HVIL System in Electric Vehicles
In the high-voltage circuit of an electric vehicle, the main electrical component of the hvil system is the high-voltage connector. However, the manual maintenance switch (MSD) is a human-operated element that serves as the electrical interface for circuit turn-on/break-off.
On the one hand, it initiates the protective measure of safely disconnecting the high-voltage electricity when the circuit of the high-voltage system is disconnected, or its integrity is damaged. On the other hand, it is used to confirm the integrity of the entire high-voltage system.
Definition of the HVIL System
High Voltage Interlocking System (HVIL for short), also known as High Voltage Interlocking Loop System.
HVIL, or High Voltage Interlock Circuit, is a safety feature for hybrid and fully electric vehicles. It protects personnel during vehicle assembly, maintenance, servicing, and operation.
The HVIL system protects anyone who may come into contact with high-voltage components at any stage of the electric vehicle lifecycle.
Gas-powered vehicles do not require HVIL. Because their electrical systems operate at low voltage and do not pose a risk to operators and service technicians. However, hybrid and electric vehicles include subsystems of batteries, wiring, adapters, and controllers. They operate at higher power levels and require additional safety features.
HVIL is a type of circuit breaker. It sends the driver an alarm or fault code if a high-voltage connection becomes loose, disconnected, or damaged during vehicle operation. HVIL also helps protect the driver and passengers in an accident.
High voltage interlock is a safety design method that uses low-voltage signals to monitor the integrity of high-voltage circuits. The HV signal is used to check all components connected to the HV harness on the EV to check the integrity and continuity of the electrical connections of each HV system circuit.
Theoretically, the low-voltage monitoring circuit is connected and disconnected before the high-voltage, with a certain time interval (e.g., 150ms). There are different designs for different projects.
Currently, it is mainly integrated into the high-voltage harness connector. That is, on the high-voltage harness connector, an additional set of low-voltage circuits is used to detect the integrity of the HVIL circuit.
Another form of high-voltage interlocking connector is more complex in design. The high-voltage interlocking structure is independent of the inner plastic housing or a separate small connector. And it is ensured by the sequential mounting relationship of the two connectors.
Role of HVIL
Role 1: Before the vehicle is powered on. The system cannot be powered on if the circuit is detected as incomplete to avoid accidents caused by false connections and other problems.
Role 2: Prevent safety accidents caused by human misoperation. During the operation of the high-voltage system. The high-voltage connection point is manually disconnected without a high-voltage interlock design.
At the moment of disconnection, the voltage of the whole circuit will all increase to the two ends of the breakpoint. The voltage breaks through the air, pulling an arc between the two devices. Although the time is short, the energy is high and may damage personnel and equipment around the breakpoint.
Role 3: Detects open high voltage circuits (which can lead to high voltage disconnection, loss of vehicle power, and affect passenger safety). And provide alarm information to the vehicle controller before the high-voltage power failure. Allow time for the vehicle system to take countermeasures.
How HVIL Works
HVIL uses a continuous low-voltage circuit to monitor all high-voltage connectors and components in an electric vehicle (EV). If the low-voltage HVIL signal is interrupted, a problem with the high-voltage system must be addressed.
When the HVIL circuit fails, it triggers a diagnostic fault code. The vehicle control panel sends an alert to inform the driver that there should be a problem with the vehicle and sends it in for prompt repair.
The fault codes also inform the service technician about the nature of the problem, allowing the technician to avoid any safety-related problems.
Two Ways of Detecting and Acting on HVIL System
1. The low-voltage system detects the connection status of the connection position in the high-voltage system.
All high-voltage connectors have a high-voltage interlock signal circuit in the docking position. However, the circuit form is not necessarily related to the high-voltage circuit. In other words, electrical A and B form a complete circuit at high voltage.
However, separate interlocking signal circuits can be set up for A and separate interlocking signal circuits for B for high-voltage interlocking. The interlocking signals of A and B can also be connected in series in one circuit.
2. The information transmission action of the low-voltage detection circuit must be ahead of the disconnection action of the high-voltage circuit.
The power pack is used for power in the high-voltage circuit. The low-voltage circuit also needs to detect power so that the low-voltage signal can be transmitted along the closed low-voltage circuit. After the low voltage signal is interrupted, the high voltage connector is loose or disconnected.
According to the overall strategy, the monitoring point or monitoring circuit is responsible for transmitting the status of the high-voltage interlock signal circuit to the VCU or BMS to complete the down power operation of the vehicle.
The high-voltage connector with high-voltage interlock function consists of low-voltage signal conductive parts, high-voltage conductive parts, housing, monitoring line, and monitor.
The general implementation of the high-voltage interlock structure is that a pair of high-voltage connectors and a pair of low-voltage connectors are fixed on the pair of male and female connectors.
The difference in length and position between the interlocking terminals and the main circuit (high-voltage) terminals enables the connection of the high-voltage terminal first and then the low-voltage terminal. When disconnecting, the low-voltage terminal is disconnected first. And then, the high-voltage terminal is disconnected.
HVIL is just one aspect of an integrated approach to safe electric vehicle power. A typical electric vehicle has thousands of connection points. If one fails, the result can be a minor inconvenience or a major life-threatening problem.
 Fault alarm. No matter the electric vehicle's state, the high-voltage interlock system should make an alarm alert to the dangerous situation when the vehicle recognizes the danger. It needs a meter or indicator to alert the driver through a sound or light alarm. So the driver can pay attention to the abnormal situation of the vehicle to deal with it in time and avoid safety accidents.
 Cut off the high-voltage source. When the electric vehicle is in the stop state, the high-voltage interlock system, in addition to identifying the serious dangerous situation in addition to the fault alarm. It should also notify the system controller to disconnect the automatic circuit breaker. So the high-voltage source is completely cut off. Avoid possible high-voltage hazards and ensure property and personal safety.
 Reduced power operation. When the electric vehicle travels at high speed, the high-voltage interlock system cannot immediately cut off the high-voltage source when recognizing a dangerous situation. It should first prompt the driver through the alarm and then let the control system reduce the operating power of the motor to bring down the vehicle's speed.
To make the whole vehicle's high voltage system operate under less load and minimize the possibility of high voltage danger. It also allows the driver to be able to park the vehicle in a safe place.
HVIL technology is important, but it is also important for OEMs to look at HVIL in the context of hybrid or fully electric vehicles. These vehicles are built to the latest safety and performance standards across the electrical infrastructure.