NCPRE | September Newsletter
Novel Bidirectional Z-source DC Circuit breaker

DC distribution is gaining attention due to its energy efficiency, high reliability and flexibility in connecting renewable distribution generation. With the emerging distributed energy resources and semiconductor technologies, DC distribution is selected for the applications such as low voltage micro grid, datacenters, naval ships, etc. The major concern while deploying DC grid is the design of protection under various fault ions. Unlike AC systems, there is absence of natural current zero crossing during breaker operations in DC systems. Also, fault propagates faster in the DC system, so the protection device should be really fast.
The circuit breakers that can be used for protecting DC systems are as follows:

1. Electromechanical circuit breaker: These are traditionally used in AC system. That can also be used for DC system after de-rating. It clears the fault in 2 to 3 cycles i.e. 60 msec.

2. Hybrid circuit breaker: Hybrid circuit: Hybrid circuit breakers are used for both AC and DC system protection. It is, basically, an integration of both electromechanical and solid state circuit breaker. Its fault clearance time is around 2 msec to 30 msec and current commutation process is really complex.

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3. Solid state circuit breaker: It uses the semiconductor device as the main switch, hence known as solid state circuit breaker. Its fault clearance time is less than 1 msec, which is faster than any other breaker.

Solid state circuit breakers commonly employ fully controlled device such as IGBT, as a main line switch. Hence, the occurrence of fault needs to be sensed and turn off command is to be provided to the switch using an external control unit, in order to clear the fault. However, use of sensors and external control unit makes system more complex.

Z-source circuit breaker is a type of solid state circuit breaker which uses SCR as the current breaking element. SCR turns ON when triggered through gate pulses and it turns OFF when the current through it falls below its holding current. Thus, no turn off command is required for SCR commutation. The Z-source breaker topologies make use of passive elements to achieve current zero crossing in power line, which assists the commutation of SCR. Therefore, Z-source circuit breaker can isolate fault without use of any sensing circuitry, unlike other solid state CBs and is capable of isolating the fault in tens of microseconds.

Z-source circuit breaker can be used to protect DC systems such as micro grid, data centers, EV battery system and naval ships. In such applications, power flows in both directions. These systems require circuit breakers which are capable of isolating the fault during either direction of power flow. Bi-directional Z-source breaker (Bi-ZSB) as shown in figure is designed and developed for similar applications. For forward direction of power flow, combination of inductors (L1 & L2 ) and capacitors (C1 & C) are utilized to turn off SCR1 during fault ion. Similarly, for reverse direction of power flow, combination of inductors (L3 & L4 ) and capacitors (C2 & C) are utilized to turn off SCR2 during fault ion. The novel circuit breaker is designed and developed for 380V, 16A. It isolates the fault within 300 microseconds.

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B. G. Fernandes
Swati Sawaliya

Vibha Surwade