Determination of Reference Voltage and Temporary Overvoltage (TOV) in SPD Type Test

Determination of Reference Voltage and Temporary Overvoltage (TOV) in SPD Type Test

 

1. Definition and Determination of Reference Voltage (U_REF)  

The reference voltage (U_REF) is a critical parameter in surge protective device (SPD) testing, reflecting the maximum continuous operating voltage (Uc) that an SPD can withstand under normal conditions. According to IEC 61643 and GB/T 18802 series standards, Uref is determined based on the system's nominal voltage(U₀) and operational characteristics.  

 

Calculation of U_REF:  

For low-voltage systems, UREF is derived from the system's nominal voltage (U₀). For example, in AC systems, UREF is typically set to 1.1 ×U₀or1.45×U₀(when voltage fluctuations do not exceed 10%). This value accounts for scenarios such as single-phase-to-neutral faults, where the voltage on other phases can rise to 1.45 times the nominal voltage due to neutral line impedance imbalances.  

 

Application in Photovoltaic Systems:  

In DC-side SPDs for photovoltaic (PV) systems, UREF (often denoted as Ucpv) must exceed the maximum open-circuit voltage (Uoc max) of the PV array under all operating conditions, ensuring SPD stability during normal operation.  

 

2. Temporary Overvoltage (TOV) Testing and U_T Determination

Temporary Overvoltage (TOV), or U_T, refers to a steady-state overvoltage caused by faults or switching operations in the power system. Its duration ranges from seconds to minutes, and SPDs must withstand this stress without degradation.  

 

Test Requirements:  

  Per GB/T 18802.11-2020 and IEC 61643, TOV testing involves applying a voltage (U_T) for 5 seconds, with the following criteria:  

  - U_T values are adjusted based on system voltage (e.g., 110V,230V or 400V for specific test setups).  

  - The SPD must maintain terminal voltage above 95% of U_T during testing to validate its performance.  

 

Relationship Between UREFand U_T:  

    

3. Key Considerations in SPD Type Testing  

System Configuration:  

  - In grounded PV systems, SPD connection modes (e.g., Type I or L) impact U_REF and U_T selection.  

  - For ungrounded systems, Y-type or delta configurations may be used to manage TOV stresses.  

 

Testing Equipment:  

  TOV test setups require specialized equipment, including transformers and control systems capable of generating precise voltage waveforms (≤3% distortion) and limiting current to 300A RMS.  

 

4. Practical Implications and Case Studies

Example Calculation:  

  For a 230V AC system with U₀= 230V:  

  U_REF = 1.1×230V =253V (Usually we take the value of 255V)

  U_T = 1.32×255V≈335V orUT =√3×255V≈440V

 

  

Conclusion

The determination of U_REF and U_T in SPD type testing is a systematic process governed by IEC 61643 and GB 18802 standards. U_REF ensures SPD reliability under normal conditions, while U_T validates resilience during transient overvoltages. Their relationship, often expressed as U_T ≈1.32×U_REF or √3×U_REF,highlights the importance of accounting for voltage fluctuations in real-world applications. Engineers must adhere to these principles, particularly in emerging fields like PV systems, to ensure long-term SPD performance and system safety.