What is a generator excitation system

What is a generator excitation system?

Generator excitation system The power supply that supplies the synchronous generator excitation current and its auxiliary equipment are collectively referred to as the excitation system. It is generally composed of two main parts: excitation power unit and excitation regulator. Excitation power unit provides excitation current to the rotor of synchronous generator; The excitation regulator controls the output of the excitation power unit according to the input signal and the given regulation criteria. The automatic excitation regulator of excitation system plays an important role in improving the stability of parallel units of power system. In particular, the development of modern power systems leads to the trend of reducing the stability limit of units, and also promotes the continuous development of excitation technology. The excitation system of synchronous generator mainly consists of two parts: power unit and regulator (device).

1. The main functions of the excitation system are:

1) Adjust the excitation current according to the change of the generator load to maintain the machine terminal voltage as the given value;

2) Control reactive power distribution among generators in parallel operation;

3) Improve the static stability of parallel operation of generators;

4) Improve the transient stability of generators running in parallel;

5) When there is a failure inside the generator, the magnetic field is removed to reduce the degree of failure loss;

6) Implement rated excitation limit and small excitation limit on the generator according to the operation requirements. Synchronous generator excitation system has a variety of forms, according to the power supply mode can be divided into separate excitation type and self-excited type two categories.

Two. Several ways for generators to obtain excitation current

1. Excitation mode of DC generator power supply:

This type of excitation generator has a special DC generator, the special DC generator is called DC exciter, the exciter is generally coaxial with the generator, the generator’s excitation winding through the slip ring installed on the large shaft and fixed brush from the exciter to obtain DC current. This excitation mode has the advantages of independent excitation current, reliable operation and reducing self-use electricity consumption, etc., is the main excitation mode of generators in the past decades, and has more mature operation experience. The disadvantage is that the excitation regulation speed is slow and the maintenance workload is large, so it is rarely used in units above 10MW.

2. Excitation mode of AC exciter power supply:

Some modern high-capacity generators use AC exciter to provide excitation current. Ac exciter is also installed on the main shaft of the generator, and its output alternating current flows through the rectifier to supply the generator rotor excitation, at this time, the excitation mode of the generator belongs to the other excitation mode, and because of the static rectifier device, it is also known as the static excitation, AC secondary exciter provides excitation current. The AC secondary exciter can be a permanent magnet machine or an alternator with a self-excited constant voltage device. In order to improve the excitation regulation speed, the AC exciter usually uses a 100-200Hz intermediate frequency generator, and the AC secondary exciter uses a 400-500Hz intermediate frequency generator. The DC excitation winding and three-phase AC winding of this generator are wound in the stator slot, the rotor only teeth and slots without winding, like a gear, therefore, it has no brush, slip ring and other rotating contact parts, with reliable work, simple structure, convenient manufacturing process and other advantages. The disadvantage is that the noise is larger, and the harmonic component of the AC potential is also larger.

3. Excitation mode without exciter:

No special exciter is set up in the excitation mode, and the excitation power is obtained from the generator itself, and then the generator itself is excited after rectification, which is called self-excited static excitation. Self-excited static excitation can be divided into two ways: self-shunt excitation and self-compound excitation. Self-shunt excitation mode it obtains excitation current through the rectifier transformer connected to the generator outlet, and supplies the generator excitation after rectification. This excitation mode has the advantages of simple junction, less equipment, less investment and less maintenance work. The self-compound excitation mode not only has no rectifier transformer, but also has a high-power current transformer in series in the generator stator loop. The function of the transformer is to provide a larger excitation current to the generator when a short circuit occurs, so as to make up for the deficiency of the output of the rectifier transformer. This exciting mode has two exciting power sources, the voltage power source obtained by the rectifier transformer and the current source obtained by the series transformer.

Iii. Generator and excitation current related characteristics

1, voltage regulation

The automatic regulating excitation system can be regarded as a negative feedback control system with voltage as the regulated quantity. The reactive load current is the main reason for the drop of generator terminal voltage. When the excitation current is constant, the generator terminal voltage will decrease with the increase of reactive current. However, in order to meet the requirements of users on power quality, the terminal voltage of the generator should remain basically unchanged, and the way to achieve this requirement is to adjust the excitation current of the generator with the change of reactive current.

2, reactive power regulation:

When the generator runs in parallel with the system, it can be considered as running with the bus bar of the infinite capacity power supply. To change the excitation current of the generator, the induced potential and stator current also change, and the reactive current of the generator also changes. When the generator runs in parallel with the infinite capacity system, the excitation current of the generator must be adjusted in order to change the reactive power of the generator. At this time, the changed generator excitation current is not what is usually called “voltage regulation”, but only changes the reactive power fed into the system.

3, reactive load distribution:

The generators running in parallel distribute the reactive current proportionally according to their rated capacity. Large capacity generators should bear more reactive load, while smaller capacity generators should provide less reactive load. In order to realize the automatic distribution of reactive load, the excitation current of the generator can be changed by the excitation device of automatic high voltage regulation to keep the terminal voltage unchanged, and the inclination of the generator voltage regulation characteristic can be adjusted to realize the reasonable distribution of reactive load of the parallel running generator.

Method of automatically adjusting excitation current

In changing the excitation current of the generator, it is generally not carried out directly in its rotor loop, because the current in the loop is very large, it is not easy to directly adjust, and the method usually used is to change the excitation current of the exciter to achieve the purpose of adjusting the rotor current of the generator. The common methods include changing the resistance of the excitation circuit of the exciter, changing the additional excitation current of the exciter, changing the on-angle of the thyristor, etc. Here mainly talk about the method of changing the thyristor on-angle, it is according to the generator voltage, current or power factor changes, and correspondingly change the on-angle of the thyristor rectifier, so the generator excitation current will change. This device is generally composed of transistors, thyristor electronic components, with sensitive, fast, no failure zone, large output power, small size and light weight and other advantages. In the case of accident, the generator overvoltage can be effectively suppressed and the magnetic field can be quickly eliminated. The automatic regulating excitation device is usually composed of measuring unit, synchronizing unit, amplifying unit, adjusting unit, stabilizing unit, limiting unit and some auxiliary units. The measured signal (such as voltage, current, etc.) is compared with the given value after being transformed by the measurement unit, and then the comparison result (deviation) is amplified by the pre-amplification unit and the power amplification unit, and is used to control the on-angle of the thyristor to achieve the purpose of adjusting the generator excitation current. The function of the synchronization unit is to synchronize the trigger pulse output of the phase shift part with the AC excitation power supply of the thyristor rectifier to ensure the correct triggering of the controlled silicon. The function of the differential adjustment unit is to make the generators running in parallel to distribute the reactive load stably and reasonably. Stability unit is a unit introduced to improve the stability of power system. The excitation system stabilization unit is used to improve the stability of the excitation system. The limiting unit is designed to prevent the generator from operating under overexcited or underexcited conditions. It must be pointed out that not every kind of automatic regulating excitation device has the above-mentioned units, and the units of a regulator device are related to the specific tasks they undertake.

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