Foreword
In recent years, distributed inverters have continued to gain popularity. With the relative maturity of core materials such as IGBTs, SiC, and GaN, and the ever-increasing demand for power density, the single-unit power output of inverters has been continuously improving. Inverters that dominate the market have transitioned from 50-60 kW to 70-80 kW, and inverters with single-unit power outputs exceeding 100 kW are poised to enter the market at any time.
The increase in single-unit power places extremely stringent demands on the overall inverter design. Leakage detection is a crucial component, requiring overcoming challenges arising from increased power: larger operating ranges, electromagnetic interference, and different leakage modes. This discussion will focus on one of these challenges: high-frequency leakage.
Several common types of ground leakage in inverters
Non-isolated PV systems may experience grounding leakage.
Because the output side is directly grounded, if someone touches any of the output wires, it will cause current to flow through the human body and the ground, forming a leakage circuit.
[Isolated PV system with ground leakage]
With the addition of an isolation transformer, neither the primary nor secondary terminals are directly connected to ground. Therefore, touching the output terminal will not create an effective leakage circuit.
High-frequency leakage to ground
The high-frequency leakage current, which is of great concern, is always present in the system circuit, regardless of whether an isolation transformer is added to the output terminal.
The principle behind this is that when the inverter switches at high frequencies, some of the output current flows through the EMIY capacitor, through the parasitic capacitance of the PV module to ground, and then back to the inverter. Therefore, the larger the Y capacitor of the EMI or the parasitic capacitance of the PV module, the larger the high-frequency leakage current to ground will be, and the more severely the inverter's output current will be affected.
Should high-frequency leakage current be handled and protected against?
(1) To understand whether high-frequency leakage current protection is needed in an inverter, we must first know what the purpose of leakage current protection is.
The general purposes of leakage current protection are as follows:
One aspect is the protection of human safety, which is set to handle short-term sudden changes, such as 30mA needing to complete the alarm protection within 0.03 seconds.
Secondly, there is fire protection for the system equipment. The protection threshold is usually set at 300mA. For equipment with higher power, the threshold will increase with the increase of power range.
Thirdly, the detection of leakage current of DC 6mA and below is conducted to detect the insulation resistance value to ground and to confirm whether the leakage current to ground of the system is normal by detecting the change in voltage to ground.
High-frequency capacitive leakage current exists in real time as the inverter operates, has a relatively large base value, and changes slowly with varying operating conditions. This clearly does not fall under the category of sudden leakage current for human safety or insulation detection. From a fire prevention perspective, high-frequency leakage current is primarily composed of short-duration odd harmonics with relatively weak energy, insufficient to cause a fire. Furthermore, these higher harmonics can be eliminated through hardware. Therefore, the classification of high-frequency capacitive leakage current remains somewhat controversial.
Since the purpose of protecting against these high-frequency capacitive leakage currents is not very clear, are there similar systems available for reference, and how do they handle them?
The operation and leakage current of mine frequency converters are similar to those of photovoltaic inverters.
Due to their special structure, mine frequency converters often caused the coal mine leakage protection system to make misjudgments in the early days. This resulted in the leakage protection system sending incorrect power-off signals to the circuit breaker under normal production conditions, posing a serious accident hazard to coal mine safety production.
The main reasons why adding a frequency converter causes the leakage protection system in coal mines to misjudge are as follows:
High-order harmonics generated inside the frequency converter cause leakage current:
The rectangular square wave generated during the rectification process of the frequency converter and the pulse square wave formed by PWM modulation during the inversion process both contain higher harmonics in addition to the fundamental wave. Therefore, the output circuit also contains both the fundamental wave and higher harmonics. Due to the presence of capacitance between the underground cable and ground, as well as parasitic capacitances between the motor casing and the windings and ground, and the presence of Y-capacitors within the machine itself, higher harmonics will generate current in the capacitors, i.e., zero-sequence current. This can cause the coal mine leakage current protection system to misjudge and issue a power-off signal.
High-frequency interference:
The high-frequency and high-pulse signals in frequency converters are even higher than those of conventional signals, making it difficult for monitoring points to distinguish between interference and normal signals. In such cases, the system struggles to guarantee the reliability of detection values, leading to misjudgments by the monitoring system. Furthermore, interference-induced system detections can be either too high or too low compared to the actual protection target. For downstream protection, this can range from minor issues like frequent protection cycles disrupting normal system operation to serious problems like failure to provide protection when necessary, causing damage to production equipment and posing significant safety hazards. High-frequency interference can be summarized as an EMC (Electromagnetic Compatibility) problem.
The relationship between EMC and leakage current will be explained in a subsequent article.
The adverse effects of high-order harmonics generated by the frequency converter itself on the entire mine system can be mitigated through hardware measures, such as installing output filters, limiting them with reactors, and isolating them with transformers.
In addition, the grounding of equipment and the detection of insulation resistance values are becoming increasingly important. They can monitor the real-time value of the equipment's resistance to ground and reflect whether there are problems such as leakage current after the equipment has been working for a long time.
Hardware correction method:
Software correction method:
After a series of hardware processing steps, most of the high-order harmonic leakage current will be removed, but a small portion will still remain. Furthermore, the leakage current will tend to smooth out. For this smoothed leakage current, the method generally adopted is to increase the inverter's protection threshold. Since the environment of each mine is different, the selection of this threshold also needs to be adjusted separately.
In the low-voltage electrical appliance industry, which specializes in leakage current detection, how is high-frequency capacitive leakage current protected?
Based on information gathered from various customer sites using residual current devices (RCDs), it was observed that in the early days, when selecting RCDs for frequency converters, tripping during machine operation was a frequent occurrence. Considering the unique operating conditions of frequency converters, a preference has shifted towards RCDs with adjustable leakage current protection. This is because the machine itself exhibits high-frequency capacitive leakage current during operation, which is referred to in the industry as "false leakage." It's called "false leakage" because it's not the actual object the RCD is designed to protect against. Its danger is relatively minor, but its presence affects the accuracy of human safety protection and fire leakage protection.
Therefore, we often see that many residual current devices (RCDs) have the function of adjusting the residual current protection threshold.
The specific adjustment needs to be determined by the manufacturer based on the existing leakage current in their own frequency converter system, combined with the total protection threshold, to allocate a certain proportion. A common proportion is:
Protection threshold (IΔn) = Actual leakage current (0.7*IΔn) + High-frequency capacitive leakage current (0.3*IΔn);
The proportion of high-frequency capacitive leakage current should not exceed 30% of the protection threshold; otherwise, the entire system will frequently alarm and fail to function properly.
Returning to the method of detecting leakage current in photovoltaic inverters:
The causes of high-frequency capacitive leakage current in photovoltaic inverters are similar to those in mining inverters. The real-time leakage current values of both are affected by parasitic capacitance and voltage variations. Considering safety regulations, the larger the power range of the inverter, the larger the total capacitance of the absorption capacitors required. While improving resistance to grid voltage surges and EMC immunity, this indirectly increases high-frequency capacitive leakage current. High-frequency leakage currents, such as those from higher harmonics, are generally filtered out using reactors or similar methods.
The internal handling of leakage current in inverters can be either software-based or hardware-based.
[Main Software Processing]:
(There are also two solutions for leakage current detection that rely more on software processing.)
All leakage current components collected are summed and calculated.
However, this solution has certain drawbacks: it is difficult to collect all leakage current signals completely; at the same time, due to the presence of high-frequency capacitive leakage, it will greatly affect the accuracy of the inverter's sudden leakage protection and continuous leakage protection.
For example:
When a high-power inverter is operating in the field, due to the large number of components connected to its front end, the base value of its high-frequency capacitive leakage current is already quite large after the entire unit starts running. Moreover, the influencing factors in the field are unpredictable, and its base leakage current consists of countless harmonics of varying magnitudes and timings. At this point, any change in the field will amplify the actual output of the leakage current detection and greatly increase the possibility of triggering a sudden leakage current. These changes include ambient temperature and humidity, cable sway, voltage variations inside the inverter, and electromagnetic interference.
Now let's revisit the testing requirements in IEC 62109. The test models don't actually require testing for high-frequency capacitive leakage. Some tests involve suddenly adding resistive leakage to a high-frequency capacitive leakage current to test the reliability of this sudden change in leakage.
To illustrate the test requirement of capacitive superimposed resistive properties in inverters, the requirements of automotive leakage protection are introduced here.
Take IEC 62752 automotive residual current protection as an example. One requirement for leakage current testing is to superimpose a 1kHz waveform onto the normal 50Hz power frequency leakage current test. The standard explicitly states the purpose of superimposing the 1kHz waveform: to simulate various interference conditions during operation. It requires that the system's protection threshold be based on 50Hz, but not affected by the 1kHz waveform. The regulations also recognize that you can filter out high-frequency interference before testing and judgment.
Comparing the two testing standards and verification methods, it is not difficult to find that the high-frequency part is used as an interference factor that affects the actual leakage current detection.
The software filters out all high-frequency components of the collected leakage current, retaining only the low-frequency to DC leakage current.
The advantage of this approach is that the system only protects against leakage currents deemed necessary, unaffected by high-frequency components. This significantly improves the accuracy of alarms for actual leakage currents. However, the disadvantages are equally apparent: software algorithms that differentiate between actual and high-frequency leakage currents are highly complex and require substantial computational resources.
[Main Hardware Processing]:
Photovoltaic inverters filter out most high-frequency interference leakage current through their hardware. However, hardware alone cannot completely eliminate it. Therefore, sensors use their integrated low-pass filters to filter out the detected signals again, revealing the leakage current that is truly of concern.
The simulation verification test is as follows:
The high-frequency leakage current remaining inside the inverter can be controlled to a reasonable level by adjusting the low-pass filter.
Client-side practical applications:
Magtron continuously upgrades and improves its products based on the latest market demands. Addressing the increasing power consumption of mainstream inverters, Magtron offers higher range, lower power consumption, and greater stability for high-power inverters. Magtron is dedicated to solving leakage problems in industrial and electric vehicle applications, safeguarding electrical equipment across various industries.
