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Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
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Grounding transformer in a system is usually connected in
Delta/delta
Star/delta
Zigzag/delta
Star/star
Star/delta
A grounding transformer, typically a zigzag transformer or a Star-Delta connected unit, is used in a power system to provide a neutral point for grounding an ungrounded system. This allows for the flow of zero-sequence currents during a single line-to-ground fault, enabling protection relays to operate.
A grounding transformer, typically a zigzag transformer or a Star-Delta connected unit, is used in a power system to provide a neutral point for grounding an ungrounded system. This allows for the flow of zero-sequence currents during a single line-to-ground fault, enabling protection relays to operate.
I0тАЛ=31тАЛ(IaтАЛ+IbтАЛ+IcтАЛ) тАФ Zero sequence current representation
Z0тАЛ=I0тАЛVLNтАЛтАЛ тАФ Zero sequence impedance presented by the grounding transformer
In a Star-Delta grounding transformer, the star point is connected to the ground. During a ground fault, the zero-sequence current flows through the star-connected windings to the fault. The delta winding acts as a path for circulating currents, ensuring that the transformer presents a low impedance to zero-sequence currents while maintaining high impedance to balanced three-phase loads.
Grounding transformers are required for ungrounded systems (like Delta systems).
They provide a path for ground fault current without carrying load current.
The delta winding balances the ampere-turns during single-phase ground faults.
Commonly used to detect earth faults in otherwise isolated systems.
Provides a stable neutral reference point
Enables simple ground fault detection and relaying
Limits transient overvoltages
Adds cost to the system
Requires maintenance of an additional transformer unit
Takes up physical space in the substation
Industrial power distribution systems
Generating stations with ungrounded layouts
Wind power collector systems
Zigzag transformers are often preferred over Star-Delta because they can perform the grounding function with fewer windings and smaller physical footprint.
The question mentions Star/Delta, which is a standard configuration; however, Zigzag is technically the most common specific design for this purpose.
B is correct тАФ A Star-Delta transformer provides a neutral path for ground fault currents in an otherwise ungrounded system.
Always remember that zero-sequence impedance is low in grounding transformers, which is the exact opposite of their role for positive or negative sequence components.