Source: Wang Zhenzhong Wang Zhenzhong's technology sharing
I. Preface
The general diesel generator allows its rated capacity of 10% ~ 20% power of the motor to start directly, if beyond this range, the diesel generator end current may exceed the stator overcurrent protection value, the throttle can not guarantee that the large motor is started to quickly open to the correct position, the excitation system is also difficult to quickly increase enough excitation current, resulting in the diesel generator speed drop, overcurrent, machine end voltage drop (see Table 1 for protection value), and finally the diesel generator will be suffocated and shut down. In this paper, a 1350kW/380V accident safety diesel generator supporting a 600,000 steam turbine generator set can be dragged down by two 132kW oil pump motors directly started in the case of power loss in the factory, and a solution is formulated.
Table 1 Partial protection list for diesel generators
Figure 1 Main wiring diagram of 380V security power supply
2. Problem analysis
A 600,000 unit of 1350kW/380V accident safety diesel generator (Perkins diesel engine, marathon generator, domestic assembly), in the event of an accident, with security A and B two sections of bus (see Figure 1 for main wiring), the maximum electric load is 600kW, the actual operating power is 400kW, the specific load list is as follows Table 2:
Table 2 List of major loads for 380V security section A/B
The 380V security A/B section is each equipped with one 132kW steam turbine AC lubricating oil pump, one is transported and one is in standby under normal circumstances, one 75kW DC oil pump motor is used as an accident backup, and the power switch of the two AC lubricating oil pump motors is a DC-controlled frame circuit breaker (it can still be closed and mechanically maintained after the AC bus loses power).
When the power of the plant loses power, the 380V security A/B section loses power at the same time, the PLC in the diesel engine local switch cabinet will send a command to jump off the security A/B section incoming line (1), (2) power switch, and start the diesel generator, about 20s The frequency and voltage of the diesel generator reach the rating, the outlet circuit breaker is closed, the security A and B section diesel engine incoming line (3) the power switch is closed, and the security A/B section bus is live.
At the same time of power loss in the plant, the pressure of the turbine lubricating oil mother pipe will be rapidly reduced, and the action value of the interlocking standby pump will be reduced to about 3s, because the DCS power supply is brought by UPS, the interlocking can still be ordered to start the standby AC lubricating oil pump, at this time, the power switch of the two oil pumps will be in the state of power loss closing, and the diesel generator will start at the same time with two oil pumps.
We consulted the diesel generator manufacturer, in this case, whether the diesel generator can be started successfully, and the answer we got is no, and it is not recommended to try, otherwise it may cause damage to the diesel generator. In order to verify the manufacturer's claim, we make theoretical calculations based on the design specifications in Table 3 below.
Table 3 Accounting of diesel generator starting capacity
3. Solution
Through the calculation in Table 3, two of the verification results do not meet the requirements, it can be seen that the capacity of the diesel generator can not meet the direct start of two 132kW oil pump motors, and the probability of unsuccessful start is very large, so we do not need to risk damaging the diesel engine to verify. In the case of power loss in the factory, many 600,000 units of diesel generators can not get up, this consequence we cannot bear, must be completely solved.
In order to prevent the occurrence of the above situation, we have made technical changes to the logic of the steam turbine lubricating oil mother pipe pressure low interlock starting standby oil pump, and added the locking condition of the 380V security A and B bus bars at the same time, and the locking standby oil pump starts, see Figure 2.
By looking at the DCS historical curve chart (see Figure 3), the 380V security section A and B power loss signals were turned on 3 seconds earlier than the low lubricating oil pressure signal of the steam turbine, and the backup oil pump was successfully locked.
Figure 2 DCS logic diagram
Figure 3 Historical DCS curves
IV. Conclusions
Because the 660,000 units of Harbin Automobile have canceled the main oil pump and used an independent AC lubrication electric pump (high power), there is this problem, while the unit that still uses the traditional coaxial main oil pump + AC lubricating oil pump for start and stop (low power) does not have this problem.
If there is a large-capacity motor designed on the 380V security section of the turbine generator set, the starting capacity must be checked, and reasonable locking measures must be taken to prevent the expansion of the accident in the case of unqualified calibration.
Another point, in the case of power loss in the security section, all the low-voltage motors controlled by the contactor will trip, the duration of the DCS interlocking contact is only 3~5s, after the diesel generator starts in 15s, the DCS interlocking will not work, that is, there will be no equipment self-starting, but it is necessary to exit the low-voltage motor controlled by the motor protector (about 100kW) The loss of pressure restart function, otherwise it will still aggravate the difficulty of starting the diesel generator, and these equipment can be gradually started by the operator according to the current situation of the unit after the security section is electrified.