Analysis and commissioning of 600MW unit safety power supply characteristics

1 Introduction

With a large number of newly built 600MW and above units put into operation in recent years, my country's thermal power generating units have entered the era of large capacity and high parameters. Correspondingly, the cost of the plant equipment of the units is getting higher and higher. Once these equipments have accidents, the losses are extremely heavy.

The emergency safety power supply for the unit plant is the last line of defense to protect the unit equipment, and the requirements for its reliability are naturally getting higher and higher.

There are two basic forms of factory-used security power supply. One is to use diesel generator sets as the security power supply of the unit, and the other is to use an independent power supply outside the factory (or other units) as the security power supply of the unit. These two methods have their own advantages and disadvantages. As a completely independent system, the diesel engine is least likely to be disturbed by factors such as the power system, which is the main reason why it is widely selected as the security power supply of the unit; however, in actual application, due to the long-term static state of the diesel engine, the lack of human intervention in daily management, maintenance, and maintenance, and the antifreeze problem of cooling water and fuel in cold environments, the success rate of diesel engines as security power supply in accidents is not high, which is its main disadvantage. In addition, even if the diesel generator set starts successfully in the event of an accident, the unit's security section will still lose power during the starting process (about 5~15s). The advantage of an independent power supply outside the factory is that it can be in a powered standby state for a long time and can be put into use immediately in the event of an accident, and the safety power supply will not be lost during the accident. However, its disadvantage is that these so-called "independent" power supplies are not completely independent of the power system, and will be difficult to guarantee in the event of a serious failure in the system. Once the independent power supply outside the factory fails, the power plant will face the serious consequence of a power failure in the safety section, which is difficult for the power plant to accept.

If the advantages of these two wiring methods are combined and both methods are used as backup power supplies for the safety power supply, it is possible to achieve complete power failure in the safety section under accident conditions, greatly improving the reliability of power supply under accident conditions. Unit 1 of Huaneng Tongchuan Power Plant is the first domestically produced air-cooled subcritical 600MW unit, which was put into operation on November 1, 2007. The plant uses an independent power supply outside the plant as the first safety power supply and a diesel generator set as the second safety power supply. It also uses an ASCO7000 automatic switching switch with a synchronization check function to achieve automatic switching under accident conditions and automatic switching back after the plant power is restored, achieving complete power failure during the entire accident process from switching from the backup power supply to recovery.

2 Tongchuan #1 unit security system

Unit 1 of Huaneng Tongchuan Power Plant is equipped with 0.4 kV security PC 1A section, security PC 1B section, and security PC standby section. The normal operation power supply of the security section is supplied by the boiler PC 1A and 1B sections of the unit. The boiler PC section, security PC section, and security PC standby section are connected by an automatic switching switch ASCO with synchronization detection function. There are two security power supplies. The first security power supply is an independent 10kV city power line from outside the plant, which is connected to the security PC standby section through a security transformer; the second security power supply is a diesel generator set. The wiring diagram of the entire security system is shown in Figure 1.

Figure 1 Schematic diagram of the Tongchuan #1 unit security system.

3 Automatic transfer switch ASCO

The automatic transfer switch ASCO 7ACTS is an electrical instantaneous single-coil excitation operation, mechanical interlocking double-throw transfer switch. This switch has a fast switching speed. Generally, the open circuit switching is completed within 1/6s, and the actual load power-off time is less than 50ms. The switch is equipped with an intelligent controller, which can perform open circuit automatic conversion (break before connect) or closed circuit automatic conversion (connect before break) according to the situation of the two power supplies. The open circuit switching is shown in Figure 2(a), and the closed circuit switching is shown in Figure 2(b).

Figure 2 Two switching modes of the automatic transfer switch.

N——working power supply; E——standby power supply; L——load.

The controller logic automatically determines whether to use open-circuit or closed-circuit conversion. If both power supplies are valid, a closed-circuit conversion is performed without interruption of the load voltage. If any power supply failure is detected, an open-circuit conversion is performed in the form of a break-before-make. The switching delay can be set to control the time when the load loses power according to user requirements.

In the open-circuit switching mode when the working power fails, the controller logic can also send a delay command signal to start the diesel generator after detecting the failure of the working power. When the working power is restored, the controller logic can send a stop command signal to the diesel generator with an adjustable delay after the closed-circuit switch is back to the working power.

The 7000 series ASCO switches also have a quasi-synchronous function, which can perform a synchronization check between the working power supply and the backup power supply. The synchronization conditions such as voltage difference, frequency difference and phase difference can be set.

ASCO switches can also be controlled by commands to prohibit switching from working to standby, or from standby to working. This function is very necessary in some special situations, such as switch maintenance.

4. Security Power Switching Logic

Since the ASCO dual power automatic transfer switch has its own logic controller, it can realize a variety of flexible working modes, the action time can be adjusted, and it has a two-way synchronous closing function. Therefore, except for the interlocking between the diesel generator export switch K0 and the protection transformer voltage side switch ZK which is realized by hard wiring, all other switching logics are realized by the ASCO switch, and there is no need for a separate PLC controller.

(1) The first security power supply logic.

Taking the logic of section A as an example, when the unit is operating normally, the security PC section A is powered by the boiler PC section A, and the switch 1ZK is in the closed state as a dead switch for a long time. The switch ZK on the low-voltage side of the first security power supply line security transformer is closed, and the security section is operated by the security transformer for a long time. When the security PC section A loses power, the 2ZK switch loses pressure and trips. After the ASCO1 switch controller detects that there is no pressure on the N side of the working branch, and there is pressure on the E side of the standby branch, the ASCO1 switch controller automatically selects the "break before connect" open circuit switching mode, first trips the N side, and then closes the E side, switching the security PC section A to the security standby section, which is powered by the first security power supply security transformer. Since the switching time of the ASCO1 switch is only about 50ms, the load of the security section will not be affected.

(2) Second safety power supply logic.

If the ASCO1 switch controller detects that there is no pressure on the N side of the working branch and the E side of the standby branch after the 2ZK switch trips, it means that a serious power grid system failure has occurred and the external independent first security power supply has also been lost. The ASCO1 switch controller immediately sends a command to start the diesel generator set. After about 10 seconds, the diesel generator starts successfully. After the diesel generator set PLC detects that the voltage frequency and amplitude at the machine end have reached the rated value, it automatically closes the output switch KO and simultaneously trips the first security power supply, and the diesel generator set supplies power to the security standby section. After the ASCO1 switch controller detects that there is pressure in the security standby section, it immediately switches to the N side first and then closes the E side in an open-circuit switching mode of "break first and connect later", switching the security PC A section to the security standby section, and the second security power supply, that is, the diesel generator, supplies power to the security section.

(3) Working power recovery logic.

For the mode of power supply by the first power supply, when the power supply of boiler PC A section is restored, the operating personnel manually closes switch 2ZK. Once the ASCO1 switch controller detects that there is pressure on the working branch, it delays for 5 minutes and checks the synchronization between the working branch and the standby branch voltage. When the synchronization conditions are met, the security PC A section is switched to the boiler PC A section power supply in a closed-circuit switching mode of "first connect and then break", realizing automatic switching back without power outage, and there is no disturbance to the load of the security PC A section.

The process of restoring the working power supply by the second power supply, that is, the diesel generator, is similar. The difference is that after the switchback is completed, the ASCO1 switch controller delays for another 5 minutes to issue a diesel generator shutdown command.

(4) Manual test logic of diesel generator set.

Since the first safety power supply exists for a long time and the diesel generator outlet circuit breaker has no synchronization function, the daily manual test of the diesel generator is not connected to the grid. The diesel engine is only started and run at no load for 10 minutes. After checking that it is running normally, it is manually stopped.

(5) Normal shutdown and boiler section maintenance logic.

During normal shutdown, the boiler PC section loses power, and the ASCO switch is prohibited from automatically switching from the main power supply to the backup power supply to prevent accidental switching to the first safety power supply or starting the diesel engine. When the boiler section is overhauled or tested, the safety section load is first switched from the ASCO switch to the backup power supply, and then the ASCO switch is prohibited from switching from the backup to the main power supply to prevent the ASCO switch from automatically switching back when the boiler section has power.

From the above logic, it can be seen that the #1 unit of Tongchuan Power Plant uses a relatively independent mains power supply as the first security power supply and a diesel generator as the second security power supply. When the ASCO automatic switching switch with synchronization function is adopted, it can be ensured that the security section will not lose power at all (about 50ms) in the event of a power failure in the plant, and the load of the security section will not be affected; and in the event of simultaneous failure of the plant power supply and the external power grid system, the security section still has a diesel generator as the final guarantee, and it only needs a short power failure (about 10s of diesel generator self-starting time) to recover, so the reliability of the security power supply is greatly improved.

5. Safety power switching test

It can be seen from the above switching logic that to ensure the realization of the above functions, there are two key points that need to be tested: ① After the security PC segment loses power, the ASCO1 switch switches the security PC segment A to the security standby segment in an open-circuit switching mode of "break first and connect later". During the switching, the security segment loses power briefly. According to the ASCO automatic switching switch manual, this time is about 50ms. Will this time cause the load of the security segment to trip? ② During the process of restoring the working power supply of the security segment, the ASCO switch switches the security PC segment A to the boiler PC segment A power supply in a closed-circuit switching mode of "connect first and disconnect later". During this process, can the ASCO1 switch correctly monitor the synchronization conditions and perform synchronization operations?

According to on-site measurements, during multiple open-circuit switching processes, the main engine AC lubricating oil pump may occasionally trip, while the rest of the safety section loads operate normally. When the working power supply of the safety section is restored, the closed-circuit switching mode of the ASCO switch takes an uncertain amount of time to check the synchronization, but it can generally be switched back smoothly within half a minute. Multiple switching tests have no effect on the safety section load operation.

6 Problem Discussion

(1) Phase angle problem of the first power supply.

According to actual measurement, during normal operation, the phase angle between the voltage of the first security power supply and the voltage of the unit security section is 28 degrees. This is because: first, the winding connection of the No. 1 security transformer is wrong. The winding connection form of the No. 1 security transformer is Yd11. If the winding connection form of the No. 1 security transformer is replaced with Yd12, the angle can be adjusted by 30 degrees; secondly, because the first security power supply is an external independent power supply, there is a certain power angle between its voltage and the AC voltage of the unit, which is about 2 degrees.

If the winding connection form of the No. 1 safety transformer can be changed, the phase angle between the first safety power supply voltage and the unit safety section voltage can be close to 0 degrees, which can achieve a faster switching speed, and the occasional tripping of the main engine AC lubricating oil pump may no longer occur.

(2) The problem of prohibiting switching of ASCO switch.

In normal shutdown and boiler section maintenance logic, the ASCO switch's prohibition switching function is required, which means that the diesel engine's automatic start function is prohibited at the same time. This is correct. However, the contact of this instruction adopts the disconnection effective form, that is, it should be conductive under normal circumstances, allowing the diesel engine to start automatically. When disconnected, the diesel engine is prohibited from starting automatically.

If for some reason the contact is disconnected and not discovered, the diesel engine cannot be started in an emergency when a fault occurs, which obviously poses a hidden danger.

Therefore, it is recommended to cancel this function and short the contacts in the ASCO switch. In order to prevent the automatic switching of the ASCO switch during normal shutdown and boiler section maintenance logic, it can be achieved by switching the isolating switch, which is stipulated in the operation and shutdown operating procedures.

7 Conclusion

(1) After adopting dual backup power sources of independent power supply and diesel generator, in the event of a factory power failure when the independent power supply is normal, the safety section will not be cut off, which is obviously better than the single diesel generator backup method and can greatly improve the power supply reliability.

(2) ASCO automatic transfer switch is used. The switch has its own controller and has the functions of checking synchronization and starting the diesel engine. There is no need to set up a separate PLC. The main wiring and secondary wiring of the system are simple.

(3) The logic of the security power supply system is complex and must be carefully tested and debugged on site to confirm that the logic is correct before it can be put into operation.