1. Adaptability to the environment
1.1 Adaptability to grid voltage fluctuations
The impact on the operation of the frequency converter when the motors on the bus start in groups or when the largest motor group on the bus starts is related to the allowable input voltage fluctuation range parameter of the frequency converter. For thermal power units, it should be ensured that the frequency converter will not stop when the bus voltage drops by 30%.
In addition, after a momentary loss of bus voltage caused by bus switching or other situations, the frequency converter should have the function of continuous or restored operation (some manufacturers call it "undervoltage restart function"). That is, when the bus voltage drops or disappears momentarily (such as during fault switching), the frequency converter should not trip or allow the motor system to run inertia; when the bus voltage returns to normal, the frequency converter can correctly adjust its output according to the captured motor speed and drive the motor to run again.
1.2 Adaptability to the on-site environment
High-voltage frequency converters are mostly installed near auxiliary equipment in the field, where dust accumulates. Dust entering the converter cabinet can lead to decreased insulation or breakdown, damaging electronic components. Dust clogging the filters also results in poor heat dissipation in the power cabinet, easily causing overheating and failure of the power modules. Some manufacturers design air filters that can be removed and cleaned during operation for easy maintenance. In hot and humid climates in the south, products with lower requirements for ambient temperature and humidity and relatively low system temperature rise should be selected to ensure safe and stable operation.
2. Ability to withstand minor malfunctions
High-voltage frequency converters have unit bypass functions, meaning that if a power unit fails, that unit should be able to automatically disconnect, allowing the entire system to continue operating with the fault. This is essentially a redundancy design technique. In this case, attention should be paid to the impact of unit bypass on the frequency converter's load-carrying capacity, mainly considering the number of power units per phase of the frequency converter and the voltage compensation of the control system.
The more units are connected in series, the higher the probability of failure, but the smaller the impact of a single unit failure on the output capability. A compromise must be made between the two. Using voltage compensation algorithms and neutral point offset algorithms can improve the load-carrying capacity of the system after unit bypass, but this method may bring problems such as common-mode voltage. The choice must be made based on the specific conditions of the equipment, such as the insulation safety of the motor.
The power supply of the control system of the high-voltage frequency converter is crucial. It should be designed with multiple control power supplies, with multiple channels serving as backups for each other and seamless switching. Redundant design of the fan cooler also helps to improve the system's immunity to disturbances.
3. Ability to withstand external failures
The impact of external faults on the input side, such as voltage drops on the busbar caused by external power grid failures, on the operation of high-voltage frequency converters. A power plant unit in Guangdong Province experienced such an incident: a momentary external power grid fault caused a flickering voltage drop on the plant's auxiliary power busbar, leading to the shutdown of the auxiliary frequency converter. Although the external power grid fault was quickly cleared, the shutdown of the critical auxiliary equipment driven by the frequency converter resulted in load shedding from the unit. Therefore, the instantaneous power outage restart function should be a reliable guarantee for improving the external fault tolerance of high-voltage frequency converters used in power plant auxiliary equipment.
The impact of external faults on the output side, such as cable breakdown short circuits or single-phase grounding or even phase-to-phase short circuits in the motor, on the high-voltage frequency converter should be considered. The high-voltage frequency converter should be equipped with a single-phase grounding fault detection function, and alarm or trip protection should be set according to the site conditions.
Statistics show that an average of about 20 high-voltage motors in Guangdong Province experience phase-to-phase short-circuit faults annually due to insulation damage. Although the probability is relatively small, the impact of short-circuit current on high-voltage frequency converters, which use power electronic devices with extremely limited overcurrent capacity, is enormous and can lead to serious equipment failures. Therefore, the ability of high-voltage frequency converters to withstand output phase-to-phase short circuits and their protection technology are crucial factors to consider when selecting equipment and ensuring its safety.
4. Equipment failure recovery time
Equipment failures can be categorized into two types: one is a self-recovering failure that occurs instantly and recovers within a short time. Variable frequency drives (VFDs) with automatic speed tracking significantly improve operational capability and reliability under these conditions. Some high-voltage VFDs operating in Guangdong Province frequently shut down during lightning strikes in the rainy season precisely because they lack this feature.
Secondly, the recovery time of the device after permanent damage or failure is improved. The modular power unit allows for the replacement of spare modules in a short time, enabling the equipment to resume operation quickly.
5. The impact of frequency converter retrofit on motor protection
High-voltage frequency converters are generally equipped with a power frequency bypass cabinet to ensure that in the event of a frequency converter failure or maintenance, the motor can be switched back to power frequency operation through the power frequency bypass cabinet, ensuring continuous production. However, this switching also brings about corresponding protection configuration issues: the switch cabinet should be equipped with transformer protection when the motor is in frequency conversion operation (because the part of the frequency converter connected to the plant power supply is an input phase-shifting rectifier transformer), while motor protection should be installed when the motor is in power frequency operation.
Therefore, during the renovation, the original motor protection should be retained as a protection device for operation at power frequency. If the frequency converter control system does not have the protection function of the input transformer, from the perspective of system safety and reasonable configuration of protection, it is necessary to install "isolation phase-shifting transformer" protection; when the motor is running at frequency, the motor protection is deactivated and the transformer protection is activated.
6. Manual bypass and automatic bypass
Manually switching the inverter's operating mode (from mains frequency to variable frequency) is complex, involves long interruptions, and significantly impacts the stability of the unit. In contrast, inverters with automatic mains-to-variable frequency switching can automatically switch to mains frequency operation in the event of a fault, ensuring the continuous operation of critical auxiliary equipment and reducing the impact on the unit and even the power grid.
However, when a motor fails, the inverter automatically switches to the mains frequency, which can exacerbate the motor's fault and risk escalating it. In practical applications, the advantages and disadvantages of the "automatic bypass switching function" should be fully considered, and ideally, the inverter control system should have the ability to distinguish between its own faults and load faults.
In addition, when automatically switching between power frequency and frequency converters, attention should be paid to the automatic switching of switchgear protection devices and the linkage adjustment of dampers or valves. After the frequency converter maintenance is completed, how to instantly switch the motor from power frequency operation to frequency converter operation is also an issue that must be considered during the modification.
7. The impact of harmonics on power grids and motors
Low-voltage frequency converters have a large amount of high-order harmonic components in their input current. These harmonics not only cause "harmonic pollution" to the power grid but also reduce the power factor of the converter's input circuit. High-voltage frequency converters, on the other hand, typically employ multiple rectification technology to reduce harmonic pollution to the power grid and improve the power factor on the converter's input side. Data shows that high-voltage frequency converters using 30-pulse phase-shifting transformers generally have a total input harmonic content of less than the national standard requirement of 4%, and their grid-side power factor can reach above 0.95.
The impact of output voltage and current harmonics on motors is mainly manifested in increasing motor torque pulsation and motor heating, thereby affecting the insulation of motor windings; common-mode voltage and bearing current will exacerbate bearing erosion and reduce mechanical life. Generally, when the number of power units connected in series per phase of a multi-unit series-type high-voltage frequency converter reaches 5 or more, the output voltage abrupt change rate (du/dt) can meet the motor insulation requirements, reduce damage to winding insulation and common-mode voltage and shaft current, and the harmonic content is low, so its impact on the motor can be disregarded.
When an electric motor operates at low speed, its heat dissipation capacity decreases. For fan and pump loads, the harmonic heating effect of a unit-series high-voltage frequency converter is minimal. Since the load current is low and the heat generation is low at low speeds, additional cooling measures are unnecessary. However, for constant torque loads, the heat generated by the motor at low speeds is similar to that at high speeds, necessitating the consideration of forced air cooling or other heat dissipation measures. Additionally, attention should be paid to bearing lubrication during low-speed operation.
8. Lifespan of the frequency converter
The lifespan of high-voltage frequency converters is primarily determined by their electrolytic capacitors, which are directly related to their operating temperature and ripple current. Ensuring optimal operating temperature, improving the heat dissipation of the power modules, and reducing the temperature rise of the power modules play a crucial role in extending system lifespan. Furthermore, reducing capacitor ripple current increases its lifespan. Therefore, the operating load conditions of the system are closely related to the lifespan of the frequency converter.
For typical variable loads, under operating conditions that meet design specifications, electrolytic capacitors have a lifespan of over 8 years, with replacement costs accounting for approximately 5% to 10% of the system investment. Some domestic manufacturers have introduced non-polarized capacitors to replace electrolytic capacitors, claiming a lifespan of up to 20 years. However, non-polarized capacitors of the same volume have significantly smaller capacitance than electrolytic capacitors, impacting system input and output harmonics, power factor, and other indicators. Currently, high-voltage frequency converters using non-polarized capacitor filtering are rarely used, and their technological maturity and operational reliability remain to be observed and warrant attention.
