Rivers in nature all have a certain slope. Water flows along the riverbed under the action of gravity. Water at high altitudes contains abundant potential energy. With the help of hydraulic structures and electromechanical equipment, the energy of water can be converted into electrical energy, that is, hydropower generation. The principle of hydropower generation is our electromagnetic induction, that is, when a conductor cuts the magnetic flux lines in a magnetic field, it will generate current. Among them, the “movement” of the conductor in the magnetic field is achieved by the water flow impacting the turbine to convert water energy into rotational mechanical energy; and the magnetic field is almost always formed by the excitation current generated by the excitation system flowing through the generator rotor winding, that is, the magnetism is generated by electricity.
1. What is the excitation system? In order to realize the energy conversion, the synchronous generator needs a DC magnetic field, and the DC current that generates this magnetic field is called the excitation current of the generator. Generally, the process of forming a magnetic field in the generator rotor according to the principle of electromagnetic induction is called excitation. The excitation system refers to the equipment that provides excitation current for the synchronous generator. It is an important part of the synchronous generator. It generally consists of two main parts: the excitation power unit and the excitation regulator. The excitation power unit provides excitation current to the synchronous generator rotor, and the excitation regulator controls the output of the excitation power unit according to the input signal and the given regulation criteria.
2. Function of the excitation system The excitation system has the following main functions: (1) Under normal operating conditions, it supplies the generator excitation current, and adjusts the excitation current according to the given law according to the generator terminal voltage and load conditions to maintain voltage stability. Why can voltage stability be maintained by adjusting the excitation current? There is an approximate relationship between the induced potential (i.e. no-load potential) Ed of the generator stator winding, the terminal voltage Ug, the reactive load current Ir of the generator, and the longitudinal synchronous reactance Xd:
The induced potential Ed is proportional to the magnetic flux, and the magnetic flux depends on the magnitude of the excitation current. When the excitation current remains unchanged, the magnetic flux and the induced potential Ed remain unchanged. From the above formula, it can be seen that the terminal voltage of the generator will decrease with the increase of reactive current. However, in order to meet the user’s requirements for power quality, the terminal voltage of the generator should remain basically unchanged. Obviously, the way to achieve this requirement is to adjust the excitation current of the generator as the reactive current Ir changes (that is, the load changes). (2) According to the load conditions, the excitation current is adjusted according to a given rule to adjust the reactive power. Why is it necessary to adjust reactive power? Many electrical equipment work based on the principle of electromagnetic induction, such as transformers, motors, welding machines, etc. They all rely on the establishment of an alternating magnetic field to convert and transfer energy. The electrical power required to establish an alternating magnetic field and induced magnetic flux is called reactive power. All electrical equipment with electromagnetic coils consumes reactive power to establish a magnetic field. Without reactive power, the motor will not rotate, the transformer will not be able to transform voltage, and many electrical equipment will not work. Therefore, reactive power is by no means useless power. Under normal circumstances, electrical equipment not only obtains active power from the generator, but also needs to obtain reactive power from the generator. If the reactive power in the power grid is in short supply, the electrical equipment will not have enough reactive power to establish a normal electromagnetic field. Then these electrical equipment cannot maintain rated operation, and the terminal voltage of the electrical equipment will drop, thus affecting the normal operation of the electrical equipment. Therefore, it is necessary to adjust the reactive power according to the actual load, and the reactive power output by the generator is related to the magnitude of the excitation current. The specific principle will not be elaborated here. (3) When a short circuit accident occurs in the power system or other reasons cause the generator terminal voltage to drop seriously, the generator can be forcibly excited to improve the dynamic stability limit of the power system and the accuracy of the relay protection action. (4) When the generator overvoltage occurs due to sudden load shedding and other reasons, the generator can be forcibly demagnetized to limit the excessive increase of the generator terminal voltage. (5) Improve the static stability of the power system. (6) When a phase-to-phase short circuit occurs inside the generator and on its lead wires or the generator terminal voltage is too high, demagnetization is carried out quickly to limit the expansion of the accident. (7) The reactive power of the parallel generators can be reasonably distributed.
3. Classification of excitation systems According to the way the generator obtains the excitation current (that is, the supply method of the excitation power supply), the excitation system can be divided into external excitation and self-excitation: the excitation current obtained from other power supplies is called external excitation; the excitation current obtained from the generator itself is called self-excitation. According to the rectification method, it can be divided into rotary excitation and static excitation. The static excitation system does not have a special excitation machine. If it obtains the excitation power from the generator itself, it is called self-excitation static excitation. Self-excitation static excitation can be divided into self-parallel excitation and self-compounding excitation.
The most commonly used excitation method is self-parallel excitation static excitation, as shown in the figure below. It obtains the excitation power through the rectifier transformer connected to the generator outlet, and supplies the generator excitation current after rectification.
Wiring diagram of self-parallel excitation static rectifier excitation system
The self-parallel excitation static excitation system mainly consists of the following parts: excitation transformer, rectifier, demagnetization device, regulation controller and overvoltage protection device. These five parts respectively complete the following functions:
(1) Excitation transformer: Reduce the voltage at the machine end to a voltage matching the rectifier.
(2) Rectifier: It is the core component of the entire system. A three-phase fully controlled bridge circuit is often used to complete the conversion task from AC to DC.
(3) Demagnetization device: The demagnetization device consists of two parts, namely the demagnetization switch and the demagnetization resistor. This device is responsible for the rapid demagnetization of the unit in the event of an accident.
(4) Regulation controller: The control device of the excitation system changes the excitation current by controlling the conduction angle of the thyristor of the rectifier device to achieve the effect of regulating the reactive power and voltage of the generator.
(5) Overvoltage protection: When the generator rotor circuit has an overvoltage, the circuit is turned on to consume the overvoltage energy, limit the overvoltage value, and protect the generator rotor winding and its connected equipment.
The advantages of the self-parallel excitation static excitation system are: simple structure, less equipment, low investment and less maintenance. The disadvantage is that when the generator or system is short-circuited, the excitation current will disappear or drop greatly, while the excitation current should be greatly increased (i.e. forced excitation) at this time. However, considering that modern large units mostly use closed busbars, and high-voltage power grids are generally equipped with rapid protection and high reliability, the number of units using this excitation method is increasing, and this is also the excitation method recommended by regulations and specifications. 4. Electric braking of the unit When the unit is unloaded and shut down, a part of the mechanical energy is stored due to the huge rotational inertia of the rotor. This part of energy can only be completely stopped after it is converted into friction heat energy of the thrust bearing, guide bearing and air. Since the friction loss of the air is proportional to the square of the linear velocity of the circumference, the rotor speed drops very quickly at first, and then it will idle for a long time at a low speed. When the unit runs for a long time at a low speed, the thrust bush may burn out because the oil film between the mirror plate under the thrust head and the bearing bush cannot be established. For this reason, during the shutdown process, when the speed of the unit drops to a certain specified value, the unit braking system needs to be put into use. The unit braking is divided into electric braking, mechanical braking and combined braking. Electric braking is to short-circuit the three-phase generator stator at the machine end outlet after the generator is decoupled and demagnetized, and wait for the unit speed to drop to about 50% to 60% of the rated speed. Through a series of logical operations, the braking power is provided, and the excitation regulator switches to the electric braking mode to add excitation current to the generator rotor winding. Because the generator is rotating, the stator induces a short-circuit current under the action of the rotor magnetic field. The electromagnetic torque generated is just opposite to the inertial direction of the rotor, which plays a braking role. In the process of realizing electric braking, the braking power supply needs to be provided externally, which is closely related to the main circuit structure of the excitation system. Various ways to obtain the electric brake excitation power supply are shown in the figure below.
Various ways to obtain the electric brake excitation power supply
In the first way, the excitation device is a self-parallel excitation wiring method. When the machine end is short-circuited, the excitation transformer has no power supply. The braking power supply comes from a dedicated brake transformer, and the brake transformer is connected to the plant power. As mentioned above, most hydropower projects use a self-parallel excitation static rectifier excitation system, and it is more economical to use a rectifier bridge for the excitation system and the electric brake system. Therefore, this method of obtaining the electric brake excitation power supply is more common. The electric braking workflow of this method is as follows:
(1) The unit outlet circuit breaker is opened and the system is decoupled.
(2) The rotor winding is demagnetized.
(3) The power switch on the secondary side of the excitation transformer is opened.
(4) The unit electric brake short-circuit switch is closed.
(5) The power switch on the secondary side of the electric brake transformer is closed.
(6) The rectifier bridge thyristor is triggered to conduct, and the unit enters the electric brake state.
(7) When the speed of the unit is zero, the electric brake is released (if combined braking is used, when the speed reaches 5% to 10% of the rated speed, mechanical braking is applied). 5. Intelligent excitation system Intelligent hydropower plant refers to a hydropower plant or hydropower station group with information digitization, communication networking, integrated standardization, business interaction, operation optimization, and intelligent decision-making. Intelligent hydropower plants are vertically divided into process layer, unit layer, and station control layer, using a 3-layer 2-network structure of process layer network (GOOSE network, SV network) and station control layer network (MMS network). Intelligent hydropower plants need to be supported by intelligent equipment. As the core control system of the hydro-turbine generator set, the technological development of the excitation system plays an important supporting role in the construction of intelligent hydropower plants.
In intelligent hydropower plants, in addition to completing basic tasks such as starting and stopping the turbine generator set, increasing and decreasing reactive power, and emergency shutdown, the excitation system should also be able to meet the IEC61850 data modeling and communication functions, and support communication with the station control layer network (MMS network) and the process layer network (GOOSE network and SV network). The excitation system device is arranged at the unit layer of the intelligent hydropower station system structure, and the merging unit, intelligent terminal, auxiliary control unit and other devices or intelligent equipment are arranged at the process layer. The system structure is shown in the figure below.
Intelligent excitation system
The host computer of the station control layer of the intelligent hydropower plant meets the requirements of the IEC61850 communication standard, and sends the signal of the excitation system to the host computer of the monitoring system through the MMS network. The intelligent excitation system should be able to connect with the GOOSE network and SV network switches to collect data at the process layer. The process layer requires that the data output by CT, PT and local components are all in digital form. CT and PT are connected to the merging unit (electronic transformers are connected by optical cables, and electromagnetic transformers are connected by cables). After the current and voltage data are digitized, they are connected to the SV network switch via optical cables. The local components are required to be connected to the intelligent terminal via cables, and the switch or analog signals are converted into digital signals and transmitted to the GOOSE network switch via optical cables. At present, the excitation system has basically the communication function with the station control layer MMS network and the process layer GOOSE/SV network. In addition to meeting the network information interaction of the IEC61850 communication standard, the intelligent excitation system should also have comprehensive online monitoring, intelligent fault diagnosis and convenient test operation and maintenance. The performance and application effect of the fully functional intelligent excitation device need to be tested in future actual engineering applications.
Post time: Oct-09-2024
