Electrical Protection Schemes Based on High Voltage Switchgear
Key Takeaways
-
Switchgear rated above 36kV AC is called high voltage switchgear and is generally installed in high voltage power systems.
-
The two classifications of high voltage switchgear are gas-insulated indoor type switchgear and air-insulated outdoor type switchgear.
-
The major classification of high voltage circuit breakers is oil circuit breakers and oil-less circuit breakers.
Power system protection using switchgear
Power generation, transmission, and distribution systems are vulnerable to rapid changes, which can sometimes turn out hazardous. The sudden opening or shorting of a circuit or a rise in current and voltage can result in losses to industries or commercial and residential electrical systems; it can even take human life, if not repaired.
In a modern power system, we limit the probability of such losses by installing switchgear. Switchgear is a form of circuit protection that safeguards equipment from overload or short circuit conditions and helps to prevent damage to a power system.
Switchgear is the amalgamation of switching and controlling devices that are used to measure, meter, or regulate the electrical quantities in a power system. A switchgear provides the basic function of electrical isolation, protection, and control.
As a power system deals with different voltage levels, so does the switchgear. Switchgears are classified into low voltage switchgears, medium voltage switchgears, and high voltage switchgears. Switchgears rated above 36kV AC are called high voltage switchgears and are generally installed in high voltage power systems.
In this article, we will identify the classifications and features of high voltage switchgear as they relate to power system protection.
High Voltage Switchgear
A high voltage circuit breaker
High voltage switchgear, otherwise called high tension switchgear, handles high voltages. As the voltage level increases, the intensity of arcing generated due to switching operations also increases. There are two types of classifications for high voltage switchgear:
-
Gas-insulated indoor
-
Air-insulated outdoor
Choosing the right type of high voltage switchgear depends on how the switchgear will be applied, what environment it needs to function in, and its ratings.
Features of High Voltage Circuit Breakers
High voltage switchgear must provide safe operation of the electrical system under conditions such as:
-
Terminal faults-The fault that occurs at the circuit breaker terminals.
-
Short-line faults-The short circuit fault that occurs within 5km of the transmission network.
-
Transformer magnetizing currents-When a transformer is energized, there will be an inrush of current, which is around ten times the normal current rating of the transformer.
-
Capacitor bank charging-Capacitor bank charging is like an overvoltage condition and electrical system switchgear should allow the capacitor bank charging without interruption.
-
Transmission line energizing-Switch gears should provide safe energizing and de-energizing of transmission lines, keeping the equipment connected to the line without damage.
-
Switching of the phase sequence-Phase sequence change particularly affects the rotating machines and the switchgear should take care of the safe operation of electrical rotating machines under such conditions.
Classification of High Voltage Circuit Breakers
The two major classifications of high voltage circuit breakers are oil circuit breakers and oil-less circuit breakers.
Oil Circuit Breakers
In oil circuit breakers, oil is used as the arc quenching medium. The advantages of using oil for arc extinction are:
-
Oil provides an insulating medium.
-
Arc energy is absorbed by the decomposing oil.
-
The gas formed has a cooling property.
-
Oil in close vicinity of the arc presents a cooling medium.
-
High dielectric strength oil flows into the arc space after the current zero.
Oil circuit breakers are bifurcated into:
Bulk oil circuit breakers- Bulk oil circuit breakers are filled with transformer oil and this medium is used to quench the arc and insulate the current-carrying parts and earthed tank.
Low-oil circuit breakers- The oil only serves the purpose of arc extinguishing—the insulating medium in this type of circuit breaker is either air, porcelain, or any other insulating materials. This type can also be called a minimum oil circuit breaker (MOCB) or small oil circuit breaker.
Oil-Less Circuit Breakers
The major types of oil-less circuit breakers are shown in the table below:
|
|
Classifications of oil-less circuit breakers
High voltage switchgear is the protection system that safeguards electrical power generation, transmission, and distribution. It can monitor the system, measure electrical quantities, isolate equipment during contingencies, and control and regulate power system parameters. If electrical equipment in a power system operates safely and reliably and is protected under short circuits or overcurrent conditions, this is often because of high voltage switchgear. To ensure a safe and reliable power system operation, high voltage switchgear should be incorporated into the system’s design.