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Appendix 1 - Description of processes/plants or systems
Power Distribution Systems

An Introduction to Power Distribution Systems

A large industrial area such as the Complex requires a significant quantity of electricity on a daily basis for a number of different applications – lighting, heating, power for equipment etc.

Electricity generated at power stations which feed into the national grid is available at high voltages (since this is the most efficient means of transporting electricity over large distances) which on arrival at site must then be gradually reduced in voltage to supply the individual requirements of the different users.

The voltage is gradually "stepped down" in electrical sub-stations to provide supplies at different voltages (say 33,000V (33kV) down to 11,000V (11kV) and eventually down to 240V which is the domestic supply voltage).

Electricity is transported in electrical cables which must be designed to the appropriate standards for the voltage to be carried and suitably protected when installed. Cables must be insulated to ensure that electricity does not "leak out" and dissipate into the surroundings. This is a potentially hazardous situation which can result in electrocution if personnel come into contact.

In addition to the protection offered by the insulation additional protection is provided by trips (relays) installed in the electrical sub-stations as circuit breakers.

Circuit breakers such as this operate when they detect unusually high levels of current flowing in the circuits that they are protecting, on the basis that excess current usually indicates that a fault exists. The amount of current flowing is continuously measured by devices called current transformers, which essentially produce a small current that is proportional to the much higher current flowing in the high voltage circuits. The current from the transformers then flows through a device called a protection relay (trip). When this relay senses that the current is too high it sends a signal to the associated circuit breaker to trip and this then cuts off the electricity supply to the distribution system downstream of the protection relay.

Under normal operation the "current" associated with the system will be low. However when there is a fault in the electrical distribution system (caused by loss of insulation, flow of electricity to ground etc.) the current in the system will significantly increase. This will be automatically detected by trips which are pre-set resulting in the safe isolation of the faulty section of the distribution system.

Cables must also be protected from external physical damage by appropriate cladding, identification and route marking etc. since injury/accidents caused by cable strikes on buried underground services are widely reported. Common methods are above ground markers or protective cable tiles which are laid on top of the cable when buried.

The HSE has published guidance on avoiding danger from underground services.

Relay Testing Procedures

Relays must be periodically tested to ensure that they are functional (a procedure known as secondary injection testing). This is essential since a non-functional relay offers no protection. In order to do this the relays are temporarily disconnected from the current transformers. Common practice is to use a proprietary test block for the relay which allows the relay to be disconnected for testing purposes. An alternative if no test block is available is to use temporary plastic inserts which should be removed after the testing has been completed in order to reinstate the protection offered by the relay system.

Failure to remove plastic inserts following testing effectively disconnects the protective trip relay from that part of the system and means that that part of the system can not be isolated. Isolation then has to be provided by other relays located further upstream in the distribution system with the resultant risk that additional sections of the distribution system to that part where the fault is present will also be tripped out.

Power supply systems are covered by the Electricity at Work Regulations (EWR). The HSE has published guidance on safe working practices for electricity.

Power Distribution at the Complex

Electricity is one of the basic utility systems and as such an extensive distribution system is required to distribute power throughout the Complex.

There are a number of different electricity supply sources (Scottish Power, the on-site power station etc.) which feed into a distribution system that supplies power to the users. The supply initially at 275kV is gradually reduced through a number of different steps (33kV, 11kV etc.) suitable to the individual users. A number of different electricity supply cables carrying electricity at different supply voltages cross the Complex to form a site wide distribution system.

In order to minimise the likelihood of power loss to individual facilities or areas of the Complex in the event of a failure in one part of the system a number of different routes are available for the distribution of power throughout the Complex. A series of electrical sub-stations are located throughout the Complex.

Protection is built into the system in the form of automatic trips (relays) installed in the electrical sub-stations which on detection of a fault are designed to isolate the relevant section of the system in order to ensure safety. By tripping out the supply to the relevant faulty section, and by having alternative distribution routes safety can be ensured whilst minimising the disruption caused.

The power distribution failure (27th May 2000) involved a fault which developed on a 33kV feeder cable. Details of the 33kV system are given below.

33kV Feeder Cable

The 33kV feeder cable was a three-phase cable constructed with three internal conductors with 33,000 volts between each conductor and earth. The three phases are traditionally identified by the colours red, yellow and blue. The internal conductors were surrounded by oil-impregnated paper insulation, with an extruded lead sheath over the paper insulation, connected to earth. The sheath itself was covered by earthed steel wire armouring which provided an outer layer of mechanical protection. The wire armouring was then covered by an outer plastic sheath providing protection against corrosion.

The cable was laid at a depth of 750mm alongside other high voltage cables and was partially covered with a reinforced concrete cable tile.

The relay trip facilities were the subject of periodic testing to ensure that they were functional but BP historically did not have proprietary test blocks as described above for the temporary isolation of the relays for testing purposes and the testing procedures followed used the installation of plastic inserts in relays for isolation purposes.