High voltage disconnectors - classification, rules of use and technique of performing operations

Disconnectors are switching devices with a visible trip point that do not have a free release mechanism. They are designed to switch on and off live sections of an electric circuit (high voltage) in the absence of load current or to change the connection scheme.

Purpose of disconnectors

Disconnectors serve to create a visible gap separating non-operating equipment from live parts. This is necessary, for example, when displaying equipment for repair in order to carry out work safely.

Disconnectors do not have arcing devices and therefore are mainly designed to switch circuits on and off in the absence of load current and are energized or even switched off.

Read more about the different disconnector designs here: How high voltage disconnectors work and are arranged

In the absence of a switch in the electrical circuit in 6-10 kV electrical installations, switching on and off by disconnectors of small currents, much less than the rated currents of the device, is allowed, as discussed below.

Requirements for disconnectors

The requirements for disconnectors from the point of view of their maintenance by service personnel are as follows:

• disconnectors must create a clearly visible open circuit corresponding to the voltage class of the installation;

• disconnector drives shall have devices for rigidly fixing blades in each of the two operating positions: on and off. In addition, they must have reliable stops, limiting the rotation of the knives to an angle greater than a given one;

• disconnectors must be switched on and off under any worst environmental conditions (eg icing);

• supporting insulators and insulating rods must withstand mechanical loads resulting from operations;

• the main blades of the disconnectors must be connected to the blades of the earthing device, which excludes the possibility of switching on both at the same time.

Classification and arrangement of disconnectors

The individual types of disconnectors 6 — 10 kV differ from each other:

• by type of installation (disconnectors for internal and external installation);
• by the number of poles (single-pole and three-pole disconnectors);
• by the nature of the movement of the blade (disconnectors of the vertical-rotating and swinging type).
• three-pole disconnectors are operated by a lever drive, single-pole disconnectors - by an operating insulating rod.

The difference in the design of disconnectors for internal and external installations is explained by the conditions of their operation. External disconnectors must have devices that break the ice crust formed during ice. In addition, they are used to switch off small load currents and their contacts are equipped with horns to extinguish the arc that occurs between diverging contacts.

Use of disconnectors to disconnect equalizing currents and small load currents

The ability of disconnectors to turn on and off the charging currents of cable and overhead lines, magnetizing currents of power transformers, equalizing currents (this is the current passing between two points of an electrically connected closed network and due to the difference in voltage and redistribution of the load during disconnection or turning on the electrical connection) and small load currents confirmed by numerous tests carried out in power systems. This is reflected in a number of directives regulating their use.

So, in closed switchgear 6-10 kV disconnectors allow switching on and off magnetizing currents of power transformers, charging currents of lines, as well as earth fault currents that do not exceed the following values:

• At voltage 6 kV: magnetizing current — 3.5 A. Charging current — 2.5 A. Earth fault current — 4.0 A.

• At a voltage of 10 kV: magnetizing current — 3.0 A. Charging current — 2.0 A. Earth fault current — 3.0 A.

The installation of insulation barriers between the poles allows to increase the current on and off by 1.5 times.

6 — 10 kV disconnectors allow switching on and off equalizing currents up to 70 A, as well as line load currents up to 15 A, provided that the operations are carried out with three-pole disconnectors for outdoor installation with mechanical drive.

Disconnectors are often equipped with stationary grounding devices, which makes it possible not to resort to the installation of portable grounding on equipment taken out for repair, and thus eliminates violations of safety rules associated with the process of installing portable grounding.

Switches for disconnectors

The variety of electrical installations results in an unlimited combination of switchgear sizes and configurations. Using foreign experience in substations, it is recommended to replace disconnectors and switches with new generation equipment — switch disconnectors.

The switch-disconnector combines disconnection and disconnection functions in one device, which makes it possible to reduce the area of ​​the substation and increase availability.

The use of switch-disconnectors reduces maintenance work and offers the following advantages:

• Almost continuous power supply to users (depending on the development of the substation or the network, maintenance may interrupt the power supply to some users).
• Reducing the risk of system failures, as the risk of failures in primary circuits during maintenance (i.e. when people are at the substation) is higher than in normal operation, because during maintenance not all equipment is in operation and there is no possibility of redundancy.
• Reduced operating costs associated with low switchgear maintenance occupancy.
• Improving personnel safety and reducing the risk of accidents, substation power outages, work errors, as all work in the substation involves a potential risk of electric shock, falling from a height, etc. Expedited disassembly of the contact device allows quick disconnection of the switch-disconnector. Thus, while the tripped switch-disconnector is being operated, other substation equipment can be energized.

Technique for carrying out operations with disconnectors

In switchgear, the operations of opening and closing disconnectors of a connection that has a switch in its circuit must be carried out after checking the off position of the switch at the place of its installation.

Before disconnecting or connecting disconnectors, it is necessary to check them from the outside. Disconnectors, actuators and blocking devices must not be damaged, which would prevent operation. Particular attention should be paid to the absence of bypass jumpers. If any defects are found, operations with live disconnectors must be carried out very carefully and only with the permission of the person who ordered the switching. Work with disconnectors under voltage is prohibited if cracks are found on the insulators.

Switching the disconnectors by hand should be quick and decisive, but without a shock at the end of the stroke.When an arc occurs between the contacts, the blades of the disconnectors should not be pulled back, because if the contacts diverge, the arc may extend, close the gap between the phases and cause a short circuit. The operation of inclusion must in all cases be carried out completely. If the contacts touch, the arc will extinguish without causing damage to the equipment.

Disconnecting the disconnectors, on the other hand, is done slowly and carefully. First, a trial run is made with the drive lever to ensure that the rods are in good working order, there is no vibration and damage to the insulators. If an arc occurs at the moment the contacts diverge, the disconnectors must be turned on immediately and do not work with them until the cause of the arc formation is clarified.

Work on single-pole disconnectors carried out using operating rods must be carried out in the order that provides the greatest safety for personnel. Let's assume that personnel mistakenly opened the disconnectors under load.

With a mixed load, it is safest to turn off the first of the three disconnectors, as it does not generate a strong arc even if the rated current is flowing through the circuit. At the moment of divergence of the contacts between them, only relatively small potential difference, because on the one hand the disconnector to be tripped will be powered by the power source, and on the other hand, approximately the same emf will operate for some time, induced by synchronous and asynchronous load motors rotating when supply in two phases, as well as due to capacitor banks installed in the distribution networks.

When the second disconnector is tripped, heavy arcing will occur on load. The third disconnect will not cut the power at all. Since tripping of the second series disconnector is the greatest danger, it should be located as far as possible from the disconnectors of the other phases. Therefore, for any arrangement of disconnectors (horizontally or vertically), the intermediate phase disconnector must always be switched off first, then when the disconnectors are arranged in a horizontal row, the end disconnectors are alternated in sequence and with the vertical arrangement of disconnectors (one above another), the upper disconnector is tripped second and the lower one is the third. …

The closing operations of single-pole disconnectors are carried out in reverse order.

In circuits containing spring-operated circuit breakers, the disconnector operations must be performed with the springs loose to avoid accidental closing of the circuit breakers during disconnector operations.

In 6-10 kV networks operating with earth fault capacitive current compensation, before turning off the magnetizing current of the transformer, in the neutral part of which the arc suppression reactor is connected, the arc suppression reactor must be turned off first of all, to avoid overvoltages that may be caused by the simultaneous opening of the contacts of the three phases.

Personal safety of personnel performing disconnector operations When performing any operation on live disconnectors, the person performing the operation (and controlling his actions - in the case of two-person switching) must first select such a location on device devicein order to avoid injury from the possible destruction and falling of the insulators of the apparatus together with the conductive elements fixed to them, and also to protect yourself from the direct effect of an electric arc when it occurs.

It is not recommended to look at the contact parts of the device during operation. However, after the completion of the operation of switching on or off, checking the position of the main blades of the disconnectors and the blades of fixed earthing switches is mandatory, because in practice there have been cases of non-disengagement of main blades, tripping of blades of fixed earthing switches on individual phases, knives falling past contact jaws, pulling rods from drives, etc. In this case, each phase of the disconnectors must be checked separately, regardless of the actual position of the vanes of other phases and the presence of mechanical connections between them.