# Analyze the classification of power transformers and the configuration principles of protection devices

2019-04-30 11:19:26 JuKe Technology (DongGuan) Co., Ltd. Read

Analyze the classification of power transformers and the configuration principles of protection devices

When it comes to power transformers, it is one of the key configurations in many power plants and substations. It can generate power loss on the line resistance when the load current passes through the line. △P=I^2*R, when I is smaller, the lower the loss, the length of the transmission line is certain, R is certain, the loss is reduced, and the power generated by the power plant is only boosted by the transformer and then transmitted to the user end. After the transformer is stepped down for the user to use, the main pressure function of the transformer is transformed. Because there are a large number of transformers, motors and other inductive components in the power line, in order to improve the power factor and reduce the reactive power, a capacitor cabinet should be added.

Power transformer classification

Power transformers are classified according to usage: boost (power plant 6.3kV/10.5kV or 10.5kV/110kV, etc.), contact (220kV/110kV or 110kV/10.5kV between substations), buck (35kV/0.4kV for power distribution or 10.5kV/0.4kV).

Power transformers are classified by phase number: single phase, three phase.

Power transformers are classified according to windings: double windings (each phase is mounted on the same core, the primary and secondary windings are separately wound and insulated from each other), and three windings (three windings per phase, the primary and secondary windings are separately wound and insulated from each other), Autotransformer (one set of winding center taps for primary or secondary output). A three-winding transformer requires that the capacity of the primary winding be greater than or equal to the capacity of the secondary and tertiary windings. The percentage of the three-winding capacity is in the order of high pressure, medium pressure, and low pressure: 100/100/100, 100/50/100, 100/100/50, and the secondary and tertiary windings are not required to be fully loaded. Generally, the tertiary winding voltage is low, and it is mostly used for near-field power supply or compensation equipment for connecting three voltage levels. Autotransformer: There are two kinds of booster or step-down. Because of its low loss, light weight and economical use, it is widely used in ultra-high voltage power grids. The commonly used model of small autotransformers is 400V/36V (24V), which is used for power supply such as safety lighting.

Power Transformers are classified according to insulating media: oil-immersed transformers (flame-retardant, non-flame-retardant), dry-type transformers, 110kV SF6 gas-insulated transformers. The power transformer cores are all core structures. The three-phase power transformer configured in the general communication engineering is a two-winding transformer.

The reliability of power transformer operation is very high. Due to the great impact caused by the transformer failure, the function of the relay protection device should be strengthened to improve the safe operation of the power system. According to the technical regulations, the configuration principles of the power transformer relay protection device are generally as follows:

(1) Gas protection should be installed for various short circuits and oil level drops inside the transformer, in which the light gas instantaneously acts on the signal, and the heavy gas instantaneously acts to disconnect the circuit breakers on each side;

(2) The longitudinal differential protection or the current quick-break protection of the multi-phase short-circuit and the winding-to-turn short-circuit of the reaction transformer winding and the lead-out line shall be installed as the main protection, and the instantaneous action shall be to disconnect the circuit breakers on each side;

(3) Overcurrent of transformer caused by external phase-to-phase short circuit, depending on transformer capacity and operation conditions and different requirements on transformer sensitivity, overcurrent protection, overvoltage protection of composite voltage start, negative sequence current and single phase can be used. Overcurrent protection or impedance protection of low voltage starting as backup protection, with time limit action on tripping;

(4) For the power network directly connected to the neutral point of 110kV and above, zero-sequence current protection and zero-sequence voltage protection shall be installed according to the specific conditions of the transformer neutral grounding operation and the insulation condition of the transformer, with time limit action on tripping;

(5) In order to protect against damage caused by long-term overload, install overload protection according to possible overload conditions, with time limit action on the signal;

(6) For the temperature rise of the transformer and the failure of the cooling system, the device acting on the signal or acting on the trip shall be installed in accordance with the provisions of the transformer standard.