Control scheme of security power supply system in DCS

The safety power supply is set up to ensure that the unit can be shut down smoothly when the whole plant is out of power. It mainly provides power for steam turbines, water pumps, coal mills, fan lubrication systems and control systems, uninterruptible power supplies (UPS), power and control DC systems, etc. At present, the accident safety power supply set up in the power supply system of large-capacity unit plants mostly uses diesel generators as backup power supplies. The control of diesel generators and the switching of safety power supplies generally use relay hard wiring control or PLC control. In view of the reliability of DCS, the safety power supply system of the 2×600MW units of the first phase of Huaneng Chaohu Power Plant was changed from PLC control to DCS control.

1. Security Power System

Figure 1 shows the safety power supply system of a unit in Huaneng Chaohu Power Plant. Each unit is equipped with MCC A section and MCC B section safety power supplies, which are respectively powered by the turbine PCA section and the turbine PCB section during normal operation. When the safety MCC A section or the safety MCC B section power supply fails, the safety MCC A section or the safety MCC B section is first switched to the boiler PCA section or the boiler PCB section for power supply. If the boiler PCA section or the boiler PCB section power supply also fails or the switching fails, the diesel generator is automatically started, so that the safety MCC A section or the safety MCC B section is powered by the diesel generator.

2. Defects and shortcomings

(1) The security power supply system is controlled by PLC, which has complex wiring. At the same time, the control signal, feedback signal, fault signal and security MCC section bus low voltage signal of the security power supply system are sent to the DCS and the local PLC control system, which makes the secondary circuit complex and reduces reliability. In order to prevent asynchronous closing, the closing signal of the security section incoming switch sent by the DCS must be sent to the local PLC control system for control, which also makes the secondary circuit more complex.

(2) The switching method for restoring normal power supply is the uninterrupted power supply parallel switching method, that is, first close the QF12 (or QF22) switch to restore the power supply of the safety section, and then disconnect the QF1 (or QF2) feeder switch of the safety PC section, which makes the synchronous check circuit and control logic more complicated, and usually the instantaneous power outage method is used for switching.

(3) When the safety section loses power, only the low voltage of the safety section bus is used as the control criterion, which can easily trigger the automatic control program of the diesel generator by mistake. Cutting off the power supply line switch of the safety section increases the risk of power failure in the safety 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 on the DCS, namely the security MCC section A and the security MCC section B. The operation modes are automatic and manual. To prevent asynchronous paralleling caused by misoperation, taking the security MCC section A as an example, after the automatic control mode of the security MCC section A is put into operation, the control logic will prohibit manual operation, that is, the QF11, QF12, QF13, QF14, and QF1 switches cannot be manually operated on the DCS. The conditions for the automatic control mode of the security MCCA section to be put into operation are: the diesel generator is in automatic control mode; the QF1 and QF14 switches are in standby state; the QF11, QF12, and QF13 switches are in the closed position; the security MCCA section is energized; the turbine PCA section is energized; and the boiler PCA section is energized.

When the operation mode switch of the security MCC A section is in the automatic control position, when the normal power supply of the security MCC A section disappears, its automatic control program will run. The automatic control program of the security MCC A section is shown in Figure 2.

(1) Determine whether the A section of the security MCC is de-energized. To prevent misjudgment leading to erroneous switching to the security section, a composite condition is used for judgment, that is, when the busbar of the A section of the security MCC and the busbar of the PCA section of the turbine are at low voltage and the switches QF11 and QF12 are in the closed position, or when the busbar of the A section of the security MCC is at low voltage and QF11 or QF12 is in the tripped position, it is determined that the A section of the security MCC is de-energized.

(2) The security MCC A section switches to the backup power supply of the boiler PCA section and issues a QF12 switch tripping command, and the QF12 switch has tripped. Confirm that the boiler PCA section is energized and the QF13 switch is in the closed position, otherwise, an instruction to start the diesel generator set is issued; an instruction to close the QF14 switch is issued and the QF14 switch has been closed, otherwise, an instruction to start the diesel generator set is issued.

(3) Determine if the security MCC A section has lost power again. The security MCC A section busbar and the boiler PCA section busbar are under voltage and the QF13 and QF14 switches are in the closed position. The security MCC A section busbar is under voltage, or the QF13 or QF14 switch is in the tripped position and 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 A section has lost power again.

(4) After the starting diesel generator set issues the QF14 switch tripping command, the QF14 switch has tripped; after the starting diesel generator set command is issued and the diesel generator set starts, the QF01 switch immediately closes automatically and the safety PC section is energized.

(5) The power supply of security MCC section A is switched to security PC section. Confirm that QF12 and QF14 switches are in the tripped position and QFO1 is in the closed position. Issue the command to close the QF1 switch, and security MCC section A is energized.