1. Design challenges in inverter bus voltage monitoring
Frequency converters change their operating frequency by controlling the switching of power electronic devices such as IGBTs. Because the actual operating bus voltage is generally high and the switching speed is fast, the voltage change rate is extremely high (dV/dt). For example, in a 10.5kV bus system, the voltage change rate can reach 12kV/μs, which poses a significant challenge to the stability of the control system. For reliable system operation, the control system must be effectively isolated from the IGBT drive system.
Similarly, to accurately control the bus voltage, we also need to monitor the bus voltage and current signals in real time. However, there is a very high potential difference between the voltage bus side and the control system, which can easily lead to control system failure. We also need an efficient isolation solution similar to the IGBT drive system to solve the above problems, namely, a reinforced insulation solution. Let's look at a practical case that shows how a reinforced insulation solution can solve the above problems.
As shown in Figure 1, when the sampling IC acquires the bus voltage, a significant common-mode potential difference exists between the sampling system on the high-voltage side and the control system on the low-voltage safety side. Combined with the switching spikes introduced by the IGBT system, this potential difference can reach several kV. Therefore, signal isolation is achieved using an optocoupler that meets the UL standard of 37.5kV AC isolation withstand voltage. Simultaneously, a matching DC/DC power supply module meeting the UL isolation withstand voltage of at least 2.5kV AC is required. It is worth noting that the high-voltage side is located at the DC/DC output terminal; this application has some specific implicit technical requirements that pose design challenges.
Traditional DC-DC converters are generally used as secondary power supplies in power systems. The preceding stage typically includes an AC-DC converter as the primary power source, while the DC-DC converter primarily serves as a secondary isolation stage, providing isolation to reduce common-mode noise and voltage conversion. In this configuration, interference from the primary AC voltage is isolated by the AC-DC converter and then further isolated by the DC-DC converter, resulting in a very stable voltage for the control system. Therefore, the aforementioned problems do not exist. The system architecture is shown in Figure 2, with both the DC/DC input and output residing within the SELV (Self-Employment Low-Voltage) safety circuit.
As shown in Figure 1, if a DC-DC converter is used to monitor high voltage, its output terminal is directly connected to the negative terminal of the bus voltage. Therefore, the DC-DC output side must simultaneously face the high potential difference between the input and output, as well as common-mode interference caused by the high voltage change rate dv/dt of the IGBT system. How to isolate these two types of interference so that they do not affect the normal operation of the control system becomes a design challenge. So, how can this design challenge be solved?
2. Strengthen insulation design scheme
We know that the AC-DC converter in Figure 2 uses a reinforced insulation design. Therefore, we must consider using a similar design in the DC-DC section of Figure 2 to enhance insulation performance. First, the transformer's insulation wire cannot be ordinary enameled wire, but rather triple-insulated wire. Second, the electrical clearance between the input and output must be designed to be longer, requiring reference to the EN60950 standard and the actual operating voltage. For example, if the operating voltage is 480V AC, the electrical clearance needs to be greater than or equal to 6.4mm. Furthermore, the design processes for transformers and PCBs are also quite different.
The above design scheme can improve the insulation effect against high potential differences. However, for high voltage change rate interference immunity design, we must consider another important design requirement—low isolation capacitance.
Interference signals caused by high voltage change rates can couple to the control side through the signal optocoupler and the parasitic capacitances of the primary and secondary sides of the DC/DC power supply. Since we have already selected the signal optocoupler, we only focus on the DC/DC design. There are potentially three interference coupling paths within the DC/DC power supply: the parasitic capacitance inherent in the transformer itself, the parasitic capacitance of the optocoupler in the feedback loop within the DC/DC power supply, and the safety Y capacitor added to improve EMI performance. The greater the combined value of these three factors, the easier it is for high-frequency interference signals to pass through the isolation barrier and enter the control system, causing malfunctions. Therefore, when designing this type of power supply, we first eliminate the safety Y capacitor, and secondly, choose a Royer topology without an optocoupler to reduce the latter two factors to zero. Simultaneously, we adopt a transformer design scheme that reduces the isolation capacitance, as shown in Figure 4, minimizing the transformer's isolation capacitance as much as possible. For example, the isolation capacitance of the power supply we designed is less than 10pF.
Our enhanced insulation design can solve the above technical problems, but it is not enough to prove that our design is reliable enough. We still need to establish a complete insulation testing system to verify the reliability of our design.
3. Insulation performance testing and verification scheme
The following are three testing and verification solutions from Mornsun for insulation performance:
3.1 Isolation withstand voltage test
Step 1: Short-circuit all terminals at both ends of the test component to prevent damage from being left unconnected. See Figure 5.
Step 2: Following the withstand voltage test standard, gradually increase the withstand voltage value from 0V until it reaches the set maximum withstand voltage, and maintain this maximum withstand voltage for one minute. See Figure 6.
If the test shows no breakdown or arcing, and the leakage current is less than the set value (e.g., 1mA), the test is acceptable. If the test fails, the tester will alarm.
The isolation withstand voltage test is simple, requiring only a standard withstand voltage tester. If the insulation system is damaged, the tester can easily detect it and trigger an alarm. Therefore, the withstand voltage test is widely used for testing the insulation characteristics of insulation systems. However, its drawbacks are also obvious: it cannot detect defects in the insulation system, thus limiting its use to a preliminary insulation testing approach.
3.2 Impulse Voltage Test
Similar to the connection method in Figure 5, replace the withstand voltage tester with a surge generator to apply a 1.2μs/50μs standard surge wave to the insulating test object. The actual test voltage is set according to the system requirements, and the test object is then judged to determine whether it is damaged. See Figures 7 and 8 respectively.
We can acquire the impulse voltage waveform using an oscilloscope, as shown in Figures 9 and 10. After the surge voltage is removed, power on the product and apply a load. If the product functions normally, it passes; otherwise, it fails. The impulse voltage test is a relatively violent test method that tests the test object's ability to withstand impulse voltage. Like the withstand voltage test, it is a destructive test, meaning that the test itself causes some damage to the test object.
3.3 Partial Discharge Test
We know that withstand voltage testing and impulse voltage testing are destructive tests. Repeated tests or test values close to the critical values of the insulation system may lead to damage or failure of the insulation system. However, the test results after the damage is assessed show that the leakage current of the insulation system does not exceed the standard, indicating that the insulation system is normal. This poses a certain risk, hence the introduction of the partial discharge testing method.
Partial discharge testing is an effective method for testing the insulation characteristics of frequency converters. The difference between it and the previous two test schemes is that it can detect minute insulation defects in the insulation system without damaging the insulation body.
The specific method involves applying a certain voltage to the insulation system and testing the residual charge over a certain period of time to determine the performance of the insulation system. Relative withstand voltage testing is a non-destructive and highly accurate insulation testing method. The test method is briefly described below, and the test process is shown in Figure 11.
(1) Referring to the requirements of UL508, the operating voltage of the frequency converter is selected as the partial discharge test voltage UPD. The partial discharge voltage is linearly increased to 1.875 times UPD and held for less than 5 seconds. For example, if the operating voltage of the frequency converter is 400V AC, the test UPD is determined by referring to the table in UL508, as shown in Table 1. The test voltage reference is 450V AC, and the first stage test voltage is 835V AC (first column in yellow).
(2) Then the voltage is linearly reduced to 1.5 times UPD (error ±5%), and the second stage test voltage is 668V AC (refer to the second column in yellow) and held for 15s.
(3) During this stage, the partial discharge charge is tested. If the tested charge is less than 10 pC, the test is passed, as shown in Figures 12 and 13. The residual charge during the entire test process is less than 2.5 pC. Otherwise, it is NG, as shown in Figure 14. The highest point during the test process reaches 175 pC.
4 Reference Design for Inverter Bus Voltage Monitoring
For the inverter block diagram in Figure 1, a suitable reference design is the Mornsun H2409S-2W-GM. The output can be directly connected to the negative terminal of the bus after the rectifier bridge, and the input is connected to a 24V SELV safety low-voltage circuit, sharing a common ground with the 5V controller. This power supply is designed based on the implicit technical requirements of the inverter, as shown in Figure 15, and its specific features are as follows:
● Meets CE requirements for high isolation withstand voltage (4242V DC) / impulse withstand voltage / partial discharge test;
● Meets UL material requirements and features a high isolation withstand voltage (4242V DC) design;
● Meets the enhanced insulation requirements for 480V AC operating voltage;
● The isolation capacitance is less than 10pF;
● Continuous output short-circuit protection.
5. Conclusion
This article introduces the technical challenges encountered in bus voltage monitoring of frequency converter systems and Mornsun's corresponding enhanced insulation solution. It also describes three methods for testing and verifying insulation performance and finally recommends a power supply reference design for an enhanced insulation system. We hope this information is helpful.
Original article address: http://www.mornsun.cn/news/NewDetail.aspx?id=291&channelid=132
