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Transformer Work Faults and Protection
If mounted on a utility pole, they are called pole-mount transformers. If the distribution lines are located at ground level or underground, distribution transformers are mounted on concrete pads and locked in steel cases, thus known as pad-mount transformers.
Distribution transformers normally have ratings less than 200 kVA although some national standards can describe units up to 5000 kVA as distribution ransformers. Since distribution transformers are energized for 24 hours a day (even when they don't carry any load), reducingiron losses has an important role in their design. As they usually don't operate at full load, they are designed to have maximum efficiency at lower loads. To have a better efficiency, voltage regulation in these transformers should be kept to a minimum. Hence they are designed to have small leakage reactance.
TRANSFORMER PROTECTION IN A SUBSTATION
The electrical equipment and circuits in a substation must be protected in order to limit the damages due to abnormal currents and over voltages.
All equipment installed in a power electrical system have standardized ratings for short-time withstand current and short duration power frequency voltage.
All equipment installed in a power electrical system have standardized ratings for short-time withstand current and short duration power frequency voltage.
The role of the protections is to ensure that these withstand limits can never be exceeded, therefore clearing the faults as fast as possible.In addition to this first requirement a system of protection must be selective. Selectivity or discrimination means that any fault must be cleared by the device of current interruption (circuit breaker or fuses) being the nearest to the fault, even if the fault is detected by other protections associated with other interruption devices.
As an example for a short circuit occurring on the secondary side of a power transformer, only the circuit breaker installed on the secondary must trip. The circuit breaker installed on the primary side must remain closed. For a transformer protected with MV fuses, the fuses must not blow.
TYPES OF FAULTS IN TRANSFORMER
There are mainly three types of faults occurred in transformer in a substation
1:Over current fault
2:Short circuit fault
3:Earth fault
2:Short circuit fault
3:Earth fault
HOW FAULT OCCURS IN A TRANSFORMER
Stresses generated by the supply
Two types of over voltages may stress and even destroy a transformer:
i. The lightning over voltages due to lightning stroke falling on or near an overhead line supplying the installation where the transformer is installed
ii. The switching over voltages generated by the opening of a circuit breaker or a load break switch for instance.
Depending of the application, protection against these two types of voltage surges may be necessary and are often ensured by means of zno surge arrestors preferably connected on the MV bushing of the transformer.
Stresses due to the load
A transformer overload is always due to an increase of the apparent power demand (kVA) of the installation. This increase of the demand can be the consequence of either a progressive adjunction of loads or an extension of the installation itself. The effect of any overload is an increase of the temperature of oil and windings of the transformer with a reduction of its life time.
The protection of a transformer against the overloads is performed by a dedicated protection usually called thermal overload relay. This type of protection simulates the temperature of the transformer’s windings. The simulation is based on the measure of the current and on the thermal time constant of the transformer. Some relays are able to take into account the effect of harmonics of the current due to non-linear loads such as rectifiers, computers, variable speed drives etc. This type of relay is also able to evaluate the remaining time before the emission of the tripping order and the time delay before re-energizing the transformer.
In addition, oil-filled transformers are equipped with thermostats controlling the
temperature of the oil.Dry-type transformers use heat sensors embedded in the hottest part of the windings insulation.Each of these devices (thermal relay, thermostat, heat sensors) generally provides two levels of detection:
i. A low level used to generate an alarm to advise the maintenance staff,
ii. A high level to de-energize the transformer.
Internal faults in oil filled transformers
In oil filled transformers, internal faults may be classified as follow:
Faults generating production of gases, mainly:
Micro arcs resulting from incipient faults in the winding insulationSlow degradation of insulation materials
Inter turns short circuit
Faults generating internal over pressures with simultaneously high level of line over currents:
Phase to earth short circuit
Phase to Phase short circuit.
These faults may be the consequence of external lightning or switching over voltage.Depending on the type of the transformer, there are two kinds of devices able to detect internal faults affecting an oil filled transformer
TRANSFORMER PROTECTION SYSTEM
Over current & Earth Fault relay for 11kV T/F panel has to be provide on LV side i.e. 11kV VCB panel as per latest VCB panel specification of HVPNL.
PROTECTION SCHEME FOR POWER TRANSFORMERS.
TRANSFORMER DIFFERENTIAL PROTECTION:
The relay shall
i) Be triple pole numerical type.
ii) Have 3 instantaneous high set over-current units.
iii) Have second harmonic restraint or other inrush proof feature and be stable under normal over fluxing condition.
iv) Have fifth harmonic by pass filter or similar other arrangement to prevent mal- operation of the relay under operating conditions.
v) Be suitable for rated current of 1A and include necessary ICTs for ratio & phase angle correction.
vi) Have operating current setting sufficiently low (15% or less) so as to cover practically the whole of the transformer against all types of faults.
vii) Have adjustable bias setting range of 20- 50%.
viii) Have maximum operating time of 30milli seconds at 5 times the rated current.
ix) Be provided with necessary terminals & links to measure current in restraining circuits & in the
operating circuits of all the phases under load without making any wiring changes.
x) Cover the lightning arrestors (proposed to be provided very close to the transformer) in the zone of protection.
Voltage signal: (0-5V continuously settable, with 50m Sec. minimum pulse duration)
Potential free contact (Minimum pulse duration of 50 m Sec.)
IRIG-B RS232C the recorder shall give alarm in case of absence of synchronizing pulse within specified time.
a) The disturbance recorder shall meet the following requirements: the frequency response shall be 5 Hz on lower side and 250 Hz or better on upper side. Scan rate shall be 1000 Hz/Channel or better. Pre–fault time shall not be less than 100 milliseconds and the post fault time shall not be less than 2 seconds (adjustable). If another system disturbance occurs during one post-fault run time, the recorder shall also be able to record the same. However, the total memory of acquisition unit shall not be less than 5.0 seconds.
b) The event logger shall meet the following requirements: The time resolution shall be 1 milli- second. Cope with up to 40 changes in any one 10milli seconds interval. The date and time should be printed to the nearest 1milli second followed by a message describing the point which has operated. Events occurring whilst a previous event is in process of being printed are to be stored to await printing.
Xii) Display resolution of differential/pickup current should be 0.01A (10mA).
PROTECTION EQUIPMENT
Vacuum circuit breaker: (VCB)
A vacuum circuit breaker is such kind of circuit breaker where the arc quenching takes place in vacuum. The technology is suitable for mainly medium voltage application.
For higher voltage take place in a vacuum chamber in the breaker which is called vacuum interrupter.
The vacuum interrupter consists of a steel arc chamber in the center symmetrically arranged ceramic insulators. The vacuum pressure inside a vacuum interrupter is normally maintained at 10 - 6 bar.
The material used for vacuum technology has been developed but not commercially viable. The operation of opening and closing of current carrying contacts and associated arc interruption current carrying contacts plays an important role in the performance of the vacuum circuit breaker.
Operation of Vacuum Circuit Breaker
The main aim of any circuit breaker is to quench arc during current zero crossing, by establishing high dielectric strength in between the contacts so that reestablishment of arc after current zero becomes impossible. The dielectric strength of vacuum is eight times greater than that of air and four times greater than that of SF6 gas. This high dielectric strength makes it possible to quench a vacuum arc within very small contact gap. For short contact gap, low contact mass and no compression of medium the drive energy required in vacuum circuit breaker is minimum. When two face to face contact areas are just being separated to each other, they do not be separated instantly, contact area on the contact face is being reduced and ultimately comes to a point and then they are finally de-touched. Although this happens in a fraction of micro second but it is the fact. At this instant of de-touching of contacts in a vacuum, the current through the contacts concentrated on that last contact point on the contact surface and makes a hot spot. As it is vacuum, the metal on the contact surface is easily vaporized due to that hot spot and create a conducting media for arc path. Then the arc will be initiated and continued until the next current zero. At current zero this vacuum arc is extinguished and the conducting metal vapor is re-condensed on the contact surface. At this point, the contacts are already separated hence there is no question of re-vaporization of contact surface, for next cycle of current. That means, the arc cannot be reestablished again. In this way vacuum circuit breaker prevents the reestablishment of arc by producing high dielectric strength in the contact gap after current zero.
O.C.B (Oil Circuit Breaker):
Mineral oil has better insulating property than air. In oil circuit breaker the fixed contact and moving contact are immerged inside the insulating oil. Whenever there is a separation of current carrying contacts in the oil, the arc in circuit breaker is initialized at the moment of separation of contacts, and due to this arc the oil is vaporized and decomposed in mostly hydrogen gas and ultimately creates a hydrogen bubble around the arc. This highly compressed gas bubble around the arc prevents re-striking of the arc after current reaches zero crossing of the cycle. The oil circuit breaker is the one of the oldest type of circuit breakers.
Operation of Oil Circuit Breaker:
The operation of oil circuit breaker is quite simple let’s have a discussion. When the current carrying contacts in the oil are separated an arc is established in between the separated contacts. Actually, when separation of contacts has just started, distance between the current contacts is small as a result the voltage gradient between contacts becomes high. This high voltage gradient between the contacts ionized the oil and consequently initiates arcing between the contacts. This arc will produce a large amount of heat in surrounding oil and vaporizes the oil and decomposes the oil in mostly hydrogen and a small amount of methane, ethylene and acetylene. The hydrogen gas can not remain in molecular form and its is broken into its atomic form releasing lot of heat. The arc temperature may reach up to 5000° K. Due to this high temperature the gas is liberated surround the arc very rapidly and forms an excessively fast growing gas bubble around the arc. It is found that the mixture of gases occupies a volume about one thousand times that of the oil decomposed. From this figure we can assume how fast the gas bubble around the arc will grow in size. If this growing gas bubble around the arc is compressed by any means then rate of de – ionization process of ionized gaseous media in between the contacts will accelerate which rapidly increase the dielectric strength between the contacts and consequently the arc will be quenched at zero crossing of the current cycle. This is the basic operation of oil circuit breaker. In addition to that cooling effect of hydrogen gas surround the arc path also helps, the quick arc quenching in oil circuit breaker.
Types of Oil Circuit Breakers:
There are mainly two types of oil circuit breakers available-
Bulk Oil Circuit Breaker or BOCB
Bulk oil circuit breaker or BOCB is such types of circuit breakers where oil is used as arc quenching media as well as insulating media between current carrying contacts and earthed parts of the breaker. The oil used here is same as transformer insulating oil.
Minimum Oil Circuit Breaker or MOCB
These types of circuit breakers utilize oil as the interrupting media. However, unlike bulk oil circuit breaker, a minimum oil circuit breaker places the interrupting unit in insulating chamber at live potential. The insulating oil is available only in interrupting chamber. The features of designing MOCB is to reduce requirement of oil, and hence these breaker are called minimum oil circuit breaker.
Bulk Oil Circuit Breaker:
Construction of Bulk Oil Circuit Breaker
The basic construction of bulk oil circuit breaker is quite simple. Here all moving contacts and fixed contacts are immerged in oil inside closed iron vessel or iron tank. Whenever the current carrying contacts are being open within the oil the arc is produced in between the separated contacts. The large energy will be dissipated from the arc in oil which vaporizes the oil as well as decomposes it. Because of that a large gaseous pressure is developed inside the oil which tries to displace the liquid oil from surrounding of the contacts. The inner wall of the oil tank has to withstand this large pressure of the displaced oil. Thus the oil tank of bulk oil circuit breaker has to be sufficiently strong in construction. An air cushion is necessary between the oil surface and tank roof to accommodate the displaced oil when gas forms around the arc. That is why the oil tank is not totally filled up with oil it is filled up to certain level above which the air is tight in the tank. The breaker tank top cover should be securely bolted on the tank body and total breaker must be properly locked with foundation otherwise it may jump out during interruption of high fault current. In these type of equipment where expansible oil is enclosed in an air tight vessel (oil tank) there must be a gas vent fitted on the tank cover. Naturally some form of gas vent always is provided on the cover of bulk oil circuit breaker tank. This is very basic features for construction of bulk oil circuit breaker.
Arc Quenching in Bulk Oil Circuit Breaker
Protective Relay:
A relay is automatic device which senses an abnormal condition of electrical circuit and closes its contacts. These contacts in turns close and complete the circuit breaker trip coil circuit hence make the circuit breaker tripped for disconnecting the faulty portion of the electrical circuit from rest of the healthy circuit. Now let’s have a discussion on some terms related to protective relay. Pickup Level of Actuating Signal: The value of actuating quantity (voltage or current) which is on threshold above which the relay initiates to be operated. If the value of actuating quantity is increased, the electromagnetic effect of the relay coil is increased and above a certain level of actuating quantity the moving mechanism of the relay just starts to move.
Types of Relays
Types of protection relays are mainly based on their characteristic, logic, on actuating parameter and operation mechanism. Based on operation mechanism protection relay can be categorized as electromagnetic relay, static relay and mechanical relay. Actually relay is nothing but a combination of one or more open or closed contacts. These all or some specific contacts the relay change their state when actuating parameters are applied to the relay. That means open contacts become closed and closed contacts become open. In electromagnetic relay these closing and opening of relay contacts are done by electromagnetic action of a solenoid. In mechanical relay these closing and opening of relay contacts are done by mechanical displacement of different gear level system. In static relay it is mainly done by semiconductor switches like thyristor. In digital relay on and off state can be referred as 1 and 0 state. Based on Characteristic the protection relay can be categorized as-
Definite time relays
Inverse time relays with definite minimum time(IDMT)
Instantaneous relays.
IDMT with inst.
Stepped characteristic.
Programmed switches.
Voltage restraint over current relay.
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