Introduction to the inverter selection strategy - Delta inverter

1. Ability to adapt to the environment

1.1 Ability to adapt to grid voltage fluctuations

When the motor on the bus starts in a group, when the largest motor unit on the bus starts, the impact on the operation of the inverter, which is related to the input voltage fluctuation range parameters promised by the inverter, and the thermal power unit should ensure that the inverter will not stop when the bus voltage drops by 30%.

In addition, after the instantaneous loss of bus voltage formed under the condition of bus switching, the inverter should have the function of continuous or rehabilitated operation (some manufacturers call it "loss of voltage restart function"), that is, when the bus voltage drops or disappears instantaneously (such as incident switching), the inverter does not trip or makes the motor system run by inertia; When the bus voltage is restored to normal, the inverter can correctly adjust its own output according to the captured motor speed, and re-drag the function of the motor to run.

Introduction to the inverter selection strategy - Delta inverter

Introduction to the inverter selection strategy - Delta inverter

1.2 Ability to adapt to the on-site environment

Most of the high-voltage inverters are installed near the auxiliary equipment on site, and there is a lot of dust, and the dust entering the inverter cabinet will cause insulation to drop or break down and damage the electronic components; Dust clogging the filter screen forms poor heat dissipation in the power cabinet, which can easily lead to overheating and failure of the power module. Some manufacturers design air filters to be replaceable and cleanable during operation, making them easy to maintain. In the high temperature and humid climate areas in the south, products with low requirements for environmental temperature and humidity and relatively low system temperature rise should be selected to ensure safe and stable operation.

2. The ability to bear its own small problems

The high-voltage inverter has the function of unit bypass, that is, when a power unit fails, the unit should be able to actively exit, and the whole system can continue to operate with faults, which is a redundant design skill. At this moment, attention should be paid to the influence of the unit bypass on the load capacity of the inverter, and the number of power units per phase of the inverter equipment and the voltage compensation of the control system should be considered first.

The more units are connected in series, the greater the probability of failure, and the smaller the impact of a single unit failure on the output ability, so the two should be compromised. If the voltage compensation algorithm and the neutral point offset algorithm are selected, the load carrying ability of the system unit after bypass can be improved, but this method may bring problems such as common mode voltage, and it is necessary to choose according to the detailed conditions of the equipment such as motor insulation safety.

The power supply of the control system of the high-voltage inverter is very important, and the multi-channel control power supply should be designed and selected, and the multi-channel backup and non-disturbance switching should be selected. The redundant design of the fan cooler also contributes to the anti-disturbance capability of the improved system.

3. Ability to withstand external problems

The impact on the operation of the high-voltage inverter caused by external faults on the input side, such as external power grid faults, when the bus voltage drops. Such an incident occurred in a power plant unit in Guangdong Province: because of the instantaneous fault of the external power grid, the voltage of the power bus of the plant flickered down, resulting in the shutdown of the auxiliary inverter. Therefore, the function of instantaneous power failure and restart should be a reliable guarantee for the improvement of the auxiliary high-voltage inverter of the power plant unit and the ability to withstand external problems.

The impact of external faults on the output side, such as cable breakdown short circuit or single-phase grounding or even phase-to-phase short circuit of the motor, on the high-voltage inverter. The high-voltage inverter should be equipped with a single-phase grounding fault detection function, and the alarm or trip maintenance can be set according to the site conditions. According to statistics, the interphase short circuit problem of high-voltage motors in Guangdong Province due to insulation damage is about 20 units per year, although the relative probability is small, but on the selection of high-voltage inverters with extremely limited overcurrent ability, the impact of short-circuit current on the equipment is huge, and may lead to serious problems of equipment damage. The ability of high-voltage inverter to withstand output phase short circuit and maintenance skills are an important factor that should be considered in equipment selection and to ensure equipment safety.

4. Repair time of equipment

Equipment faults are divided into two types: one is the instant self-healing fault, once this fault is presented, it can be recovered in a relatively short period of time, and the frequency conversion equipment with the function of active tracking of speed can significantly improve the operation ability and reliability in this fault situation. Some of the high-voltage inverters in operation in Guangdong Province frequently shut down when lightning strikes occur during the thunderstorm season, which is caused by the lack of this function;

The second is the recovery time of the equipment after the occurrence of permanent damage, and the modularization of the power unit can replace the standby module in a short time, so that the equipment can be restored and operated in a short time.

5. The impact of frequency conversion transformation on motor maintenance

The high-voltage inverter is generally equipped with a power frequency bypass cabinet to ensure that when the inverter is faulty or overhauled, the switching motor of the power frequency bypass cabinet will resume the power frequency operation to ensure continuous production. However, this kind of switching also brings the problem of corresponding maintenance equipment: the switch cabinet should be equipped with transformer maintenance under the condition of frequency conversion operation of the motor (because the internal connection part of the inverter and the factory electricity is the input phase-shift rectifier transformer), and the motor maintenance should be installed during the power frequency operation.

Therefore, during the transformation, the maintenance of the original motor should be preserved, as the maintenance equipment during the power frequency operation, if the inverter control system does not have the maintenance function of the input transformer, from the perspective of system safety and reasonable equipment maintenance, it is necessary to install the "barrier phase-shifting transformer" maintenance; When the motor is running at variable frequency, exit the motor maintenance and invest in transformer maintenance.

6. Manual bypass and active bypass

Manually switching the operation mode of the inverter (power frequency - frequency conversion) has the problems of chaotic operation and long stop time that have a great impact on the stability of the unit. The inverter with active switching of power frequency conversion can actively switch to power frequency operation when something goes wrong, which ensures the continuous operation of important auxiliary equipment and reduces the impact on the unit and even the power grid.

However, when the motor fails, the inverter actively switches to the power frequency, which will aggravate the fault of the motor and have the risk of amplifying the fault. In the detailed application, the pros and cons of the "active bypass switching function" should be fully considered, and it is best that the inverter control system has the ability to distinguish its own faults and load faults.

In addition, attention should be paid to the active switching of switchgear maintenance equipment and the linkage conditioning of dampers or valves during other active power frequency conversion switching. After the frequency conversion overhaul is completed, how to make the motor switch from the power frequency operation to the frequency conversion operation status instantaneously is also a problem that must be paid attention to during the transformation.

7. The influence of harmonics on power grids and motors

The input current of the low-voltage inverter has a large high-order harmonic component, which forms "harmonic pollution" to the power grid and reduces the power factor of the input circuit of the inverter. The high-voltage inverter generally uses multiple rectification skills to reduce the harmonic pollution of the power grid and improve the power factor on the input side of the inverter. Some materials show that the high-voltage inverter with 30 pulse phase shifter transformer is basically less than 4% of the national standard, and the power factor on the grid side can reach more than 0.95.

The influence of output voltage and current harmonics on the motor is mainly reflected in the addition of the ripple of the motor torque and the heating of the motor, and then affects the insulation of the motor winding; The common mode voltage and bearing current will aggravate the electrostatic corrosion of the bearing and reduce the mechanical life. Generally, when the number of power units per phase series of multi-unit series high-voltage inverter reaches 5 or more, the mutation rate of output voltage (du/dt) can meet the requirements of motor insulation, reduce the harm to winding insulation and common mode voltage shaft current, and the harmonic content is low, so its impact on the motor can be ignored.

When the motor runs at low speed, the heat dissipation ability of the self-cooled motor will decrease, and the harmonic heating effect of the high-voltage inverter with the unit series connection is small, and the motor does not have to consider the additional heat dissipation method because of the low load current and less heat generation when the motor is running at low speed; However, with regard to constant torque loads, the heat of the motor during low-speed operation is close to that of high-speed operation, so it is necessary to consider heat dissipation methods such as installing forced air cooling during low-speed operation. Together, attention should also be paid to the lubrication of the bearing at low speeds.

8. The life of the inverter

The life of high-voltage inverter equipment is mainly determined by the electrolytic capacitor, which is directly related to its operating temperature and ripple current. Ensure the operating environment temperature, improve the heat dissipation effect of the power module, and reduce the temperature rise of the power module to improve the life of the system. The ripple current of other capacitors reduces its lifetime, so the operating load condition of the system is closely related to the life of the inverter.

For general variable loads, the service life of electrolytic capacitors is more than 8 years, and the cost of replacing capacitors accounts for about 5%-10% of the system's investment under the condition that the operating environment is satisfactory to the design. Domestic manufacturers have launched non-polar capacitors to replace electrolytic capacitors, which are said to have a life of up to 20 years, but the capacity of the same volume of non-polar capacitors is much smaller than that of electrolytic capacitors, which has an impact on the system input, output harmonics, power factors and other goals.

Introduction to the inverter selection strategy - Delta inverter

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