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Installation-commission-maintenance part 6

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Topic includes

- Transformer maintenance
- LT switch 
- Lightening Arrestor
- It's types
- Surge arrestor
- Cross arm
- Gang operated switch
- Electrical Earthing

Transformer maintenance

Here are 3 maintenance tests you should run on your transformer on a daily basis:

Oil levels of MOG( Magnetic Oil Gauge) of the main tank and conservator tank. Always maintain to keep oil filled up to the desired level in MOG.
Replace the silica gel if its color changes to pink.
If any leakage is detected seal it.


How often should transformer oil be changed?


DGA or Dissolve Gas Analysis of transformer Oil should be performed, annually for 132 KV transformer, once in 2 years for the transformer below 132 KV transformer and in 2 years interval for the transformer above 132 KV transformer.

Electrical transformers are an expensive and important part of any machinery. So for getting optimal performance and increased product life, it is best to conduct regular maintenance checkups on your machinery.

There are basically two types of transformer maintenance. One is done on a regular basis and the other is done at the time of urgency. If maintenance checkups are done on a routine basis then you may not have to undergo breakdown maintenance and hence save downtime.

There are a lot of different types of maintenance checks to be conducted on a power transformer. Here are some of the main transformer maintenance actions that have to be performed:

Transformer Maintenance Checks on a Monthly Basis

Oil level in the oil cap must be checked on a monthly basis so that it doesn’t drop below a fixed limit and hence avoid damage due to it.
Keep the breathing holes in the silica gel breather clean to ensure proper breathing action at all times.
If your electrical transformer has oil filling bushing, make sure that the oil is filled up to the correct level.
Daily Basis Maintenance Testing and Checking

The air fans, oil pumps along with other items that are used to cool down a transformer and control circuit must be inspected annually.

Make sure that you clean all the bushings of your electrical transformer with only soft cotton cloth annually.
Oil condition of OLTC should be carefully examined on an annual basis. For that take an oil sample from drain valve and test it for moisture content (PPM) and dielectric strength (BDV). If the BDV value is found low and the PPM value high, then the oil needs to be replaced.
Make sure to clean out the inside of all of the marshalling boxes annually. Check proper functioning of the space and illumination heaters. All of the terminal connections of control and relay wiring need to be tightened at least once a year.

All the control switches, alarms and relays along with their circuits, Remote Tap Changer Control Panel and Relay and Control Panel have to be cleaned with a proper cleaning agent.
Examine all the pockets for the Winding Temperature Indicator and Oil Temperature Indicator if they have the necessary level of oil and make sure to top it up if required.
The proper function of Buchholz and Press Release Device relay need to be checked on a yearly basis.
Make sure to measure the resistive value of the earth connection and rizer should be measured annually with a clamp on the earth resistance meter.

Transformer Maintenance on a Half Yearly Basis


Your electrical transformer needs to be checked every 6 months for IFT, DDA, flash point, sludge content, acidity, water content and dielectric strength along with how resistant it is to transformer oil.

Maintenance of Current Transformer


A current transformer is a vital part of any equipment installed in an electrical substation for protection purposes as well as for electrical measurement. Insulation resistance of the CT has to be checked on an annual basis.

Thermo vision scanning of primary terminals and the top dome of a live CT must be conducted on an annual basis.
All CT secondary connections should be examined, cleaned and tightened each year to ensure that the secondary currents have the lowest resistance path.


LT switch 

Lighting switch
The LT-Series toggle switches are AC rated illuminated toggle switches featuring a three-color lighting sequence from a single lamp. These lighted toggle switches contain neoprene bushing seals for dust and moisture protection. ... LT-Series switches fit the industry standard .


Lighting arrestor


A lightning arrester (also called lightning diverter) is a device used on electric power systems and telecommunication systems to protect the insulation and conductors of the system from the damaging effects of lightning. The typical lightning arrester has a high-voltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrester, in most cases to earth.

What is the working principle of lightning arrester?

Lightning Arrester Working Principle
When a voltage surge traveling along the conductor reaches the point at which a lightning arrestor is installed it breaks down the insulation of the arrester momentarily, allowing the voltage surge to discharge to ground.

What are the types of lightning arrester?

The lightning arrestor is mainly classified into twelve types. These types are;

Road Gap Arrester. Sphere Gap Arrester.
Their types are explained below in details.
Rod Gap Arrester. ...
Sphere Gap Arrester. ...
Horn Gap Arrester. ...
Multiple- Gap Arrester. ...

Rod Gap Arrester

It is one of the simplest forms of the arrester. In such type of arrester, there is an air gap between the ends of two rods. The one end of the arrester is connected to the line and the second end of the rod is connected to the ground. The gap setting of the arrester should be such that it should break before the damage. When the high voltage occurs on the line, the gap sparks and the fault current passes to the earth. Hence the equipment is protected from damage.

The difficulty with the rod arrester is that once the spark having taken place it may continue for some time even at low voltages. To avoid it a current limiting reactor in series with the rod is used. The resistance limits the current to such an extent that it is sufficient to maintain the arc. Another difficulty with the road gap is that the rod gap is liable to be damaged due to the high temperature of the arc which may cause the rod to melt


. Sphere Gap Arrester

In such type of devices, the air gap is provided between two different spheres. One of the spheres is connected to the line, and the other sphere is connected to the ground. The spacing between the two spheres is very small. A choking coil is inserted between the phase winding of the transformer and spheres is connected to the line.

Horn Gap Arrester

It consists of two horns shaded piece of metal separated by a small air gap and connected in shunt between each conductor and earth. The distance between the two electrodes is such that the normal voltage between the line and earth is insufficient to jump the gap. But the abnormal high voltage will break the gap and so find a path to earth.

Multiple- Gap Arrester

The multiple gap arrester consists a series of small metal cylinder insulated from one another and separated by an air gap. The first and the last of the series is connected to ground. The number of gaps required depends on the line voltage.

Expulsion Type Lightning Arrester

Expulsion type arrester is an improvement over the rod gap in that it seals the flow of power frequency follows the current. This arrester consists of a tube made up of fibre which is very effective, isolating spark gap and an interrupting spark gap inside the fibre tube

Valve Type Lightning Arrester

Such type of resistor is called nonlinear diverter. It essentially consists a divided spark gap in series with a resistance element having the nonlinear characteristicDuring the slow voltage variations, there is no sparks-over across the gap. But when the rapid change in voltage occurs, the potential is no longer evenly graded across the series gap. The influence of unbalancing capacitance between the sparks gaps and the ground prevails over the grounded resistance. The impulse voltage is mainly concentrated on the upper spark gap which in spark over cause the complete arrester to spark over to.

What is difference between lightning arrester and surge arrester?

Surge arrester is used to protect the circuit or electrical equipment from high voltage transient or electrical surges. ... Lightning arrester works like surge arrester from outside of the conductor. They absorb the electrical surges in the transmission tower itself. Lightning rod is a pre protection device.

Is lightning DC or AC?

Lightning cannot be AC, since the wave shape of Lightning current is not sine-wave shaped voltage. A typical lightning current waveform looks something like this as shown below. This complete event (A, B and C) is a single strike. Therefore, Lightning is neither DC nor AC

What is Ferranti surge absorber?

Ferranti Surge Absorber. ... To reduce the steepness of the wavefront of a surge generally, surge absorbers are used. A surge absorber is a protective device which reduces the steepness of wavefront of a surge by absorbing surge energy

What is a surge absorber?

Surge absorbers are protective devices used to absorb the complete surge i,e. due to lightening surge or any transient surge in the system unlike the lightening arrestor in which a non-linear resistor is provided which provides a low resistance path to the dangerously high voltages on the system to the earth.

How does a surge diverter work?

Surge diverters work by capturing the excess electricity that would otherwise be absorbed into the house's electrical circuit. They will help protect the property by diverting this power away from the house by “shunting” it to the earth. ... Any electricity over 260 volts will automatically be diverted to the ground.

Cross Arm


The function of line support (pole) is to support the line conductors at a safe distance from ground whereas the function of cross arms is to keep the conductors at a safe distance from each other and from the pole.
Cross Arm is a cross-piece fitted to the pole top end portion by means of brackets, known as pole brackets, for supporting insulators.

Cross arms are of various types such as MS channel, angle iron or wooden. These may be straight, U-shaped, U-shaped or zig-zag shaped, as shown in the below image.

Wooden cross arms are commonly employed on 11 KV and 33 KV lines. These are made of sal wood, seasoned Sheridan wood or creosoted firewood.

Grains of wood cross arms should be length-wise and completely free from knots.

Wooden cross arms are preferred owing to their insulating property which provides safety to line staff and minimizes flash-over due to the bridge.

What is gang operated switch?

The most commonly used switch in small to medium substations is a gang-operated, air-break disconnect switch. "Gang-operated" because the three separate switches for each phase are operated as a group from a single control; "air-break" because the switch operates in air rather than in another medium, such as oil.

What are the four types of switches?
The types of switches are classified into four types namely:
SPST (Single Pole Single throw)
SPDT (single pole double throw)
DPST (double pole, single throw)
DPDT (double pole double throw)

Electrical Earthing

Definition: The process of transferring the immediate discharge of the electrical energy directly to the earth by the help of the low resistance wire is known as the electrical earthing. The electrical earthing is done by connecting the non-current carrying part of the equipment or neutral of supply system to the ground.

Mostly, the galvanised iron is used for the earthing. The earthing provides the simple path to the leakage current. The shortcircuit current of the equipment passes to the earth which has zero potential. Thus, protects the system and equipment from damage.

Types of Electrical Earthing

The electrical equipment mainly consists of two non-current carrying parts. These parts are neutral of the system or frame of the electrical equipment. From the earthing of these two non-current carrying parts of the electrical system earthing can be classified into two types.

Neutral Earthing
Equipment Earthing.

Neutral Earthing
In neutral earthing, the neutral of the system is directly connected to earth by the help of the GI wire. The neutral earthing is also called the system earthing. Such type of earthing is mostly provided to the system which has star winding. For example, the neutral earthing is provided in the generator, transformer, motor etc.

Equipment  Earthing
Such type of earthing is provided to the electrical equipment. The non-current carrying part of the equipment like their metallic frame is connected to the earth by the help of the conducting wire. If any fault occurs in the apparatus, the short-circuit current to pass the earth by the help of wire. Thus, protect the system from damage.

Importance of Earthing

The earthing is essential because of the following reasons

The earthing protects the personnel from the shortcircuit current.
The earthing provides the easiest path to the flow of shortcircuit current even after the failure of the insulation.
The earthing protects the apparatus and personnel from the high voltage surges and lightning discharge.

Earthing can be done by electrically connecting the respective parts in the installation to some system of electrical conductors or electrodes placed near the soil or below the ground level. The earthing mat or electrode under the ground level have flat iron riser through which all the non-current-carrying metallic parts of the equipment are connected.


When the fault occurs the fault current from the equipment flows through the earthing system to the earth and thereby protect the equipment from the fault current. At the time of the fault, the earth mat conductors rise to the voltage which is equal to the resistance of the earth mat multiplied by a ground fault.

The contacting assembly is called earthing. The metallic conductors connecting the parts of the installation with the earthing are called electrical connection. The earthing and the earthing connection together called the earthing system.


Dielectric

A Dielectric or dielectric material is an electrical insulator that can be polarized by an applied electric field . 
when a dielectric material is placed in an electric field ok, electric charges do not flow through the the material as they do in electrical conductor but only shifts from their average equilibrium position causing dielectric polarization.
Because of dielectric polarization positive charges are displayed in the direction of the field and negative charges shift in the direction opposite to the field.


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