High-Voltage Interlock (HVIL) is a critical safety design mechanism in electric vehicles that ensures the integrity of high-voltage electrical connections. This system uses low-voltage signals to monitor all components connected with high-voltage wiring harnesses, detecting any disruptions or faults in the high-voltage system circuits. When a circuit is disconnected or its integrity is compromised, HVIL triggers appropriate safety measures to prevent potential hazards.
Key Functions of High-Voltage Interlock
Detect High-Voltage Circuit Looseness
HVIL systems continuously monitor high-voltage connections for any signs of looseness or disconnection. This is crucial because loose connections can lead to high-voltage power failure, causing the vehicle to lose power and compromising riding safety. Before complete power failure occurs, HVIL provides early warning information, allowing the vehicle system time to implement countermeasures.
Pre-Power-On Testing
Before the vehicle's high-voltage system is energized, HVIL performs a comprehensive integrity check of all high-voltage circuits. If any circuit is detected to be incomplete or faulty, the system prevents power-on, thereby avoiding accidents caused by virtual connections and other electrical issues.
Prevent Human Misoperation
During high-voltage system operation, manual disconnection of high-voltage connections without proper safeguards can generate dangerous high-voltage arcs. HVIL prevents this by ensuring that high-voltage connections cannot be disconnected while energized, protecting both personnel and equipment from potential harm.
Scope of HVIL Protection
High-voltage interlock systems provide comprehensive electrical safety coverage for multiple critical components in electric vehicles, including:
HVIL Implementation Mechanisms
High-voltage interlock systems operate through two primary levels of implementation: mechanical and controller-based approaches.
Mechanical Level Implementation
At the mechanical level, all high-voltage connectors are equipped with mechanical interlocks. The HVIL pin is specifically designed to connect only after the high-voltage pin is securely engaged during connection, and to disconnect first during disconnection. This sequencing ensures that whenever the HVIL pin is connected, the high-voltage pin remains securely engaged, maintaining system safety.
Controller Level Implementation
At the control level, HVIL uses low-voltage signals to continuously monitor the integrity of high-voltage circuits. The low-voltage monitoring circuit is designed to connect before the high-voltage circuit is disconnected, providing necessary lead time for safety measures. This lead time can be customized based on specific project requirements but must always ensure that appropriate safety measures are implemented before any high-voltage circuit disruption occurs.
HVIL Monitoring Principle
A typical HVIL monitoring circuit applies a predefined current (typically a few milliamps) to the high-voltage interlock circuit and measures the returned current. If the measured current matches the predefined value, all connectors in the high-voltage system are securely connected, and it is safe to activate the high-voltage system.
Conversely, if a connector is disconnected or loose, causing the measured current to decrease (or drop to 0A), this indicates a problem with the high-voltage connector status. When such an issue is detected, the HVIL monitoring unit relays this information to the Electronic Control Unit (ECU), which controls the high-voltage relay (typically managed by the battery management system) to deactivate the high-voltage system.
HVIL Implementation Architecture
The following example illustrates a typical high-voltage interlock implementation architecture in electric vehicles:
Two primary circuits work in tandem:
- The high-voltage interlock circuit within the battery pack itself
- A comprehensive high-voltage interlock circuit connecting the VCU (Vehicle Control Unit), motor system, PTC (Positive Temperature Coefficient) heater, compressor, and charger
In this architecture, the Battery Management System (BMS) detects the high-voltage interlock status of the battery and communicates this state to the VCU. The VCU supplies 12V power to the other high-voltage interlock circuit and monitors its status. By combining information from both circuits, the VCU determines the overall high-voltage interlock status of the entire vehicle.
HVIL Strategy Implementation
The specific high-voltage interlock strategy typically follows this logic:
- The VCU's high-voltage interlock output pin outputs a 12V high level
- If the high-voltage interlock input pin acquires a high level exceeding 8V, the powertrain's high-voltage interlock is considered normal
- If the VCU receives a "normal" BMS high-voltage interlock status via CAN message, the battery system's high-voltage interlock is considered normal
- The VCU determines the overall vehicle high-voltage interlock status based on the logical "AND" relationship between these two conditions
- The high-voltage interlock judgment time is typically around 150ms
HVIL Status Logic Table
| VCU Detects HV Interlock Status | Battery System High Voltage Interlock Status | High Voltage Interlock Status of the Whole Vehicle |
|---|---|---|
| 0 (Normal) | 0 (Normal) | 0 (Normal) |
| 0 (Normal) | 1 (Fault) | 1 (Fault) |
| 1 (Fault) | 0 (Normal) | 1 (Fault) |
| 1 (Fault) | 1 (Fault) | 1 (Fault) |
Note: 0 = HV interlock normal; 1 = HV interlock fault status
Fault Handling Strategies
Different high-voltage interlock faults require specific troubleshooting methods:
| Serial Number | Fault Name | Troubleshooting Method |
|---|---|---|
| 1 | BMS High Voltage Interlock Fault | Prohibit high-voltage power-on or power off the whole vehicle high-voltage system |
| 2 | VCU High Voltage Interlock Fault | Prohibit high-voltage power-on or power off the whole vehicle high-voltage system |
Additionally, the high-voltage interlock circuit should provide clear indications when the high-voltage circuit can be directly disconnected. The vehicle must alert the driver through instrument indicators or audible warnings, suggesting appropriate post-treatment measures to ensure maximum high-voltage safety.
HVIL Detection Circuit Schemes
There are two primary schemes for high-voltage interlock detection circuits:
DC Source Scheme
This approach involves applying an external DC source across the entire HVIL loop and diagnosing the state of high-voltage connectors by detecting voltage at measurement points V1 and V2. When high-voltage connectors are properly plugged in, the HVIL terminal is short-circuited, and the voltage at detection points remains stable. When connectors are disconnected, the HVIL terminal opens, causing detectable voltage changes at measurement points.
PWM (Pulse Width Modulation) Scheme
This method generates an 88Hz PWM signal using a controllable switch (such as a MOSFET) and a timer. The voltage at points V1 and V2 is monitored to diagnose high-voltage connector status. When connectors are properly engaged, the HVIL terminal is short-circuited, allowing normal PWM signal transmission. When connectors are disconnected, the HVIL terminal opens, interrupting the PWM signal.
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