3-Phase Converter SCR Commutation and Voltage Notching

In a 3-phase converter circuit, Silicon Controlled Rectifiers (SCRs), also known as thyristors, are essential electronic switches. They control the flow of electrical power by turning on when triggered and turning off when the current through them naturally falls to zero or reverses.

Understanding Commutation in 3-Phase Converters

Commutation is the process where current transfers from one conducting SCR to another. In AC-based converter systems, commutation typically occurs naturally due to the AC supply waveform. The specific situation described involves the transition period where one SCR is turning ON, and concurrently, an SCR in a different phase of the 3-phase system is turning OFF.

The Consequence: Voltage Notching Explained

During this commutation interval, when an SCR in one phase turns off and an SCR in another phase turns on, there's a brief moment. Due to the inherent inductance or impedance of the source, the two phases involved in the commutation become momentarily interconnected. This creates a temporary path that effectively short-circuits the source phases for a very short duration.

This rapid, transient connection leads to a characteristic dip or interruption in the voltage waveform across the terminals experiencing the commutation. This specific phenomenon is known as voltage notching. The shape and depth of the notch are influenced by factors like source voltage, firing delay angles, and source impedance.

Distinguishing Voltage Notching from Other Phenomena

It is important to differentiate voltage notching from other power quality issues:

• Voltage Sag: A voltage sag is a longer-duration decrease in voltage magnitude, lasting typically for several cycles to a minute. Voltage notching is an instantaneous, sharp dip.
• Harmonic Distortion: While the process of SCR switching and commutation inherently introduces harmonics into the system, voltage notching is the specific visual signature of the transient event itself on the voltage waveform, rather than the overall frequency distortion.
• Voltage Swell: A voltage swell is a temporary increase in voltage magnitude, the opposite of the dip observed during voltage notching.

Therefore, the occurrence of turning off an SCR in one phase while turning on an SCR in another phase during commutation in a 3-phase converter directly results in voltage notching.