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1. Buchholz Relay- Transformer Protection 
2. Basic Principle of Auto Transformer


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Equivalent Inductance of Series Connected Inductor

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When two or more  inductors  are connected in such a way that same current flows through them, then it is said that they are connected in series. Like series connection of  resistors , series connected inductors can be replaced by an equivalent inductor. Let us assume that ‘n’ inductors having inductance of L 1 , L 2 , L 3 ,…L n  Henry are connected in series as shown in figure below. Since same current I is flowing through each of the inductors therefore voltage drop across inductors L 1 , L 2 , L 3 , …..L n  will be L 1 di/dt, L 2 di/dt, L 3 di/dt, …..L n di/dt respectively. Therefore from Kirchhof’s voltage law, V = L 1 di/dt + L 2 di/dt + L 3 di/dt + ……..+ L n di/dt     = di/dt (L 1  + L 2  + L 3  +…….+L n )  …………….(1) If the series connected inductors are replace by a single equivalent inductor Le then V = L e di/dt  …………………….(2) Comparing equation (1) and (2) we get, L e  = L 1  + L 2  + L 3  +…….+L n “It shall be kept in mind that the above equation for e
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Buchholz Relay- Transformer Protection

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Buchholz relay is a gas actuated protection relay which is generally used in large oil immersed transformers of rating more than 500 kVA. It is used for the protection of a Transformer from the faults occurring inside the transformer. A typical Buchhloz Relay is shown below. The gas actuated protective relay is designed to detect faults as well as to minimize the propagation of any damage, which might occur within oil-filled Transformers. The Buchholz relay is therefore particularly effective in case of: Short-circuited core laminations Broken-down of core bolt insulation Overheating of some part of the windings Bad contacts Short circuits between phases, turns Earth faults-puncture of bushing insulators inside tank Buchholz relay `can prevent the development of conditions leading to a fault in the transformer, such as the falling of the oil level due to leaks, or the penetration of air as a result of defects in the oil circulating system.
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