Protection of overhead and cable lines in 3-10 kV networks with isolated neutral

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Section 3. Protection and automation

Relay protection. Protection of overhead and cable lines in 3-10 kV networks with isolated neutral

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Encyclopedia of radio electronics and electrical engineering / Rules for the installation of electrical installations (PUE)

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3.2.91. For lines in networks of 3-10 kV with an isolated neutral (including with a neutral grounded through an arcing reactor), relay protection devices against multi-phase short circuits and from single-phase short circuits to earth must be provided.

3.2.92. Protection against multi-phase faults should be provided in a two-phase version and included in the same phases throughout the network of a given voltage to ensure that in most cases of double earth faults only one fault location is disconnected.

The protection must be performed as one-, two- or three-relay, depending on the requirements of sensitivity and reliability.

3.2.93. On single lines with one-sided supply from multi-phase faults, as a rule, two-stage current protection should be installed, the first stage of which is made in the form of a current cutoff, and the second - in the form of overcurrent protection with an independent or dependent time delay characteristic.

On non-reacted cable lines with one-sided power supply, extending from the busbars of power plants, current cutoffs must be performed without time delay and their zone of action must be determined from the condition of disconnecting short circuits, accompanied by residual voltage on the buses of these power plants below 0,5-0,6 nominal. To fulfill the specified condition, it is allowed to perform non-selective protection in combination with AVR or AVR devices that correct fully or partially the non-selective protection action. It is allowed to install the indicated cut-offs also on lines extending from substation busbars and supplying large synchronous electric motors.

If on unreacted cable lines with one-sided power supply, extending from the buses of power plants, current cutoffs cannot be applied due to the requirements of selectivity, then to ensure speed it is allowed to provide protection according to 3.2.94, clause 2 or 3. The use of these protections is also allowed for working lines own needs of thermal power plants.

On reacted lines, the switches of which are not designed to cut short circuits up to the reactor, current cutoffs are not allowed.

3.2.94. On single lines with two-way supply, with or without bypass connections, as well as on lines included in a ring network with one point of supply, it is recommended to apply the same protections as on single lines with one-way supply (see 3.2.93), performing directed them if necessary.

In order to simplify the protections and ensure their selective action, it is allowed to apply automatic division of the network into radial sections at the time of damage with its subsequent automatic restoration.

If non-directional or directional current step protection does not provide the required speed and selectivity, it is allowed to provide the following protections:

1) remote protection in the simplest version;

2) transverse differential current protection (for double cable lines);

3) longitudinal differential current protection for short sections of lines; if it is necessary to lay a special cable only for longitudinal differential protection, its length should not exceed 3 km.

For the protections specified in paragraphs 2 and 3, current protection should be provided as a backup protection.

3.2.95. When performing protection of 3-10 kV parallel lines, one should be guided by the instructions for parallel lines in 35 kV networks (see 3.2.104).

3.2.96. Protection against single-phase earth faults must be made in the form:

selective protection (establishing a damaged direction) acting on the signal;
selective protection (determining the damaged direction), acting on shutdown when it is necessary for safety reasons; protection must be installed on the supply elements in the entire electrically connected network;
isolation control devices; in this case, the search for a damaged element should be carried out by special devices; it is allowed to search for a damaged element by turning off the connections one by one.

3.2.97. Protection against single-phase earth faults must be carried out, as a rule, using zero-sequence current transformers. Protection in the first place must respond to established ground faults; it is also allowed to use devices that register short-term short circuits, without ensuring the repetition of the action.

Protection against single-phase earth faults, acting on disconnection without time delay according to safety requirements (see 3.2.96), should disconnect only the element supplying the damaged section; at the same time, protection should be provided as a backup, performed in the form of zero-sequence protection with a time delay of about 0,5 s, acting to turn off the entire electrically connected network - a busbar system (section) or a supply transformer.

Increasing the power frequency current specifically to ensure the operation of protection in a network with a neutral grounded through an arcing reactor (for example, by detuning the reactor), as a rule, is not allowed to be provided.

See other articles Section Rules for the installation of electrical installations (PUE).

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