ENGINEERING ARTICLES

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 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 e...

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

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      ...

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.

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 e...

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.

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.

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 disconn...

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 dist...