Large generator excitation system special requirements

With the development of major equipment and technology in the country, the stand-alone capacity of generator sets is getting larger and larger, and the importance of generator excitation systems has become increasingly apparent. The task of the excitation system is to maintain the voltage of the generator terminal or the specified control point at a given level, stably distribute the reactive power of the parallel operation units, and improve the dynamic stability of the power system.

1 Explanation of excitation method

At present, there are two kinds of excitation methods for large-scale generators in China: a. Self-excited static excitation system. b. Brushless excitation system.

The determination of the method is directly related to the selection of the generator.

Comparison of two different excitation methods:

(1) Brushless Excitation System: 1 Only has the ability of positive top voltage. 2 The excitation response is related to the time constant of the exciter (>200ms). 3 Demagnetization time is the natural deactivation time constant. 4 PMG (exciter) can provide excitation power when the system is short circuited. 5 The shafting of the unit was lengthened and the maintenance of the machine increased. 6 No slip ring (reduction of carbon brush maintenance and dust).

(2) Static Excitation System: 1 The ability to have a bidirectional top voltage. 2 Bidirectional fast response capability (<20ms). 3 through the demagnetization resistance and inverter operation to achieve rapid demagnetization. 4 The size of the excitation transformer is only related to the capacity of the excitation. 5 No rotating parts, short shaft (small torsional vibration), reliable operation. 6 The power rectifier cabinet does not have to be shut down. 7 Excitation parameters such as Uf, If can be measured directly.

2 Explanation of the excitation transformer

At present, there are mainly two kinds of excitation transformers for the excitation system of domestic large-scale generators: a. Epoxy casting dry (Dry). b. Oil.

The determination of the method is directly related to the capacity of the transformer and the special requirements of certain users. Comparison of two different excitation transformers:

(1) Dry-type excitation transformer: 1 epoxy casting, dry type, indoor type, natural air cooling; 2 insulation class F; 3 limited capacity; 4 can be supplied in three-phase or three single-phase manner; 5 Easy installation and maintenance

(2) Oil-immersed excitation transformer: 1 transformer coil oil-immersed cooling; 2 insulation class B; 3 unlimited capacity; 4 can only be supplied in three-phase mode; 5 installation, maintenance workload.

3 Temperature rise description of the excitation transformer

The excitation transformer temperature rise of the excitation system is related to the following conditions:

a. Design margin

If the user does not have special requirements, we have different requirements for the temperature rise of transformers with different insulation levels when ordering the excitation transformer. Usually the insulation of class F is 100K in the case of self-cooling (ambient temperature is 40°C). Insulation 125K. This temperature rise means that the temperature rise of the rated operation of the transformer will not exceed this value, and the transformer can operate continuously for a long period of time without causing damage to the transformer within this temperature rise range.

b. Ambient temperature

The temperature rise mentioned above is based on the ambient temperature at 40°C. If the working environment temperature of the excitation transformer is higher than this value, it is required to keep the temperature rise or a higher temperature rise while the capacity is unchanged, which will inevitably lead to an increase in the volume of the transformer and an increase in the price. Or require a larger capacity transformer or forced air cooling to solve.

c. The size of the load

The magnitude of the excitation of the excitation system is related to the reactive load carried by the unit. The greater the load, the higher the temperature rise of the transformer. With the completion of large domestic backbone transmission grids and reasonable power structure, the entire power system has become more and more stable. At present, most of the domestic power plants are far away from the load center. Because most of the power plants in most cases operate with a small amount of reactive power or in-phase operation due to long-distance power transmission, the load rate of the excitation system is relatively low. Normally, its operation cannot reach the rated state. In the design and selection of the transformer, its capacity is determined based on the rated requirements of the engine with considerable margins.

4 Explanation of Winding Operation of Rectifier

At present, the most popular domestic excitation system rectifier device cooling methods are the following: a. water cooling; b. forced ventilation cooling; C. natural air flow cooling (self-cooling).

Judging from the operation of the unit, most of the rectifier units in the domestic excitation system of the generator set generally adopt forced air cooling.

Rectification device forced cooling method features:

At present, the most popular domestic excitation system rectification device forced cooling method fan layout has the following types: a. cabinet bottom; b. cabinet top; c. centralized layout.

Regardless of the method, its purpose is to take away the heat generated inside the rectifier to reduce the temperature of the rectifier. For thyristors, the operating temperature of the SCR directly determines the output current when the electrical design is complete. Therefore, the reliability of the wind turbine operation directly determines the reliability of the rectifier.

Measures to ensure reliable cooling of the forced cooling mode rectifier device: a. fan backup; b. fan power backup; c. temperature monitoring; d. good ventilation duct.

In view of the current domestic certain users of the rectifier device in all fans after stopping the wind to ensure that the excitation system can run with a specific load for a certain period of time the special requirements. Need to explain from the following aspects.

(1) Purpose of stopping the wind: The main purpose of the current user request for shutting down the wind is to improve the reliability of the excitation system, ie, it is considered that the excitation can be maintained when the wind turbine of the rectifier device is faulty.