Understanding the conducting states of active and passive switches in an inverter circuit used for power system applications

Bi-directional inverter circuits are commonly used in many applications in power systems, such as in FACTS devices, energy storage systems and active power filters. Many power engineering students however find it difficult to understand the conducting states of the active switches (transistors, IGBTs) and the passive switches (the anti-parallel diodes connected across the active switches). It seems a mystery to them that power can flow from a lower voltage ac system to a higher voltage dc system to charge the energy storage system. This paper proposes a simple chart that intuitively explains how to determine the states (either conducting or non-conducting) of the active and passive switches of an inverter. The flowchart helps students to understand the cyclic changes in the topology of the circuit in a switching cycle. The flowchart by nature is general and can be used for any type of control methods such as sinusoidal PWM and hysteresis control. The application of the use of this chart is also discussed for a battery energy storage system.