Grid Connection Equipment and Technology Best Practice and Philosophy

Transcription

1 Grid Connection Equipment and Technology Best Practice and Philosophy CONTENTS 1 INTRODUCTION GENERATOR GRID CONNECTION - PRINCIPLES SYNCHRONIZATION AND CONNECTION 1 3 SYNCHRONIZATION VOLTAGE FLUCTUATION SYNCHRONISATION LIMITS SYNCHRONISATION OF INDUCTION MACHINES SYNCHRONISATION OF SYNCHRONOUS MACHINES VARIABLE SPEED DRIVES INTERCONNECTION PROTECTION INTERCONNECTION PROTECTION OUTLINE UNDER AND OVER VOLTAGE PROTECTION UNDER AND OVER FREQUENCY PROTECTION TIME DELAY ON VOLTAGE AND FREQUENCY PROTECTION ISLANDING HAZARDS OF ISLANDING UNEARTHED OPERATION LOWER FAULT LEVELS OUT OF SYNCHRONISATION RECLOSURE VOLTAGE LEVELS QUALITY OF SUPPLY RISK TO MAINTENANCE PERSONNEL ISLANDING SUMMARY LOSS OF MAINS PROTECTION RATE OF CHANGE OF FREQUENCY VOLTAGE VECTOR SHIFT INTER TRIPPING OTHER POSSIBLE METHODS 7 4 INTER CONNECTION PROTECTION IMPLEMENTATION INTERCONNECTION PROTECTION SETTINGS INTERCONNECTION PROTECTION TESTING OTHER SYSTEM TESTING 8

2 1 Introduction The issues to be covered in this section are: Grid connection principles Synchronisation requirements and methods Interconnection protection Islanded operation meaning and hazards Loss of Mains protection Protection implementation and testing 1.1 Generator Grid Connection - Principles The distribution network should not be unacceptably affected by the generator. That is to say, the quality of supply to customers should be maintained at all times. This is the prime DNO requirement. Conversely, the generating equipment is not to be damaged by the distribution system. The generator should be able to operate, and export power to the grid as intended. Safety should be maintained at all times for the generator, distribution system and customers 2 Synchronization and Connection A generator must be synchronized with the grid prior to connection. Specific regulations must be followed for the interconnection arrangements and protection, for example G59 or G83 in the UK. Other requirements that must be complied with include: Electrical safety, earthing, voltage regulation, harmonics 3 Synchronization Synchronisation means the minimisation of the difference in: magnitude (voltage) frequency phase angle between the corresponding phases of the generator output and the grid supply prior to the connection of the two supplies. Synchronisation MUST occur before a generator can be connected to the grid Synchronisation can be achieved either manually or automatically, Automatic synchronisation is preferred. It is very unlikely that new installations will include only manual synchronisation. If manual synchronisation is suggested for a particular reason, its safe and reliable operation should be seriously considered and implemented carefully. DR-S6-RevA.doc Section 6 Page 1

3 3.1 Voltage Fluctuation There will normally be a voltage fluctuation on the distribution line when a generator is synchronised. During Synchronisation of a generator, the induced voltage fluctuation on the grid should: 1. not normally exceed 3% at the Point of Common Coupling, and 2. should meet the requirements for voltage step and flicker 3.2 Synchronisation limits The normal limits specified for allowing synchronisation are: Phase angle +/- 20 degrees Maximum voltage difference 7% Maximum slip frequency 0.44 % These are the maximum settings of a synch check relay Synch Check relay A synch check relay must be used to check that the synchronisation of the generator and the grid is within the specified limits. The relay must operate on at least two, and preferably all three phases to ensure phase rotation is correct. The use of synch check relays does not apply to induction generators. The requirement for a synch check relay in addition to automatic synchronisation systems is: 1. to act as a backup during synchronisation and 2. to act as a backup to ensure a generator will not connect to a dead distribution line 3.3 Synchronisation of Induction machines An Induction generator is easily synchronised and connected with the grid. It just needs to be run up to synchronous speed and connected BUT there will be a large inrush current and consequent drop in voltage. Some standards require that there is limited inrush current The two main methods of synchronization are: 1. Use an electronic soft start unit to motor the generator up to the synchronous speed. This method can be used to control inrush current. A standard motor controller is used to run the generator up to the synchronous speed and is then bypassed by a contactor. 2. Use the turbine shaft power (with the control valve very slightly open), to mechanically drive the generator up to synchronous speed. Once the generator is at the synchronous speed, the generator may be connected to the grid. It is best if the generator speed is slightly above the synchronous speed as the generator will then export power as it slows down to the synchronous speed. 3.4 Synchronisation of Synchronous Machines A Synchronous machine needs to be running at synchronous speed and has the additional requirement that the generator s output voltage waveform should be in phase with the grid DR-S6-RevA.doc Section 6 Page 2

4 voltage waveform. Put another way, the phase angle between the generator output and the grid supply must be within specified limits. Not only should the generator s output voltage waveform be in phase with the grid voltage waveform but, during the process of synchronizing, the rate of change in phase angle between the grid and the generator must be within specified limits. A Synch Check relay is used to ensure that the generator is connected to the grid only when it is in synch. Synchronisation can be achieved by either: 1. Controlling the turbine power so that the generator speed will slowly pass through the synchronous speed and hoping that the voltages will pass through being in phase this needs good turbine speed control. 2. Control of the load on the generator to achieve synchronisation. This can practically be achieved with an Electronic Load Control (ELC) system. ELC systems are described in another section of this guide. This method of synchronization permits a very smooth an accurate way of connecting the generator to the grid. 3.5 Variable Speed Drives A variable speed drive arrangement can be used to maintain a constant output frequency whilst allowing the generator speed to vary. This can be useful with low head reaction turbines which only run efficiently if the speed of the turbine matches the flow if the flow changes, the speed of the turbine needs to change to maintain efficiency. Such turbines include the Propellor turbine and the Archemedies screw type turbines (such as one implemented by GP Electronics at the Dart Country Park). Using a variable speed drive, the turbine can be run at it s optimum operating speed for the available flow and head. A pair of variable speed motor drives are used to connected back-toback with one half of the system running in regenerative breaking mode. Fig Variable speed drive example DR-S6-RevA.doc Section 6 Page 3

5 3.6 Interconnection Protection Interconnection Protection describes the protection required between the generator and the distribution system. The Interconnection Protection is required to ensure the principles outlined in the Introduction for generator connection are complied with. The required Interconnection Protection will depend on a number of factors, including: 1. Individual and cumulative generation capacity 2. Generator type 3. Relative size compared to captive load 3.7 Interconnection Protection Outline The primary types of interconnection protection required for embedded generators include: Under and Over Voltage Under and Over Frequency Loss of Mains (Islanding) protection includes: ROCOF true ROCOF Voltage Vector Shift Intertripping Neutral Voltage Displacement Under and over voltage protection Under and over voltage protection is required to: 1. Ensure the grid system is operating within specification and is suitable for connection (e.g. it will not damage the generator). 2. Act as a backup Loss of Mains protection Typical settings: Under voltage -10% of nominal voltage Over voltage +10% of nominal voltage Under and over frequency protection Under and over frequency protection is required to: 1. Ensure the grid system is operating within specification and is suitable for connection (e.g. it will not damage the generator) 2. Act as a backup Loss of Mains protection Typical settings: Under frequency -6% of nominal frequency Over frequency +1% of nominal frequency DR-S6-RevA.doc Section 6 Page 4

6 3.7.3 Time delay on voltage and frequency protection A time delay is usually allowed for the operation of voltage and frequency protection. This time delay is to minimise nuisance tripping due to grid system disturbances. The maximum time delay must be less than the minimum operating time of auto-reclose devices used on the distribution line. Typical maximum time delay is 0.5 secs this includes the operation time of any relays and breakers in the control and protection system. Generator operators should make use of this time delay to minimise nuisance tripping. 3.8 Islanding An Islanded situation is where the generator(s) and some local loads have become disconnected from the main distribution system. The usual situation is a section of distribution line (or the complete distribution line or lines), becomes disconnected from the main distribution / transmission system. The generator(s) output may balance with the load demand. 3.9 Hazards of Islanding The hazards created during islanded operation are: 1. Unearthed operation of the distribution system 2. Lower fault levels 3. Out of synchronisation reclosure 4. Voltage levels outside statutory limits 5. Reduction in quality of supply 6. Risk to maintenance personnel Unearthed operation Usually the distribution line is only earthed at the grid sub station. At the generating site the high voltage (HV) side of the transformer is in delta configuration with no earth. When the distribution line, and its generator(s), are disconnected from the grid sub-station the line is not earthed. This is a potential hazard as line voltages may become excessive in a fault. This is detected by Neutral Voltage Displacement protection Lower Fault Levels In an islanded condition the fault level will be reduced. The fault level contribution from the grid sub station has been lost. The reduction in fault level will affect the operation of protection on the distribution line. There may not be sufficient current generated in a fault condition to operate the protection Out of Synchronisation Reclosure During islanded operation the island will not maintain synchronisation with the grid system. When the grid system attempts to reconnect the islanded distribution line to the grid system there will an out of synchronisation siituation at the reconnection point. If out of synchronisation reclosure occurs, there will be sudden and potentially large power flows and voltage disturbances. This may cause damage to distribution, generator and consumer equipment DR-S6-RevA.doc Section 6 Page 5

7 The use of synch check or dead line check relays on auto reclose breakers will protect against out of synchronisation reclosure. If auto reclosers are to be used on distribution lines that have emebedded generators connected to them, the reclose time should be at least twice the maximum disconnection time of any generator protection systems Voltage Levels The voltage levels of an islanded distribution line are controlled by the generator voltage regulators, the generated power and the load demand. Voltage levels may go outside statutory or normal operating limits Quality of supply The quality of supply of an islanded distribution line will generally be reduced. It will be subject to voltage variations as there is limited generating capacity and may be little control on the output of this capacity Risk to Maintenance Personnel There is a risk to the distribution system maintenance personnel from islanded operation. For example, a line may be disconnected for maintenance purposes and the maintenance personnel may assume the line will become, or remain, dead. Should a generator continue to be connected, or become connected later, this will be a hazard for them Islanding summary Due to the hazards listed the operation of an islanded situation is generally not allowed. There may be an advantage to consumers, and generating companies, to allow islanded operation. This is the maintenance of supply when a line has been disconnected. This advantage is small and the hazards outweigh the advantages. It may be allowed in special situations with appropriate additional protection 3.10 Loss of Mains Protection Loss of Mains (LoM) Protection is designed to sense when the generator, and any local loads, have become disconnected from the main grid system that is to say, when an island has been created. Normal forms of Loss of Mains protection for G59 sized systems are: 1. Rate of Change of Frequency usually referred to by the terms RoCoF or df/dt 2. Voltage Vector Shift usually referred to by the term dv 3. Inter tripping Rate of Change of Frequency When an island occurs, there is usually an imbalance between the generator output and local captive load. The generator will therefore speed up or slow down. This causes a change of frequency. The change of frequency can be detected by a Rate of Change of Frequency relay (RoCoF or df/dt). Settings of the relay required depend upon the relative size of generation, captive load and system inertia. The first two items are difficult to quantify. A typical setting is 0.15 Hz/second. DR-S6-RevA.doc Section 6 Page 6

8 Voltage Vector Shift At the moment when an island occurs, there will be a jump in the voltage waveform as the current flow is disturbed. The size of the waveform jump will depend upon the current flow and system characteristics. This jump is known as Voltage Vector Shift. It can be single or three phase usually single phase is asked for to detect loss of a single overhead line. Typical settings are 6 to 8 degrees. Setting requirement will be as the same as given for Rocof, however, it will depend on the generation and location Inter tripping Inter tripping is a communication connection between the distribution sub-station and the generator. If there is a trip at the sub-station (e.g. caused by a line fault) this will signal to the generator to disconnect as an island has formed. Whilst is very reliable, it is expensive to make arrangements for and implement. This is because it requires fail safe, reliable communications. In order to arrange for this, the distributed network operator (DNO might need a fixed leased line. Usually this kind of arrangement is only made for larger generation schemes of several MW or more Other possible methods Other methods of islanding protection that may be considered for some particular situations include: 1. Low power or reverse power protection 2. Fault throwing causing a line fault which will operate other forms of protection 3. Reverse VAR protection- detecting a flow of reactive power from the generator to the grid. Requires the generator to normally import reactive power and have stable power factor control 4 Inter connection Protection Implementation Inter connection protection necessitates the use of standard relays or proprietary systems. Standard protection relays are available from several manufacturers such as Selco, Areva, P&B Engineering and SEG/AVK. Nowadays, the equipment is always digital. Proprietary systems have been developed for specific applications. These are analogue or digital systems. From a specification view point, always use relays designed and manufactured to IEC standards e.g. IEC 255 to ensure performance and compatibility. It is also good practice to have a direct operation of the breaker / contactor by the protection relay. If relying on a PLC system then the PLC code may need to be proved and verified which could be time consuming and expensive Interconnection Protection settings The standards and connection agreement will specify protection settings. Loss of Mains relays are a common source of nuisance tripping. For example, local grid disturbances (particularly on weak distribution lines), can cause voltage dips and vector shifts. This may cause problems for a generator loosing generation time and may be a reason to discuss possible changes to settings. A common cause of tripping can be a problem when a fault on one distribution line causes tripping on other lines. Some forms of islanding protection are more subject to common cause tripping. DR-S6-RevA.doc Section 6 Page 7

9 4.2 Interconnection Protection testing An on-site test will normally be required by the Network Operator when the system is first grid connected. Testing is required to prove the operation of the interconnection protection and verify settings. Testing is undertaken by the generator and witnessed by the Network Operator engineer. It is the responsibility of the generator to provide a suitable test procedure and test equipment. It is normally the Generator operator s responsibility to maintain and re-test the protection equipment at regular intervals (typically every 3-5 years). In order to do this, the following preparations will need to be made before the day of the test: 1. Discuss and agree tests to be undertaken and test procedures with the Network Operator engineer before hand 2. Check through the test form to be completed by the Network Operator engineer 3. Provide single line protection drawings to the Network Operator engineer 4. Use calibrated test equipment 5. Agree where signals are to be injected and protection operation is to be measured Tests will include: Over and Under Voltage Over and Under Frequency Loss of Mains ROCOF and / or Voltage vector shift Loss of supply disconnect main supply Loss of phase disconnect one phase of the supply. On an LV connected system the Utility can usually pull one supply fuse. On an HV connected system the Generator may have to add a facility to disconnect one phase Loss of supply to the protection relay 4.3 Other System Testing It is the generator operator s responsibility to undertake testing of all other electrical equipment. These tests will include at least the following: 1. Cable integrity, connections and fixing 2. Insulation resistance 3. Earthing continuity 4. Earth system resistance 5. Control system operation 6. Sensor operation 7. Generator AVR and PFC setup and commissioning A clear record must be made of all tests and the test procedures should comply with the appropriate legislation. This is a large subject which is beyond the scope of this section. David Roberts MorbenHydro November 2008 DR-S6-RevA.doc Section 6 Page 8