Zig Zag transformer
ZIg Zag transformer is a special purpose transformer used in power system, which is also called as “interconnected start winding”. Even though this type of transformer connection is not useful in transforming power, it has many features combining Star and Delta type winding connections.
Zig Zag transformer winding connections
Zig Zag transformer has six coils in which three are outer coils and three are inner coils as shown in the figure. The outer coil windings are called as ZIG winding and inner coil windings are connected as ZAG winding. The zig winding of one phase is connected in series with the zag winding of another phase so it is called interconnected star winding where two star winding coils are interconnected each other. In each phase two coil windings will have same number of turns but they are wounded in opposite directions to cancel the mismatch voltages. Following are the connections of zig zag winding coils
1) The outer coil of phase ‘a’ is connected with the inner coil of phase ‘b’
1) The outer coil of phase ‘b’ is connected with the inner coil of phase ‘c’
1) The outer coil of phase ‘c’ is connected with the inner coil of phase ‘c’
The inner coil second terminals are connected together and connected to neutral terminal to pass the zero sequence current components.
The interconnection of winding of different phases introduces 30 phase shift between zig winding and the corresponding line to neutral voltage. The zig-zag winding have 15.47% more turns compared to conventional transformers to get the same magnitude of voltages. Hence the cost of zig zag transformer is high and it is essential in some applications to be used.
Zig Zag Transformer applications or uses
Grounding Transformer
It can be used as earthing transformer in a delta connected (no neutral terminal) system or an ungrounded start connected (three terminal star) where in neutral is not available for grounding. The zig zag transformer used for earthing of delta connected transformer.
In delta connected transformer there will not be path to zero sequence components and no protection can be performed for these components which increases and stress and heating in the windings. The zig zag transformer provides a neutral for the proving a path to zero sequence components during line to ground fault and allows the protection to be operated due to this fault. In the obsence of grounded neutral, voltages of healthy would increase line to line voltage level, stressing the insulation connected to equipment. Thug Zig zag transformer not only helps in protection it also reduces the voltages stress under symmetrical fault conditions.
2) Power Electronic converters
In power electronic converters the zig zag transformer is used to eliminate the DC magnetizing component presented due to improper firing angles. The improper firing angles of power electronic components (SCR) may introduce DC magnetizing component and this is canceled in each limb of zig zag transformer due to opposite direction of DC magnetizing component of currents flowing in the windings on the same limb.
3) Earthing reference or earthing transformer
Zig zag transformer offers low impedance path to zero sequence components under fault conditions so it can be perfectly used as earthing transformer with and earthing reference. If the earthing current has to be limited under fault conditions, a suitable resistor can be placed in zig zag neutral terminal.
4) Harmonic voltages presented in system to some extent can be cancelled in zigzag windings due to opposite connection of winding coils.
Advantage of Zig zag transformer
The following are the advantages of zigzag transformer compared to Scott transformer and other grounding transformers
- Less cost compared to Scott transformer
- Provides low impedance to zero sequence currents
- Harmonic voltages suppression
- It provides perfect isolation between ground and component
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