Summary:
The security power supply is set to ensure that the unit can be shut down smoothly when there is a power outage in the whole plant. It is mainly for steam turbines, feed water pumps, coal mills, fan lubrication systems and control systems, uninterruptible power supplies (UPS), power and control DC systems, etc. Provide power. At present, the accident security power supply set in the plant power system of the unit system large-capacity unit mostly uses a diesel generator as a backup power supply. The control of the diesel generator and the switch of the security power supply generally use relay hard-wired control or PLC control. In view of the reliability of DCS, the security power supply system of the 2 & TImes; 600MW unit of Huaneng Chaohu Power Plant Phase I was changed from PLC control to DCS control.
1. Security Power Supply System
Figure 1 shows the security power supply system of one unit of Huaneng Chaohu Power Plant. Each unit is equipped with MCC A section and MCC B section security power supply, which are respectively powered by the steam turbine PCA section and steam turbine PCB section during normal operation. When the power supply of the security MCC section A or the security MCC section B loses power, first cut the security MCC section A or the security MCC section B to the boiler PCA section or boiler PCB section for power supply, if the boiler PCA section or boiler PCB section power supply also loses power or If the switch fails, the diesel generator is automatically started, so that the security MCC section A or the security MCC section B is powered by the diesel generator.
2. Defects and deficiencies
(1) The control mode of the security power supply system adopts PLC control, and the wiring is complicated. At the same time, the control signal, feedback signal, fault signal and low voltage signal of the busbar of the security MCC section are sent to the DCS and local PLC control system. The secondary circuit is complicated and the reliability is reduced. In order to prevent non-synchronized closing, the closing signal of the incoming line switch of the security section must be sent to the local PLC control system for control by the DCS, which also makes the secondary circuit more complicated.
(2) The normal power supply switching mode is the uninterrupted power supply parallel switching mode, that is, first close the QF12 (or QF22) switch to restore the power supply of the security section, and then disconnect the QF1 (or QF2) feeder switch of the security PC section to check the loop and control at the same time The logic is more complicated, and the method of instantaneous power failure is usually used for switching.
(3) Power outage in the security section only uses the low voltage of the bus in the security section as the control criterion, which can easily trigger the automatic control program of the diesel generator; cutting off the power supply line switch of the security section increases the risk of power outage in the security section.
3. Improvement of security power supply system control
3.1 Automatic start control logic
The operation of the system is selected by the operation mode switch. There are two operation mode switches of the security MCC section A and the security MCC section B on the DCS. The operation modes include automatic mode and manual mode. In order to prevent the non-synchronized parallel caused by misoperation, take the security MCC section A as an example. After the automatic control mode of the security MCC section A is put in, the control logic will prohibit manual operation, that is, the DCS cannot manually operate QF11, QF12, QF13, QF14, QF1 switch. The permission conditions for the automatic control mode of the security MCCA section are: the diesel generator is in the automatic control mode; the QF1, QF14 switches are in the standby state; the QF11, QF12, QF13 switches are in the closing position; the security MCCA section is live; the steam turbine PCA section is live; the boiler PCA Segment charged.
When the operation mode switch of the security MCC section A is in the automatic control position, when the normal power supply of the security MCC section A disappears, its automatic control program runs. The automatic control procedure of the security MCCA section is shown in Figure 2.
(1) Judging the power loss of the security MCC section A In order to prevent misjudgment leading to the erroneous switch to the security section, a composite condition is adopted, that is, the security MCC section A bus and the steam turbine PCA section bus are of low voltage and the QF11 and QF12 switches are in the closed position, or When the bus voltage of the security MCC section A is low and QF11 or QF12 is in the trip position, it is determined that the security MCC section A is powered off.
(2) The security MCC section A is switched to the boiler PCA section backup power supply to issue the QF12 switch trip command and the QF12 switch has tripped, confirm that the boiler PCA section is live and the QF13 switch is in the closed position, otherwise issue the command to start the diesel generator set; issue the QF14 switch The closing command and the QF14 switch have been closed, otherwise a command to start the diesel generator set is issued.
(3) Judging that the MCC section A of the security MCC has lost power again. The MCC section A bus and the boiler PCA section bus are low voltage and the QF13 and QF14 switches are in the closing position. If the QF13 and QF14 switch protection is not activated (if the QF13 or QF14 switch protection is activated, the diesel generator automatic control program will be exited), it is determined that the security MCC section A loses power again.
(4) The QF14 switch has tripped after the start diesel generator set issued the QF14 switch trip command; after the start diesel generator set command was issued and the diesel generator set was started, the QF01 switch was automatically closed immediately, and the security PC section was energized.
(5) Switch the security MCC section A to the security PC section for power supply. Confirm that the QF12 and QF14 switches are in the trip position and QFO1 is in the closing position. Issue the QF1 switch closing command and the security MCC section A is energized.
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