TIDUEZ8 May   2021

 

  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Description
    1. 1.1 Insulation Monitoring
    2. 1.2 Isolation Capacitance
    3. 1.3 IEC 61557-8 Standard for Industrial Low-Voltage Distribution Systems
    4. 1.4 Key System Specifications
  7. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Highlighted Products
      1. 2.2.1 TPSI2140
      2. 2.2.2 AMC3330
      3. 2.2.3 TPS7A24
      4. 2.2.4 REF2033
      5. 2.2.5 TLV6001
    3. 2.3 Design Considerations
      1. 2.3.1 Resistive Bridge
      2. 2.3.2 Isolated Analog Signal Chain
        1. 2.3.2.1 Differential to Single-Ended Conversion
        2. 2.3.2.2 High-Voltage Measurement
        3. 2.3.2.3 Signal Chain Error Analysis
      3. 2.3.3 PCB Layout Recommendations
  8. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
      1. 3.1.1 Connectors
      2. 3.1.2 Default Jumper Configuration
      3. 3.1.3 Prerequisites
    2. 3.2 Software Requirements
    3. 3.3 Test Setup
    4. 3.4 Test Results
  9. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Documentation Support
    3. 4.3 Support Resources
    4. 4.4 Trademarks
  10. 5About the Authors

System Description

The rapid adoption of electric vehicles in the market, along with the democratization of solar energy solutions, is increasing the demand on systems to ensure safe energy transmission.

Currently, high-voltage batteries of around 400 V are used as storage elements in electric cars, and there is a strong trend emerging towards higher voltage batteries, which allow for faster charge times. DC fast chargers supply power to the battery management system in the electric vehicle (EV) bypassing the onboard battery charger. This translates into high-voltage DC lines flowing directly from the electric vehicle supply equipment (EVSE) to the vehicle. In the case of solar string inverters, there are high-voltage DC lines coming from the photovoltaic (PV) string panels of up to 1 kV.

User protection is necessary in these kinds of high-voltage DC distribution systems. Death can occur with currents above 10 mA. All parts of the system are isolated to protective earth through high-ohmic paths to avoid high leakage current flowing to the user if contact is made with high-voltage lines.

Designers must consider the isolation requirements that apply to achieving basic or reinforced isolation (these can be determined based on line and peak voltages). Ensuring proper operation of the isolation barrier is compulsory in avoiding accidents.



Figure 1-1 Isolation Barrier in DC Unearthed Distribution Systems.

Degradation or loss of isolation could occur due to many factors such as deterioration of wire harnesses, general aging of power handling components, or peak electrical stress on semiconductors. A single point of failure in regards to isolation does not have much impact on the operation of the system. However, it does become a potential life risk when operators contact the high-voltage operating environment.



Figure 1-2 Isolation Barrier Leakage in Unearthed DC Distribution Systems

Unearthed power distribution systems such as DC fast charging stations and solar string inverters, must be compliant with safety standards such as the IEC 61557-8: “Electrical safety in low voltage distribution systems up to 1 000 V a.c. and 1 500 V d.c.”, which is further specified in IEC 61851-23 for DC fast charging stations.

These safety standards force monitoring of the isolation barrier at regular intervals during energy transfer to ensure proper operation. In EVSE, charging protocols also establish insulation monitoring tests prior to charge. The idea is to prevent isolation barrier breakdowns that could lead to a fatal short.

As per the previously-mentioned standards, warning (500 Ω / V d.c. - 20 mA); and fault (100 Ω / V d.c. - 10 mA) thresholds are set for the isolation barrier resistances. While the isolation barrier resistances do not fall under those limits, a proper condition is proven and no actions are expected.

If warning states are detected, visual indications through the Human-Machine Interface (HMI) trigger and then control actions are executed by the central control unit. If a fault state is detected, energy distribution stops.



Figure 1-3 Insulation Monitoring Device in DC Unearthed Distribution Systems

This design comes sized to 400-V systems to make it convenient for DC fast charging stations and solar string inverters. Small resistance ratio modifications are needed in case higher voltages lines are expected.