What Is a Zigzag Transformer?

The grounding transformer is called grounding transformer for short. According to the filling medium, the grounding transformer can be divided into oil type and dry type. According to the number of phases, the grounding transformer can be divided into three-phase grounding transformer and single-phase grounding transformer.

(1) The zero-sequence impedance is low to ensure the output of the zero-sequence current;

(2) High excitation impedance to reduce no-load current;

(3) Low no-load loss to save energy consumption in daily operation ^[6]

In order to meet the needs of the power distribution network using arc suppression coil ground compensation, and also to meet the needs of substation power and lighting loads, the selection of Z-type connection transformers requires reasonable setting of the main parameters of the grounding transformer ^[6] .

(1) Rated capacity

The capacity of the primary side of the grounding transformer is matched with the capacity of the arc suppression coil. According to the current capacity specifications of arc suppression coils, it is recommended to set the capacity of the grounding transformer to 1.05-1.15 times the capacity of the arc suppression coils. For example, a 200kVA arc suppression coil is equipped with a grounding transformer with a capacity of 215kVA.

(2) Neutral point compensation current

Total current flowing through the neutral point of the transformer during a single-phase fault:

In the above formula: U is the distribution network line voltage (V); Zx is the impedance of the arc suppression coil (); Zd is the primary zero sequence impedance of the grounding transformer ( / phase); Zs is the system impedance (); the neutral point The duration of the compensation current should be the same as the continuous working time of the arc suppression coil, which is 2 hours according to regulations.

(3) Zero sequence impedance

Zero-sequence impedance is an important parameter for grounding transformers, and it has an important impact on relay protection to limit single-phase grounding short-circuit current and suppress over-voltage. For the zigzag (Z type) without secondary coil and the star / open delta connection grounding transformer, there is only one impedance, that is, the zero-sequence impedance, so that the manufacturing department can meet the requirements of the power department.

(4) Loss

Loss is an important performance parameter of a grounded transformer. For a grounded transformer with a secondary coil, its no-load loss can be the same as a double-winding transformer with the same capacity. As for the load loss, when the secondary side is running at full load, the load loss is smaller than the load loss of a double-winding transformer with the same capacity as the secondary side due to the lighter load on the primary side.

(5) Temperature rise

According to the national standard, the temperature rise of the grounding transformer has the following provisions:

1) The temperature rise under the rated continuous current shall comply with the requirements of the national standard for dry-type transformers for general power transformers, but it is mainly applicable to grounding transformers that often carry loads on the secondary side;

2) When the duration of the short-term load current is within 10s (this situation mainly occurs when the neutral point is connected to the resistor), its temperature rise should meet the national standard power transformer's limit for temperature rise under short-circuit conditions. ;

3) When the grounding transformer is operated together with the arc suppression coil, the temperature rise shall meet the requirements for the temperature rise of the arc suppression coil:

For windings with a continuous current rating of 80K, it is mainly suitable for grounded transformers with star / open delta connection;

For a winding with a maximum current rating of 2h, the specified temperature is 100K. This situation meets the working conditions of most grounding transformers;

For windings with a maximum flow time of 30 minutes, the specified temperature is 120K.

The starting point of the above provisions is based on the maximum temperature of the hot spot of the winding under the most severe conditions not exceeding 140 ° C to 160 ° C to ensure the safe operation of the insulation and not to seriously endanger the insulation life ^[10] .