The composition of the shunt static generator excitation system
Generator since shunt static Excitation system Mainly by the Automatic excitation regulator 、Excitation transformer, thyristor rectifier bridge and the excitation device, rotor overvoltage protection and destroy the magnetic device, etc, the principle of connection as shown in figure 1 to 13.At present, the hydropower stations using the shunt static excitation system.
1. The excitation control device
Including excitation control device Automatic voltage regulator And the excitation control circuit.Since the shunt excitation system of large unit of automatic voltage regulator (AVRType), use the computer Digital voltage regulator.Excitation regulator of measuring the voltage of the power pattern, and comparing with the given value (set point), when the voltage of the machine is higher than the given value, the increase of the thyristor control Angle, decrease the exciting current, the voltage of the power pattern in return to set point.When the voltage of the machine is less than the given value, reduce the thyristor control Angle, increasing the excitation current, maintains pattern generator terminal voltage to the set value.
2. The magnetic and rotor over voltage protection
Generator magnetic is destroyed by the magnetic switch and peg magnetic resistance, safe and fast cutting rotor excitation current and absorb energy.Nonlinear magnetic resistance destroy way in large generators, especially a lot of application in the hydroelectric generating set.To destroy the magnetic resistance, the domestic general USES high-energy zinc oxide (ZnO) valve plates, and abroad using silicon carbide (SiC) resistance.For using linear resistance or adopt arcing out the magnetic grid way, must set up a separate rotor overvoltage protection device (crossover).And using nonlinear resistance drop-out, can both of rotor overvoltage protection.
3. The excitation transformer
Excitation transformer excitation energy for excitation system.To excitation variable secondary side switch automatically, if the switch linkage cutting out the magnetoresistance, destroy the magnetic switch is communication;If no linkage magnetoresistance, this switch is only played the role of isolation, isolating switch is communication.Communicate out the magnetic switch and ac isolating switch, can isolate the excitation device after downtime and excitation transformer circuit, convenient maintenance test.
Excitation transformer can be set up over current protection, temperature protection.Capacity of the larger oil-immersed transformer excitation also sets the gas protection.Small capacity excitation transformer generally they have no protection.Transformer high voltage side of the connection must be included in the range of differential protection of generator.
The early oil-immersed transformer excitation transformer are generally adopted.In recent years, with the progress of dry type transformer manufacturing technology and considering the influence of factors such as fire prevention, maintenance, are now using epoxy resin transformer.For the large capacity of excitation transformer, usually adopts three single-phase a mix of dry type transformer.Excitation transformer connection group, usually adopts Y/delta group.Just like normal distribution transformer, excitation transformer short circuit voltage drop of 4% ~ 8%.
4. The thyristor rectifier bridge
Since the shunt excitation system in the high power rectifier device adopts three-phase bridge type connection.The advantage of this connection is semiconductor devices under low voltage, high utilization rate of the excitation transformer.Three-phase bridge circuit can use half controlled or fully-controlled bridge.Both increase the ability of magnetic, but in the reduction of magnetic, half controlled bridge can only control the excitation voltage to zero, and all control bridge in the runtime can produce negative excitation voltage inverters, the exciting current rapid drop to zero, and the energy feedback to the power grid and generators.Now since the shunt excitation system in almost all adopt fully-controlled bridge.
Thyristor rectifier bridge using phased approach.For three-phase fully-controlled bridge, when the load is inductive load, control Angle between 0 ~ 90 o o for rectifier state (forward voltage and forward current);Control Angle in 90 ~ 150 o o (control theory Angle can reach 180 o, considering the actual commutation overlap Angle, and the trigger pulse width to a certain extent, so the general maximum Angle of control, take 150 o) for the inverter state between (produce negative voltage and forward current).So when the generator load changes, by changing the thyristor control Angle to adjust the size of the excitation current, to ensure the machine of generator terminal voltage constant.
For large excitation system, to guarantee enough of the exciting current, using several parallel rectifier bridge.The selecting principle of rectifier bridge parallel branch route for: (N + 1), N number to ensure the normal order of the generator excitation rectifier bridge.That when a rectifier bridge exit because of malfunction, will not affect the normal field of excitation system.Some data (N - 1) expression of excitation rectifier cabinet redundancy, its meaning with the (N + 1).
