Aug 31, 2020
Discuss the treatment of high frequency leakage in the inverter
In recent years, distributed inverters have continued to be hot, and core materials including IGBT, SiC, GaN and other core materials have become relatively mature, and power density requirements have continued to rise. Therefore, the single-unit power kilowatts of inverters have also been continuously improved. The inverter, which dominates the market, has a power transition from 50-60KW to 70-80KW. Inverters with a single power of hundreds of kilowatts are also ready to go to the market.
The increase in the power of a single machine has made the overall design of the inverter very strict. Leakage detection is a very core piece. It needs to overcome the problems brought about by the increase of power: long range, electromagnetic interference, different leakage modes, etc. This time we will discuss one of the high-frequency leakage.
Several types of common ground leakage of inverters
【Non-isolated PV system to earth leakage】
Since the output side is directly grounded, if someone touches any line of the output terminal, it will cause current to flow through the human body and the earth to form a leakage loop.
【Isolated PV system to ground leakage】
After adding the isolation transformer, the primary and secondary terminals are not directly connected to the ground. Touching the output terminal at this time will not form an effective leakage loop.
【High frequency to ground leakage】
The high-frequency leakage, which is of great concern, is not affected by whether the output terminal is added with isolation transformer or not, and always exists in the system loop.
The principle of its generation: When the inverter is switched at high frequency, part of the output current will flow through the EMI Y capacitor through the parasitic capacitance of the PV component to the earth, and then flow back to the inverter. Therefore, only the Y capacitor or PV The greater the parasitic capacitance of the component, the greater the high-frequency leakage current to the ground generated, and the more serious the output current of the inverter is affected.
(1) To understand whether the high-frequency leakage in the inverter needs protection, first of all, we must know what is the purpose of leakage protection?
There are generally several protection purposes for leakage current:
The first is the protection of human safety, which is set as a short-term mutation, such as 30mA, to complete the alarm protection within 0.03S.
The second is the protection of system equipment from fire. Usually the protection threshold is set to 300mA. If the power of the equipment is larger, the threshold will increase with the increase of the power section.
The third is the detection of the leakage current of DC 6mA and below. Its purpose is to detect the insulation resistance value to ground, and to confirm whether the leakage current of the system to ground is normal by detecting the change of the ground voltage.
The high-frequency capacitive leakage exists in real time with the operation of the inverter, the basic value is large, and it changes slowly with the change of working conditions, which obviously does not belong to the sudden leakage and insulation detection to protect the safety of the human body. From the perspective of fire protection, high-frequency leakage is more composed of odd harmonics with a short time, and its energy is relatively weak, not enough to cause a fire. And these higher harmonics can be removed by hardware. There is some controversy about the location of high-frequency capacitive leakage.
Since the protection purpose of these high-frequency capacitive leakage is not very clear, is there a similar system for reference, and how do they deal with it?
The working conditions of the mine inverter and the leakage generation are similar to the photovoltaic inverter.
Due to its special structure, the mine frequency converter often caused the coal mine leakage protection system to make a misjudgment in the early stage, which caused the leakage protection system to send an incorrect power-off signal to the circuit breaker under normal production conditions, which caused serious damage to coal mine safety production. Hidden dangers of accidents.
The main reasons for the misjudgment of the coal mine leakage protection system caused by the frequency converter are as follows:
1. High-order harmonics generated inside the inverter cause leakage current:
The rectangular square wave generated during the rectification process of the inverter and the pulse square wave formed by PWM modulation during the inverter process also contain high harmonics in addition to the fundamental wave, so that the output line also contains the fundamental wave and high harmonics , Due to the existence of the underground cable-to-ground capacitance, and there are parasitic capacitances between the motor casing and the windings to the ground, and there are Y capacitors inside the machine itself. High-order harmonics will generate current on the capacitor, that is, zero-sequence current, which will make the coal mine leakage protector system misjudge and send out a power-off signal;
2. High frequency interference:
The high frequency and high pulse in the frequency converter are higher than conventional signals, and it is difficult to distinguish whether this is an interference signal or a normal signal at the monitoring point. In this case, the system is difficult to guarantee the reliability of the detection value, which leads to errors in the monitoring system. Judgment action. And compared with the actual protection target point, the system detection caused by interference will be both high and low. For the back-end protection, the lighter one, the frequent protection affects the normal operation of the system. In serious cases, if the protection is not protected, the production equipment will be damaged, and there will be major accidents. High frequency interference is summarized as a problem of EMC electromagnetic interference.
Regarding the relationship between EMC and leakage current, the author will explain in a subsequent article.
The high-order harmonics generated by the frequency converter itself have an adverse effect on the entire mine system and can be handled by hardware. Such as: installation of output filters, reactor restrictions, transformer isolation and other methods.
In addition, whether the equipment is grounded or not, and the detection of the insulation resistance value is becoming more and more important. It can monitor the real-time value of the equipment to the ground impedance, and can reflect whether the equipment has leakage current after working for a long time.
Hardware modification method:
Software revision and revision:
After a series of hardware processing, most of the high-order harmonic leakage will be removed, but a small part will still be left. And it will tend to be smooth. For the smooth leakage at this time, the method of increasing the protection threshold of the inverter is generally adopted to operate. The environment of each mine is different, so the choice of this threshold also needs to be debugged separately.
(2) In the low-voltage electrical industry that specializes in leakage detection, how do you protect against high-frequency capacitive leakage?
The author has collected relevant information and learned from various customer sites where the leakage protection is used. In the early days, when the inverter was selected for leakage protection, the phenomenon of machine tripping during operation often occurred. After considering the particularity of the inverter's working conditions, when choosing a leakage protection for the inverter, everyone is more inclined to choose a leakage protection device with an adjustable leakage protection function. The reason is that the machine itself has high-frequency capacitive leakage during operation, which is called false leakage in the industry. The reason for calling it false leakage is that it is not the real object of leakage protection. It is less harmful, and its existence affects the accuracy of human body safety protection and fire leakage protection. Therefore, we can often see the function of adjustable leakage protection threshold on many leakage protection.
The specific adjustment manufacturer needs to allocate a certain proportion according to the predetermined leakage current of its own inverter system, combined with the size of the total protection threshold. The general ratio is:
Protection threshold (IΔn) = true leakage current (0.7*IΔn) + high-frequency capacitive leakage (0.3*IΔn);
It is required that the proportion of the high-frequency capacitive leakage part is not higher than 30% of the protection threshold, otherwise the entire system will frequently alarm and fail to work normally.
Return to the detection method of leakage current in the photovoltaic inverter:
The causes of high-frequency capacitive leakage of photovoltaic inverters are similar to those of mine inverters. The real-time leakage current value of both will be affected by parasitic capacitance and its own voltage change. Taking into account safety regulations, the larger the power section of the inverter, the larger the total capacity of the absorption capacitor that needs to be used. While improving the ability of resisting grid voltage shock and EMC immunity, it also indirectly increases high-frequency capacitive leakage. For the treatment of high frequency leakage such as high-order harmonics, reactors are generally used to filter out.
The internal handling of the leakage current in the inverter is divided into software processing and hardware processing.
[Main software processing]:
(There are also two schemes for the leakage detection of partial software processing)
① Accumulate all the collected leakage components.
However, this scheme has certain drawbacks: it is difficult to collect all leakage current signals completely; at the same time, due to the existence of high-frequency capacitive leakage, it will greatly affect the accuracy of inverter sudden leakage protection and continuous leakage protection.
When a larger power inverter is working in the field, due to the large number of components connected to the front end, the basic value of the high-frequency capacitive leakage generated by the whole machine is already very large after the whole machine is running. Moreover, the influencing factors on site are uncertain, and the basic leakage is composed of countless harmonics of different sizes and times. At this time, any site change will amplify the true output of the leakage detection, and it is very likely to trigger a sudden leakage. The changes here include environmental temperature and humidity, cable wind swing, voltage changes inside the inverter, and electromagnetic interference.
At this time, let's look at the testing requirements in IEC62109. In fact, there is no requirement to detect high-frequency capacitive leakage in the test model. Others add resistive leakage suddenly on the basis of high-frequency capacitive leakage to test the reliability of sudden leakage at this time.
In order to illustrate the test requirement of capacitive superposition resistance in the inverter, the requirement of vehicle leakage protection is introduced here
For example, IEC62752 car leakage protection. One of the requirements of the leakage current detection project is to superimpose a 1KHz waveform on the basis of the normal detection of the power frequency 50Hz leakage current. The standard clearly states the purpose of superimposing 1KHz: to simulate various interference conditions in operation. The protection threshold of the system in the test is required to be based on 50Hz, but it cannot be interfered by the 1KHz waveform. The regulations also recognize that you can filter out high-frequency interference in advance and then make detection judgments.
Comparing the two test standards and verification methods, it is not difficult to find that the high frequency part is used as the interference quantity that affects the real leakage detection.
① Filter out all high-frequency parts of the collected leakage software, and only keep the low-frequency to DC leakage.
The advantage of this method is that the system only protects the leakage current that is considered to be protected, and is not affected by the high-frequency part. Its accuracy of real leakage alarms will be greatly improved. But the shortcomings are also obvious. The algorithm for distinguishing real leakage from high-frequency leakage by software is more difficult and takes up a lot of calculation.
[Main hardware processing]:
The photovoltaic inverter filters most of the high-frequency interference leakage through its own hardware. However, hardware alone cannot completely remove it. At this time, the sensor filters out the detected signal again through its own internal low-pass filter. The leakage current that really needs attention is reflected in the main form.
The simulation verification test is as follows:
Regarding the remaining high-frequency leakage inside the inverter, it can be controlled to a reasonable position by adjusting the low-pass filter.
Client practical application:
SAFESAVE is constantly upgrading and improving based on the latest real-time market demand. In view of the rising market of mainstream inverter stand-alone power. Matching high-power machines with larger range, lower power consumption, and higher stability. SAFESAVE is committed to solving various leakage problems in industrial and electric vehicles, and escorting power equipment in various industries.