The current characteristics of high-voltage electrical networks are not comparable to those of their domestic counterparts. Consequently, in an emergency situation, more powerful devices than standard automatic devices are needed to disconnect equipment and extinguish arcing.
Sensitive gas-insulated circuit breakers (ESBs) are used as protective structures, which can be controlled both manually and by means of automation. We have described in detail the design features and principle of operation of the devices. We have given recommendations for installation, wiring and maintenance.
- Definition and use of gas-insulated circuit-breakers
- The design of gas-insulated circuit-breakers
- Column and tank devices
- The arc quenching principle
- What is the drive for?
- Benefits and disadvantages of using EVs
- Rules for connecting and maintaining EVs
- Conclusions and useful video on the subject
Definition and application of elegas
Elegas is sulphur hexafluoride which is classified as electric gas. Because of its insulating properties it is used extensively in the manufacture of electrical devices.
In its neutral state gas is a colourless and odourless non-flammable gas. Compared to air, it has a high density (6.7) and a molecular weight five times higher than air.
One of the advantages of gas-air is its resistance to external manifestations. It does not change its characteristics under any conditions. If disintegration occurs during the electric discharge, a full-fledged, necessary recovery soon follows.
The secret is that the molecules of elegas bind electrons and form negative ions. The \"electronegativity\" quality has endowed 6-fluorine sulfur with a characteristic such as electrical strength.
In practice, the electrical strength of air is 2-3 times weaker than the same property of gas-air. It is also fire safe as it is non-flammable and has a cooling capacity.
When it became necessary to find a gas to extinguish an electric arc, the properties of SF6 (sulphur hexafluoride), carbon 4-chloride and freon were studied. The tests won the SF6
The characteristics listed above made SF6 particularly suitable for electric devices:
- magnetic induction power transformers;
- completed type switchgears;
- High voltage lines connecting remote installations;
- High voltage circuit breakers.
But some properties of gas-insulated gas have led to improvements in breaker design. The main disadvantage concerns the gaseous phase transition to the liquid phase, and this is possible at certain ratios of pressure and temperature parameters.
For the equipment to work without interruption, it is necessary to provide comfortable conditions. Let's assume that for gas-insulated equipment to function at -40º, the pressure must not exceed 0.4 MPa and the density must be less than 0.03 g/cm³. In practice, if necessary, the gas is heated, which prevents the transition to the liquid phase.
If we compare gas-insulated devices with their counterparts of other types, they are closest in design to oil-insulated devices. The difference is in the filling of the arc-quenching chambers.
The oil-type breakers use an oil mixture as the filler, while the gas-insulated ones use 6-fluoride sulfur. The advantage of the second version is durability and minimum maintenance.
Scheme of a column-type gas-insulated switch. The arc-quenching modules, mounted on a high post, are at the top, the control cabinet at the bottom
The type of arc-quenching depends on many factors, among which the rated current and voltage and the operating conditions of the device are decisive.
There are four types of EBs:
- with electromagnetic blowing;
- with blowing in gas-air - with 1 pressure stage;
- with longitudinal blowing - with 2 pressure stages;
- with autogenerating blowing.
While in air devices the gas enters the atmosphere during the arc quenching process, in gas-insulated devices it remains in an enclosed space filled with a gas mixture. A small overpressure is maintained.
Column and tank units
There are two types of gas-insulated installations in practice:
- tank units;
- column units.
Differences concern both the construction features and the arc quenching principle. Their external design is similar to that of their light-oil counterparts: They consist of two functional parts - an arc-quenching circuit and a contact part, and have the same overall dimensions.
Breakers are designed for 220 V operation and are single-phase devices. An example of a column-type gas-insulated circuit breaker is the LF 10 Schneider Electric.
The equipment can be controlled in two different ways: manually, when adjustment and control is done with mechanical devices, and remotely, automatically
Bull-type gas-insulated devices are smaller in size and have a multi-phase drive. This distribution allows better control and smooth adjustment of the voltage parameters.
One of the advantages of tank EVs is the ability to withstand increased loads. This quality is ensured by the current transformer incorporated into the design
Alstom Grid's DT2-550 F3 gas-insulated unit is an example of a tank unit. Such devices have proven positive in 500 kV power systems.
The design is assembled and equipped to function without failure in low temperatures (critical), high humidity, and in regions with seismic activity and excessive atmospheric contamination.
The arc quenching principle
How the device operates, let us take the Chinese CHINT LW36 breaker as an example.
The spring acts on the cylinder dynamic elements and they go down. All contacts, except the arc-quenching contacts, open. When the arc-quenching contacts, through which the current flows, are also disconnected, an electric arc occurs.
Hot gas moves into the heat chamber, the check valve is triggered. When the gas from the heat chamber is blown into the gap, the arc is extinguished.
If small currents are switched off, the pressure in the heat chamber is insufficient, so pressure from the compression chamber is attracted (it is always higher). The check valve opens, the gas flows unobstructed into the gap and extinguishes the arc when it passes through zero.
The internal layout and operation of moving, fixed valves, decompression, check valves. Position 1 - on; position 2 - high current shutoff; position 3 - low current shutoff; position 4 - instrument shutoff
Modern column units have improved features. Maintenance is reduced to a minimum and switching life is increased. Elegasic switches feature low noise levels, reliable mechanics, and ease of installation and testing.
Tank models are adjusted by means of an actuator and transformers. The spring or spring-hydraulic actuator controls the on/off processes, the level of arc hold.
What is the actuator for?
The actuator is designed to perform all operations related to switching on/off or holding the unit in a certain position. The diagram shows exactly where the actuator can be located. Usually it is a ground surface or a low support allowing the operator easy access to the control devices.
The tank switch design: 1 - porcelain or plastic modules; 2 - transformers; 3 - tank with gas extinguishing unit; 4 - gas chamber; 5 - hydraulic drive; 6 - metal frame; 7 - gas-insertion socket
The drive consists of a switching mechanism, latching device - latch, release mechanism. The switch-on process must be carried out as fast as possible in order to avoid welding of the contacts.
High forces are applied during the switch-on process to overcome the friction forces of all the elements involved. The disconnection is simpler and consists in reversing the movement of the latch, which ensures the switch-on and its retention.
There are several activation/deactivation methods:
For low power systems manual operation is used. In this case the power of one operator is sufficient. The manual mechanisms are usually switched off automatically. The spring actuator is also actuated manually, but sometimes low-power electric motors are used.
The traditional location of the actuator is near the mounting metal frame. The integrity and functioning of the mechanism is ensured by a solid metal casing - drawer with operator friendly door
The application of the electromagnetic actuator requires more energy, therefore a direct current source of about 58A with voltage 220V is needed. A manual lever is available as a back-up tripping mechanism. Electromagnetic devices are reliable, so they are successfully used in areas with harsh winters. The disadvantage is the need for a powerful battery.
Pneumatic actuator differs in that instead of an electromagnet, the main working element is a cylinder/piston pair. Thanks to compressed air, the switching speed is much higher than with previous models.
Benefits and disadvantages of using EV
Elegal switches, like other types of electric switchgear, have a number of advantages and disadvantages. When selecting an installation, make the necessary calculations and, in addition to the technical characteristics and design features, consider the pros and cons of the models.
Universal use in high voltage systems
Rugged and durable design
Maintains high voltage current
Silenium type switches operate in harsh environments with occasional vibration, low temperatures (heated), in fire-hazardous areas.
Disadvantages include the high cost of the gas-air filler, the specificity of installation on a panel or foundation, the need for a certain qualification of the operating staff.
Rules for connecting and maintaining EVs
All activities concerning installation, switching on/off, repair and maintenance of gas-insulated devices are subject to strict rules, which are regulated by PUE 1.8.21.
The installation must be checked for minimum pressure in the gas-filled chamber, otherwise the switch will malfunction. To prevent damage, an alarm is installed that is triggered when the pressure parameters are critically low. The pressure level can be monitored with a pressure gauge.
The drive cabinet is equipped with heating elements that effectively prevent condensation on the mechanism elements. The operator must ensure that the heaters are constantly on.
The unit is inspected every day during daylight hours and about 2 times a month during dark hours. If an emergency shutdown has occurred for any reason, an unplanned inspection is required
The external protection should be checked, dirt should be removed, damage should be repaired. If contacts get hot, find out the cause.
If there is a crackling, suspicious noise, the source must be identified. The metal mounting structure is also part of the earthing circuit, so its integrity must be checked.
The pressure gauge must be read. The pressure must be within the norm calculated by the manufacturer. It is necessary to check the functioning of the control and monitoring devices, and if one or more elements are out of order, take measures - replace or send for repair.
If the gas pressure has dropped, the chamber should be refilled with gas-insulating gas. You can find out how gas-insulated circuit breakers are built, how the arcing is quenched and what kind of units there are by this helpful and informative video.
Conclusions and helpful video
How gas-insulated circuit breakers are built, what principle they use to quench the arc and what kind of units they are, you can read in this helpful and informative video.
Video #1. Overview of gas-insulated circuit breakers with a description of their construction and operating principle:
Video #2. Installation features:
Video #3. How the switch is mounted:
Elegas switches leave the factory in full operational readiness and are designed to operate in a variety of climates, from tropical to cold, and are therefore actively used by industrial companies in various countries.