In recent years, through the analysis of the static excitation system of large and medium-sized synchronous generators on the system transient stability, relay protection and shafting torsional vibration, it is generally believed that this excitation method has fast response speed, simple wiring and convenient maintenance. The operation is reliable, and the length of the shafting of the unit can be shortened to improve the shafting characteristics. At the same time, as the static excitation system of the imported large-capacity unit is continuously put into operation in various local power grids, the country is currently in the stage of popularization and application. However, it is undeniable that static excitation systems have been used since the 1950s and are now quite popular. For example, the units using static excitation systems in the Canadian power system account for nearly 80% of the total installed capacity. ABB, Mitsubishi, GE and other companies are all In the production of static excitation systems, in comparison, domestic manufacturers have some gaps in their key technologies compared with their foreign counterparts, such as reliability design, intelligent current sharing technology, shaft voltage absorption, and remote diagnosis.
1 Redundancy mode Currently, static excitation systems generally use digital regulators (AVR) and adopt dual-channel configuration. Each channel has two main adjustment modes: constant voltage and constant current, but static excitation of large and medium synchronous generators. The system has its particularity, that is, it does not have a manual cabinet like the AC exciter excitation system, so it puts higher requirements on its reliability. On the basis of improving the overall reliability of the excitation equipment, it is considered to adopt a redundant design in both the regulator and the power circuit.
1.1 AVR with dual-channel AVR with manual manual adjustment can automatically track between channels and constant voltage/constant current mode, and automatically switch when there is a fault. Usually, the AVR runs in the constant voltage mode of channel 1. If a fault occurs, it cuts to the constant voltage mode of channel 2. If the fault occurs again and channel 1 is not repaired, it will cut to the constant current mode of channel 2. (Manual Mode), if the fault occurs again, the AC exciter excitation system can be switched to the manual spare cabinet operation, but for the static excitation system, because the output is high voltage and large current, it is usually not manual for economical considerations. In the case of a similar failure, the excitation system will directly exit the operation, which is quite unfavorable for the safe and reliable operation of the unit. In addition, the constant current mode in the dual channel regulator is usually not completely independent. For example, the constant voltage/constant current mode CPU is shared, and the control program is also in the same program memory. When the main control board fails, the result is The constant voltage/constant current mode is not available at the same time. Based on this situation, the AVR with manual manual adjustment should be used with a completely independent CPU. The control functions are completely independent and have the following functions: automatic tracking/automatic switching, synchronous loop and pulse generation completely independent, power supply loop independent, I The /O interface is independent; in addition, the backup channel should have certain protection functions, such as excitation current inverse time protection, overcurrent transient protection and minimum excitation current limit. It can be seen that in this configuration, each adjustment channel has a constant voltage mode, a constant current mode, and a completely independent backup manual mode.
1.2 Power circuit redundancy design and intelligent current sharing technology Static excitation system for large and medium-sized generators, the power bridge usually adopts (1) redundant design, that is, n bridges run in parallel, when a controllable bridge exits, It can still meet the rated operating conditions and strong excitation conditions. In view of the multi-bridge parallel operation, most of the current AC equal length cables and reactors are used to achieve current sharing. It can be seen that all are static current sharing measures. When the power component characteristics change and the excitation current is deviated, only manual adjustment is made. The method and adjustment during operation are not conducive to the safe operation of the equipment, so dynamic intelligent current sharing technology should be adopted.
In order to achieve dynamic current sharing, the output current of each controllable bridge should be measured first, and the current sharing between the controllable bridges should be realized without relying on the AVR. This requires a controllable bridge with intelligent bridge control unit to achieve current measurement. Trigger angle adjustment and other functions, at the same time can achieve bridge temperature measurement, cooling fan control. It can be seen that each controllable bridge is actually an independent intelligent component, which can be connected to the AVR through a field bus such as ARCnet, CANBUS, etc. In this way, the configuration of the number of controllable bridges is also very flexible. Can meet the requirements of different excitation capacity.
In summary, if a manual AVR and intelligent power bridge are used, the control part is actually four completely independent channels, and the controllable bridge can be configured according to the principle of (n-2), and the manual manual adjustment is actually implemented. The function of the manual cabinet is realized, which greatly improves the operational reliability.
The 2-axis voltage absorption static excitation system is also a new shaft voltage source due to the influence of the thyristor arc-changing, which will generate an axis voltage of up to 60V. Practice has shown that the conventional grounded carbon brush can not effectively eliminate the high-frequency shaft voltage component. If effective measures are not taken to prevent the shaft surface and the support will cause electrical corrosion, thus affecting the safe operation of the unit. It is generally considered that the shaft voltage value is less than 20V. In order to achieve this goal, in addition to the improvement of the conventional grounded carbon brush, a dedicated shaft voltage suppression circuit should be provided inside the excitation device to effectively reduce the shaft voltage value. An effective method is to add a symmetric RC filter. It should be noted that this RC circuit has an influence on the grounding detection of the generator rotor, and the grounding detection device should be re-tuned during the on-site debugging process.
3 Cooling fan configuration With the continuous development of electronic technology and thyristor technology, the overall reliability of the excitation system has been greatly improved. In comparison, the cooling fan part is the weakest link, usually 1 fan. The service life is about 40,000 hours. In some nuclear power plants abroad, after the equipment has been in operation for a certain period of time, all fans will be replaced to improve the reliability of equipment operation. At present, two sets of fans are commonly used in the domestic standby mode. When the fan is faulty, it is replaced, but this is not conducive to the dynamic maintenance of the fan. If an intelligent bridge control unit is used, the actual running time of each group of fans can be recorded, and replaced when it is near the service life, which greatly facilitates the maintenance work.
4 loss pulse detection technology At present, the methods of pulse loss detection mainly include bus pulse loss detection and detection in the secondary of pulse transformer, or several methods at the same time, but in the event of thyristor gate failure or thyristor out of control In the case of the situation, the above methods are invalid. The most effective method should be to detect the current of the AC input of each control bridge. By analyzing the current waveform, it is judged whether the pulse is lost and which bridge arm is out of pulse. This can avoid the occurrence of false detection.
5 Fieldbus and remote diagnostic excitation systems are usually connected to the DCS. At this stage, the hard contact is mainly used, but this is not possible on the DCS. 39. Modification of the internal parameters of the excitation system and recording, etc., it is now required that the excitation system has a communication interface with the DCS, but the problem is that such interfaces are rarely put into use at present, and only the interface is reserved for later use. In the specific design, it is best to use the common internal interface plus the corresponding communication protocol conversion interface form, so in the actual use process, you can not update the main control program of the AVR, but choose different conversion interfaces to meet the requirements. Such as the use of Modbus, Piofibus and other communication protocol interfaces.
In view of the importance of the excitation equipment, the excitation equipment manufacturer is required to solve the practical problems encountered by the user in the shortest time. With the development of the Internet technology, the excitation equipment can be remotely connected through a dedicated debugging tool, so that the excitation equipment is produced. The manufacturer can realize remote control and diagnosis of the excitation equipment, such as reading the internal setting parameters of the equipment and modifying it, and simultaneously recording and analyzing in real time, thereby greatly shortening the maintenance time.
6 Conclusion In this paper, based on the current situation of domestic excitation equipment, several new technology applications based on equipment operation reliability are proposed. It has certain value for the development and research of excitation equipment and equipment selection.
(Continued from page 34) 3 points of view 5% capacity superheater small bypass can meet the cold, warm and extremely hot start requirements of the unit, can also be used for two shifts and peak shift operation, so it is a A simple, feasible, investment-saving startup bypass system. However, if the starting time is long, the operation mode of stopping or stopping the furnace must be adopted, and the operation mode of stopping the furnace or the power consumption of the factory can not be realized, otherwise the reheater is in a dry burning state. In addition, even if the unit is under heavy load, the PCV valve will take off. If the load is large, the safety valve will also take off.
After the boiler is ignited, the combustion rate must be strictly controlled before the turbine is rushed to control the flue temperature of the furnace outlet not higher than 538C, so as to ensure that the reheater does not burst due to dry burning. Therefore, the operation and maintenance personnel must ensure that the furnace outlet smoke temperature probe is always in a good and trouble-free state.
(3) When the hot state starts, the steam is reversed due to the reheat steam temperature. Although the medium pressure cylinder of the steam turbine allows the negative deviation to start, the metal temperature at the first stage blade of the steam turbine must be guaranteed. The mismatch is limited to a range of 56 to 110, otherwise it will have a greater impact on the life of the turbine.
(4) All the steam traps of the main steam pipeline shall have a flow capacity of not less than 2% of the total steam flow of the BMCR leading to the condenser. In all cases where the pipeline is displaced from a cold state to a hot state, the main steam pipeline shall be sloped. To these hydrophobic devices. It works in conjunction with a 5% superheater bypass to establish a 7% BMCR flush flow and corresponding burn rate to meet startup needs.
(5) According to the calculation data provided by GEC-ALSTOM and CE Company for the 660MW unit of Shajiao C Power Plant, during the expected life of not less than 25 years, the unit is allowed to operate as follows: % rated power) 4000 times according to calculation The total life lost to the turbine rotor and boiler drum under operating conditions were 65% and 9.77%, respectively, and the above data is fully acceptable. This means that only a small bypass of 5% capacity superheater can be used to ensure that the expected service life of the unit is 30 years.
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