Commutation Failure in LCC HVDC Systems

What is Commutation?

Commutation refers to the process of switching the direction of current flow in the DC circuit.

How is it related to HVDC?

Line-commutated converters (LCC) use thyristors (also known as silicon-controlled rectifiers or SCR) for rectifying and inverting the current. They rely on the natural commutation of the thyristors to switch from one phase to another.

In LCC HVDC converters, commutation relies on the natural zero-crossing point of the AC voltage waveform. When the voltage across the thyristor naturally reverses polarity (crosses zero), it turns off, allowing the next thyristor in line to turn on. This process is called line commutation.

Commutation Failure & HVDC Systems

Commutation failure occurs when the thyristor fails to turn off at the natural zero-crossing point. In an HVDC system, it is a critical operational issue that can occur due to various reasons, including:

• If the firing angle (the point in the AC cycle when the thyristor is triggered) of one thyristor overlaps with the natural zero-crossing point, it can lead to commutation failure.
• Sudden disturbances or voltage imbalances in the AC system can affect the natural zero-crossing point, making it challenging for the thyristor to turn off properly.
• Faults in the DC system, such as a short circuit, can disrupt the commutation process.
• Rapid changes in the DC load or the clearing of DC faults can create conditions where commutation failure may occur.

Role of Reactive Power Compensation in Avoiding Commutation Failure

In HVDC systems, maintaining stable and synchronized AC voltage levels is crucial for proper commutation. AC voltage disturbances, such as voltage sags or swells, can affect the natural zero-crossing points of the AC voltage waveform. Reactive power compensation devices such as synchronous condensers and static VAR compensators (SVCs) can be used to quickly respond to these disturbances by injecting or absorbing reactive power, helping to maintain a stable voltage profile, and preventing commutation failure.

To consider the HVDC power in the Effective Short Circuit Ratio (ESCR) formula, you would need to account for the contribution of the HVDC system to the short-circuit MVA available in the AC system. This involves considering the rated power of the HVDC system as part of the available short-circuit power.

Possible Solutions to Commutation Failure in relation to ESCR

ESCR is relevant for understanding the interaction between the HVDC system, the AC grid, and the need for reactive power compensation. A higher Effective Short Circuit Ratio (ESCR) for the AC system typically indicates that an LCC HVDC system will be less prone to commutation failure. ESCR is given by the formula.

ESCR=MVASC-QC/PHVDC

• A higher ESCR value indicates that the AC system has a greater capacity to deliver fault current during short-circuit conditions. This means the AC grid is robust and can maintain its voltage levels even during fault events. In other words, a higher ESCR suggests that the AC system can support the demands imposed by the HVDC system, including fault currents.
• The strength of the AC grid is crucial for maintaining stable voltage levels and ensuring that the natural zero-crossing points of the AC voltage waveform are well-defined and consistent. A strong AC grid minimizes voltage disturbances, harmonics, and other factors that can disrupt the commutation process.
• In an AC system with a higher ESCR, there is a reduced risk of voltage disturbances or voltage dips during fault conditions. This means that the AC voltage waveform is less likely to deviate significantly from the expected zero-crossing points, making it easier for the thyristors in the LCC HVDC converter to commutate effectively.

In the operational domain, you need to find a trade-off between Short Circuit MVA and Power Transfer through HVDC (assuming New Reactive Power Compensation cannot be implemented in a very short duration) to avoid regular commutation failures. EMTP-based studies considering various operating conditions can also be very helpful in the Operational Planning Time Domain.