Proc. of Int. Conf. on Control, Communication and Power Engineering 2010
the setting of restraint is adequate to prevent the relay operation. In fig.8, various protection schemes are represented for power transformer. Also, differential protection scheme for 13.8 kV, breaker failure for 115 kV may exist.
2
115 kV 50/51 P,N,Q
400:5
52
400:5
52
52
Fig. 6. Sudden Pressure relay 63 49
G. Turn- to- turn fault protection: Due to electromechanical /electromagnetic forces on the winding, chafing or cracking of insulation between turns takes place. Ingress of moisture content in to the oil can also be a contributing factor. When the transformer is subjected to lightning surge, the steep–fronted surges may hit the end windings causing the puncture in the insulation. It thus leads to short circuit. Phase differential relay may detect a turn-to-turn fault protection as this fault changes the transformer turns ratio. The fault case is given in fig.7.
18/24/30 MVA 400:5
RG 400A 20 Ω 10 Sec
RG 51G
87N
Trip Dir 51 P.Q,N Partial Differential
2000:5 Bus BU
67/51 P,Q,N 32
52
E Short Turn
52
27 P, 59P/N, 47
L O A D
2000:5 51 P.Q,N
Fig.7. Turn-Turn fault
Bus Tie 52
13.8 kV
2000:5
H. Excessive high voltage protection: The relay offers high impedance to CTs. Hence, CTs are required to develop an extremely high voltage. To limit this voltage, a voltage dependent resistor (VDR) is normally mounted across the relay to prevent the external flashover. It is usually preferred in polluted environments. For maximum efficiency, transformers are operated near the knee of their saturation curve. So, at voltage above about 110% of rated, the exciting current becomes very high. These large currents can destroy the units. The protection against over voltage is included in the regulating and control devices. I.
Harmonic restraint protection:
The magnetizing inrush current has a high harmonic content, particularly second harmonic. The second harmonic can be used to restrain and thus desensitize the relay during energization. While setting the relay in such case, it should be made sensitive to internal fault. In harmonic restraint relay unit, a second harmonic blocking filter is connected in operating coil circuit, whereas a second harmonic pass filter in restraint coil circuit. Thus, the predominant second harmonic characteristics of an inrush current produce ample restraint. It requires minimum operating energy. The circuit is designed to hold open its contact when secondary harmonic component is higher than 15% of fundamental. For all inrush currents,
F1
F2
F3
F4
a) Relay units: 50-Instantaneous, 51-Time over current, 49-Thermal, 63-Sudden Pressure, 67-Directional over current, 87-Differential b) 52AC circuit breaker c) RG – Ground resistance Fig.8. Various protection for power transformer.
J. Thermal Protection The conventional thermal relays measure the oil temperature and transformer current to calculate hot-spot temperature. It is usually supplied as a part of transformer. It is used for monitoring and alarm, but may be used for tripping. K . Gas Detection: Gas detection devices can be applied only to the transformer units built with conservator tanks. A gas accumulator device commonly known as Buchholz relay is connected between the main and conservator tanks. Failure of winding insulation will result in some form of arcing. It collects any gas rising through oil. It decomposes the oil into hydrogen, acetylene, methane. It can detect the core fault. The intense localized heat damages the winding insulation. Localized heating can also precipitate a breakdown of oil into gas. One part of this device accumulates gas over a time period and provides sensitive indication of low energy arcs. It is used for alarm. The other part of this device
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