Secondary circuits: concept, definition, purpose, operating principle, installation and application

Kinds

Secondary circuits come in several varieties. So, these include voltage and current circuits. They are distinguished by the presence of devices for measuring current, power, and voltage.

There is also an operational type. It facilitates the transmission of current to the main actuators. Secondary circuits of this type are represented by electromagnets, contactors, automated switches, fuses, keys, and so on.

The current circuit that comes from the CT for measurements is most often used to power:

Instruments that display and measure ammeters, wattmeters, varmeters, and so on.
Protective relay systems: remote, against short circuits, against switch failures and others.
Devices for regulating power flows, emergency automation.
A number of devices included in the alarm or blocking system.

In addition, a current circuit is used when there is a need to power devices for converting alternating current into direct current, which are used as sources of operational current.

You are master

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

3.4.2. The operating voltage of the secondary circuits of the connection, which is not connected to other connections and the equipment of which is located separately from the equipment of other connections, should not be higher than 1 kV. In all other cases, the operating voltage of the secondary circuits should be no higher than 500 V.

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

3.4.3. In power plants and substations, control cables with semi-solid aluminum conductors should be used for secondary circuits. 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, at power plants for secondary switching and lighting of chemical water treatment facilities, wastewater treatment, utility and auxiliary structures, mechanical workshops and start-up boiler houses, control cables with aluminum conductors should be used;

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

3) differential busbar protection and failure backup devices for 110 – 220 kV circuit breakers, as well as system emergency automation equipment;

4) technological protection of thermal power plants;

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

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

In industrial plants, control cables with aluminum-copper or semi-solid aluminum conductors should be used for secondary circuits. Control cables with copper conductors should be used only in secondary circuits located in explosive zones of classes B-I 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 special group I category, and also in secondary circuits with an operating voltage not higher than 60 V with a diameter of cable cores and wires up to 1 mm (see also 3.4.4).

3.4.4. According to the condition of mechanical strength:

1) the cores of control cables for screw connection to the terminals of panels and devices must have a cross-section of at least 1.5 square meters. mm (and when using special clamps - at least 1.0 sq. mm) for copper and 2.5 sq. mm. mm for aluminum; for current circuits - 2.5 sq. mm for copper and 4 sq. mm for aluminum; for non-critical secondary circuits, for control and signaling circuits, screw connection of cables with copper conductors with a cross-section of 1 square meter is allowed. mm;

2) in circuits with an operating voltage of 100 V and above, the cross-section of the copper conductors of the cables connected by soldering must be at least 0.5 square meters. mm;

3) in circuits with an operating voltage of 60 V and below, the diameter of the copper cores of the 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 (by screw or soldering) is allowed only to fixed elements of the equipment. The 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 made with flexible (stranded) cores.

3.4.5. The cross-section of the cores of cables and wires must meet the requirements for their protection against short-circuit without time delay, permissible long-term currents in accordance with Chapter. 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 operate in the accuracy class:

for settlement meters - according to Ch. 1.5;

for power measuring transducers used to input information into computing devices - according to Ch. 1.5, as for technical metering meters;

for panel devices and current and power measuring transducers used for all types of measurements - not lower than accuracy class 3;

for protection, usually within a 10% error (see also Chapter 3.2).

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

to metering meters and power measuring converters used to enter information into computing devices - no more than 0.5%;

to the estimated meters of intersystem power transmission lines - no more than 0.25%;

to technical metering meters - no more than 1.5%;

to panel devices and power sensors used for all types of measurements - no more than 1.5%;

to protection and automation panels - no more than 3% (see also Chapter 3.2).

When the specified loads are powered together via common conductors, their cross-section must be selected according to the minimum permissible voltage loss standards.

3. For operational current circuits, the voltage loss from the power source should be:

to the device panel or to control electromagnets that do not have boost - no more than 10% at the highest load current;

to control electromagnets that have a threefold or greater boost - no more than 25% at the boost current value.

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

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

To avoid an increase in the inductive reactance of the cable cores, the wiring of the secondary circuits of the current and voltage transformers must be done so that the sum of the currents of these circuits in each cable is equal to zero in any mode.

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

3.4.7. Cables should generally be connected to clamp assemblies. Connecting two copper wires of a cable under one screw is not recommended, and two aluminum wires are not allowed.

Cables may be connected directly to the terminals of instrument transformers or individual devices.

The design of the clamps must match the material and cross-section of the cable cores.

3.4.8. Connecting control cables in order to increase their length is permitted 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 sealed couplings.

Cables with a non-metallic sheath or with aluminum conductors should be connected on intermediate rows of clamps or using special couplings 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 installation and protection should be selected taking into account the requirements of Chapter. 2.1 – 2.3 and 3.1 to the extent that they are not changed by this chapter. When laying wires and cables over hot surfaces or in places where the insulation may be exposed to oils and other aggressive environments, special wires and cables should be used (see Chapter 2.1).

Wires and cable cores that have non-light-resistant insulation must be protected from exposure to light.

3.4.11. Cables of secondary circuits of voltage transformers of 110 kV and above, laid from the voltage transformer to the switchboard, 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 higher along the entire length of the route should be laid side by side. For circuits of devices and devices that are sensitive to interference from other devices or nearby circuits, shielded wires must be used, as well as control cables with a common shield or cables with shielded conductors.

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

for single-wire conductors connected with screw terminals, 1.5 sq. mm;

for single-wire conductors connected by soldering, 0.5 sq. mm;

for stranded conductors connected by soldering or screwing using special lugs, 0.35 sq. mm; 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 square meters. mm, but not less than 0.2 sq. mm;

for cores connected by soldering in circuits with a voltage not exceeding 60 V (control panels and consoles, telemechanics devices, etc.) – 0.197 sq. mm (diameter – not less than 0.5 mm).

The connection of single-wire conductors (by screw or soldering) is allowed only to fixed elements of the equipment. The connection of cores to movable or removable elements of equipment (detachable connectors, removable blocks, etc.) should be done with flexible (stranded) cores.

Mechanical loads on the places where wires are soldered are not allowed.

For transitions to device doors, stranded wires with a cross-section of at least 0.5 square meters must be used. mm; It is also allowed to use wires with single-wire conductors with a cross-section of at least 1.5 square meters. mm, provided that the wiring harness 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 Chapter. 1.3, and for circuits coming from current transformers, in addition, thermal resistance. For installation, wires and cables with insulation that does not support combustion should be used.

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

3.4.13. Connections of devices to each other within the same panel should, as a rule, be made directly without connecting the connecting wires to intermediate terminals.

The terminals or test blocks must contain circuits in which testing and checking apparatus and instruments are required to be included. 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 terminals for trip circuits, voltage circuits, etc.);

required to include portable test and measuring apparatus if test blocks or similar devices are not available;

several cables become one cable or the circuits 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 terminals there should not be any clamps in close proximity to one another, the accidental connection of which could cause the connection to be turned on or off or a short circuit in the operational current circuits or in the excitation circuits.

When placing equipment related to different types of protection or other devices of the same connection on a panel (in a cabinet), the supply of power from the operational current poles through the terminal assemblies, as well as the wiring of these circuits throughout the panel, must be carried out independently for each type of protection or device. If linings are not provided in the trip circuits from individual protection sets, then the connection of these circuits to the output protection relay or circuit breaker trip circuits should be carried out through separate terminals of the terminal assembly; in this case, connections along the panel of these 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, providing (except for the cases specified in 3.4.7) without disconnecting wires and cables, disconnection from the source of operational current, voltage and current transformers with the possibility of preliminary short-circuiting current circuits; connection of testing devices for checking and adjusting devices.

Relay protection and automation devices that 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 operating personnel.

3.4.17. Clamp assemblies, auxiliary contacts of switches and disconnectors and devices must be installed, and grounding conductors mounted in such a way that accessibility and safety of servicing the assemblies and devices of secondary circuits is ensured without removing the voltage from primary circuits with voltages above 1 kV.

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 isolation transformer) feeding these circuits.

Insulation monitoring of operational direct and alternating current circuits should be provided at each independent source (including isolation transformers) that is not grounded.

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

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

Operating current supply to the relay protection and switch control circuits of each connection must be provided, as a rule, through separate circuit breakers or fuses not connected to other circuits (alarm, electromagnetic blocking, etc.). Shared power supply of control circuits and position signaling lamps of the controlled device is allowed.

For connections of 220 kV and above, as well as for generators (units) with a capacity of 60 MW and more, separate power supply with operating current (from different fuses, circuit breakers) must be provided for the main and backup protections.

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

3.4.20. Relay protection, automation and control devices for critical elements must have constant monitoring of the state of the operating current supply circuits. Monitoring can be carried out using separate relays or lamps or using devices provided to monitor the health of the circuit of subsequent operation of switching devices with remote control.

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

Monitoring the health of the subsequent operation circuit must be carried out in the presence of an auxiliary contact of the switching device. In this case, monitoring the serviceability of the shutdown circuit must be carried out in all cases, and monitoring the serviceability of the switching circuit must be carried out on switches of critical elements, short circuiters and on devices switched on under the influence of automatic transfer transfer devices (ATS) or telecontrol.

If the parameters of the drive enable circuits do not provide the ability to monitor the serviceability of this circuit, monitoring is not performed.

3.4.21. In electrical installations, as a rule, an automatic signal must be provided about a violation of the normal operating mode and about the occurrence of any malfunctions.

Checking the serviceability of this alarm system should include periodic testing.

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

3.4.22. Operating current circuits in which false operation of various devices is possible from overvoltage during the operation of switching 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 combining 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 no higher than 1 kV with a shunt resistance of 100 Ohms to drain the static charge.

The 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.

Grounding of the secondary windings of a 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 the grounded secondary circuits of several voltage transformers of one switchgear with a common grounding busbar. If the specified 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 working grounding of one of the poles of the operational current network is not provided, 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 ungrounded conductors after assembling the terminals, with the exception of the zero-sequence circuit (open delta) of voltage transformers in networks with high ground 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 (switches, detachable connectors, etc.).

The installation of devices that can create a break 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 ground fault currents without capacitive current compensation (for example, on the generator voltage of a generator-transformer unit, on the auxiliary voltage of power plants and substations), if necessary, protection against overvoltage should be provided in the event of spontaneous neutral displacements. Protection can be achieved 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 must be provided.

It is allowed to perform mutual redundancy between linear voltage transformers if they have sufficient power for the secondary load.

3.4.28. Voltage transformers must have monitoring of the health of voltage circuits.

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

Regardless of the presence or absence of the protection circuits of the specified devices, the following signals must be provided:

when disconnecting circuit breakers - using their auxiliary contacts;

in case of malfunctions of relay repeaters of bus disconnectors - with the help of monitoring devices for open control circuits and relay repeaters;

for voltage transformers, in the circuit of the high 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 disruption of contact connections of wires, false operation of relays, as well as against premature wear of devices and devices.

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

How are they built

Installation of secondary circuits is carried out taking into account a number of rules. Thus, each device can be connected to 1 or several current sources. This is determined taking into account the power consumption, the required accuracy, and length.

If we are talking about a multi-winding transformer, the secondary circuit is an independent source of current. All secondary devices that are connected to the CT of one phase are connected to the secondary winding in a certain order. The devices and connecting circuits must form a closed system. You cannot open the secondary circuit of a current transformer if there is current in the primary. Therefore, circuit breakers and fuses are never installed in it.

Preparation and laying of wires.

All secondary devices (panels and remote control panels, protection, alarms and automation, cabinets, assemblies) are supplied by the manufacturers fully assembled, including installation of secondary circuits with devices and instruments that have undergone inspection, adjustment and testing.
Finished elements of electrical wiring of secondary circuits within one device (panel, cabinet, etc.) end with sets of clamps intended for connecting cores of connecting wires and control cables to them. Wiring of secondary circuits is an important part of the electrical installation, therefore, during installation, they are subject to high demands on the quality of work, as well as on the reliability of all contact connections.

Secondary circuits within switchboard panels, relay cabinets, switchgear chambers are made with insulated wires with aluminum or copper conductors (permitted by the PUE in some cases).

According to the conditions of mechanical strength, aluminum conductors of cables and wires connected to the terminals of devices and devices must have a cross-section of at least 2.5 mm2, copper - at least 1.5 mm2. For non-critical secondary circuits in electrical installations with voltages up to 1000 V, control and signaling circuits of electrical installations of industrial enterprises, it is allowed to connect copper conductors with a cross-section of 1 mm2 to the terminals of devices and devices.

In circuits with voltages up to 60 V, the diameter of the copper cores of the cables connected by soldering must be at least 0.5 mm. External connections of the secondary circuits of panels, cabinets, chambers with each other and with blocking, measuring and signaling devices of electrical equipment are carried out with control cables. Less commonly, these connections are made with insulated wires, protected from mechanical damage by steel or other pipes, boxes, trays, etc.

Recently, for the installation of electrical wiring of secondary circuits on switchboards and consoles, new aluminum-copper AMPV wires with a cross-section of 1.5-10 mm2 with polyvinyl chloride insulation, an aluminum core with a copper sheath, have been produced. It is allowed to use them temporarily to test wires under operating conditions.

Installation of secondary circuits, as well as other circuits and devices, begins with a review of design drawings and diagrams and their compliance with the requirements of industrial installation. Project documentation contains: explanatory note; diagrams of electrical connections and connections; schematic electrical diagrams of external and internal connections of electrical devices; general view drawings of boards, panels, cells; working drawings of cable distribution and cable magazine; lists of inscriptions and lists of elements indicating positional and letter designations of names, types, technical data, numbers of cabinets, panels, consoles; custom specifications; lists of MEZ products and drawings of non-standard components and structures.

When preparing for installation in the MEZ, they assemble assemblies and packages of wires, manufacture and complete supporting and fastening structures, products and parts for laying wires and cables of secondary circuits. In the process of installing secondary circuits, different methods of laying packages and wire flows are used: with rigid fastening to the panel, freely hanging packages without attachment to the base, on strings, in boxes, on trays, perforated profiles, tracks and “directly”.

Rice. 1. Preparing a stream of wires using a universal template (a) and bending them onto a plane using a wooden plate (b) or an aluminum bracket (c)

Packages and streams of wires are prepared and assembled in the MEZ according to measurement sketches using templates. In Fig. 1, a-c show the preparation of a stream of wires on a wooden plate using universal templates, their packaging and bending. Using such templates and rearranging the studs, you can prepare threads and packages of wires according to various schemes. To manufacture several identical flows or jumpers according to the same scheme, simple templates are used, made of electrical cardboard, plywood or other sheet material and representing a mock-up of part or the entire mounted panel.

When forming wire flows, comply with the requirements of the instructions: maintain a bending radius for flexible single- and multi-wire wires of at least five diameters, avoid crossing wires during branches, and if necessary, cross them at the exit from the main flow or directly at the device; perform turns equally and at right angles; They bandage wires in streams in straight sections with a pitch of 150-200 mm, as well as in all places where wires exit.

Let's look at the most commonly used methods of laying wires. The laying of wires in freely hanging packages (Fig. 2, a) is carried out without fastening to the panel; the packages are suspended on the connecting terminals of devices and devices. This method is used for this panel with a short length of wires and a vertical arrangement of rows of stacked clamps, which greatly simplifies installation and reduces the complexity of installation. However, connecting the control cable cores in this case becomes somewhat more complicated. Wires of freely hanging packages are laid at a distance of at least 10 mm from the surface.

Rice. 2. Laying wires of secondary circuits: a - freely hanging packages, b - on a string, c - “directly”; 1 — package of wires, 2 — insulating gasket, 3 — buckle strip, 4 — device output, 5 — panel, 6 — device

When mounting on a string (Fig. 2, b), the wires of the secondary circuits are pre-connected into one common bundle. A string is placed in the middle of the bundle, which is a straightened steel wire with a diameter of 5 mm with a polyvinyl chloride tube put on it. The wire has a thread at one end. A bandage of perforated polyvinyl chloride tape, secured with a polystyrene button or buckle strip, is applied to the bundle of wires every 175–200 mm (for horizontal laying blanks) and every 250–300 mm (for vertical laying blanks). The bundles of wire prepared in this way are transferred to the panel and attached to brackets pre-welded to the panel. Tensioning the wire into a string and aligning the bundle is done by screwing a nut on one end of the wire, while its other end is bent on the bracket. The wire cores are connected in the usual way. The string can also be made of a 2X20 mm steel strip with PVC tape wound along its entire length.

Laying wires with rigid fastening to the panel is rarely used. To fasten the wire flows, thin tin strips-buckles are used, welded to the steel sheets of the panels by spot electric welding. When marking on the panels, strips are welded at two points, the length of which should be slightly greater than twice the width of the flow of wires being fixed.

When laying wires on the panels, strips of electrical cardboard or varnished cloth are glued along the flow path. The wires are additionally insulated from the metal strips with varnished cloth or electrical cardboard. Having laid the thread, the strips are bent towards its center, the ends are threaded into the holes of the buckles, pulled together with pliers and slightly bent in different directions. Excess strips are cut off with scissors, the remaining ends are bent as far as possible with a wooden mandrel, hitting it with a light hammer.

The distance between the points of attachment of the wire flow to the panel in straight sections is taken to be 175-200 mm horizontally and 250-300 mm vertically. Wires are connected only in type-set terminals or at the terminals of devices and devices (connecting wires between terminals is not allowed). Within the same panel, the devices are connected to each other without connecting the connecting wires to the terminals. It is not allowed to connect copper and aluminum wires in one clamp with one screw. Connections between the terminals of the devices are made with permanent jumpers that sequentially go around the screws of the connected terminals using limiting star washers.

Laying wires “directly” (Fig. 2, c) reduces the complexity of installation and is used for panels on which devices made with front connection are installed. With this method of laying the wires, they do not intersect at the terminals and are easily replaced in case of damage or changes in the circuits; the panels have a beautiful appearance, since the wiring nodes are located at the back and are additionally covered with a decorative cover. The maintenance conditions for the panels are simplified, since the devices and type-setting clamps are mounted on the front side of the panel. In the panels, at a distance of 40 mm from each clamp, holes with a diameter of 10 mm are drilled into which insulating sleeves are inserted. The wires are laid “directly” along the back side of the panel, pulling from hole to hole. The ends of the wires are pulled through the insulating sleeves to the front side of the panel, where their strands are connected to the clamps. At the intersection points, the wires are tied together with a bandage made of insulating tape.

This method requires the installation of all devices and devices intended for front connection on the front side of the panel, which is not always possible. Therefore, it has not yet received widespread use.

If there are a larger number of wires in the flow, boxes and perforated trays are used for their installation, which are connected to each other with bolts or welding into a continuous electrical circuit and attached to the panels with brackets on screws (4-5 mm in diameter) or welding. Boxes and trays must have anti-corrosion painting or coating. Wires in boxes are laid without fastening and additional insulation with a box fill factor of 0.7. Wires in trays in horizontal sections are not secured, but in vertical sections they are secured after 1 m.

For laying secondary circuit wires, perforated bases on profiles and tracks are also used. The tracks are a metal strip 150-200 mm wide and 0.5-1 mm thick with perforations along the length. The wires are secured in one row across the entire width of the profile (track).

Protection

To protect personnel when faults occur in the secondary circuit, for example when the insulation between the primary and secondary structure is cut off, protective earthing connections are installed. This is done at the points closest to the TT, on the clamps. Isolation of the secondary circuit is also important in the case when several CTs are connected to each other, and it is fixed at one point. Grounding is provided by a fuse-discharger whose voltage rating does not exceed 1000 V.

Be sure to take into account the characteristics of the primary system, in particular the ability to power both lines of 2 bus systems. For this reason, the secondary currents from the CT, which is supplied to the relays and primary connection devices, are added. But this does not take into account the differential protection of busbars and breaker failure.

If the connections are not currently functioning and are subject to repair, then the working cover is removed from the test block. This leads to the fact that the secondary circuits of the current transformers are closed and grounded. At the same time, the circuits that went to the protective relays are subject to rupture.

Secondary circuit devices. Relay protection and system automation elements

Automatic devices, in particular relay protection, are needed where a quick response to a change in operating mode and an immediate command to turn off or turn on the corresponding circuits are required. So, for example, during a short circuit, when the current in a number of circuits increases sharply, it is necessary to immediately turn off the damaged section of the system in order to reduce the size of the destruction and not interfere with the operation of adjacent undamaged circuits. Such a command can only be given by an automatic device that responds to changes in current, direction of power and other factors and closes the control circuits of the corresponding switches.

Automatic shutdown of system elements must be selective. This means that in the event of damage at any cost, only the damaged circuit must be disconnected by the switches closest to the location of the damage. The rest of the system must not be disrupted. So, for example, when there is a short circuit at point K1 (Fig. 2), the current flows through the circuits of generators, step-up transformers, damaged and undamaged lines. However, only the damaged line on both sides must be disconnected. The station will remain connected to the system via another line.

In the event of damage to a generator or transformer, only the damaged element must be disconnected. In Fig. 2, areas of the system that must be disconnected in the event of damage are delimited by dotted lines. Each section is switched off by one or two switches. In the event of damage to the circuit breaker, two adjacent sections must be disconnected.

Fig.2. Electrical diagram of the station and network section. Dotted lines delimit sections of the station and network that are subject to disconnection in case of damage.

The selectivity of relay protection is ensured in various ways, for example, by appropriate selection of the time or current of operation of the protection of adjacent sections of the network, the use of relays that respond to the direction of power, etc.

The circuit shutdown time during a short circuit is composed of the relay protection response time and the circuit breaker shutdown time, calculated from the moment the shutdown command is given until the arc goes out at the breaker breaks.

They strive to reduce the shutdown time of the main lines of the system as much as possible so as not to disrupt the stability of the parallel operation of power plants. The shutdown time of the newest switches is two periods and the relay protection time is another 0.5 periods. The total shutdown time is therefore 2.5 periods. For distribution networks, 2.5-cycle tripping is not required. Here simpler protections and slower-acting switches are used, the cost of which is much lower. The total shutdown time is several tenths of a second or more.

Automatic restart

Automatic devices for re-closing (reclosure) of overhead lines after their protection has been disconnected are intended to quickly restore the operation of the line after a disconnection. The effectiveness of re-closing overhead lines is based on the fact that most short circuits are associated with lightning discharges and lead to overlapping of insulators along the surface. After the line is automatically turned off, the electrical strength of the air gap is quickly restored and when turned on again, the line remains in operation.

Initially, the command to restart was given manually by the person on duty at the control panel. Later, the switching operation began to be automated. Currently, automatic re-closing, single and double, is widely used. It helps to increase the reliability of power supply, especially when feeding consumers via single lines.

The total automatic re-closing time is calculated from the submission of the relay protection command to open the circuit breaker until the re-closure of its contacts. It should be as small as possible so as not to disrupt the work of consumers, but at the same time sufficient to deionize the arc gap at the point of overlap. The restart time depends on the mains voltage and the speed of the switch. In double-reclosing devices, the minimum time is selected for the first switching on from the condition of deionization of the arc gap. If the first switch-on is unsuccessful and the line is disconnected again, a second switch-on occurs with an interval of several seconds.

Automatic reserve input

Automatic devices for turning on a backup circuit (ATS) must automatically turn on a backup transformer or a backup unit to replace the one disabled by the protection, and also automatically connect the busbar section (with the corresponding load) that has lost power to an adjacent section provided with power, in order to quickly restore power supply. The break in the power supply should be relatively small, no more than 0.5 s, so that electric motors that have lost power do not have time to stop, and after power is restored they can quickly return to normal operation.

About voltage circuits

Voltage circuits that come from voltage transformers are used to power:

Measuring devices that indicate and record data - voltmeters, frequency meters, wattmeters.
Energy meters, oscilloscopes, telemetering devices.
Relay protective systems – remote, directional and others.
Automated devices, emergency automation, power flows, blocking devices.
Organs that control the presence of tension.

They are also used to power rectifiers, which act as sources of direct operating current.

About grounding

Grounding for protection is always inserted into the secondary circuit. This is done by combining the corresponding device with one of the phase wires or the zero point of the secondary system. Grounding is done at a point that is as close as possible to the VT terminal assemblies or next to its terminals.

In the wires on the grounded phase on the secondary circuit, work on installing automatic switches between it and the grounding point of the switch is not carried out. The terminals of the voltage transformer windings that were grounded are not connected. The cores of the control cables are laid to their destination - for example, to bus bars. The terminals that have been grounded on different voltage transformers are also not connected.

During use, the voltage transformer may be damaged, the secondary circuits of which are protected and connected to automation devices, measurements, and so on. To avoid damage, backups are made.

If there is a circuit that includes a double busbar system, the transformers back up each other mutually when one of the transformers is taken out of service. If the circuit has 2 busbar systems, in the process of switching the connection from one system to the second, the voltage circuits are automatically switched.

Always exclude the possibility that the grounded circuits of both transformers will connect. This is extremely important. Practice proves that if this happens, the operation of the protective relay system and automatic devices will be seriously disrupted.

It is always necessary to ensure that the detachable contacts are in good condition, as well as the secondary voltage and operating current circuits that extend from them.

PUE: Chapter 3.4 Secondary circuits

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

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

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

3) differential protection of busbars and failure redundancy devices for 110-220 kV circuit breakers, as well as system emergency control equipment;

4) technological protection of thermal power plants;

5) with an operating voltage not higher than 60 V with a diameter of cable cores 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, control cables with aluminum-copper or semi-solid aluminum conductors should be used for secondary circuits. 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 in blast furnace and converter shops, the main line of crimping and continuous high-performance rolling mills, electrical receivers of special group I category, as well as in secondary circuits with an operating voltage not higher than 60 V with a diameter of cable cores and wires up to 1 mm (see also 3.4.4).

3.4.4. According to the condition of mechanical strength:

1) the cores of control cables for screw connection to the terminals of panels and devices 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, screw connection of cables with copper conductors with a cross-section of 1 mm2 is allowed;

2) in circuits with an operating voltage of 100 V and above, the cross-section of the copper conductors of the cables connected by soldering must be at least 0.5 mm2;

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

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

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

to metering meters and power measuring converters used to enter information into computing devices - no more than 0.5%; to settlement meters of intersystem power lines - no more than 0.25%; to technical metering meters - no more than 1.5%; to panel devices and power sensors used for all types of measurements - no more than 1.5%; to protection and automation panels - no more than 3% (see also Chapter 3.2.).

When the specified loads are powered together via common conductors, their cross-section must be selected according to the minimum permissible voltage loss standards.

3. For operational current circuits, the voltage loss 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; to control electromagnets that have a threefold or greater boost - no more than 25% at the boost current value.

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

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

3.4.7. Cables should generally be connected to clamp assemblies. Connecting two copper wires of a cable under one screw is not recommended, and two aluminum wires are not allowed.

Cables may be connected directly to the terminals of instrument transformers or individual devices.

The design of the clamps must match the material and cross-section of the cable cores.

Cables with a non-metallic sheath or with aluminum conductors should be connected on intermediate rows of clamps or using special couplings 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 installation and protection should be selected taking into account the requirements of Chapter. 2.1-2.3 and 3.1 to the extent that they are not changed by this chapter. When laying wires and cables over hot surfaces or in places where the insulation may be exposed to oils and other aggressive environments, special wires and cables should be used (see Chapter 2.1).

Wires and cable cores that have non-light-resistant insulation must be protected from exposure to light.

for single-wire conductors connected with screw terminals, 1.5 mm2;
for single-wire conductors connected by soldering, 0.5 mm2;
for stranded conductors connected by soldering or screwing using special tips, 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 conductors connected by soldering in circuits with a voltage not exceeding 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 (by screw or soldering) is allowed only to fixed elements of the equipment. The connection of cores to movable or removable elements of equipment (detachable connectors, removable blocks, etc.) should be done with flexible (stranded) cores.

Mechanical loads on the places where wires are soldered are not allowed.

For transitions to device doors, stranded wires with a cross-section of at least 0.5 mm2 must 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 wiring harness operates only in torsion.

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

3.4.13. Connections of devices to each other within the same panel should, as a rule, be made directly without connecting the connecting wires to intermediate terminals.

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 terminals for trip circuits, voltage circuits, etc.);
required to include portable test and measuring apparatus if test blocks or similar devices are not available;
several cables become one cable or the circuits 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.

Relay protection and automation devices that 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 operating personnel.