2.1- PRINCIPLES OF OPERATIONS OF CIRCUIT BREAKERS
2.2- SOME TYPES OF CIRCUIT BREAKERS
2.3- THE WAYS OF OPERATING MECHANISMS
Circuit breakers differ from switches in that they not only manually make and break the circuit and carry their normal current, but they also are capable of making and breaking the circuit under the severest system conditions.
Breaking or making the circuit under load conditions represents no real problem for a circuit breaker since the interrupted current is rather low and the power factor is high. Under short circuit conditions the current may reach a value of tens of thousands of amperes at a power factor as low as 0.1. It is the duty of a circuit breaker to interrupt such current as soon as possible to avoid equipment damage. Improper circuit breaker operation can result not only in damage to equipment, but in enormous economic loss if generation is interrupted for long and frequent periods. Loss of system stability is a consequence of slow fault clearance by a circuit breaker. Fault clearance time has been immensely reduced during the last 50 years due to the high technology adopted in circuit breaker design and the use of static relays. Fault clearance times of the order of 2 to 3 cycles have been achieved of which the circuit breaker arcing time occupies to 1 cycle.
2.1- PRINCIPLES OF OPERATION OF CIRCUIT BREAKERS
The circuit breaker is manly composed of:
i. The contacts: the main contacts responsible for continuously carrying the current and the arcing contacts between which the arc strikes when the breaker opens or closes the circuit. They are usually made of copper, aluminum or silver plated copper.
ii. The operating mechanism: which moves the contacts for making or breaking the circuit.
iii. The energy-storage device: in which energy is stored while the breaker is closed. The release of this energy enables fast separation of the breaker contacts. Such a device is usually a heavy spring made of bronze, copper or stainless steel. In air-blast breakers the compressed air represents the stored energy.
iv. The holding mechanism: it holds the breaker in the closed state and releases the stored energy when needed.
v. The tripping mechanism: it initiates the opening of the breaker by releasing the stored energy.
vi. The arc-interrupting device: it is the part of the breaker responsible for extinguishing the arc between these contacts during the breaking operation.
2.2- SOME TYPES OF CIRCUIT BREAKERS
Circuit breakers are classified according to the dielectric medium in which their contacts arc enclosed.
A switching medium is intended for arc interruption as well as insulating the high voltage components from the earthed ones. The main switching media in common use nowadays are air, Oil, sulphur-hexaflouride (SF6), and vacuum. With the rapid advancement in the field of solid state devices, solid-state breakers have promising future.
The different types of circuit breakers are:
a) Air circuit breakers (Air-Magnetic CB).
b) Oil circuit breakers.
c) Air-blast circuit breakers
d) SF6 circuit breakers.
e) Vacuum circuit breakers
f) Gas-blast circuit breakers
The construction and principle of operation of each type will be outlined in the following sections.
2.2.1- Air Circuit Breakers
The arc established between the contacts of air circuit breakers is interrupted in atmospheric air. They are commonly used in medium and high voltage systems in the voltage range 415V to 17.5 KV with a normal current up to 3000 A. The air-magnetic CB uses a pafiv air & magnetic field to stretch and extinguish the arc that is formed when the CB is opened energized circuit. Due to their simple construction and maintenance they may replace oil circuit breakers, of the same ratings, in systems where fire hazards are not tolerable. In heavy industy having large electric motors with frequent starting, air circuit breakers supersede oil breakers due to oil contamination. They are also extensively used in electric furnaces. They may be hand or power operated with springs or solenoids.
2.2.2- Oil Circuit Breakers
Circuit breaking in oil has been adopted since the early stages of circuit breakers manufacture. The oil in oil-filled breakers serves the purpose of insulating the live parts from the earthed ones and provides an excellent medium for arc interruption. Oil circuit breakers of the various types are used in almost all voltage ranges and ratings. However, they are commonly used at voltages below 115KV leaving the higher voltages for air blast and SF6 breakers. The contacts of an oil breaker are submerged in insulating oil, which helps to cool and extinguish the arc that forms when the contacts are opened.
Oil circuit breakers are classified into two main types namely: bulk oil circuit breakers and minimum oil circuit breakers.
The methods of arc control and interruption is different from one type to the other.
A.Bulk Oil Circuit Breakers:
Bulk oil circuit breakers are widely used in power systems from the lowest voltages up to 115KV. However, they are still used in systems having voltages up to 230KV.
The contacts of bulk oil breakers may be of the plain-break type, where the arc is freely interrupted in oil, or enclosed within arc controllers.
Plain-break circuit breakers consist mainly of a large volum of oil contained in a metallic tank. Arc interruption depends on the head of oil above the contacts and the speed of contact separation. The head of oil above the arc should be sufficient to cool the gases, mainly hydrogen, produced by oil decomposition. A small air cushion at the top of the oil together with the produced gases will increase the pressure with a subsequent decrease of the arcing time
B. Minimum Oil Circuit Breakers:
Bulk oil circuit breakers have the disadvantage of using large quantity of oil. With frequent breaking and making heavy currents the oil will deteriorate and may lead to circuit breaker failure. This has led to the design of minimum oil circuit breakers working on the same principles of arc control as those used in bulk oil breakers. In this type of breakers the interrupter chamber is separated from the other parts and arcing is confined to a small volume of oil. Fig. (2) shows one pole of a minimum oil circuit breaker. The lower chamber contains the operating mechanism and the upper one contains the moving and fixed contacts together with the control device. Both chambers are made of an insulating material such as porcelain. The oil in both chambers is completely separated from each other. By this arrangement the amount of oil needed for arc interruption and the clearances to earth are roused. However, conditioning or changing the oil in the interrupter chamber is more frequent than in the bulk oil breakers. This is due to carbonization and sludging from arcs interrupted chamber is equipped with a discharge vent and silica gel breather to permit a small gas cushion on top of the oil.
Single break minimum oil breakers arc available in the voltage range 13.8 to 34.5KV.
2.2.3 Air Blast Circuit Breakers:
The principle of arc interruption in air blast circuit breakers is to direct a blast of air, at high pressure and velocity, to the arc. Fresh and dry air of the air blast will replace the ionized hot gases within the arc zone and the arc length is considerably increased. Consequently the arc may be interrupted at the first natural current zero. In this type of breaker, the contacts are surrounded by compressed air. When the contacts are opened, the compressed air is released in forced blast through the arc to the atmosphere extinguishing the arc in the process.
2.2.4 SF6 Circuit Breakers:(Gas– Buffer breakers)
Sulphur hexafluoride has proved its-self as an excellent insulating and arc quenching medium. The physical, chemical, and electrical properties of SF6 are more superior than many of the other media. It has been extensively used during the last 30 years in circuit breakers, gas-insulated switchgear (GIS), high voltage capacitors, bushings, and gas insulated transmission lines. In SF6 breakers the contacts are surrounded by low pressure SF6 gas. At the moment the contacts are opened, a small a mount of gas is compressed and forced through the arc to extinguish it. See fig. (3).
2.2.5 Vacuum Circuit Breakers:
The advantages of vacuum as an insulant and arc interrupting medium has been known for many years. A vacuum of the order of 10 -5 torr has higher dielectric strength and better arc interruption ability than other media such as air, other gases or oil. The rate of recovery of the dielectric strength in a vacuum breaker is 10 to 20 times that achieved in gas blast breaker. In addition to these advantages vacuum breakers are compact, do not need long contact separation (2 to 10mm) or maintenance, and they do not suffer from transient recovery over voltages when interrupting capacitive or small inductive currents. Also, the power needed to close or open a vacuum breaker is much less than that needed for other types of breakers.
Vacuum circuit breakers are now commonly used in the medium voltage up to 34.5KV, replacing air blast and oil breakers. However, they are used in few places for 132 KV power systems. In this case the breaker is constructed of several series interrupter heads arranged as in other types of circuit breakers.
Arc extinction in vacuum is different from that in air, oil, and other gases. The contacts of vacuum breakers are enclosed in sealed vacuum envelop or bottle. Because the contacts opened and closed in vacuum, any arcing that may occur can not be sustained and usually extinguished quickly. )
2.2.6 Gas-blast Circuit Breaker:
Its contacts are also surrounded with low pressure SF6 gas. But this type of breakers maintain the circuit reservoir of gas under high pressure. When the breaker contacts ate opened, the high pressure gas is released through the arc and sudden blast to extinguish it.
2.3 THE WAYS OF OPERATING MECHANISMS
There are four basic ways of operating mechanism and each type of circuit breakers is operated by one way or a variation of these operating mechanism ways. The operating mechanism is considered to be the operation of opening the contacts of the breaker and close them as required later on.
2.3.1 Solenoid Operating Mechanism
In this mechanism, a solenoid coil provides the force needed to close the breaker contacts and at the same time, compresses the opening spring. The compressed opening spring provides the force needed to open the breaker contacts.
2.3.2 Spring Operating Mechanism
A spring in this mechanism operates to compress or change the closing spring. The closing spring provides the force needed to close the breaker contacts and in the same time compresses or charges the opening spring. In this case the opening spring is enclosed in covered pipe near the top of the breaker. The spring is connected to the breaker contacts and the spring mechanism to provide the force needed to open the breaker contacts.
2.3.3 Pneumatic Operating Mechanism
In this mechanism, a reservoir of compressed air provides the force needed to close the breaker contacts and at the same time charge the opening spring. The opening spring gives the force needed to open the breaker contacts.
2.3.4 Hydraulic Operating Mechanism
In this mechanism, a pressurized fluid gives the force to close the breaker contacts and at the same time charge the opening spring. The opening spring which is enclosed in covered pipe near the top of the breaker provides the force needed to open the breaker contacts.
Inspections and Tests Made for the CB’s
3.1- INSPECTIONS OF CB’S
3.2- TESTS OF CB’S
Inspection and test are two main aspects that they should be worked out to every electrical equipment including the CB’s which are very important electrical equipments to protect the electrical power system. Those two aspects are done to ensure that the CB’s are in a good condition so that they will function properly when called upon to operate.
3.1 INSPECTION OF CB’S
Inspection of the CB’s is very important activity that should be carried out to check the status of the CB according to the company standards (SMS). There are some items to be inspected as in the following table.
Table 1 Inspection Steps
ITEM # ITEM REQUIRED ACTION
1 Tank Oil Level/SF6 Pressure Check level and color.
2 Operations Counter Record reading
3 Compressor Record run time reading. Blow down receiver; record excessive moisture.
4 Air Pressure/Hydraulic Pressure Record reading
5 Bushings Check condition for drips and cracks, duct contamination. Check oil level.
6 Cabinets Check that seals are in good condition. Check operation of lights and heaters. Check for hydraulic leaks.
3.2 TESTS OF CB’S
The safe operation of power CB is very important in power system. It is therefore essential to develop methods which can detect the CB problems early in their development and prevent a catastrophic failure which could cause a power outage and mitigate the extent of change to other equipments in the system, thus reducing the cost and duration of the repair. The most common methods that are usually done to achieve the safe operation of CB will be discussed in the following sections.(see the appendix)
3.2.1 Timing Test
The purpose of this test is to determine if the breaker can make and break circuit under normal load conditions and if it can interrupt circuits under fault conditions. In principle, the test is performed by operating the breaker and measuring its electrical and mechanical performance. There are two main parts of this test, which are timing and travel. For the timing part of the test, contact test cables are connected from the test instrument across the breaker contact terminals to determine when the breaker contacts open and close. For the travel part of the test, a cable is connected from the test instrument to a mechanism called a transducer. The transducer is connected to breaker operating rod to detect the movement or the travel of the operating rod when the breaker is operated. When the breaker is operated for the test, the timing test instrument draws lines or traces on a moving sheet of paper. These traces provide good gain information about the operation of the breaker including:
1. Contact make and break times.
2. Operating rod travel distance.
3. Operating rod travel velocity and some other CB operating characteristics.
The timing test is typically performed for several types of breaker operations. The traces for each operation can be analyzed to determine how the breaker is performing in comparison with the manufacture specifications. If the breaker performance does not fall with acceptable ranges, the breaker will need to be properly adjusted before it can be put in service.
3.2.2 Contact Resistance Test
Resistance in closed contacts of circuit breaker can have a number of causes including arcing deposit, loss of complete connection. Contact resistance creates heat that can reduce the life of contacts and evenly to breaker failure. The purpose of contact resistance testing is to detect unacceptably high contact resistance levels before failure occurs. In principle, the test is performed by passing direct current through the closed contacts of breaker and measuring the voltage drop across the contacts. The test instrument uses the current and voltage valves to calculate and display the contact resistance. If the contact resistance exceeds an acceptable limit, the contact may have to be cleaned or replaced.
3.2.3 Insulation Resistance Test
When the circuit breaker contacts are opened, the breaker insulation should provide a high resistance to prevent current from flowing. The purpose of insulation resistance test is to detect unacceptably low levels of insulation resistance before poor or weakened insulation leads to failure. In principle, the test is performed by applying a high DC voltage to one of the breaker’s bushing terminals with the breaker’s contact is opened. Then leakage current is measured either to ground or across the open contacts to the other pushing terminal. The test instrument uses DC voltage and leakage current values to calculate and display the insulation resistance. If the insulation resistance is lower than an acceptable minimum, the cause of low insulation resistance must be identified and corrected.
3.2.4 Power Factor Test
The purpose of power factor test is to determine the condition of the electrical insulation of the circuit breaker. In principle, the test is performed by applying a high AC voltage across the breaker bushing terminals. With the breaker’s contact open, leakage current is measured across the open contact in both directions. With the contact closed, leakage current is measured across the closed circuit to ground. The test instrument measurements are mAmp and Watt. These values are documented and they are used to perform calculations to determine the power factor of the breaker insulation. If the power factor is not within an acceptable range, the breaker may have to be closely monitored or retested on regular basis or the bushing may have to be replaced. Regardless of the test on breaker, the power factor test is also typically performed on each of the bushings.
3.3.5 Hi-Pot Test
The purpose of this test is to check the condition of the electrical insulation of the circuit breaker at its rated voltage. In principle, the test is performed by applying a rated AC or DC voltage across the breaker bushing terminals. With the breaker contacts open, leakage current is measured across the open contacts in both directions. With the contacts close, leakage current is measured across the closed circuit to ground. The test instrument indicates the applied voltage and measures and indicates the leakage current. If the CB insulation is weak, the slack of the applied rated voltage may result in flash over or insulation failure. If this happens, the test set automatically shut itself off. If the test instrument indicates that leakage current exceeds an allowable level, the cause must be identified and corrected. If the insulation fails during the test, the failed component must be replaced.
3.2.6 Dielectric Test
An oil sample has to be taken from each tank in the oil circuit breaker (OCB). This sample is tested according to the dielectric test. The test measures the breakdown voltage of the oil sample. Special equipment is used to do the test. It consists of two rods emerged in the oil sample with 2.5 mm between the two rods. The voltage between the two rod is increased until the breakdown voltage is increased. Breakdown voltage is usually from 35KV to 50 KV. This test is repeated six times. The first reading is neglected and the average of the rest readings calculated. This test gives an approximate indication of how good is the isolation of the oil inside the tank of the breaker. If the oil is below standards, an arrangement for oil filtering shall be done to clean the oil from any moisture, because moisture in the oil decreases its breakdown voltage. Hence, its ability to isolate and to extinguish the arc will also decrease.