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.

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Testing and Commissioning of Substation DC System

Objective

Power substation DC system consists of battery charger and battery. This is to verify the condition of battery and battery charger and commissioning of them.

Test Instruments Required

Following instruments will be used for testing:

Multimeter. (Learn how to use it)

Battery loading unit (Torkel-720 (Programma Make) or equivalent.

The Torkel-720is capable of providing a constant current load to the battery under test.

Commissioning Test Procedure

1.Battery Charger 

1. Visual Inspection: The battery charger cleanliness to be verified. Proper cable termination of incoming AC cable and the outgoing DC cable and the cable connection between battery and charger to be ensured. A stable incoming AC supply to the battery charger is also to be ensured.
2. Voltage levels in the Float charge mode and the Boost charge mode to be set
3. according to specifications using potentiometer provided.
4.  Battery low voltage, Mains ‘Off”, charger ‘Off’ etc., conditions are simulated and
5. checked for proper alarm / indication. Thus functional correctness of the battery charger is ensued.
6. Charger put in Commissioning mode for duration specified only one time during initial commissioning of the batteries. (By means of enabling switch.)
7. Battery charger put in fast charging boost mode and battery set boost charged for the duration specified by the battery manufacturer.
8. After the boost charging duration, the battery charger is to be put in float charging (trickle charge) mode for continuous operation.
9. Some chargers automatically switch to float charge mode after the charging current reduces below a certain value.
10. Voltage and current values are recorded during the boost charging and float charging mode.

This test establishes the correct operation of the battery charger within the specified voltage and current levels in various operational modes.

2. Battery Unit

 Mandatory Condition: The battery set should have been properly charged as per the commissioning instructions of the battery manufacturer for the duration specified

.
Visual Inspection: Cleanliness of battery is checked and the electrolyte level checked as specified on the individual cells. The tightness of cell connections on individual terminals should be ensured.

The load current, minimum voltage of battery system, ampere-hour, duration etc., is preset in the test equipment using the keypad.

It is to be ensured that the set value of the current and duration is within the discharge capacity of the type of cell used. Also the total power to be dissipated in the load unit should be within the power rating of the battery load kit.

Individual cell voltages to be recorded before the start of the test.

Battery chargers to be switched off/load MCB in charger to be switched off
Loading of the battery to be started at the specified current value.

 Individual cell voltages of the battery set are to be recorded every half an hour.
 It is to be ensured that all the cell voltages are above the end-cell voltage specified by the manufacturer.

If any of the cell voltages falls below the threshold level specified by the manufacturer, this cell number is to be noted and the cell needs to be replaced.

Test set automatically stops loading after set duration (or) when minimum voltage reached for the battery set.

Test to be continued until the battery delivers the total AH capacity it is designed for.Value of AH and individual cell voltages to be recorded every half an hour.         

Acceptance Limits

This test establishes the AH capacity of battery set at required voltage.

The acceptance limit for the test is to ensure the battery set is capable of supplying the required current at specified DC voltage without breakdown for the required duration.

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TYPES OF BUS BAR SYSTEM

TYPES OF BUS BAR SYSTEM

1 Single Busbar System

Single busbar system is as shown below in figure 

Single Busbar System

a. Merits

1. Low Cost

2. Simple to Operate

3. Simple Protection

b. Demerits

1. Fault of bus or any circuit breaker results in shut down of entire substation.

2. Difficult to do any maintenance.

3. Bus cannot be extended without completely deenergizing substations.

c. Remarks

1. Used for distribution substations up to 33kV.

2. Not used for large substations.

3. Sectionalizing increases flexibility.

2 Main & Transfer Bus bar System

Main & Transfer Bus is as shown below in figure 

a. Merits

1. Low initial & ultimate cost

2. Any breaker can be taken out of service for maintenance.

3. Potential devices may be used on the main bus.

b. Demerits

1. Requires one extra breaker coupler.

2. Switching is somewhat complex when maintaining a breaker.

3. Fault of bus or any circuit breaker results in shutdown of entire substation.

c. Remarks

1. Used for 110kV substations where cost of duplicate bus bar system is not justified. 

3 Double Bus bar Single Breaker system

Double Bus Bar with Double Breaker is as shown below in figure 

a. Merits

1. High flexibility

2. Half of the feeders connected to each bus

b. Demerits

1. Extra bus-coupler circuit breaker necessary.

2. Bus protection scheme may cause loss of substation when it operates.

3. High exposure to bus fault.

4. Line breaker failure takes all circuits connected to the bus out of service.

5. Bus couplers failure takes entire substation out of service.      

c. Remarks

Most widely used for 66kV, 132kv, 220kV and important 11kv, 6.6kV, 3.3kV

Substations.

4 Double Bus bar with Double breaker System

 Double Bus Bar with Double breaker system is as shown below in figure 

a. Merits

1. Each has two associated breakers

2. Has flexibility in permitting feeder circuits to be connected to any bus

3. Any breaker can be taken out of service for maintenance.

4. High reliability

b. Demerits

1. Most expensive

2. Would lose half of the circuits for breaker fault if circuits are not connected to both the buses.

c. Remarks

1. Not used for usual EHV substations due to high cost.

2. Used only for very important, high power, EHV substations.

5 Double Main Bus & Transfer Busbar System

Double main bus & transfer bus system is as shown below in figure

a. Merits

1. Most flexible in operation

2. Highly reliable

3. Breaker failure on bus side breaker removes only one ckt. From service

4. All switching done with breakers

5. Simple operation, no isolator switching required

6. Either main bus can be taken out of service at any time for maintenance.

7. Bus fault does not remove any feeder from the service

b. Demerits

1. High cost due to three buses

c. Remarks

1. Preferred by some utilities for 400kV and 220kV important substations.

6 ONE & HALF BREAKER SCHEME

a. Merits

1. Flexible operation for breaker maintenance.

2. Any breaker can be removed from maintenance without interruption of load.

3. Requires 1 1/2 breaker per feeder.

4. Each circuit fed by two breakers.

5. All switching by breaker.

6. Selective tripping.

b. Demerits

1. One and half breakers per circuit, hence higher cost

2. Any breaker can be removed from maintenance without interruption of load.

c. Remarks

1. Used for 400kV & 220kV substations.

2. Preferred.

7 RING OR MESH ARRANGEMENT

a.      Merits

Bus bars gave some operational flexibility.

b.      Demerits

1. If fault occurs during bus maintenance, ring gets separated into two sections.

2. Auto-reclosing and protection complex.

3. Requires VT’s on all circuits because there is no definite voltage reference point.

4. Breaker failure during fault on one circuit causes loss of additional circuit because of breaker failure.

These VT’s may be required in all cases for synchronizing live line or voltage indication

c.       Remarks

 Most widely used for very large power stations having large no. of incoming and outgoing lines and high power transfer.

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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.

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