Control Protection of SHP and MHP plantCONTENTS
HYDRO SYSTEM CONTROL
1 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 5 6 6 6 7 7 7 7 7 7 8 8 8 8 10 11 12 12 12
1.1 MECHANICAL CONTROL 1.1.1 ELECTRICAL ACTUATORS 1.1.2 HYDRAULIC ACTUATORS 1.2 PLC CONTROL 1.3 SYNCHRONISATION 2 2.1 2.2 2.3 3 CONTROL PANEL SPECIFICATION CONTROL PANEL COMPONENTS CONTROL PANEL DESIGN AND CONNECTIONS METERING SYNCHRONOUS GENERATORS OPERATION
3.1 VOLTAGE CONTROL 3.2 AUTOMATIC VOLTAGE REGULATORS - AVRS 3.3 POWER FACTOR AND REACTIVE POWER 3.4 POWER FACTOR FOR VOLTAGE CONTROL 3.5 POWER FACTOR CONTROL 3.5.1 SYNCHRONOUS MACHINES POWER FACTOR CONTROL 3.5.2 INDUCTION MACHINE POWER FACTOR CONTROL 3.6 ELECTRICAL GENERATOR PROTECTION 3.7 GENERATOR INSULATION PROTECTION 3.8 GENERATOR MECHANICAL PROTECTION 3.8.1 BEARING PROTECTION USING TEMPERATURE SENSORS. 3.8.2 GENERATOR OVER SPEED 4 4.1 4.2 5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 SYSTEM EARTHING LV CONNECTED HV CONNECTED ELECTRONIC LOAD CONTROL THYRISTOR SYSTEMS - ADVANTAGES AND DISADVANTAGES FAST SWITCHING IGBT BASIC ELC ARRANGEMENT ELC BURST FIRING OPERATION PULSE WIDTH MODULATION FREQUENCY PROTECTION STAND ALONE SYSTEM - SYNCHRONOUS GENERATOR WITH ELC ELC BALLAST LOADS
PERMANENT MAGNET GENERATORS
Hydro System ControlA 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. 2. 3. 4. 5. 6. 7. 8. 9. 10. Intake e.g. screen cleaning and level monitoring Turbine operation: Turbine spear valve or vane control Main Inlet Valve Bearings Generator Excitation Synchronisation Power Factor 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. 2. 3. 4.5.
Can provide linear or rotary actuation Can be pulsed or continuous operation Can provide very slow and small movements reliably Can provide accurate position and torque feedback signals and indications Relatively expensive for good quality units
1.1.2 Hydraulic actuators 1. 2. 3. 4. 5. 6.7.
Usually provide linear actuation Can be pulsed or continuous operation Speed of operation dependant upon oil temperature (can use expensive temperature compensated flow control valves) Difficult to achieve very small movements 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 The potential for oil pollution from leakages needs to taken into account by the specification of bunds or other protection systems. Are relatively cheap
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.
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 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.
Control panel specificationHydro 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.
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. 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. 2. Providing generator protection functions e.g. overcurrent and earth fault. These functions are usually provided using a suitable circuit breaker and / or protection relays Providing a connection between the generator and the load / grid usually using a contactor or motorised circuit breaker Providing terminals / connections for the power cabling
3. 4. Contactors compared with motorised breakers for connection The choice mainly depends on system size: 1.
2. 3. 2.3
The smallest motorised circuit breakers are around 400 kVA, and basic units can be derated to about 60%. These will be Air Circuit Breakers (ACBs) usually referred to as Moulded Case Circuit Breakers (MCCB) Note that the maximum reasonable contactor size is 500 kVA A breaker of some sort will usually be required to provide over current protection
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. 2. 3. 4. 5. Three phase volts and current Power, power factor and cummulative power often by individual phases Frequency Peak values of current and voltage Harmonic distortion informationSection 5 Page 3
Synchronous Generators OperationSynchronous 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.Excitation from AVR DC voltage AVR
Voltage signal and supply to AVR
Exciter Stator Winding
Magnetic flux linkage A.C. current Exciter Rotor Winding D.C Current
Magnetic flux linkage
Main rotor winding
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 - AVRs Modern AVRs are solid state devices usually supplied by the generator manufacturer. AVRs 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. 2. Use surge arrestors on the line to a power house. 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).
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.
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.
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 AVRs to operate at exactly the same voltage, and any difference will manifest as circulating current.DR-S5-RevA.doc Section 5 Page 5
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 its 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. 2. 3. 4. 5. 6. 3.7 Overcurrent protection on the generator breaker Earth fault protection on the generator breaker (and on generator earth during run up) Core (winding) temperature monitoring Limitation on the maximum excitation current Stability limitations Pole slipping protection, for larger generators
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...