Section 3. Protection and automation

Chapter 3.4. Secondary circuits

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

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3.4.1. This chapter of the Rules applies to secondary circuits (circuits of control, signaling, control, automation and relay protection) of electrical installations.

3.4.2. The operating voltage of the secondary circuits of the connection, which has no connection with other connections and the equipment of which is located separately from the equipment of other connections, should not exceed 1 kV. In all other cases, the operating voltage of the secondary circuits must not exceed 500 V.

The design of the devices to be connected must comply with environmental conditions and safety requirements.

3.4.3. In power plants and substations for secondary circuits, control cables with aluminum conductors made of semi-solid aluminum should be used. Control cables with copper conductors should only be used in secondary circuits:

1) power plants with generators with a capacity of more than 100 MW; at the same time, control cables with aluminum conductors should be used at power plants for secondary switching and lighting of chemical water treatment facilities, treatment, utility and auxiliary facilities, mechanical workshops and starting boilers;

2) switchgear and substations with a higher voltage of 330 kV and above, as well as switchgear and substations included in intersystem transit transmission lines;

3) differential busbar protections and devices for redundant failure of 110-220 kV circuit breakers, as well as system emergency automatics;

4) technological protection of thermal power plants;

5) with an operating voltage of not more than 60 V with a diameter of the cores of cables and wires up to 1 mm (see also 3.4.4);

6) power plants and substations located in explosive zones of classes BI and B-Ia.

In industrial plants for secondary circuits, control cables with aluminum-copper or aluminum conductors made of semi-solid aluminum should be used. Control cables with copper conductors should be used only in secondary circuits located in explosive zones of classes BI and B-Ia, in secondary circuits of mechanisms of blast furnace and converter shops, the main line of crimping and continuous high-performance rolling mills, electrical receivers of a special category I category, as well as in secondary circuits with an operating voltage of not more than 60 V with a diameter of the cores of cables and wires up to 1 mm (see also 3.4.4).

3.4.4. According to the condition of mechanical strength:

1) the conductors of control cables for screw connection to the clamps of panels and apparatuses must have a cross section of at least 1,5 mm2 (and when using special clamps - at least 1,0 mm2) for copper and 2,5 mm2 for aluminum; for current circuits - 2,5 mm2 for copper and 4 mm2 for aluminum; for non-critical secondary circuits, for control and signaling circuits, it is allowed to connect cables with copper conductors with a cross section of 1 mm2 under the screw;

2) in circuits with an operating voltage of 100 V and higher, the cross section of the copper cores of cables connected by soldering must be at least 0,5 mm2;

3) in circuits with an operating voltage of 60 V and below, the diameter of the copper cores of cables connected by soldering must be at least 0,5 mm. In communication devices, telemechanics and the like, linear circuits should be connected to screw terminals.

The connection of single-wire conductors (for screw or soldering) is allowed only to fixed elements of the equipment. Connection of cores to movable or removable elements of equipment (plug-in connectors, removable blocks, etc.), as well as to panels and devices subject to vibration, should be carried out with flexible (stranded) cores.

3.4.5. The cross section of the conductors of cables and wires must meet the requirements of their protection against short circuit without time delay, permissible continuous currents in accordance with Ch. 1.3, thermal resistance (for circuits coming from current transformers), as well as ensure the operation of devices in a given accuracy class. In this case, the following conditions must be met:

1. Current transformers together with electrical circuits must work in the accuracy class:

• for settlement meters - according to Ch. 1,5;
• for measuring power transducers used to enter information into computing devices - according to Ch. 1.5, as for technical accounting meters;
• for switchboards and current and power measuring transducers used for all types of measurements - not lower than accuracy class 3;
• for protection, as a rule, within 10% error (see also ch. 3.2.).

2. For voltage circuits, voltage losses from a voltage transformer, provided that all protections and devices are turned on, should be:

• up to settlement meters and power transducers used to enter information into computing devices - no more than 0,5%;
• to settlement meters of intersystem transmission lines - no more than 0,25%;
• to technical accounting meters - no more than 1,5%;
• to switchboards and power sensors used for all types of measurements - no more than 1,5%;
• up to protection and automation panels - no more than 3% (see also chapter 3.2.).

When the specified loads are fed together through common conductors, their cross section must be selected according to the minimum of the allowable voltage loss rates.

3. For control current circuits, voltage losses from the power source should be:

• to the device panel or to control electromagnets that do not have forcing - no more than 10% at the highest load current;
• up to control electromagnets having threefold and greater forcing - no more than 25% at the forcing current value.

4. For voltage circuits of AVR devices, the voltage loss from the voltage transformer to the measuring device must be no more than 1%.

3.4.6. In one control cable, it is allowed to combine control, measurement, protection and signaling circuits for direct and alternating current, as well as power circuits that feed low-power electrical receivers (for example, electric motors for valves).

In order to avoid an increase in the inductive resistance of the cable cores, the wiring of the secondary circuits of the current and voltage transformers must be carried out so that the sum of the currents of these circuits in each cable is equal to zero in any modes.

It is allowed to use common cables for circuits of different connections, with the exception of mutually redundant ones.

3.4.7. Cables should normally be connected to terminal assemblies. It is not recommended to connect two copper conductors of a cable under one screw, and two aluminum conductors are not allowed.

Cables may be connected directly to the outputs of measuring transformers or individual devices.

The design of the clamps must correspond to the material and cross section of the cable cores.

3.4.8. Connection of control cables in order to increase their length is allowed if the length of the route exceeds the construction length of the cable. The connection of cables with a metal sheath should be carried out with the installation of hermetic couplings.

Cables with a non-metallic sheath or with aluminum conductors should be connected on intermediate rows of clamps or using special sleeves designed for this type of cable.

3.4.9. Secondary circuit cables, cable cores and wires connected to terminal assemblies or devices must be marked.

3.4.10. Types of wires and cables for secondary circuits, methods of their laying and protection should be selected taking into account the requirements of Ch. 2.1, 2.3 and 3.1 to the extent that they are not modified by this chapter. When laying wires and cables on hot surfaces or in places where the insulation can be exposed to oils and other aggressive media, special wires and cables should be used (see Chap. 2.1).

Wires and cable cores with non-light-resistant insulation must be protected from light exposure.

3.4.11. Cables of the secondary circuits of voltage transformers of 110 kV and above, laid from the voltage transformer to the shield, must have a metal sheath or armor, grounded on both sides. Cables in the circuits of the main and additional windings of one voltage transformer of 110 kV and above along the entire length of the route should be laid side by side. For circuits of instruments and devices that are sensitive to pickups from other devices or circuits passing nearby, shielded wires must be used, as well as control cables with a common shield or cables with shielded cores.

3.4.12. Installation of DC and AC circuits within switchboard devices (panels, consoles, cabinets, boxes, etc.), as well as internal wiring diagrams for switches, disconnectors and other devices, according to the conditions of mechanical strength, must be made with wires or cables with copper conductors section not less than:

• for single-wire conductors connected with screw clamps, 1,5 mm2;
• for single-wire conductors attached by soldering, 0,5 mm2;
• for stranded conductors connected by soldering or screwed with special lugs, 0,35 mm2; in technically justified cases, it is allowed to use wires with stranded copper conductors, connected by soldering, with a cross section of less than 0,35 mm2, but not less than 0,2 mm2;
• for cores connected by soldering in circuits with a voltage of not more than 60 V (control panels and consoles, telemechanics devices, etc.) - 0,197 mm2 (diameter - not less than 0,5 mm).

The connection of single-wire conductors (for screw or soldering) is allowed only to fixed elements of the equipment. Connection of cores to movable or removable elements of equipment (plug-in connectors, removable blocks, etc.) should be carried out with flexible (stranded) cores.

Mechanical loads on the places of soldering of wires are not allowed.

For transitions to the doors of devices, multi-wire wires with a cross section of at least 0,5 mm2 should be used; it is also allowed to use wires with single-wire conductors with a cross section of at least 1,5 mm2, provided that the wire bundle works only in torsion.

The cross section of wires on switchboard devices and other factory-made products is determined by the requirements for their protection against short circuits without time delay, permissible current loads in accordance with Ch. 1.3, and for circuits coming from current transformers, in addition, and thermal resistance. For installation, wires and cables with flame retardant insulation should be used.

The use of wires and cables with aluminum conductors for the internal installation of switchboards is not allowed.

3.4.13. The connections of the devices to each other within the same panel should be carried out, as a rule, directly without removing the connecting wires to the intermediate terminals.

The terminals or test blocks must be connected to the circuits in which it is required to include test and verification devices and devices. It is also recommended to output circuits to a number of terminals, the switching of which is required to change the operating mode of the device.

3.4.14. Intermediate clamps should only be installed where:

• the wire goes into the cable;
• circuits of the same name are combined (assembly of clamps for trip circuits, voltage circuits, etc.);
• Portable test and measuring apparatus are required to be included if test blocks or similar devices are not available;
• several cables merge into one cable or the chains of different cables are redistributed (see also 3.4.8).

3.4.15. Terminals belonging to different connections or devices must be separated into separate terminal assemblies.

On the rows of clamps, there should not be clamps in close proximity to one another, the accidental connection of which can cause the connection to be turned on or off or a short circuit in the control current circuits or in the excitation circuits.

When placing on a panel (in a cabinet) equipment related to different types of protection or other devices of the same connection, the power supply from the poles of the operating current through the terminal assemblies, as well as the wiring of these circuits across the panel, must be performed independently for each type of protection or devices. If there are no overlays in the trip circuits from individual sets of protection, then the connection of these circuits to the output relay of the protection or the trip circuits of the circuit breaker should be carried out through separate terminals of the terminal assembly; at the same time, panel connections of the indicated circuits should be made independently for each type of protection.

3.4.16. To carry out operational checks and tests in protection and automation circuits, test blocks or measuring clamps should be provided that provide (except for the cases specified in 3.4.7) without disconnecting wires and cables, disconnection from the auxiliary current source, voltage and current transformers with the possibility of preliminary short-circuiting current circuits; connection of test devices for checking and adjusting devices.

Relay protection and automation devices, which are periodically taken out of operation due to the requirements of the network mode, selectivity conditions, and other reasons, must have special devices for taking them out of operation by operational personnel.

3.4.17. Terminal assemblies, auxiliary contacts of switches and disconnectors and devices should be installed, and grounding conductors should be mounted so that the accessibility and safety of servicing the assemblies and devices of secondary circuits without de-energizing primary circuits with a voltage above 1 kV is ensured.

3.4.18. The insulation of equipment used in secondary circuits must comply with the standards determined by the operating voltage of the source (or isolating transformer) that feeds these circuits.

Insulation control of operational direct and alternating current circuits should be provided for each independent source (including isolating transformers) that does not have grounding.

The insulation monitoring device must provide a signal when the insulation drops below the set value, and at direct current - also measure the value of the insulation resistance of the poles. Insulation control may not be performed with an unbranched operating current network.

3.4.19. The auxiliary current supply of the secondary circuits of each connection should be carried out through separate fuses or circuit breakers (the use of the latter is preferable).

The supply of operational current to the circuits of relay protection and control of switches of each connection should be provided, as a rule, through separate circuit breakers or fuses not connected to other circuits (alarm, electromagnetic blocking, etc.). Joint power supply of control circuits and lamps for signaling the position of the controlled vehicle is allowed.

For connections of 220 kV and above, as well as for generators (units) with a capacity of 60 MW or more, separate power supply with operational current (from different fuses, automatic switches) of the main and backup protections should be provided.

When circuit breakers and fuses are connected in series, the latter must be installed in front of the circuit breakers (on the power supply side).

3.4.20. Relay protection devices, automation and control of critical elements must have permanent monitoring of the state of the power supply circuits with operational current. Control can be carried out by using separate relays or lamps or by means of devices provided for monitoring the health of the circuit of the subsequent operation of switching devices with remote control.

For less critical devices, power control can be carried out by giving a signal about the disconnected position of the circuit breaker in the control current circuit.

The follow-up circuit must be checked for proper operation if it contains an auxiliary contact of the switching device. At the same time, the health check of the shutdown circuit must be carried out in all cases, and the health check of the switching circuit must be carried out on the switches of critical elements, short circuits and on devices switched on under the action of automatic transfer devices (ATS) or telecontrol.

If the parameters of the drive enabling circuits do not provide the possibility of monitoring the health of this circuit, monitoring is not performed.

3.4.21. In electrical installations, as a rule, automatic signaling of a violation of the normal operating mode and the occurrence of any malfunctions should be provided.

Checking the serviceability of this alarm should be provided for by its periodic testing.

In electrical installations operating without constant personnel on duty, a signal must be provided to the personnel location.

3.4.22. Control current circuits in which false operation of various overvoltage devices is possible during the operation of closing electromagnets or other devices, as well as during ground faults, must be protected.

3.4.23. Grounding in the secondary circuits of current transformers should be provided at one point on the terminal assembly closest to the current transformers or on the terminals of the current transformers.

For protections that combine several sets of current transformers, grounding must also be provided at one point; in this case, grounding is allowed through a breakdown fuse with a breakdown voltage of not more than 1 kV with a shunt resistance of 100 Ohm to drain the static charge.

Secondary windings of intermediate isolating current transformers may not be grounded.

3.4.24. The secondary windings of the voltage transformer must be grounded by connecting the neutral point or one of the ends of the winding to a grounding device.

The earthing of the secondary windings of the voltage transformer must be carried out, as a rule, at the terminal assembly closest to the voltage transformer or at the terminals of the voltage transformer.

It is allowed to combine grounded secondary circuits of several voltage transformers of one switchgear with a common grounding bar. If these busbars belong to different switchgears and are located in different rooms (for example, relay boards of switchgears of different voltages), then these busbars, as a rule, should not be connected to each other.

For voltage transformers used as sources of operational alternating current, if the working grounding of one of the poles of the operational current network is not provided, the protective grounding of the secondary windings of the voltage transformers must be carried out through a breakdown fuse.

3.4.25. Voltage transformers must be protected from short circuits in secondary circuits by automatic switches. Circuit breakers should be installed in all unearthed conductors after assembling the clamps, with the exception of the zero sequence (open delta) circuit of voltage transformers in networks with high earth fault currents.

For unbranched voltage circuits, circuit breakers may not be installed.

In the secondary circuits of the voltage transformer, it must be possible to create a visible break (knife switches, detachable connectors, etc.).

Installation of devices that can create a gap in the conductors between the voltage transformer and the grounding point of its secondary circuits is not allowed.

3.4.26. On voltage transformers installed in networks with low earth fault currents without compensation for capacitive currents (for example, on the generator voltage of the generator-transformer unit, on the auxiliary voltage of power plants and substations), if necessary, overvoltage protection should be provided for spontaneous neutral displacements. Protection can be implemented by including active resistances in an open delta circuit.

3.4.27. In the secondary circuits of linear voltage transformers of 220 kV and above, redundancy from another voltage transformer should be provided.

It is allowed to perform mutual redundancy between linear voltage transformers with sufficient secondary load capacity.

3.4.28. Voltage transformers must have voltage circuit health monitoring.

Relay protection, the circuits of which are fed by voltage transformers, must be equipped with the devices specified in 3.2.8.

Regardless of the presence or absence in the protection circuits of these devices, signals must be provided:

• when disconnecting circuit breakers - with the help of their auxiliary contacts;
• in case of violations of the operation of the relay-repeaters of bus disconnectors - using devices for monitoring the breakage of control circuits and relay-repeaters;
• for voltage transformers, in the circuit of the higher voltage windings of which fuses are installed, in case of violation of the integrity of the fuses - with the help of central devices.

3.4.29. In places subject to shocks and vibrations, measures must be taken against the violation of the contact connections of wires, false operation of the relay, as well as against premature wear of apparatus and instruments.

3.4.30. The panels must have inscriptions on the serviced sides indicating the connections to which the panel belongs, its purpose, the serial number of the panel in the shield, and the equipment installed on the panels must have inscriptions or markings according to the diagrams.

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

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