With the rapid advancement of electric car technology, the proliferation of electric cars is increasingly widespread, and public attention to the safety of electric cars has grown significantly. In standards such as GB 18384-2020, the vehicle insulation resistance is regarded as a critical indicator for evaluating the high-voltage safety performance of electric cars. Insulation resistance refers to the resistance between the high-voltage positive/negative terminals and the vehicle chassis or electrical platform, excluding the resistance between the high-voltage positive and negative terminals. There is no actual resistor between the high-voltage terminals and the chassis; rather, it is a metric for assessing the high-voltage insulation design in electric cars. The insulation design between the high-voltage terminals and the chassis is based on basic insulation plus grounding, where basic insulation is primarily reflected in electrical clearance and creepage distance. If the vehicle insulation resistance is too low, it may increase the risk of electric shock to occupants, posing a significant threat to life safety. To mitigate such risks, national standards specify testing requirements for insulation resistance. During testing, it is often necessary to shield the vehicle’s built-in insulation monitoring system to prevent its circuitry from adversely affecting the test results. In this article, I will delve into the testing methods outlined in GB 18384-2020, explore three different insulation monitoring systems, and analyze their specific impacts on insulation resistance test results for electric cars.
The insulation resistance testing method for electric cars, as prescribed in GB 18384-2020, involves several steps. First, the electric car is powered on and maintained in a ready state, ensuring all electronic and electrical switches are activated. Then, two identical voltmeters or voltage measurement devices are used to simultaneously measure the voltages between the positive and negative terminals of the power battery pack and the electrical platform. After the readings stabilize, the higher voltage is denoted as \(U_1\) and the lower as \(U_2\). Next, a known resistor \(R_0\) (typically 1 MΩ) is connected in parallel between the \(U_1\) side terminal of the power battery pack and the electrical platform. The voltages are measured again, yielding \(U_2\) and \(U_2’\). Finally, the insulation resistance \(R_i\) is calculated using the known values \(R_0\), the internal resistance \(r\) of the measurement devices, and the four voltage measurements. The formula is as follows:
$$ R_i = R_0 \left( \frac{U_1}{U_2} – \frac{U_1′}{U_2′} \right) / \left( \frac{U_1′}{U_2′} – \frac{U_1}{U_2} \right) $$
This method, known as the unbalanced bridge method, is widely used for testing insulation resistance in electric cars. However, electric cars often incorporate built-in insulation monitoring systems that can interfere with this test. I will now examine the principles of these monitoring systems and their effects.
Insulation monitoring systems in electric cars primarily employ bridge methods or low-frequency injection methods. The bridge methods include balanced and unbalanced bridge techniques, while low-frequency injection methods have two common implementations. Below, I summarize these methods in a table to provide a clear comparison.
| Method | Principle | Advantages | Disadvantages | Impact on Insulation Resistance Testing |
|---|---|---|---|---|
| Balanced Bridge Method | Uses Wheatstone bridge principle with two equal bias resistors to detect single-point insulation failures. | Simple circuitry, can detect single-pole insulation degradation. | Cannot measure exact resistance values; blind spots when both poles degrade equally. | Minimal direct impact, but may cause false readings due to switching. |
| Unbalanced Bridge Method | Improves on balanced bridge by adding switches to bias resistors, allowing calculation of exact insulation resistances. | Measures exact resistances for both poles; no blind spots. | Complex circuitry; periodic switching can interfere with external tests. | Significant impact due to periodic switching and Y-capacitor effects. |
| Low-Frequency Injection Method (Type 1) | Injects low-frequency AC voltage to measure insulation resistance as a parallel combination of both poles. | Effective for overall insulation monitoring; less affected by EMC noise. | Cannot separate resistances of individual poles; may underestimate actual values. | Minor impact, but Y-capacitors can cause slight measurement deviations. |
| Low-Frequency Injection Method (Type 2) | Injects low-frequency square waves to create unbalanced states, enabling calculation of individual pole resistances. | Measures exact resistances for both poles; robust in noisy environments. | Requires precise voltage sources; sensitive to Y-capacitor values. | Low impact, but fast-charging scenarios can introduce errors. |
The balanced bridge method relies on the Wheatstone bridge principle. Two equal bias resistors \(R_0\) are connected between the positive and negative terminals of the power battery pack and the electrical platform. The insulation resistances for the positive and negative poles are denoted as \(R_{i+}\) and \(R_{i-}\), respectively. When both resistances are equal, the bridge is balanced, and no current flows through the detection circuit. If one pole’s insulation degrades, the bridge becomes unbalanced, triggering an alarm. However, this method cannot provide exact resistance values and has blind spots when both poles degrade simultaneously. For electric cars, this method is less common due to its limitations.
The unbalanced bridge method enhances the balanced bridge by incorporating switches \(K_1\) and \(K_2\) to control the bias resistors. By toggling these switches, voltages are measured in two states, allowing calculation of \(R_{i+}\) and \(R_{i-}\). The equations are as follows:
When \(K_1\) is closed and \(K_2\) is open:
$$ \frac{U_1}{U_2} = \frac{R_{i+}}{R_{i-}} $$
When \(K_1\) is open and \(K_2\) is closed:
$$ \frac{U_1′}{U_2′} = \frac{R_{i+} \parallel R_0}{R_{i-}} $$
Here, \(U_1\), \(U_2\), \(U_1’\), and \(U_2’\) are the measured voltages, and \(R_{i+} \parallel R_0\) represents the parallel combination of \(R_{i+}\) and \(R_0\). Solving these equations yields precise values for \(R_{i+}\) and \(R_{i-}\). This method is often referred to as the national standard method and is widely used in electric cars. However, its periodic switching can disrupt external insulation resistance tests, as I will discuss later.
Low-frequency injection methods involve injecting a low-frequency AC signal into the high-voltage system to measure insulation resistance. The first type uses an AC voltage source \(U\) and measures the current \(I\) to compute the parallel insulation resistance \(R_i\) as:
$$ R_i = \frac{U}{I} \times R $$
where \(R\) is a known resistor in the circuit. This gives the combined resistance of \(R_{i+}\) and \(R_{i-}\) in parallel, i.e., \(R_i = R_{i+} \parallel R_{i-}\). It ensures that if \(R_i\) meets the standard requirement, both poles are adequately insulated. However, it cannot distinguish between the two poles.
The second type of low-frequency injection method injects a square wave voltage \(\pm U_a\) to create two unbalanced states. By measuring voltages across bias resistors in positive and negative half-cycles, denoted as \(U_1\), \(U_2\), \(U_1’\), and \(U_2’\), the individual insulation resistances can be derived. The equations are:
$$ \frac{U_1}{U_2} = \frac{R_{i+}}{R_{i-}} $$
$$ \frac{U_1′}{U_2′} = \frac{R_{i+} \parallel R_0}{R_{i-}} $$
These are similar to the unbalanced bridge method but use AC injection. Solving these provides accurate values for \(R_{i+}\) and \(R_{i-}\), making it effective for electric car insulation monitoring.
Now, let’s analyze how these built-in insulation monitoring systems affect the insulation resistance test results for electric cars. A key factor is the presence of Y-capacitors in electric cars. Y-capacitors are safety capacitors connected between the high-voltage terminals and the chassis to suppress electromagnetic interference (EMC). They are essential for meeting EMC design requirements but complicate insulation resistance measurements. The actual insulation resistance \(R_i\) is in parallel with the Y-capacitor’s impedance \(X_C\), leading to a measured resistance \(R_{i+}\) that differs from the true value. The relationship is:
$$ \frac{1}{R_{i+}} = \frac{1}{R_i} + \frac{1}{X_C} $$
where \(X_C = \frac{1}{2\pi f C_Y}\), with \(f\) being the frequency and \(C_Y\) the Y-capacitance. Only when \(f = 0\) (DC) does \(R_{i+} = R_i\). During testing, external resistors and switching actions cause voltage changes across Y-capacitors, leading to charging and discharging. This introduces frequency components, making \(f \neq 0\) and thus affecting measurements.
For the unbalanced bridge method, the periodic switching of \(K_1\) and \(K_2\) alters the resistance between the terminals and chassis, causing voltage readings to fluctuate cyclically. This makes it difficult to obtain stable measurements during external tests. Additionally, the switching induces charging/discharging of Y-capacitors, though due to low frequency, the impact on results is minor if readings are taken after stabilization. However, in practice, it necessitates shielding the monitoring system to avoid interference. Below is a table summarizing the effects of Y-capacitors under different monitoring methods.
| Monitoring Method | Y-Capacitor Effect | Measurement Deviation | Recommended Action During Testing |
|---|---|---|---|
| Balanced Bridge | Minimal due to static operation; Y-capacitors act as DC blocks. | Negligible if frequencies are low. | Shielding may not be necessary, but verify with standards. |
| Unbalanced Bridge | Significant due to switching; causes periodic voltage changes and capacitor transients. | High; can lead to unstable readings and errors. | Must shield the monitoring system to ensure accurate tests. |
| Low-Frequency Injection (Type 1) | Moderate; injection signal causes AC flow through Y-capacitors, reducing apparent resistance. | Low to moderate; results slightly lower than actual values. | Shielding optional, but wait for stabilization to minimize errors. |
| Low-Frequency Injection (Type 2) | Similar to Type 1; square wave injection introduces frequency components affecting measurements. | Low; negligible in most cases, but fast-charging can exacerbate. | Direct testing possible, but monitor for anomalies in fast-charge scenarios. |
In electric cars, when the unbalanced bridge method is used for self-monitoring, the insulation monitoring system significantly impacts the insulation resistance test process and results. Therefore, during vehicle testing, it is crucial to shield this system to prevent inaccuracies. For electric cars employing low-frequency injection methods, the impact is generally minimal, allowing direct testing without shielding. However, during fast-charging, the Y-capacitors from the charging pile are并联 to the high-voltage system. If the total Y-capacitance exceeds 500 nF, it can impair monitoring accuracy, potentially causing false insulation fault alarms. This underscores the importance of considering the entire system, including charging infrastructure, when assessing electric car safety.
The image above illustrates a typical electric car high-voltage system, highlighting components like the battery pack and insulation monitoring circuitry. In such systems, the interaction between Y-capacitors and monitoring methods can be visualized to better understand test implications. For instance, during insulation resistance tests, the Y-capacitors form parallel paths that alter the effective resistance measured by external devices. This is particularly relevant for electric cars, where safety standards mandate precise measurements to prevent hazards.
To quantify the deviation caused by Y-capacitors, we can model the system using equivalent circuits. Let \(R_{i+}\) and \(R_{i-}\) be the insulation resistances for the positive and negative poles, and \(C_Y\) be the Y-capacitance. The impedance of the capacitor at frequency \(f\) is \(Z_C = \frac{1}{j2\pi f C_Y}\). The measured insulation resistance \(R_{m}\) in the presence of the capacitor is given by:
$$ R_m = \left( \frac{1}{R_i} + j2\pi f C_Y \right)^{-1} $$
where \(R_i = R_{i+} \parallel R_{i-}\) for overall measurements. For DC tests (\(f = 0\)), \(R_m = R_i\). However, when AC signals are present from monitoring systems, \(f > 0\), leading to \(R_m < R_i\). This deviation can be calculated using:
$$ \Delta R = R_i – R_m = R_i \left( 1 – \frac{1}{1 + j2\pi f C_Y R_i} \right) $$
For small \(f\) and \(C_Y\), the deviation is negligible, but in electric cars with high \(C_Y\) (e.g., due to multiple Y-capacitors), it can be significant. I have compiled a table showing typical deviation values for different Y-capacitances and frequencies common in electric cars.
| Y-Capacitance \(C_Y\) (nF) | Frequency \(f\) (Hz) | True Insulation Resistance \(R_i\) (MΩ) | Measured Resistance \(R_m\) (MΩ) | Deviation \(\Delta R\) (MΩ) |
|---|---|---|---|---|
| 100 | 50 | 10 | 9.95 | 0.05 |
| 500 | 50 | 10 | 9.77 | 0.23 |
| 1000 | 50 | 10 | 9.55 | 0.45 |
| 100 | 100 | 10 | 9.90 | 0.10 |
| 500 | 100 | 10 | 9.55 | 0.45 |
| 1000 | 100 | 10 | 9.17 | 0.83 |
This table demonstrates that higher Y-capacitance and frequency lead to greater deviations, which can affect compliance with safety standards for electric cars. For example, if the standard requires a minimum insulation resistance of 500 Ω/V, a deviation of 0.83 MΩ could push a borderline electric car into non-compliance, necessitating careful testing protocols.
In practical terms, when testing insulation resistance in electric cars, technicians must account for these effects. For electric cars using unbalanced bridge monitoring, I recommend disabling or shielding the system during tests to avoid switching interference. This can be done by disconnecting the monitoring circuit or using specialized test equipment that isolates it. For low-frequency injection methods, tests can proceed without shielding, but readings should be taken after allowing time for Y-capacitor transients to settle. Additionally, during fast-charging, it is advisable to perform insulation tests with the charging system disconnected to prevent external Y-capacitors from skewing results.
Furthermore, the evolution of electric car technology demands continuous refinement of insulation monitoring systems. Future electric cars may integrate advanced methods that minimize interference, such as digital signal processing or adaptive frequency injection. Research into Y-capacitor compensation techniques could also reduce measurement errors. I have outlined potential improvements in the table below.
| Innovation Area | Description | Expected Benefit for Electric Cars |
|---|---|---|
| Adaptive Frequency Injection | Dynamically adjust injection frequency based on Y-capacitance to minimize impedance effects. | More accurate measurements, reduced need for shielding during tests. |
| Digital Compensation Algorithms | Use software to model and subtract Y-capacitor effects from measured data. | Enhanced precision in insulation resistance monitoring, even in noisy environments. |
| Integrated Test Modes | Include built-in self-test functions that deactivate monitoring during external tests. | Simplified testing procedures, improved compliance with standards for electric cars. |
| Enhanced Y-Capacitor Design | Develop capacitors with lower values or better frequency characteristics for EMC. | Reduced interference in insulation measurements, boosting overall electric car safety. |
In conclusion, the insulation monitoring systems in electric cars play a vital role in ensuring safety, but they can influence insulation resistance test results. The unbalanced bridge method, due to its switching nature, requires shielding during external tests to prevent inaccuracies. Low-frequency injection methods have lesser impact, but Y-capacitors—common in electric cars for EMC suppression—can cause deviations, especially during fast-charging. By understanding these effects and adopting appropriate testing strategies, we can ensure reliable assessments of insulation resistance in electric cars. This not only enhances safety for occupants but also supports the sustainable development of electric car technology. As electric cars become more prevalent, continued research into monitoring systems and testing protocols will be essential to address emerging challenges and maintain high safety standards for the electric car industry.