a bearing.

Some consideration should be given to the generator bearings during installation of the

electro-mechanical plant at the site. It is important to avoid shock loading on bearings and

windings and to make sure adequate provision is made to support the generator bearings

during transport.

3.8.2 Generator Over speed

When a turbine is un-loaded, the shaft power it is producing will cause it to run into an over

speed condition. The amount of over speed depends on the type of turbine and the particular

design. Generators must be rated to accommodate this over speed – otherwise it is likely that

the rotor windings will be forced out under larger than normal centrifugal force and make

contact with the stator resulting in a broken generator. Some generators can be supplied with

over speed banding on their rotors – at a cost. Most smaller generators will tolerate over

speed for short periods.

4 System Earthing

We will consider the two normal cases of so called low voltage connections – LV and high

voltage connections HV connected systems.

4.1 LV connected

In this case, the scheme operator / owener will need to provide a station earth. All metalwork

shall be connected to this earth. The distributed network operation (utility), will (usually)

provide a four wire connection with the neutral connected to earth at their transformer.

4.2 HV connected

In this case there will usually be 2 earths:

The HV earth – for earthing the HV breaker and cabling

The LV earth for earthing metalwork, LV star point etc.


These two earths must be physically separate. Often the HV earth is taken in insulated cable

about 50m away from the power house. If the two earths are physically close, or connected,

the site becomes a “hot zone”. In this situation the issues of step and touch potentials must be

addressed.

5 Electronic Load Control An Electronic Load Control (ELC) is a system to balance the power produced by a generator

with the total load so that frequency (speed) is kept stable. ELCs are mostly used in stand

alone systems, but can also be used to control synchronisation to the grid. There are two basic

types: i) Thyristor based and ii) Fast switching transistors that usually use FETs.

Thyristor load controller can feature either “Phase angle” control or “Burst firing” control.

5.1 Thyristor Systems - Advantages and Disadvantages

Table 5.1 below summarises the advantages and disadvantages.

Phase angle thyristor Burst firing thyristor

Fast response, chops each half cycle Slower response – requires a large number

of cycles to operate over

Voltage spike on generator waveform every

half cycle. Can cause electro magnetic

interference – requires filtering

No voltage spike as switching is done at

zero volts – no filtering required

Fine resolution – each half cycle can be

chopped

Limited resolution – can only switch on / off

whole cycles

Robust Thyristor technology can withstand voltage spikes

5.2 Fast Switching IGBT

Can provide a very rapid response. Fast switching (kHz or above) means a smooth output

waveform. IGBTs are still not as robust as thyristors in withstanding voltage spikes. Note that

IGBTs require very careful circuit layout design and manufacture to avoid the effects of stray

capacitances. In theory IGBTs should be better than thyristor based systems, but there is not

enough service experience yet to see what level of failure there is in the service environment.

5.3 Basic ELC arrangement

The diagram below shows the utline arrangement for an ELC system


ELC operation - ballast load voltage

The following graphs show the ballast voltage at a variety of thyristor firing angles. The

coloured solid line shows the ballast load voltage waveform. The dotted line shows the main

load voltage waveform.

Firing angle = 30 degrees


Electronic Load

Controller

Main Load

Ballast Load

Generator

Waveform

Waveform



5.4 ELC burst firing operation

In Burst Firing mode the thyristor(s) are turned on for a number of whole cycles. Switching is

at zero voltage and usually controlled over periods of one or two seconds. Over one second

there are 50 cycles – the thyristor(s) can be turned on for between 1 and 50 cycles per 50

cycles – resolution is 2%. The generator speed will vary during each second, at the beginning

of each second it will slow down, then when the thryistor(s) turns off it will speed up. ELC

burst firing operation can be used for synchronisation however, it is not suitable for stand

alone system control.

The graphs below show the ballast voltage using burst firing thyristor operation. The graphs

use a 10 cycle period for clarity. Normally a 50 cycle period is used in practice.

20% on time

-1

0

1


40% on time

80% on time

5.5 Pulse width modulation

Control of the ballast load power is done at high frequency using high power transistors

(IGBT’s) to control power to the ballast. The relative width of pulse is varied to control the

ballast power. Pulse width modulation operates at high frequency – several kHz or above. The

same arrangement is commonly used in motor speed controllers.

Advantages:

Smooth power waveform for consumers

Common electronic components

Disadvantages:

Very susceptible to voltage transients and spikes – damage due to lightning is common.

IGBT’s are not as robust as good quality Thryristors.

-1

0

1

-1

0

1


5.6 Frequency Protection

The supply to consumers needs to be protected against frequencies outside a pre-set range and

to prevent the generator running at low speed. If the hydro is overloaded the turbine speed and

generator frequency will fall. Generator volts will fall and the hydro AVR will try to increase

voltage by increasing excitation. It is possible for the generator to get stuck at low speed

(around 45 Hz) and the rotor winding damaged by over excitation.

5.7 Stand Alone System - Synchronous generator with ELC

The diagram below shows the general arrangement for stand alone operation.

5.8 ELC Ballast Loads

Ballast loads must have a total capacity greater than the maximum output of the turbine /

generator. About 10-15% greater is ideal. Ballast loads must always be available – if they are

air heaters, then the thermostats must be removed. (This can contradict the Wiring

Regulations which state that a heater must have an over temperature cut out.) Heating

elements designed for water heating – for domestic use, or as part of a wet heating system -

can be ideal but a changeover switch with air heaters must be provided as a back up for when

the water is hot.

6 Permanent Magnet Generators

Permanent Magnet Generators (PMG) are made using permanent magnets on the rotor, rather

then windings. The advantage is that no rotor excitation is required and therefore the

construction is simple. The disadvantage is that there is no control of the rotor magnetism that

is to say the rotor excitation. The electrical output from the stator will be an A.C voltage. The

frequency will depend on the speed of rotation of the rotor.

Electronic LoadController Main Loads

Ballast Load

Air and / orwater heaters

Generator Contactor

Frequencyprotection


The voltage output will also depend on the speed of the rotor and the magnetic flux linkage

between the rotor and the stator.

PMG systems are often used for small battery charging systems where frequency is not

important. The output is rectified to D.C using a simple diode arrangement – a bridge

rectifier. PMGs can be efficient for small generators of a few hundred Watts to a few kW.

Often the air gap can be adjusted to control the flux linkage, and hence optimise output for a

given rotor speed.

David Roberts

MorbenHydro

November 2008

Documents

Control Protection Micro Hydro Plant