TIDUEZ8C december   2022  – june 2023

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Insulation Monitoring
    2. 1.2 Impact of Parasitic Isolation Capacitance
    3. 1.3 IEC 61557-8 Standard for Industrial Low-Voltage Distribution Systems
    4. 1.4 Key System Specifications
  8. 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 Loss of PE Detection
      4. 2.3.4 Insulation Monitoring on AC Lines
      5. 2.3.5 PCB Layout Recommendations
  9. 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 Software
    4. 3.4 Test Setup
    5. 3.5 Test Results
  10. 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
  11. 5About the Author
  12. 6Revision History

1 System Description

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

Currently, high-voltage (HV) 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 HV DC lines flowing directly from the electric vehicle supply equipment (EVSE) to the vehicle. In the case of solar string inverters, there are HV DC lines coming from the photovoltaic (PV) string panels of up to 1 kV.

User protection is necessary in these kinds of HV DC distribution systems. All HV parts of the system are isolated to protective earth through high-omic paths. This insulation limits the maximum leakage current. International standards demand that the leakage current must be limited to 10 mA, to avoid personal injury from contact with the system. The insulation monitoring device monitors this insulation resistance and initiates a shutdown in case the insulation resistance is not sufficient.

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


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

Degradation or loss of isolation can occur due to many factors such as deterioration of the 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, but is a potential hazard when operators come in contact with the HV operating environment.


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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 demand monitoring of the isolation barrier at regular intervals during energy transfer. In EVSE, charging protocols also establish insulation monitoring tests prior to charge. The idea is to prevent isolation barrier breakdowns that can lead to a fatal short.

As per the previously-mentioned standards, warning (500 Ω / V d.c. - 2mA); 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.


GUID-20210809-SS0I-GLHH-HVJ3-QR4ZXSPW52WH-low.svg

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

This design is designed for 400-V systems by default, but can be adjusted to voltages up to 1000 V by modifying the resistor network of the switched-in resistive branch.