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1 Introduction In the thermal power plant, in order to guarantee the normal work bus voltage, a standby power automatic input (hereinafter referred to as BZT) device is usually used. The device is an automatic device that ensures that the bus voltage is normal after the busbar loses voltage or the working power switch trips, and it ensures that the bus voltage is normal. It is divided into fast switching and slow switching. Which one is used depends on the inherent operating time of the switch and the amplitude of the decay of the residual voltage of the busbar. If the BZT moves too fast, the working transformer and the standby transformer will operate in parallel for a short time, which may generate a large circulating current and cause an impact on the transformer; or if the residual voltage of the bus is too high and the standby power supply is superimposed, the bus line may overvoltage and damage the rotation of the working bus. equipment. If the BZT motion is too slow, the motor on the busbar restarts. Due to the different acceleration characteristics of different auxiliary engines, the safe operation of the unit may be jeopardized.
With the development of modern control technology, the hardware and software reliability and response speed of DCS have been greatly improved. At the same time, the gradual integration of electrical quantity control into DCS has become an urgent need for DCS retrofit of a large number of thermal power units. In view of the importance of BZT, previous BZT functions were realized by dedicated devices. Through constant exploration, accumulation and improvement, Beijing Heshish System Engineering Co., Ltd. used a unique loop control board in the DCS transformation of a 200MW unit. Fully realized BZT function.
2 Functions to achieve 2.1BZT Platform Power plant BZT devices used for power plants are generally relayed by conventional relays, sometimes refused to move or misoperation. In order to meet the reliability and rapidity of the electrical circuit, the transformation adopts the fast loop control board of the distributed control system of the Holmes Model MACSI to implement logic control.
The loop control board is a set of templates with independent fast calculation function, including HS2F43A function board, HS2T44A conditioning board and HS2T302A terminal board. The board provides 10 digital inputs, 6 digital outputs, 6 analog inputs, 4 analog outputs, a total of 26 I/Os, a minimum control calculation and a channel scan cycle of 25ms, and one board supports page functions. Block configuration, 80 board-level database points, the main function modules supported are: four operations: addition, subtraction, multiplication, division, square root; logical operations: AND, OR, NOT; control operations: PID adjustment, servo amplification Combine servo, timer, counter, operator, first-order lag, second-order lag.
2.2 BZT logic power plant factory power switch intrinsic operating time is about 200ms, plus the relay's inherent operating time, bus residual voltage vector and standby power between exactly the largest, so we use slow switching BZT, delay time setting More than 1s. The BZT action must satisfy the following conditions:
1 Work bus voltage loss for any reason The work bus voltage is monitored by the voltage relays 1YJ and 2YJ connected to the two phases AB and BC. When the 1YJ and 2YJ contacts are closed at the same time, it indicates that the busbar has a low voltage. The IYJ and 2YJ contacts are connected in series to prevent the PT from breaking in one phase.
2 Standby power supply The voltage relay 3YJ on the standby voltage supply is closed, indicating that the backup power supply has voltage. There is no voltage for the backup power supply to prevent the BZT from operating.
3PT broken or overhauled BZT.
The 4BZT device is only used once to prevent the fault in the busbar from aggravating the fault. The relay using time delay returns ensures that the BZT only operates once.
The following takes the 6kV plant power system BZT as an example to illustrate its implementation process in the DCS. All the logic of BZT is completed by a page function block. The program page operation cycle is 25ms. The used function blocks are: hysteresis-lag timer and RS flip-flop. The access I/O points include:
DI: Work bus voltage 1YJ, 2YJ, backup bus voltage 3YJ, PT broken lock, work power switch, opening position.
DO: work equipment power supply interlock switch BK, BZT action, jump work branch. 
All the BZT logic is operated on the loop control board. Using its fast computational characteristics, it is possible to more accurately set the time constants of the timers and achieve fast response. In addition, the loop control board has independent computing capability and does not completely depend on the main control unit (DPU). Thus, even if all the DPUs of the field control station are lost, the BZT logic can continue to execute, further improving the reliability of the BZT.
3 Conclusion After several tests on the spot and actual operation of the unit proves that it is feasible to use the loop control board to fully realize the BZT function in the DCS, it not only improves the reliability of the equipment power supply, but also reduces the maintenance workload on the equipment. It is generally welcomed by power plant operation and maintenance personnel.
Abstract: The use of the loop control board in the DCS fully implements the BZT function and replaces the existing relay lap BZT device. Practice has proved that this method is simple and reliable. Figure 1 shows the logic diagram. Under normal conditions, the BK input, the standby bus has a voltage of 3YJ, the PT disconnection is blocked, and the working power switch is closed. In the event of a fault, the working bus voltages 1YJ and 2YJ decrease. After the hysteresis timer tl delays, the working branch is jumped off. The working branch opening position is 1, and the RS flip-flop is 1 in the timer t2. At this time, the BZT operation Backup power supply. The setting time of the timer tl is 1 s, and the BZT delays 1 s during the bus low voltage to prevent the impact on the motor. The t2 setting time of the timer is 750ms, and it is ensured that the BZT closing pulse is a short pulse and only once.