Operating current systems at electrical substations

September 20, 2012 at 10:00

The combination of power supplies, cable lines, power buses of switching devices and other elements of operational circuits constitutes the operational current system of a given electrical installation. Operating current at substations serves to power secondary devices, which include operational protection circuits, automation and telemechanics, remote control equipment, emergency and warning alarms. In case of disturbances in the normal operation of the substation, the operational current is also used for emergency lighting and power supply to electric motors (especially critical mechanisms).

Classification of operational current systems at electrical substations

The following operational current systems are used at substations:

1. direct operating current - an operating circuit power supply system in which a rechargeable battery is used as a power source; ;
2. alternating operational current - a power supply system for operational circuits, in which measuring current transformers of protected connections, measuring voltage transformers, and auxiliary transformers are used as the main power sources. Pre-charged capacitors are used as additional pulsed power sources;
3. rectified operating current - a system for powering operational circuits with alternating current, in which alternating current is converted into direct (rectified) using power supplies and rectifying power devices. Pre-charged capacitors can be used as additional pulsed power sources;
4. mixed operating current system - an operating circuit power supply system in which different operating current systems are used (direct and rectified, alternating and rectified).

In operational current systems there are:

• dependent power supply, when the operation of the operational circuits power supply system depends on the operating mode of a given electrical installation (substation);
• independent power supply, when the operation of the operational circuits power supply system does not depend on the operating mode of the electrical installation.

Principle of operation

Charger – generates rectifying voltage, charges batteries and powers consumers, coping with the following types of loads:

• Constant – equipment power supply
• Temporary – in emergency situations
• Short-term – when starting electrical appliances

Batteries - from them current flows to the substation in cases where for one reason or another (primarily due to an accident) there is no current there. The battery operating time of devices depends on the number and capacity of the connected batteries.

SHUOT and AUOT - operational current distribution cabinets identify damaged elements and turn them off using selective circuit breakers. It is important to use switches from only one manufacturer.

Application areas of various auxiliary current systems

Direct operating current is used at 110-220 kV substations with busbars of these voltages, at 35-220 kV substations without busbars at these voltages with oil switches with an electromagnetic drive, for which the possibility of switching on from rectifier devices has not been confirmed by the manufacturer.

Alternating operating current is used at 35/6(10) kV substations with 35 kV oil circuit breakers, at 35-220/6(10) and 110-220/35/6(10) kV substations without circuit breakers on the high voltage side, when the circuit breakers 6(10)-35 kV are equipped with spring drives.

Rectified operational current should be used: at 35/6(10) kV substations with 35 kV oil circuit breakers, at 35-220/6(10) kV and 110-220/35/6(10) kV substations without circuit breakers on the high voltage side when the switches are equipped with electromagnetic drives; at 110 kV substations with a small number of oil circuit breakers on the 110 kV side.

A mixed system of direct and rectified operating current is used to reduce the battery capacity through the use of power rectifier devices to power the circuits of the electromagnets for turning on the oil switches. The feasibility of using this system must be confirmed by technical and economic calculations.

A mixed system of alternating and rectified operating current is used: for substations with alternating operating current when installing switches with an electromagnetic drive at the power inputs, to power the switching electromagnets of which power rectifier devices are installed; for substations 35-220 kV without circuit breakers on the high voltage side, when reliable operation of protection from power supplies in case of three-phase short circuits on the medium or high voltage side is not ensured.

In this case, the protection of transformers is carried out on alternating current using pre-charged capacitors, and the remaining elements of the substation - on rectified operating current.

Types of SOPT by structure

The system can have 2 types of structures:

• Centralized – 1 set of DC sources to power a separate consumer group
• Decentralized – 2 or more sets

One of the most popular SOPT configurations today is the operating current control device. It includes 3 cabinets:

• operational current and control
• rechargeable batteries (SHAB)
• distribution of direct operating current (ShR or ShVR)

From a design point of view, SHUOT and AUOT are complete low-voltage cabinet-type devices. Cabinets can have a high degree of protection (to ensure the safety of the equipment installed inside). Measuring instruments and lighting fixtures are installed on the façade to monitor the condition of switching devices and alarm signals. Inside the cabinets are divided into compartments:

• Busbar and distribution bus
• Functional equipment
• Cable connections

Protection against short circuits and overloads in switchboards is provided by switching devices. They create a 3-level protective system:

1. Protection of power input circuits
2. Protection of power distribution circuits by groups of electrical receivers
3. Protection of power supply circuits of direct consumers

DC consumers

All energy consumers powered by a battery can be divided into three groups:

1. Constantly switched on load – devices of control, blocking, signaling and relay protection devices, constantly flowing current, as well as a permanently switched part of emergency lighting. The constant load on the battery depends on the power of the always-on alarm and emergency lighting lamps, as well as on the types of relays. Since the constant loads are small and do not affect the choice of battery, in the calculations we can approximately take for large substations 110-500 kV the value of a constantly switched load of 25 A.
2. Temporary load – appearing when the alternating current disappears during emergency mode – load currents of emergency lighting and DC motors. The duration of this load is determined by the duration of the accident (estimated duration 0.5 hours).
3. A short-term load (lasting no more than 5 s) is created by switching on and off currents of drives of switches and automatic machines, starting currents of electric motors and load currents of control, interlocking, signaling and relay protection devices, briefly flowing current.

The main advantages of using SOPT at facilities

• Ease of operation, inspection and maintenance of secondary switching circuits (they are placed in a separate compartment) and primary circuit devices
• Ease of adaptation to the specific needs of a particular customer
• Possibility of using circuits with stationary and withdrawable circuit breakers of selective and non-selective types, as well as using protection, alarm and control equipment based on modern relay equipment and microprocessor devices
• Easy installation - minimum installation work, since COPT can already be supplied in the form of separate cabinets or fully assembled

You can verify these and other advantages of COPT for yourself by purchasing operational direct current systems produced by RUSELT. We offer you a variety of design solutions and guarantee reliability and high functionality of the equipment.

AC system

With alternating operating current, the simplest way to power the tripping electromagnets of circuit breakers is to directly connect them to the secondary circuits of current transformers (circuits with direct-acting relays or with deshunting of the tripping electromagnets when the protection is triggered). At the same time, the limit values ​​of currents and voltages in the current protection circuits must not exceed permissible values, and the current tripping electromagnets (relays of RTM, RTV or TEO types) must provide the necessary sensitivity of protection in accordance with the requirements of the Electrical Electrical Regulations. If these relays do not provide the required protection sensitivity, the trip circuits are powered from pre-charged capacitors.

At substations with alternating operating current, the power supply for automation, control and signaling circuits is provided from auxiliary buses through voltage stabilizers.

Sources of alternating operating current are auxiliary transformers and measuring current and voltage transformers that supply power to secondary devices directly or through intermediate links - power supplies, capacitor devices. AC operating current is centrally distributed and therefore does not require a complex and expensive distribution network. However, the dependence of the power supply of secondary equipment on the presence of voltage in the main network, the insufficient power of the sources themselves (current and voltage measuring transformers) limits the scope of application of operational alternating current.

Current transformers serve as reliable sources for powering short-circuit protection panels; Voltage transformers and auxiliary transformers can serve as sources of protection against damage and abnormal conditions that are not accompanied by deep voltage drops, when high voltage stability is not required and power interruptions are acceptable.

Voltage stabilizers are designed for:

1. maintaining the required voltage of operational circuits during AFC operation, when a simultaneous reduction in frequency and voltage is possible;
2. separation of operational circuits and other circuits of the substation’s own needs (lighting, ventilation, welding, etc.), which significantly increases the reliability of operational circuits.

What does SOPT consist of?

First, it’s worth saying a little about terminology. This abbreviation can be deciphered as a set of converting, storage and distribution devices of electricity necessary to supply direct operating current to all secondary switching devices connected to it. Power supply is provided:

Thus, taking into account the request that comes from the operating party, all components included in the COPT may have different designs and parameters. However, in most cases, the DC operating system includes the following elements:

Rectified operating current system

To rectify alternating current:

Stabilized power supplies of type BPNS-2 together with current ones of type BPT-1002 - for powering protection, automation, and control circuits.

Unstabilized power supplies of the BPN-1002 type are used to power signaling and interlocking circuits, which reduces the branching of operational current circuits and makes it possible to supply the full power of stabilized units for tripping protection and tripping circuit breakers.

BPN-1002 blocks instead of BPNS-2 - for powering protection, automation, and control circuits, when the possibility of their use is confirmed by calculations and stabilization of the operating voltage is not required (for example, in the absence of AFC).

Power rectifier devices PM on UKP and UKPK with inductive storage - for powering the switching electromagnets of oil switch drives. The inductive storage ensures that the switch is turned on in the event of a short circuit with dependent power supply to the switching circuits.

Unstabilized power supplies BPZ-401 are used to charge capacitors, which are used to turn off separators, turn on short circuiters, turn off 10(6) kV circuit breakers with minimum voltage protection, as well as turn off 35-110 kV circuit breakers when the power supply is insufficient.

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Purpose

Direct operating current is used at distribution points (DP) 6(10) kV, as well as at all substations 35 kV and above at newly installed facilities [1].

Alternating operating current is used at distribution points (DP) 6(10) kV, at existing substations 35/6(10) kV, and substation 35-220 kV without circuit breakers on the high voltage side.

Rectified operational current is used at existing 35/6(10) kV substations, and 35-220 kV substation without circuit breakers on the high voltage side.

Thus, the most promising at the moment is the operational direct current system, despite the fact that its use requires the installation of rechargeable batteries (AB), which increases the cost of the structure and necessitates the organization of a direct current network.

Composition of SOPT

In general, an operational direct current system consists of the following components:

Charger (charger).

DC switchgear (DCB) is a complete low-voltage cabinet-style switchgear designed for connecting power sources (AB and charger) and distributing DC electricity among groups of electrical receivers.

An operational direct current system can have a centralized or decentralized structure. In a decentralized COPT, two or more galvanically isolated sets of direct current sources are used to provide power to individual groups of electrical receivers; in a centralized one, one is used.