The policy on protection systems for 132, 33 and 11/6.6kV is detailed in the electricity policy document EPD350. The protection of secondary distribution substations through standard settings is detailed in code of practice CP331. The following is a brief summary of the protection philosophy on the distribution network.
132kV network protection
At 132kV main protection will be fully discriminative for phase and earth faults and have an operating time not to exceed 200ms. This will generally be provided by distance or biased differential protection relays. Backup protection will be provided by inverse time overcurrent and earth fault relays, which in general will be non-discriminative. Transformer protection will also be fully discriminative for phase and earth faults, typically biased differential, although this may be reliant on a non-duplicated intertripping scheme (fully monitored). Busbar protection will be a fast acting fully discriminative high impedance scheme for phase and earth faults with backup from line and plant inverse time relays as well as additional relays fitted to bus couplers and bus sections.
33kV network protection
At 33kV, the main protection for phase and earth faults is mainly unit protection, but some distance protection is used, particularly in rural areas. The general aim is for 200ms clearance time except for zone two clearance on distance protection in some locations. Backup protection is standard inverse time overcurrent and earth fault relays. Feeder transformers incorporate balanced earth fault at the source with restricted earth fault, and standby earth fault on the Low Voltage (LV) side at the remote end. Neutral voltage displacement would be incorporated for overhead line fed transformers. Intertripping will be used for fast fault clearance on plant, but where pilots are not available then a fault thrower will be used to ensure remote end tripping. Busbar protection at 33kV assumes the use of metal clad switchgear and therefore only requires earth fault protection. The standard is a main discriminatory system for each bus zone with a check system for the whole board, usually of the frame leakage type.
High Voltage (HV) network protection
At HV there will be only a main overcurrent and earth fault protection of the inverse time type. Clearance time for a fault close to the primary will generally be 1s. Where faster clearance times are required, a high set facility can be utilised. Where feeders contain large sections of overhead line there will usually be auto-reclose fitted incorporating instantaneous, inverse time overcurrent and earth fault trips as well as sensitive earth fault protection. Backup protection for feeders is the standby earth fault protection on the primary transformer which will usually trip only the affected half of the board. Busbar protection is not applied at HV but a form of busbar blocking with circuit breaker fail protection is utilised on modern installations with microprocessor based relays.
Automatic voltage control relays are utilised on transformers at grid and primary substations. The method of control utilised in the relay and settings employed will vary depending on the location, age and configuration of the network. Differing types of voltage control have been utilised; however the modern schemes utilise a combination of circulating current control with the ability to change to reverse reactance where necessary for operational reasons. Line drop compensation is applied to give the optimum voltage characteristic for the network.
Generator Neutral Voltage Displacement (NVD) protection
We would only consider the application of NVD for a generation connection at High Voltage or Extra High Voltage. This would only be required in the unusual circumstances of a generator and its controls being considered capable of supporting our network demand in islanded mode.