SF6 CIRCUIT BREAKERS FOR MODERN POWER SYSTEM PROTECTION

Electrical faults give rise to abnormal operating conditions and can damage or disrupt the power system in many ways. It is necessary that the faulty section should be immediately disconnected so that the normal operation of the rest of the system is maintained. The protective relay should immediately detect the fault and initiate the operation of circuit breaker or breakers.

What is Circuit Breaker?

A circuit breaker is an automatic switch designed to protect an electrical circuit from damage caused by heavy currents, mainly caused due to overloading, short circuit, or earth fault. The basic function of a circuit breaker is to interrupt the current flow and isolate the faulty section from the rest of the network. Circuit breakers are made in different sizes ranging from the low voltage and low current circuit breaker used in low voltage residential, commercial and industrial premises to very high voltage and high current breakers used in electrical power stations and generating stations.

When a heavy current is interrupted, an electric arc is initiated. This arc must be quenched in a safe and reliable manner so that the breaker contacts are safe and the gap between the contacts can regain the dielectric strength in the desired time span. Different circuit breakers use different arc quenching medium and are accordingly classified as oil circuit breakers, air blast circuit breakers, vacuum circuit breakers and SF₆ circuit breakers.    

Properties of Sulphur hexafluoride (SF₆) gas:

Sulphur hexafluoride (SF₆) is a chemically stable, odourless, inert, non-inflammable and non-toxic gas. This gas has a high dielectric strength and outstanding arc quenching properties. At atmospheric pressure, the dielectric strength of SF₆ gas is about 2.5 times of air and may increase up to 5 times. 

SF₆ and its decomposition products are electro-negative. This property permits electron capture at relatively higher temperature. The ability of an atom to attract and hold electrons is called “electro-negativity”. Thus, the dielectric strength rises rapidly which enables the breaker to withstand the recovery voltage even under extreme switching conditions.

Construction of SF₆ circuit breaker:

Double pressure breaker is the early design of SF₆ circuit breaker and its operating principle is the same as that of air blast circuit breaker. Because of its complicated construction and the need for various auxiliary equipments such as compressors, control device etc., this type of SF₆ circuit breaker has become obsolete.

The puffer type or single pressure type SF₆ circuit breaker is the most popular and is available in the voltage range 3.6 kV to 765 kV. In such SF₆ breakers, the SF₆ gas is compressed by a moving cylinder and is released through a nozzle to quench the arc. Figure shows the working principle of single pressure type SF₆ circuit breaker. 

The operating mechanism, may be pneumatic or hydraulic, and is installed on the base. This operating mechanism is connected to the movable contacts located in the interrupter with the help of insulated rod of fibre glass. The interrupter and support insulator are filled with SF₆ gas at a pressure of about 5 kg/cm².

Figure shows the interrupter of a puffer type SF₆ circuit breaker in fully closed and a position in which contacts are separating. The moving cylinder or puffer cylinder and the moving contacts are coupled together. As a result, when the contacts are separated, the trapped SF₆ gas is compressed. This compressed gas is released axially through a nozzle. The gas removes the heat of the arc by axial convection and radial dissipation. The arc diameter reduces with the decrease of current and becomes very small during current zero and thus the arc is extinguished. Due to the electro-negativity and low arc time constant of SF₆ gas, it rapidly regains the dielectric strength after final current zero.     

Application of SF₆ circuit breaker

SF₆ circuit breaker has the ability to interrupt high fault currents, magnetizing and capacitive currents without too much over-voltages. Thus, it can perform duties like clearing line faults, and switching of capacitors, transformers and reactors. Because of the various advantages mentioned above, SF₆ circuit breakers are preferred for voltages above 132 kV.    

Initiation of Arc in Electric Circuit Breakers and it’s Interruption theory

Initiation of Arc in an Electric Circuit Breaker:

The electric arc is a type of electric discharge between the contacts of the circuit breaker. Arc plays an important role in the behavior of an electric circuit breaker. A circuit breaker should be capable of extinguishing the arc without getting damaged.

As the contacts of a circuit breaker begin to separate, the voltage is appreciable and the distance of separation is very small. Therefore, a large voltage gradient occurs at the contact surface. When the voltage gradient attains a sufficiently high value (10⁶ V/cm) electrons are dragged out of the surface causing ionization of the particles between the contacts. The emission of electrons because of the high value of voltage gradient is known as field emission.

Although this high voltage gradient exist only for a fraction of micro-seconds, but a large number of electrons are liberated from the cathode because of this. These electrons move towards the positive contact i.e. anode at a very rapid pace. On their way to anode, these electrons collide with the atoms and molecules of the gases and vapour existing between the contacts. Hence, each liberated electron tends to create other electrons. If the current is high, which is certainly in case of an electric fault, the discharge attains the form of an arc.  

The temperature of arc is high enough and causes thermal ionization. The liberation of electrons because of high temperature is called thermal emission. Thus, in an electric circuit breaker, an arc is initiated because of field emission but is maintained due to thermal ionization.

Arc Extinction in Circuit Breakers:

As per the above discussion, in every electric circuit breaker the arc is there to be formed. So there should be ways and methods to extinct or quench the arc. There are two methods of arc extinction. They are:

1.       High resistance method, and

2.       Low resistance or Current zero Interruption method.

High Resistance Method of Arc Extinction:

In this method, the arc resistance is increased with time to such a high value that the current is reduced to a value insufficient to maintain the arc. The rate at which the resistance is increased or the current is decreased should be such that no harmful voltages are induced in the system.

The resistance of an arc can be increased by any or all of the following de-ionization methods:

1.       Lengthening the arc by increasing the gap between the contacts.

2.    Constraining the arc; the arc is constrained to a very narrow channel. The arc resistance increases with the decrease in the cross section of the arc.

3.    Cooling the arc; the voltage required to maintain ionization increases with the decrease in temperature.

4.     Splitting the arc; the arc resistance can be increased by splitting the arc into a number of smaller arcs in series.

Now the question arises where, I mean, in which circuit breaker, this method of arc extinction is used? Since the energy dissipated in the arc is high and therefore, the high resistance method of arc extinction is employed only in low and medium power AC circuit breakers and in DC circuit breakers.

Low Resistance or Current Zero Method of Arc Extinction:

In AC circuits there is a natural zero of current present in the system. This property of AC circuits is used for the interruption purpose and the current is not allowed to rise again after a zero occurs. It is undesirable to cut-off the current at any other point, other than natural zero, because this may induce a high voltage in the system. 

The current zero method of circuit interruption is adopted only in AC circuits and is employed in all modern AC circuit breakers.

The two main theories explaining current zero interruption are:

1.       Energy Balance or Cassie theory: According to this theory, the interruption or re-establishment of the arc is an energy balance process. If the rate at which the heat is generated between the breaker contacts is lower than the rate at which heat is dissipated the arc will be extinguished or else it will re-strike. The amount of heat generated is variable and depends on the separation of contacts. Initially when the contacts are about to open the heat generated is zero as the re-striking voltage is zero. The heat generated is again zero when the contacts are fully open as the space between the breaker contacts has become de-ionized and hence the resistance is very high. The heat generation rises to a maximum in between the two states of the breaker contacts. This theory is also known as Cassie theory.

2.       Slepian theory: Current zero is the stage where the degree of ionization is minimum. The ionization at current zero depends upon the voltage appearing across the breaker contacts. This voltage is known as re-striking voltage. If at this current zero, the electrons and ions between the contact space can be removed either by recombining them with neutral molecules or by sweeping them away by forcing the insulation at a faster rate than the rate of ionization, the arc will be interrupted. The recombination can be accelerated by cooling and by increasing the pressure in the arc space. This theory was given by Dr. J. Slepian and therefore called the Slepian theory.

Protective Relays and their functional characteristics

An electric power system should ensure the availability of electrical energy without interruption to every load connected to the system. The most severe electrical failures in a power system are the shunt faults which are characterized by an increase in system current, reduction in voltage, power factor and frequency. 

"Protective relays and relaying systems detect abnormal conditions like faults in the electrical circuits and operate automatic switchgear to isolate faulty equipment/equipments from the system as quickly as possible."

Thus a protective relay is an automatic device which senses an abnormal condition in an electrical circuit and closes its contacts. These contacts in turn close the circuit breaker trip coil circuit, thereby it opens the circuit breaker and the faulty part of the line or equipment is disconnected from the rest of the system. 

The protective relays should operate only the concerned circuit breaker so as to disconnect the faulty equipment from the system as quickly as possible without affecting the healthy section. Relays are tested thoroughly to ensure that they will operate correctly and will assist the related circuit breaker to clear a fault, only within their specified zone. Figure 1 shows the wiring connection of Relay and Circuit Breaker.  

Fig.1: Wiring connection of Relay and Circuit Breaker.

The protective relays do not eliminate the possibility of fault occurrence. It can take action only after the fault has occurred. Situation would be ideal if the protection system could anticipate and prevent fault occurrence but this is next to impossible except where the original cause of fault creates some effect which can operate a protective relay. However Buchholz relay is one of such devices which can anticipate and prevent major faults.

Functional Characteristics of Protective Relays:

A protective relay should have certain qualities. These essential qualities are:

1.      Reliability – A protective should be reliable and must operate when it is required. Every component and circuit which is involved in the operation of the relay is vital and should be considered as a potential source of failure. Failure can be reduced by reliable design well supported by regular and thorough maintenance. Some features of design and manufacture which make relays inherently reliable are good contact material, high contact pressures, dust free enclosures, well braced joints and robust construction.

2.      Selectivity – The protective relay must be capable of selecting the part of the system which is faulty and should isolate it from the healthy one i.e. a relay should differentiate between the faulty part and the healthy part.

3.      Speed – A protective relay must be quick acting and fast otherwise may result in damage to the equipment and the system. The operating time of a relay is of the order of 30 to 100 ms, depending upon the fault level. Also the relays should not be extremely fast (less than 10 ms), otherwise it may result in undesired operation during transient faults such as lightning surges.

4.      Sensitivity – Sensitivity of a protective relay refers to the smallest value of actuating quantity at which the protective relay starts operating in relation with the minimum value of fault current in the protected zone.