Abstract : High-voltage frequency converters play a vital role in energy conservation in modern industry. However, due to their high cost and important location, they require various protection measures to ensure normal operation. This article introduces the various protection devices and principles of the Ipower series frequency converters manufactured by Harbin Jiuzhou Electric, as well as solutions to common faults, providing a reference for the daily inspection and maintenance of frequency converters.
Keywords : High-voltage frequency converter protection fault handling
1. Introduction
The Ipower series high-voltage frequency converter is an AC-DC-AC voltage source converter with a multi-unit series structure. It achieves sinusoidal input and output voltage and current waveforms through multiple superposition technology, effectively controlling harmonics and reducing pollution to the power grid and load. It is an environmentally friendly high-voltage frequency converter that does not require filters. Simultaneously, it has complete protection devices and measures to protect the frequency converter and load, eliminating and avoiding losses caused by various complex operating conditions, thus creating greater benefits for users.
2. Protection of high-voltage frequency converters
2.1 Incoming line protection for high-voltage frequency converters
Incoming line protection protects both the user's incoming line and the frequency converter. This includes lightning protection, grounding protection, phase loss protection, reverse phase protection, imbalance protection, overvoltage protection, transformer protection, and so on. These protection devices are typically installed at the input of the frequency converter. Before operating the frequency converter, it is essential to ensure that the incoming line protection is functioning correctly.
2.1.1 Lightning protection is achieved through surge arresters installed in bypass cabinets or at the input terminals of frequency converters. A surge arrester is an electrical device that can release the energy of lightning or, concurrently, the operating overvoltage energy of the power system, protecting electrical equipment from transient overvoltage hazards and interrupting follow current to prevent system grounding short circuits. The surge arrester is connected between the frequency converter's input line and ground, in parallel with the protected frequency converter. When the overvoltage value reaches the specified operating voltage, the surge arrester immediately trips, allowing charge to flow, limiting the overvoltage amplitude, and protecting the equipment insulation. After the voltage value returns to normal, the surge arrester quickly returns to its original state to ensure normal system operation and prevent damage from lightning strikes.
2.1.2 Ground fault protection is achieved by installing a zero-sequence current transformer at the inverter's input terminal. The principle of zero-sequence current protection is based on Kirchhoff's current law: the algebraic sum of the complex currents flowing into any node in a circuit equals zero. Under normal conditions of the line and electrical equipment, the vector sum of the currents in each phase is zero. Therefore, the secondary winding of the zero-sequence current transformer has no signal output, and the actuator does not operate. When a ground fault occurs in a certain phase, the vector sum of the currents in each phase is not zero. The fault current induces magnetic flux in the toroidal core of the zero-sequence current transformer, generating an induced voltage on the secondary side of the transformer. This voltage is fed back to the main monitoring box, which then issues a protection command, thus achieving the purpose of ground fault protection.
2.1.3 Phase loss, phase reversal, unbalance, and overvoltage protection. Phase loss, phase reversal, unbalance, and overvoltage protection are primarily achieved by the inverter's input voltage feedback board or voltage transformer acquiring the input voltage, and then using the CPU board to perform calculations to determine if there is a phase loss, phase reversal, input voltage imbalance, or overvoltage. This is because input phase loss, phase reversal, voltage imbalance, or overvoltage can easily cause transformer burnout, power unit damage, or motor reverse rotation.
2.1.4 Transformer Protection. The Ipower series high-voltage frequency converter consists of three main parts: a transformer cabinet, a power unit cabinet, and a control cabinet. The transformer is a split-type dry-type transformer that converts high-voltage AC power into a series of low-voltage phases at different angles to power the power units. The transformer can only be cooled by air cooling; therefore, transformer protection is mainly based on temperature control to prevent overheating and coil burnout. Temperature probes are placed in the three-phase coils of the transformer, with one end connected to a temperature control device. This device can automatically start the fan at the bottom of the transformer, set alarm temperatures, and trip temperatures. It also displays the temperature of each phase coil. If the temperature reaches the alarm or trip value, the temperature controller sends a signal to the PLC, displays the alarm information on the user interface, and the PLC will then perform alarm or trip protection.
2.2 Protection on the output side of Ipower series high-voltage frequency converters
The output protection of the Ipower series high-voltage frequency converter provides protection for the output side of the frequency converter and the load, including output overvoltage protection, output overcurrent protection, output short circuit protection, motor over-temperature protection, etc.
2.2.1 Output Overvoltage Protection. Output overvoltage protection is achieved by sampling the output voltage through the output-side voltage sampling board. If the output voltage is too high, the system will automatically trigger an alarm.
2.2.2 Output Overcurrent Protection. Output overcurrent protection determines whether an overcurrent has occurred by detecting the output current collected by the Hall effect sensor.
2.2.3 Output Short Circuit Protection. This refers to the protective measures taken when a short circuit fault occurs between phases of the motor stator windings and their leads. If the inverter detects an output short circuit, it will immediately issue a shutdown signal to the power unit, stopping its operation.
3 Troubleshooting Methods for Ipower Series High Voltage Frequency Converters
The Ipower series high-voltage frequency converters feature highly intelligent computing capabilities and comprehensive fault detection circuits, providing precise fault location and clear indications on the main control interface. In practical applications, we have found that common faults can be categorized as control channel abnormalities, IGBT overcurrent, overvoltage faults, etc. This section analyzes common faults, their causes, and solutions.
3.1 Control channel malfunction
Control channel malfunctions are usually caused by fiber optic communication issues between the PWM board and the power unit board, and generally fall into the following categories:
1. Poor contact at the fiber optic connection point or detachment of the fiber optic connector;
2. Dust accumulates inside the fiber optic signal transmitter/receiver;
3. The optical fiber broke;
4. The fiber optic communication control board is damaged;
In the event of a fiber optic fault, the first step is to determine whether the fault lies with the power unit or the controller. This can be done by swapping the fibers. Swap the fiber corresponding to any power unit on the same phase of the fiber optic board in the controller with the fiber reporting the fault. If the fault is still located in the original position on the monitoring interface after powering on again, the fiber optic board is damaged. Conversely, if the fault has changed position, the power unit is faulty, and replacement or repair of the faulty power unit should be considered.
3.2 Causes and solutions for IGBT overcurrent faults
IGBTs are the most critical power devices in high-voltage frequency converters. As high-power composite devices, IGBTs are susceptible to lock-up and damage under overcurrent conditions. To improve system reliability, several measures are implemented to prevent damage due to overcurrent. The common causes of IGBT overcurrent failures include the following:
1. Inverter output short circuit;
2. The IGBT inside the power unit is damaged and broken down;
3. Damaged drive detection circuit
4. The detection circuit is interfered with;
The detection method involves locating the corresponding module based on the fault location displayed on the monitoring interface, disassembling it, and checking whether the IGBT is damaged. The method for judgment is to locate the positive terminal V+ and the negative terminal V- of the DC bus inside the power unit. Connect the black probe of a multimeter to V+ and the red probe to U and V respectively. Using the diode setting, it should display a value of about 0.4V. If the reading is reversed, it will display infinity. Connect the red probe to V- and repeat the above steps. The same result should be obtained. Otherwise, it can be determined that the IGBT is damaged and needs to be replaced.
3.3 Causes and solutions for overvoltage faults
Overvoltage is generally caused by overvoltage on the power input side. Under normal circumstances, grid voltage fluctuations are within -10% to +10% of the rated voltage. However, in special cases, because the DC bus voltage rises with the power supply voltage, the inverter will trip due to overvoltage protection when the voltage rises to the protection value. To avoid input-side overvoltage, the transformer taps can be adjusted. This method is only suitable when the on-site voltage is consistently high. Alternatively, adding an absorption device on the power input side can reduce the overvoltage factor on the inverter's input side.
4 Conclusion
High-voltage, high-power frequency converters are playing an increasingly crucial role in industrial production, making their daily maintenance even more important. Only by understanding the various protection functions and troubleshooting methods of high-voltage frequency converters can we properly handle various problems that occur during operation. With the continuous development of technology, the functions and protection of high-voltage frequency converters will become more complete, and my country's high-voltage frequency converters will also move to the forefront of the world!
References:
[1] Jiuzhou Electric IPOWER Inverter Technical Manual, Harbin Jiuzhou Electric 2004
[2] Application Manual for High Voltage Frequency Converters, Zhong Mingzhen and Zhao Xiangbin, 2009, 04, Machinery Industry Press
About the author:
Yu Mingtao (1982-), male, Bachelor's degree, Engineer, currently the head of the frequency converter testing group at Jiuzhou Electric's medium-voltage frequency converter testing station.
