Cooling Methods of a Transformer

Why it is needed to cool down the transformer?

No transformer is truly an 'ideal transformer' and hence each will incur some losses, most of which get converted into heat. If this heat is not dissipated properly, the excess temperature in transformer may cause serious problems like insulation failure. It is obvious that transformer needs a cooling system.

Transformers can be divided in two types as

1.               Dry type transformers.
2.               Oil immersed transformers.

Different cooling methods of transformers are

 For dry type transformers

• Air Natural (AN)

• Air Blast

For oil immersed Transformers

• Oil Natural Air Natural (ONAN)

• Oil Natural Air Forced (ONAF)

• Oil Forced Air Forced (OFAF)

• Oil Forced Water Forced (OFWF)

Cooling Methods for Dry Type Transformers

Air Natural or Self Air Cooled Transformer:  This method of transformer cooling is generally used in small transformers (upto 3 MVA). In this method the transformer is allowed to cool by natural air flow surrounding it.

Air Blast:   For transformers rated more than 3 MVA, cooling by natural air method is inadequate. In this method, air is forced on the core and windings with the help of fans or blowers. The air supply must be filtered to prevent the accumulation of dust particles in ventilation ducts. This method can be used for transformers upto 15 MVA.

Cooling Methods for Oil Immersed Transformers

Oil Natural Air Natural (ONAN):    This method is used for oil immersed transformers. In this method, the heat generated in the core and winding is transferred to the oil. According to the principle of convection, the heated oil flows in the upward direction and then in the radiator. The vacant place is filled up by cooled oil from the radiator. The heat from the oil will dissipate in the atmosphere due to the natural air flow around the transformer. In this way, the oil in transformer keeps circulating due to natural convection and dissipating heat in atmosphere due to natural conduction. This method can be used for transformers upto about 30 MVA.

Oil Natural Air Forced (ONAF):   The heat dissipation can be improved further by applying forced air on the dissipating surface. Forced air provides faster heat dissipation than natural air flow. In this method, fans are mounted near the radiator and may be provided with an automatic starting arrangement, which turns on when temperature increases beyond certain value. This transformer cooling method is generally used for large transformers upto about 60 MVA.

Oil Forced Air Forced (OFAF):   In this method, oil is circulated with the help of a pump. The oil circulation is forced through the heat exchangers. Then compressed air is forced to flow on the heat exchanger with the help of fans. The heat exchangers may be mounted separately from the transformer tank and connected through pipes at top and bottom as shown in the figure. This type of cooling is provided for higher rating transformers at substations or power stations.

Oil Forced Water Forced (OFWF):   This method is similar to OFAF method, but here forced water flow is used to dissipate hear from the heat exchangers. The oil is forced to flow through the heat exchanger with the help of a pump, where the heat is dissipated in the water which is also forced to flow. The heated water is taken away to cool in separate coolers. This type of cooling is used in very large transformers having rating of several hundred MVA.

Share

Blogger Widget

Maintenance Of SF6 Gas Circuit Breakers

Sulfur Hexa fluoride (SF6) is an excellent gaseous dielectric for high voltage power applications. It has been used extensively in high voltage circuit breakers and other switchgears employed by the power industry.

          Applications for SF6 include gas insulated transmission lines and gas insulated power distributions.

         The combined electrical, physical, chemical and thermal properties offer many advantages when used in power switchgears.

      Some of the outstanding properties of SF6 making it desirable to use in power applications are:

 High dielectric strength

Unique arc-quenching ability

Excellent thermal stability

Good thermal conductivity

Properties Of SF6 (Sulfur Hexafuoride) Gas

A) Toxicity:- SF6 is odorless, colorless, tasteless, and nontoxic in its pure state. It can, however, exclude oxy­gen and cause suffocation. If the normal oxygen content of air is re­duced from 21 percent to less than 13 percent, suffocation can occur without warning. Therefore, circuit breaker tanks should be purged out after opening.

B) Toxicity Of Arc Products:- Toxic decomposition products are formed when SF6 gas is subjected to an elec­tric arc. The decomposition products are metal fluorides and form a white or tan powder. Toxic gases are also formed which have the characteristic odor of rotten eggs. Do not breathe the vapors remaining in a circuit breaker where arcing or corona dis­charges have occurred in the gas.

Evacuate the faulted SF6 gas from the circuit breaker and flush with fresh air before working on the circuit breaker.

C) Physical Properties:- SF6 is one of the heaviest known gases with a den­sity about five times the density of air under similar conditions. SF6 shows little change in vapor pressure over a wide temperature range and is a soft gas in that it is more compressible dynamically than air.

The heat trans­fer coefficient of SF6 is greater than air and its cooling characteristics by convection are about 1.6 times air.

D) Dielectric Strength:- SF6 has a di­electric strength about three times that of air at one atmosphere pressure for a given electrode spacing. The dielectric strength increases with increasing pressure; and at three atmospheres, the dielectric strength is roughly equivalent to transformer oil. The heaters for SF6 in circuit breakers are required to keep the gas from liquefying because, as the gas liquifies, the pressure drops, lowering the dielectric strength.

      The exact dielectric strength, as compared to air, varies with electrical configuration, electrode spacing, and electrode configuration.

E) Arc Quenching:- SF6 is approxi­mately 100 times more effective than air in quenching spurious arcing. SF6 also has a high thermal heat capacity that can absorb the energy of the arc without much of a temperature rise.

F) Electrical Arc Breakdown:- Because of the arc-quenching ability of SF6, corona and arcing in SF6 does not occur until way past the voltage level of onset of corona and arcing in air. SF6 will slowly decompose when ex­posed to continuous corona.

All SF6 breakdown or arc products are toxic. Normal circuit breaker operation produces small quantities of arc products during current interruption which normally recombine to SF6.

Arc products which do not recombine, or which combine with any oxygen or moisture present, are normally re­moved by the molecular sieve filter material within the circuit breaker.

*------Handling Nonfaulted SF6-------*

The procedures for handling nonfaulted SF6 are well covered in manufacturer’s instruction books. These procedures normally consist of removing the SF6 from the circuit breaker, filtering and storing it in a gas cart as a liquid, and transferring it back to the circuit breaker after the circuit breaker maintenance has been performed.

No special dress or precautions are required when handling nonfaulted SF6.

*---------Handling Faulted SF6---------*

Toxicity

FAULTED SF6 GAS – Faulted SF6 gas smells like rotten eggs and can cause nausea and minor irritation of the eyes and upper respiratorNormally, faulted SF6 gas is so foul smelling no one can stand exposure long enough at a concentration high enough to cause permanent damage.

SOLID ARC PRODUCTS – Solid arc products are toxic and are a white or off-white, ashlike powder. Contact with the skin may cause an irritation or possible painful fluoride burn. If solid arc products come in contact with the skin, wash immediately with a large amount of water. If water is not available, vacuum off arc products with a vacuum cleaner.

Clothing and safety equipment requirements

When handling and re­ moving solid arc products from faulted SF6, the following clothing and safety equipment should be worn:

COVERALLS – Coveralls must be worn when removing solid arc products. Coveralls are not required after all solid arc products are cleaned up. Disposable coveralls are recommended for use when removing solid arc products; however, regular coveralls can be worn if disposable ones are not available, provided they are washed at the end of each day.

HOODS – Hoods must be worn when removing solid arc products from inside a faulted dead-tank circuit breaker.

GLOVES – Gloves must be worn when solid arc products are hah-died. Inexpensive, disposable gloves are recommended. Non-disposable gloves must be washed in water and allowed to drip-dry after use.

BOOTS – Slip-on boots, non-disposable or plastic disposable, must be worn by employees who enter eternally faulted dead-tank circuit breakers. Slip-on boots are not required after the removal of solid arc products and vacuuming. Nondisposable boots must be washed in water and dried after use.

SAFETY GLASSES – Safety glasses are recommended when handling solid arc products if a full face respirator is not worn.

RESPIRATOR – A cartridge, dust-type respirator is required when entering an internally faulted dead-tank circuit breaker. The respirator will remove solid arc products from air breathed, but it does not supply oxygen so it must only be used when there is sufficient oxygen to support life. The filter and cartridge should be changed when an odor is sensed through the respirator.

The use of respirators is optional for work on circuit breakers whose in­ terrupter units are not large enough for a man to enter and the units are well ventilated.

Air-line-type respirators should be used when the cartridge type is ineffective due to providing too short a work time before the cartridge becomes contaminated and an odor is sensed.

When an air-line respirator is used, a minimum of two working respirators must be available on the job before any employee is allowed to enter the circuit breaker tank.

Disposal of waste

All materials used in the cleanup operation for large quantities of SF6 arc products shall be placed in a 55­ gal drum and disposed of as hazardous waste.

Share

Blogger Widget

TYPES AND FUNCTIONS OF SUB-STATION

Types of Sub-station

Substations are of three types. They are:

>Transmission Substation

>Distribution Substation

>Collector Substation

Transmission Substation

A transmission substation connects two or more transmission lines. The simplest case is where all transmission lines have the same voltage. In such cases, the substation contains high-voltage switches that allow lines to be connected or isolated for fault clearance or maintenance. A transmission station may have transformers to convert the voltage from voltage level to other, voltage control devices such as capacitors, reactors or Static VAR Compensators and equipment such as phase shifting transformers to control power flow between two adjacent power systems. The largest transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, many circuit breakers and a large amount of protection and control equipment (voltage and current Transformers, relays and SCADA systems). Modern substations may be implemented using International Standards such as IEC61850.

Distribution Substation

A distribution substation transfers power from the transmission system to the distribution system of an area. It is uneconomical to directly connect electricity

consumers to the high-voltage main transmission network, unless they use large amounts of power. So the distribution station reduces voltage to a value suitable for local distribution. The input for a distribution substation is typically at least two transmission or sub transmission lines. Input voltage may be, for example, 400KV or whatever is common in the area. Distribution voltages are typically medium voltage, between 33 and 66 kV depending on the size of the area served and the practices of the local utility. Besides changing the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems. Distribution substations may also be the points of voltage regulation, although on long distribution circuits (several km/miles), voltage regulation equipment may also be installed along the line.

Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching and, switching and backup systems on the low-voltage side. Most of the typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side.

 Collector substation

In distributed generation projects such as a wind farm, a collector substation may be required. It somewhat resembles a distribution substation although power flow is in the opposite direction. Usually for economy of construction the collector system operates around 35 KV, and the collector substation steps up voltage to a transmission voltage for the grid. The collector substation also provides power factor correction, metering and control of the wind farm.

Functions of the substation

a. To Change voltage from one level to another.

b. To Regulate voltage to compensate for system voltage changes.

c. To Switch transmission and distribution circuits into and out of the grid system.

d. To Measure electric power quantity flowing in the circuits.

e. To Connect communication signals to the circuits.

f. To Eliminate lightning and other electrical surges from the           system.

g. To Connect electric generation plants to the system.

h. To Make interconnections between the electric systems of more than one utility. 

Substation Transformer Type

Further, transmission substations are mainly classified into two types depending on changes made to the voltage level. They are:

a. Step-Up Transmission Substations.

b. Step-Down Transmission Substations.

a. Step-Up Transmission Substation

A step-up transmission substation receives electric power from a nearby generating facility and uses a large power transformer to increase the voltage for transmission to distant locations.

There can also be a tap on the incoming power feed from the generation plant to provide electric power to operate equipment in the generation plant.

b. Step-Down Transmission Substation

Step-down transmission substations are located at switching points in an electrical grid. They connect different parts of a grid and are a source for sub transmission lines or distribution lines.

Layout

a. Principle of Substation Layouts

Substation layout consists essentially in arranging a number of switchgear components in an ordered pattern governed by their function and rules of spatial separation.

b. Special Separation

i. Earth Clearance: This is the clearance between live parts and earthed structures, walls, screens and ground.

ii. Phase Clearance: This is the clearance between live parts of different phases.

iii. Isolating Distance: This is the clearance between the terminals of an isolator and the connections.

iv. Section Clearance: This is the clearance between live parts and the terminals of a work section. The limits of this work section, or maintenance zone, may be the ground or a platform from which the man works. 

c. Separation of maintenance zones

Two methods are available for separating equipment in a maintenance zone that has been isolated and made dead.

i. The provision of a section clearance

ii. Use of an intervening earthed barrier The choice between the two methods depends on the voltage and whether horizontal or vertical clearances are involved.

i. A section clearance is composed of the reach of a man taken as 8 feet plus an Earth clearance.

ii. For the voltage at which the earth clearance is 8 feet the space required will be the same whether a section clearance or an earthed barrier is used.

Maintenances

Maintenance plays a major role in increasing the efficiency and decreasing the breakdown. The rules and basic principle are discussed.

Separation by earthed barrier = Earth Clearance + 50mm for barrier + Earth Clearance Separation by section clearance = 2.44m + Earth clearance

i. For vertical clearances it is necessary to take into account the space occupied by the equipment and the need for an access platform at higher voltages.

ii. The height of the platform is taken as 1.37m below the highest point of work.

Maintenance is done through two ways:

a. By Establishing Maintenance Zones.

b. By Electrical Separations.

a. Establishing Maintenance Zones

Some maintenance zones are easily defined and the need for them is self evident as in the case of a circuit breaker. There should be a means of isolation on each side of the circuit breaker, and to separate it from adjacent live parts when isolated either by section clearances or earth barriers.

b. Electrical Separations

Together with maintenance zoning, the separation, by isolating distance and phase clearances, of the substation components and of the conductors interconnecting them constitute the main basis of substation layouts. There are at least three such electrical separations per phase that are needed in a circuit:

i. Between the terminals of the bus bar isolator and their connections.

ii. Between the terminals of the circuit breaker and their connections.

iii. Between the terminals of the feeder isolator and their connections.

Share

Blogger Widget