VFD (Variable-frequency Drive) intermediate DC circuit overvoltage solution

Treatment of return pump system

(1) The failure of the return pump is caused by air mixed in the slurry and idling. In terms of the electrical system, it is mainly caused by the return pump being in a closed-loop operating state. Change the return pump VFD (Variable-frequency Drive) from the closed-loop operating state. This fault can be solved by operating in open loop mode. That is, on the FOXBORO distribution system in the main control room, four material level parameters of high level, higher level, lower material level, and low material level are set for the return trough, so that the reflux pump runs at a low speed at a lower material level. When operating at a high material level, the VFD (Variable-frequency Drive) acceleration and deceleration time is relatively extended to 60 seconds, so that the return pump variable frequency speed regulation system is not affected by instantaneous changes in slurry flow.

(2) In terms of technology, a buffer tank is added from the return outlet of each bag filter to the return tank to change the sudden change of the return flow and reduce the impact of flow changes on the VFD (Variable-frequency Drive).

(3) After the above treatment, the return pump has been able to operate stably, completely solving the overvoltage problem of the intermediate DC circuit. The processing of 2 pairs of feed pump systems. The original system of 8 bag filters is divided into 2 groups, 4 feed pumps and 2 return pumps are one group. Normally, each group has 2-3 feed pumps and 1 return pump. The pump is running, and each group has one return pump as a backup. Each group of 6 devices is on a section of the AC bus. Since each group has one return pump running for a long time, the operation is very stable after the above treatment. We use the return pump frequency conversion system to provide a discharge path for the energy generated in the intermediate DC circuit during regenerative braking of the Jinke pump frequency conversion system.

(4) The AC voltage on the power supply side of one feed pump and two return pumps VFD (Variable-frequency Drive) is 380V, and the intermediate DC voltage is about 310V. They are basically the same as each other, and will fluctuate due to the influence of load and frequency adjustment. . If the intermediate DC loops of each VFD (Variable-frequency Drive) are directly connected, increasing the system capacity of the intermediate DC loop will bring many benefits, such as: the intermediate DC voltage is relatively constant, and the energy storage capacity is increased after the capacitors are connected in parallel; the system can be improved The power factor and system power efficiency reduce the harmonic current of the power grid of the test instrument; the capacities of each VFD (Variable-frequency Drive) can be complementary, and the excess energy generated by the regenerative braking state can be used by other VFD (Variable-frequency Drive) Absorption and transformation will not cause overvoltage tripping of the VFD (Variable-frequency Drive) intermediate DC circuit during normal operation, optimizing the dynamic characteristics of the system, etc. However, since the VFD (Variable-frequency Drive) uses an uncontrollable rectifier bridge on the power supply side, the current flowing through the rectifier bridge is uncontrollable and may even exceed the current it can withstand and cause damage: the intermediate DC circuit is directly shared and a short-circuit fault occurs in the system. At this time, due to the less protection and poor sensitivity of the rectifier bridge, each VFD (Variable-frequency Drive) cannot be removed in time, which will cause the fault to further expand and cause greater losses. Therefore, when the intermediate DC loop is directly connected, the frequency conversion speed control system will become unreliable. The main purpose of using diodes and contactors in the circuit is to prevent the frequency conversion system from causing malfunctions due to the shared DC bus, thereby achieving the purpose of eliminating malfunctions without reducing the reliability of the VFD (Variable-frequency Drive) by passing test instruments.

(5) The diode of the feed pump VFD (Variable-frequency Drive) circuit is mainly to make the operation of each feed pump VFD (Variable-frequency Drive) independent of each other and not affect each other. The energy generated by the regenerative braking state is Normal operation of the return pump consumes energy to achieve full utilization.

The contactor is used to realize the mutual connection between the VFD (Variable-frequency Drive) and the common DC bus when the VFD (Variable-frequency Drive) is running and isolate each other during power outage and maintenance; to promptly cut off the connection with the common DC bus when a fault occurs.

(6) The two return pumps are used for normal operation. In order to prevent the two return pumps from running at the same time under special circumstances, the VFD (Variable-frequency Drive) of the two return pumps is installed between the intermediate DC loop and the common DC bus. Diodes are added to isolate the two return pumps from each other and reduce faults.

(7) Under normal circumstances, the feed pump variable frequency speed regulation system feeds power to the return VFD (Variable-frequency Drive) intermediate DC loop. Since the Jinke pump VFD (Variable-frequency Drive) is larger than the return pump, there is a certain margin. , it will not cause damage to the feed pump VFD (Variable-frequency Drive) during normal operation. Only when the rectifier bridge of the return pump is broken or the common DC bus is short-circuited, a fault current will flow into the intermediate DC bus. In this case In this case, this circuit mainly relies on the various protections of the VFD (Variable-frequency Drive) itself, that is, using the VFD (Variable-frequency Drive) fault signals 3 0C and 30B to make contactors added in each VFD (Variable-frequency Drive) circuit. Take action to promptly cut off the VFD (Variable-frequency Drive) and the common bus to avoid larger faults.

(8) The diode added in the loop is an ordinary rectifier diode ZP20. The average on-state current is 20A, and the surge current it can withstand can reach 150-200A. The intermediate DC voltage of each VFD (Variable-frequency Drive) is basically around 310V. Although the excess current generated in the regenerative braking state is larger in magnitude, it is shorter in duration. The current the diode bears is different from that of the resistor braking unit directly connected to both ends of the intermediate DC bus. Choosing a ZP20 diode plus a radiator can meet the circuit requirements.

(9) Since the added contactor will cut off the possible large current between the common DC bus and the VFD (Variable-frequency Drive) under fault conditions, CJ-50 AC contactors are selected for 1 to 6 km.

(10) In order to prevent the intermediate DC circuit of the return pump VFD (Variable-frequency Drive) from overvoltage tripping when a possible large current is injected, take: ① Take measures on the FOXBORO distribution system in the main control room to prevent each group from running 2-3 bag filters are washed and unloaded at the same time, so that each bag filter is unloaded and washed at an interval of 3-5 minutes. ② Install a set of resistor braking units on the shared DC bus to minimize the probability of overvoltage in the DC circuit in the middle of the return pump VFD (Variable-frequency Drive) and ensure the safety of the bag filter frequency conversion system.

(11) During the construction, the possible short circuit of the intermediate DC circuit was considered, and high-quality insulated rubber-sheathed cables were selected as connecting lines. The insulation was locally strengthened and the probability of ground short circuit of the intermediate DC circuit was reduced.

(12) The VFD (Variable-frequency Drive) operating environment is good. 10 VFD (Variable-frequency Drive) units have dedicated VFD (Variable-frequency Drive) rooms. 2 cabinet air conditioners are installed indoors, which are clean and tidy, and are conducive to ventilation. The test instrument VFD (Variable-frequency Drive) dissipates heat and reduces the probability of VFD (Variable-frequency Drive) failure.