Protection of overhead and cable lines in 20 and 35 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 20 and 35 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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3.2.98. For lines in networks of 20 and 35 kV with isolated neutral, relay protection devices against multi-phase short circuits and from single-phase short circuits to earth must be provided.

3.2.99. Protection against multi-phase short circuits should be provided in a two-phase two-relay design and included in the same phases throughout the entire network of a given voltage to ensure that in most cases of double earth faults only one fault location is disconnected. In order to increase the sensitivity to damage behind transformers with a star-delta winding connection, it is allowed to perform three-relay protection.

Protection against single-phase earth faults should be performed, as a rule, with action on the signal. An insulation monitoring device may be used for protection.

3.2.100. When choosing the type of basic protection, one should take into account the requirements for ensuring the stability of the operation of the power system and the reliable operation of the consumer in the same way as this is taken into account for the protection of 110 kV lines (see 3.2.108).

3.2.101. On single lines with one-sided supply from multi-phase faults, predominantly stepped current protection or stepped current and voltage protections should be installed, and if such protections do not meet the requirements for sensitivity or fault disconnection speed (see 3.2.108), for example, at head sections, remote step protection mainly with current start. In the latter case, as an additional protection, it is recommended to use current cutoff without time delay.

For lines consisting of several consecutive sections, for the sake of simplicity, it is allowed to use non-selective stepped current and voltage protection in combination with alternate auto-reclosing devices.

3.2.102. On single lines fed from two or more sides (the latter - on lines with branches), both with and without bypass links, as well as on lines included in a ring network with one power point, it is recommended to apply the same protection , as on single lines with one-way supply (see 3.2.101), making them directional if necessary, and remote lines - with start from a resistance relay. In this case, non-selective disconnection of adjacent elements is allowed in case of a short circuit in the "dead" voltage zone of the power direction relay, when the current cutoff used as additional protection (see 3.2.101) is not set, for example, due to its insufficient sensitivity. Protection is installed, as a rule, only on those sides from where power can be supplied.

3.2.103. On short single lines with two-way power, when required by the condition of speed of action, it is allowed to use longitudinal differential protection as the main one. In this case, the length of the cable laid specifically for this protection should not exceed 4 km. To control the serviceability of the auxiliary protection wires, special devices should be provided. In addition to the longitudinal differential protection, one of the protections in 3.2.102 shall be applied as a backup.

3.2.104. On parallel lines fed from two or more sides, as well as on the supply end of parallel lines with one-sided supply, the same protections as on the corresponding single lines can be used (see 3.2.101 and 3.2.102).

To speed up fault disconnection, especially when using current step protection or step current and voltage protection, additional protection with power direction control in a parallel line can be applied on lines with two-sided power supply. This protection can be made in the form of a separate transverse current directional protection or only in the form of an acceleration circuit of the installed protections (overcurrent, remote) with control of the power direction in the parallel line.

At the receiving end of two single-ended parallel lines, as a rule, transverse directional differential protection should be used.

3.2.105. If the protection according to 3.2.104 does not meet the speed requirements (see 3.2.108), and protection with control of the direction of power in a parallel line is not applicable or undesirable, as the main protection (when two parallel lines are operating) on two parallel lines with two-way supply and on the supply end of two parallel lines with unilateral supply, transverse differential directional protection should be used.

In this case, in the mode of operation of one line, as well as as a backup when operating two lines, stepwise protection according to 3.2.101 and 3.2.102 should be used. It is allowed to turn on this protection or its individual stages for the sum of the currents of both lines (for example, a reserve stage in order to increase its sensitivity to damage on adjacent elements). It is also allowed to use transverse differential directional protection in addition to step current protection to reduce the time of fault disconnection on the protected lines, if, according to the speed of action condition (see 3.2.108), its installation is not necessary.

In some cases, on short parallel lines, it is allowed to use longitudinal differential protection (see 3.2.103).

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

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