APPLICATION OF DIFFERENTIAL PROTECTION

Most differential protection relays are current differential relays in which vector difference between the current entering the winding and current leaving the winding is used for sensing and relay operation.

Differential protection principle is used I the following applications.

§ Protection of the generator, protection of generator transformer unit.

§ Protection of transformer.

§ Protection of feeder (transmission line) by pilot wire differential protection.

§ Protection of transmission line by phase comparison carrier current protection.

§ Protection of large motor.

§ Bus-zone protection.

DIFFERENTIAL PROTECTION OF POWER TRANSFORMER

Differential protection is based on the fact that any fault within electrical equipment would cause the current entering it, to be different, from that leaving it. Thus, we can compare the two currents either in magnitude or in phase or both and issue a trip output if the difference exceeds a predetermined set value. This method of detecting faults is very attractive when both ends of the apparatus are physically located near each other. A typical situation, where this is true, is in the case of a transformer, a generator or a bus bar. In the case of transmission lines, the ends are too far apart for conventional differential relaying to be directly applied.

Differential protection is a unit-type protection for a specified zone or piece of equipment. It is based on the fact that it is only in the case of faults internal to the zone that the differential current (difference between input and output currents) will be high. However, the differential current can sometimes be substantial even without an internal fault. This is due to certain characteristics of current transformers (different saturation levels, nonlinearities) measuring the input and output currents, and of the power transformer being protected.

with the exception of the inrush and over excitation currents, most of the other problems, can be solved by means of the percent differential relay, which adds to the normal differential relay two restraining coils fed by the zone-through current, by proper choice of the resulting percent differential characteristic, and by proper connection of the current transformers on each side of the power transformer.

TRANSFORMER NAMEPLATE RATING

Following are the minimum information and Data which to be shown on a transformer nameplate. The standards require the following information and data for transformers rated above 500 kVA.

• Name of manufacturer
• Serial number
• year of manufacture
• Number of phases
• kVA or MVA rating
• Frequency
• Voltage ratings.
• Tap voltages.
• Connection diagram.
• Cooling class
• Rated temperature in °C
• Polarity (for Single Phase Transformers)
• Phasor or vector diagram (For Polyphase or Three Phase Transformers)
• % impedance.
• Approximate mass or weight of the transformer
• Type of insulating liquid.
• Conductor material of each winding.
• Oil volume (of each transformer Container/Compartment)
• Instruction for Installation and Operation

COOLING METHODS OF POWER TRANSFORMER

As the size and capacity of the transformer increased, the associated cooling arrangement become more powerful and sophisticated. So, by definition, the transformer cooling system is such arrangement for power transformers, which limits the generated heat into a safe value by means of proper dissipation of generated heat. Different cooling system is used for different types of transformers, and they are discussed as follows.

Generally, two types of transformers are there according to the use of insulating oil, namely Dry Type Transformers and Oil Immersed Type Transformers. In oil immersed type, the transformer core is immersed into the transformer oil. Different types of cooling are needed for these two categories. In dry type transformers, air is used as the coolant medium but in oil immersed transformer (as the size and ratings both are high), both air and transformer oil are used as the coolant medium.

Dry Type Transformers	Oil Immersed Transformers
Air Natural Type	Oil Natural Air Natural type (ONAN)
Air forced Type	Oil Natural Air forced Type (ONAf)
	Oil forced Air forced Type (OfAf)
	Oil Directed Air forced Type (ODAf)
	Oil forced Water forced Type (OfWf)

Air Natural Type:

This cooling method is used in dry type transformer with smaller ratings. As the name implies, the natural circulation of atmospheric air is used in this technique. This type of transformers is also referred as self-cooled transformer. When the transformer is operated in full load, then the temperature of the transformer becomes greater than the ambient air temperature. So, by convection process, the light and heated air gets replaced by the heavy and comparatively cool surrounding air. In this way, the generated heat is dissipated via the natural air circulation process. But this type of cooling arrangement provides satisfactory operation for low voltage transformers only.

Air forced type:

This cooling method is also used in dry type transformer but the application is also implemented in oil immersed transformers. As the name implies, in addition with natural air circulation, cool air with high velocity is provided to the core. High speed fans are provided with the transformer, and by the rotation of this fan high velocity of air is subjected to the transformer. This additional air force ensures quicker heat dissipation of the transformers. The fans are automatically controlled, that is when the temperature of the transformer core goes beyond the safe limit than all the fans are switched ON. Air forced cooling method provides better performance than natural air cooling, but additional cost is associated for the fans.

Oil Natural Air Natural (ONAN) Cooling Of Transformer:

This cooling system is used in oil immersed type transformers. This is the simplest way of cooling of oil immersed transformers. We know that, the transformer core is immersed in transformer oil. When the transformer is heated up, then temperature of oil near to the transformer core is also raised. So, the light and heated oil flows in upward direction and comparatively cool and heavy oil takes the vacuum places by natural convection process. And the heated oil releases its temperature into the atmosphere. In this way, a natural oil circulation cycle is generated and this cycle continues until the transformer temperature is tapped into a safe limit. In this method, the surface area of the oil tank is usually larger, as more surface area provides more quick heat dissipation process. But to provide more surface areas, several hollows tubular plates are attached with the transformer and they are termed as radiator. When the hot oils are circulating through the radiators, they get more surface area so the cooling rate is much higher. Also the light and heated air gets replaced by the heavy and comparatively cool surrounding air by natural process. In this way, the generated heat is dissipated via the natural air circulation process as well as natural oil circulation.

Oil Natural Air forced (ONAF) Cooling of Transformer:

The word ‘forced’ signifies that, air is forcefully applied to the transformer. High speed fans are provided with this type of cooling system. In larger rated oil immersed transformers, natural air and oil cooling is not sufficient. So, additional air force is applied to the radiator by means of those fans and this method provides quicker heat dissipation of the transformers as compared to natural oil and air cooling. All fans are automatically controlled, whenever the temperature of the transformer goes beyond the safe limit than all the fans are switched ON. But here the oil circulation process made by natural convection, that is no oil pumps are provided for this type of cooling. This method provides better performance than natural oil and air cooling, but there is an additional cost due to the fans.

Oil forced Air forced (OfAf) Cooling of Transformer:

Actually, for very large rated oil immersed transformers, heat generated is quite high. Therefore, some special cooling techniques are applied in order to provide sufficient heat dissipation. In Oil forced Air forced cooling system, both oil and air are circulated at high speed by some additional configuration. High speed fans are connected to provide additional air flow of high velocity and oil pumps are provided to circulate the oil at high velocity. So, hot oil is circulated inside the main transformer tank at larger velocity, so the rate of cooling is further increased. Therefore, in oil forced air forced cooling system; both the oil and air are forcefully applied to achieve more fast cooling process.

Oil Directed Air forced Cooling of Transformer:

This is the updated version of Oil forced Air forced cooling method. Here the Oil and Air both are applied forcefully, but the hot oil follows a specific route for flowing. Convection channels are made closer to the winding of the transformer and the transformer oil is passed through those channels. In this way, superior heat dissipation is occurred.

Oil forced Water forced Cooling of Transformer:

Water is far better coolant than atmospheric air. So, in this method water is used as the oil coolant instead of natural air. Here, the flow of hot oil is directed to a heat ex-changer where water shower is applied. So, here the oil is cooled at faster rate than natural air cooling.

Therefore, from the above discussion, we understood the necessity of transformer cooling and also learned about the various types of transformer cooling.

TYPES OF POWER TRANSFORMER FAILURES

The electrical winding and the magnetic core in a transformer are subject to a number of different forces during operation, for example.

a) Expansion and contraction due to thermal cycling.
b) Vibration.
c) Local heating due to magnetic flux.
d) Impact forces due to through fault current.
e) Excessive heating over loading or inadequate cooling.

These forces can cause deterioration and failure of the winding electrical insulation.

WORKING PRINCIPLE OF POWER TRANSFORMER

The main principle of operation of a transformer is mutual inductance between two circuits which is linked by a common magnetic flux. A basic transformer consists of two coils that are electrically separate and inductive, but are magnetically linked through a path of reluctance. The working principle of the transformer can be understood from the figure.

As shown above the transformer has primary and secondary windings. The core laminations are joined in the form of strips in between the strips you can see that there are some narrow gaps right through the cross-section of the core. These staggered joints are said to be ‘imbricated’. Both the coils have high mutual inductance. A mutual electro-motive force is induced in the transformer from the alternating flux that is set up in the laminated core, due to the coil that is connected to a source of alternating voltage. Most of the alternating flux developed by this coil is linked with the other coil and thus produces the mutual induced electro-motive force. The so produced electro-motive force can be explained with the help of Faraday’s laws of Electromagnetic Induction as

If the second coil circuit is closed, a current flow in it and thus electrical energy is transferred magnetically from the first to the second coil.

The alternating current supply is given to the first coil and hence it can be called as the primary winding. The energy is drawn out from the second coil and thus can be called as the secondary winding.

In short, a transformer carries the operations shown below:

Ø Transfer of electric power from one circuit to another.

Ø Transfer of electric power without any change in frequency.

Ø Transfer with the principle of electromagnetic induction.

Ø The two electrical circuits are linked by mutual induction.

MAIN CONSTRUCTIONAL PARTS OF POWER TRANSFORMER

There are three main parts of a power transformer.

1. Primary winding of transformer – which produces magnetic flux when it is connected to electrical source.

2. Magnetic Core of transformer – the magnetic flux produced by the primary winding, will pass through this low reluctance path linked with secondary winding and creates a closed magnetic circuit.

3. Secondary Winding of transformer – the flux, produced by primary winding, passes through the core, will link with the secondary winding. This winding is also wound on the same core and gives the desired output of the transformer.

DIFFERENTIAL PROTECTION OF A POWER TRANSFORMER

Power transformers need to be protected from damages caused by internal and external faults. Power transformer can be protected from the internal faults by Differential Relay. This task is performed by relay protection which detects the fault situation and gives command to the relevant circuit breaker(s) to disconnect the faulty equipment from the rest of the power system. Power transformers are normally protected by differential protection relays.

Principle of Differential Protection

Principle of Differential Protection scheme is one simple conceptual technique. The differential relay actually compares between primary current and secondary current of power transformer, if any unbalance found in between primary and secondary currents the relay will actuate and inter trip both the primary and secondary circuit breaker of the transformer.

TYPES OF FAULTS ON POWER TRANSFORMER

Two types of faults occur in power transformer i.e. external and internal electrical faults.
External Faults in Power Transformer:

(a) External Short – Circuit of Power Transformer:

The short – circuit may occur in two or three phases of electrical power system. The level of fault current is always high enough. It depends upon the voltage which has been short – circuited and upon the impedance of the circuit up to the fault point.

(b) High Voltage Disturbance in Power Transformer:

High Voltage Disturbance in Power Transformer has two types;

I) Transient Surge Voltage II) Power Frequency over Voltage

I) Transient Surge Voltage:

High voltage and high frequency surge may arise in the power system due to any of the following causes;

(A) Arcing ground if neutral point is isolated.

(B) Switching operation of different electrical equipment.

(C) Atmospheric Lightening Impulse.

II) Power Frequency over Voltage:

There may be always a chance of system over voltage due to sudden disconnection of large load. Though its amplitude is increased, but its power frequency remains same. Over voltage in the system causes an increase in stress on the insulation of transformer. This stress happens due to increase in flux magnitude, which can strike transformer’s steel body & causes damage.

Internal Faults in Power Transformer:

The principle faults which occur inside a power transformer are categorized as;

(1) Insulation breakdown between winding and earth.

2) Insulation breakdown in between different phases.

3) Insulation breakdown in between adjacent turns i.e. inter – turn fault.

4) Transformer core fault.

POWER TRANSFORMER AND ITS IMPORTANCE

POWER TRANSFORMER

“A Power transformer is a static machine used for transforming power from one circuit to another circuit without changing Power and frequency”.

Power transformers used between the generator and the distribution circuits, and these are usually rated at 500 KVA and above. Power systems typically consist of a large number of generation locations, distribution points, and interconnections within the system or with nearby systems, such as a neighboring utility. The complexity of the system leads to a variety of transmission and distribution voltages. Power transformers must be used at each of these points where there is a transition between voltage levels.

Importance of a Power Transformer:

Power is the product of current and voltage (P=V x I). For a given amount of power, a low voltage requires a higher current and higher voltage requires a lower current. Since metal conducting wires have a certain resistance, some power will be wasted as heat in the wires of the distribution system. This power loss is given by

P _loss = R*I²

 Thus, it is overall transmitted power is same and given the constraints of practical conductor sizes, a low-voltage, higher current based electricity distribution system will have a much greater power loss than a high voltage, low current based one.

Electrical power has passed through a series of transformers by time it finally reaches the consumer. Power transformers are available for step-up operation, primarily used at the generator and referred to as generator step-up transformers, and for step-down operation, mainly used to feed distribution circuits.

Power transformers come in a range of sizes from a palm-sized transformer inside mobile phones chargers to a huge Giga-volt-amperes rated unit used to interconnect parts of national power grid.