Circuit breakers--Industrial Electrical Power Systems

Introduction

Where fuses are unsuitable or inadequate, protective relays and circuit breakers are used in combination to detect and isolate faults. Circuit breakers are the main making and breaking devices in an electrical circuit to allow or disallow flow of power from source to the load. These carry the load currents continuously and are expected to be switched ON with loads (making capacity). These should also be capable of breaking a live circuit under normal switching OFF conditions as well as under fault conditions carrying the expected fault current until completely isolating the fault side (rupturing/breaking capacity). Under fault conditions, the breakers should be able to open by instructions from monitoring devices like relays. The relay contacts are used in the making and breaking control circuits of a circuit breaker, to prevent breakers getting closed or to trip breaker under fault conditions as well as for some other interlocks.

Protective relay-circuit breaker combination

The protective relay detects and evaluates the fault and determines when the circuit should be opened. The circuit breaker functions under control of the relay, to open the circuit when required. A closed circuit breaker has sufficient energy to open its contacts stored in one form or another (generally a charged spring). When a protective relay signals to open the circuit, the store energy is released causing the circuit breaker to open.

Except in special cases where the protective relays are mounted on the breaker, the connection between the relay and circuit breaker is by hard wiring.

++++ indicates schematically this association between relay and circuit breaker.

From the protection point of view, the important parts of the circuit breaker are the trip coil, latching mechanism, main contacts and auxiliary contacts.

The roles played by these components in the tripping process is clear -- and the following step by step procedure takes place while isolating a fault (the time intervals between each event will be in the order of a few electrical cycles i.e. milliseconds):

• The relay receives information, which it analyzes, and determines that the circuit should be opened.

• Relay closes its contacts energizing the trip coil of the circuit breaker.

• The circuit breaker is unlatched and opens its main contacts under the control of the tripping spring.

• The trip coil is deenergized by opening of the circuit breaker auxiliary contacts.

Circuit breakers are normally fitted with a number of auxiliary contacts, which are used in a variety of ways in control and protection circuits (e.g. to energize lamps on a remote panel to indicate whether the breaker is open or closed).

Circuit breaker; Opening spring (compressed); Auxiliary contacts; To alarm and indication circuits; Latch Trip coil; Tripping contact; Evaluating circuits and/or mechanism; Information relative to main circuit--Main circuit--Main contacts (closed)--Relay Tripping battery

++++ Relay-circuit breaker combination

Purpose of circuit breakers (switchgear)

The main purpose of a circuit breaker is to:

1. Switch load currents
2. Make onto a fault
3. Break normal and fault currents
4. Carry fault current without blowing itself open (or up!) i.e. no distortion due to magnetic forces under fault conditions.

The important characteristics from a protection point of view are:

1. • The speed with which the main current is opened after a tripping impulse is received
2. • The capacity of the circuit that the main contacts are capable of interrupting.

The first characteristic is referred to as the 'tripping time' and is expressed in cycles.

Modern high-speed circuit breakers have tripping times between three and eight cycles.

The tripping or total clearing or break time is made up as follows:

• Opening time: The time between instant of application of tripping power to the instant of separation of the main contacts.

• Arcing time: The time between the instant of separation of the main circuit breaker contacts to the instant of arc extinction of short-circuit current.

• Total break or clearing time: The sum of the above.

Fault occurs --Trip coil energized-- Fault extinguished --Relay pickup --Relay reset CB opening time CB arcing time Total fault clearing time

++++ Total fault clearing time--The second characteristic is referred to as 'rupturing capacity' and is expressed in MVA.

System volts SC current MVA rating (breaking capacity)

Typical rupturing capacities of modern circuit breakers are as follows: kV MVA kA

The selection of the breaking capacity depends on the actual fault conditions expected in the system and the possible future increase in the fault level of the main source of supply.

In the earlier sections we have studied simple examples of calculating the fault currents expected in a system. These simple calculations are applied with standard ratings of transformers, etc., to select the approximate rupturing capacity duty for the circuit breakers.

Behavior under fault conditions

Before the instant of short-circuit, load current will be flowing through the switch and this can be regarded as zero when compared to the level of fault current that would flow.

IASYM (2.55 × ISYM) ISYM I Steady state; Arc extinguished; Current conditions Instant of shortcut Zero volts actually across switch contacts when switch closed Voltage conditions Instant of contact opening Steady-state volts that would exist across contacts when switch open ARC VOLTS System volts DC Transient RECOVERY VOLTS

++++ Behavior under fault conditions

Arc

The arc has three parts:

1. Cathode end (-ve): There is approximately 30-50 V drop due to emission of electrons.

2. Arc column: Ionized gas, which has a diameter proportional to current. Temperature can be in the range of 6000-25,000 °C.

3. Anode end (+ve): Volt drop 10-20 V. When short-circuit occurs, fault current flows, corresponding to the network parameters.

The breaker trips and the current is interrupted at the next natural current zero. The network reacts by transient oscillations, which gives rise to the transient recovery voltage (TRV) across the circuit breaker main contacts.

All breaking principles involve the separation of contacts, which initially are bridged by a hot, highly conductive arcing column. After interruption at current zero, the arcing zone has to be cooled to such an extent that the TRV is overcome and it cannot cause a voltage breakdown across the open gap.

Three critical phases are distinguished during arc interruption, each characterized by its own physical processes and interaction between system and breaker.

High current phase

This consists of highly conductive plasma at a very high temperature corresponding to a low mass density and an extremely high flow velocity. Proper contact design prevents the existence of metal vapor in the critical arc region.

Thermal phase

Before current zero, the diameter of the plasma column decreases very rapidly with the decaying current but remains existent as an extremely thin filament during the passage through current zero. This thermal phase is characterized by a race between the cooling of the rest of the plasma and the reheating caused by the rapidly rising voltage. Due to the temperature and velocity difference between the cool, relatively slow axial flow of the surrounding gas and the rapid flow in the hot plasma core, vigorous turbulence occurs downstream of the throat, resulting in effective cooling of the arc.

This turbulence is the dominant mechanism, which determines thermal re-ignition or interruption.

Dielectric phase

After successful thermal interruption, the hot plasma is replaced by a residual column of hot, but no longer electrically conducting medium. However, due to marginal ion-conductivity, local distortion of the electrical field distribution is caused by the TRV appearing across the open break. This effect strongly influences the dielectric strength of the break and has to be taken into account when designing the geometry of the contact arrangement.

Types of circuit breakers

The types of breakers basically refer to the medium in which the breaker opens and closes. The medium could be oil, air, vacuum or SF6. The further classification is single break and double break. In a single break type only the busbar end is isolated but in a double break type, both busbar (source) and cable (load) ends are broken. However, the double break is the most common and accepted type in modern installations.

Arc control device

A breaker consists of moving and fixed contact, and during the breaker operation, the contacts are broken and the arc created during such separation needs to be controlled. The arc control devices, otherwise known as turbulator or explosion pot achieves this:

1. Turbulence caused by arc bubble.

2. Magnetic forces tend to force main contacts apart and movement causes oil to be sucked in through ports and squirted past gap.

3. When arc extinguished (at current zero), ionized gases get swept away and prevents restriking of the arc. Fixed contact Ports Moving contact Turbulator

++++ Arc control device

Oil circuit breakers

In modern installations, oil circuit breakers, which are becoming obsolete, are being replaced by vacuum and SF6 breakers. However there are many installations, which still employ these breakers where replacements are found to be a costly proposition. In this design, the main contacts are immersed in oil and the oil acts as the ionizing medium between the contacts. The oil is mineral type, with high dielectric strength to withstand the voltage across the contacts under normal conditions.

(a) Double break (used since 1890).

(b) Single break (more popular in earlier days as more economical to produce - less copper, arc control devices, etc.). Arc energy decomposes oil into 70% hydrogen, 22% acetylene, 5% methane and 3% ethylene. Arc is in a bubble of gas surrounded by oil.

Busbar end Cable end

++++ Double break oil circuit breaker -- Busbar end -- Cable end

++++ Single break oil circuit breaker

Oil has the following advantages:

• Ability of cool oil to flow into the space after current zero and arc goes out

• Cooling surface presented by oil

• Absorption of energy by decomposition of oil

• Action of oil as an insulator lending to more compact design of switchgear.

Disadvantages:

• Inflammability (especially if there is any air near hydrogen)

• Maintenance (changing and purifying)

In the initial stages, the use of high-volume (bulk) oil circuit breakers was more common. In this type, the whole breaker unit is immersed in the oil. This type had the disadvantage of production of higher hydrogen quantities during arcing and higher maintenance requirements. Subsequently these were replaced with low oil (minimum oil) types, where the arc and the bubble are confined into a smaller chamber, minimizing the size of the unit.

Air break switchgear

Interrupting contacts situated in air instead of any other artificial medium. Arc is chopped into a number of small arcs by the Arc-shute as it rises due to heat and magnetic forces. The air circuit breakers are normally employed for 380~480 V distribution.

++++ Air break switchgear

SF6 circuit breakers

Sulfur-hexafluoride (SF6) is an inert insulating gas, which is becoming increasingly popular in modern switchgear designs both as an insulating as well as an arc-quenching medium.

Gas insulated switchgear (GIS) is a combination of breaker, isolator, CT, PT, etc., and are used to replace outdoor substations operating at the higher voltage levels, namely 66 kV and above.

For medium- and low-voltage installations, the SF6 circuit breaker remains constructionally the same as that for oil and air circuit breakers mentioned above, except for the arc interrupting chamber which is of a special design, filled with SF6.

To interrupt an arc drawn when contacts of the circuit breaker separate, a gas flow is required to cool the arcing zone at current interruption (i.e. current zero). This can be achieved by a gas flow generated with a piston (known as the 'puffer' principle), or by heating the gas of constant volume with the arc's energy. The resulting gas expansion is directed through nozzles to provide the required gas flow.

The pressure of the SF6 gas is generally maintained above atmospheric; so good sealing of the gas chambers is vitally important. Leaks will cause loss of insulating medium and clearances are not designed for use in air.

Vacuum circuit breakers and contactors

Vacuum circuit breakers and contactors were introduced in the late 1960s. A circuit breaker is designed for high through-fault and interrupting capacity and as a result has a low mechanical life. On the other hand, a contactor is designed to provide large number of operations at typical rated loads of 200/400/600 A at voltages of 1500/3300/6600/11 000 V.

The following table illustrates the main differences between a contactor and a circuit breaker.

Contactor Circuit Breaker Interrupting capacity Current rating Contact gap at 11 kV Contact force Mechanical life 4.0 kA 400/630 A 6.0 mm 10 kg 1-2.5 million 40 kA 630/3000 A 16.0 mm 80 kg 10,000

Hence, it’s necessary to use back-up fuses when contactors are employed to take care of the high fault conditions. Vacuum breakers are also similar in construction like the other types of breakers, except that the breaking medium is vacuum and the medium sealed to ensure vacuum.

--- give the components of a vacuum circuit breaker.

1 Upper connection; 7 Opening spring; 2 Vacuum interrupter; 8 Shift lever; 3 Lower connection 9 Mechanism housing with spring operating mechanism; 4 Roller contact (swivel contact for 630A) 10 Drive shaft; 5 Contact pressure spring; 11 Pole tube; 6 Insulated coupling rod; 12 Release mechanism

++++ General construction of a vacuum circuit breaker

++++ Diagrammatic representation – coil; Push-off springs; Pressure spring; Vacuum bottle; Flexible braid; Moving armature

Main connectors:

The modern vacuum bottle, which is used in both breakers and contactors, is normally made from ceramic material. It has pure oxygen-free copper main connections, stainless steel bellows and has composite weld-resistant main contact materials. A typical contact material comprises a tungsten matrix impregnated with a copper and antimony alloy to provide a low melting point material to ensure continuation of the arc until nearly current zero.

Because it’s virtually impossible for electricity to flow in a vacuum, the early designs displayed the ability of current chopping i.e. switching off the current at a point on the cycle other than current zero. This sudden instantaneous collapse of the current generated extremely high-voltage spikes and surges into the system, causing failure of equipment.

Another phenomenon was pre-strike at switch on. Due to their superior rate of dielectric recovery, a characteristic of all vacuum switches was the production of a train of pulses during the closing operation. Although of modest magnitude, the high rate of rise of voltage in pre-strike transients can, under certain conditions produce high-insulation stresses in motor line end coils.

Subsequent developments attempted to alleviate these shortcomings by the use of 'softer' contact materials, in order to maintain metal vapor in the arc plasma so that it did not go out during switching. Unfortunately, this led to many instances of contacts welding on closing. Restrike transients produced under conditions of stalled motor switch off was also a problem. When switching off a stalled induction motor, or one rotating at only a fraction of synchronous speed, there is little or no machine back emf, and a high voltage appears across the gap of the contactor immediately after extinction. If at this point of time the gap is very small, there is the change that the gap will break down and initiate a restrike transient, puncturing the motor's insulation. Modern designs have all but overcome these problems. In vacuum contactors, higher operating speeds coupled with switch contact material are chosen to ensure high gap breakdown strength, produce significantly shorter trains of pulses.

In vacuum circuit breakers, operating speeds are also much higher which, together with contact materials that ensure high dielectric strength at a small gap, have ensured that pre strike transients have ceased to become a significant phenomenon. These have led to the use of vacuum breakers more common in modern installations.

Horizontal 0.2ms/DIV Vertical 13.5 kV/DIV 18m cable

++++ Typical pre-strike transient at switch on of 6.6 kV 200 kW motor 90m cable Horizontal 1ms/DIV Vertical 7 kV/DIV

++++ Switch off of stalled 6.6 kV 200 kW motor-escalating restrike on R phase

Types of mechanisms

The mechanisms are required to close and break the contacts with high speed. Following are the types of mechanisms employed.

1. Hand operated: Cheap but losing popularity. Speed depends entirely on operator. Very limited use in modern installations that too for low-voltage applications only.

2. Hand operated spring assisted: Hand movement compresses spring over top dead center. Spring takes over and closes the breaker.

3. Quick make: Spring charged-up by hand, then released to operate mechanism.

4. Motor wound spring: Motor charges spring, instead of manual. Mainly useful when remote operations are employed, which are common in modern installations because of computer applications.

5. Solenoid: As name implies.

6. Pneumatic: Used at 66 kV and above. Convenient when drying air is required.

Dashpots

In oil circuit breakers, when the breaker is closed, if the operation is not damped then contact bounce may occur and the breaker may kick open. Dashpots prevent this. They also prevent unnecessary physical damage to the contacts on impact. Their use of course depends on the design.

Contacts

Fixed contacts normally have an extended finger for arc control purposes. Moving contacts normally have a special tip (Elkonite) to prevent burning from arcing.

Comparison of insulating methods for CBs Property Air Oil SF6 Vacuum Number of operations Medium Low Medium High; 'Soft' break ability Good, Good, Good, Fair Monitoring of medium N/A Manual test Automatic Not possible: Fire hazard risk None, High, None, None Health hazard risk None Low; Low None Economical voltage range Up to 1 kV 3.3-22 kV 3.3-800 kV 3.3-36 kV 7.7 Comparison of breaker types.

Following curve gives the requirement of electrode gaps for circuit breakers with different insulating mediums.

++++ Influence of electrode gap for different mediums.

The following table highlights the features for different types of circuit breakers.

Factor -- Safety Size Maintenance Environmental factors; Endurance

Oil Breakers --

Risk of explosion and fire due to increase in pressure during multiple operations Emission of hot air and ionized gas to the surroundings Quite large; Medium Regular oil replacement; Replacement of arcing contacts Humidity and dust in the atmosphere can change the internal properties and affect the dielectric; Below average; Average

Air Breakers --

Vacuum/SF6 --

No risk of explosion

Smaller; Minimum lubrication for control devices; Since sealed, no effect due to environment; Excellent.