In domestic applications, the high-voltage (HV) windings of transformers are generally connected in a Y (star) configuration, while the connections for medium-voltage (MV) and low-voltage (LV) windings are determined by specific system conditions. These conditions refer to the relationship between the voltage phasors of the HV transmission system and those of the MV or LV transmission systems. For instance, if the LV system is a distribution system, the connection method is determined in accordance with standard regulations.
The HV winding is frequently connected in a Y configuration because the phase voltage is 57.7% of the line voltage, allowing for a lower voltage per turn.
1). The voltage phasors of domestic 500, 330, 220, and 110 kV transmission systems are in phase with one another. Therefore, for three-phase, three-winding transformers or three-phase autotransformers with the following voltage ratios, both the HV and MV windings utilize a star connection. For transformers with a three-limb core structure, the LV winding may be connected in either a star or delta configuration; this choice depends on whether the LV system's voltage phasor is in phase with, or lags by 30 electrical degrees behind, the voltage phasors of the MV and HV systems.
500/220/LV kV - YN, yn0, yn0 or YN, yn0, d11
220/110/LV kV - YN, yn0, yn0 or YN, yn0, d11
330/220/LV kV - YN, yn0, yn0 or YN, yn0, d11
330/110/LV kV - YN, yn0, yn0 or YN, yn0, d11
2). Domestic 60 kV and 35 kV transmission systems exhibit two different phase angles. For example, a 220/60 kV transformer might use a YNd11 connection, whereas a 220/69/10 kV transformer might use a YN, yn0, d11 connection; the two resulting 60 kV transmission systems differ by a phase angle of 30 electrical degrees. When 220/110/35kV transformers utilize the YN, yn0, d11 connection configuration, and 110/35/10kV transformers also utilize the YN, yn0, d11 configuration, the voltage phasors of the two 35kV transmission systems will differ by 30 electrical degrees.
Therefore, the connection configuration for the 60kV and 35kV windings must be determined carefully to ensure compliance with transmission system voltage phasor requirements. The configuration is selected based on the relative relationship of the voltage phasors. Otherwise, even if the capacity and voltage ratios are correct, the transformer cannot be used; an incorrect connection prevents the transformer from being connected to the transmission system grid.
3). Domestic 10kV, 6kV, 3kV, and 0.4kV transmission and distribution systems also exhibit two types of phasor phase relationships. In the Shanghai area, there are instances where the voltage phasor difference between the 10kV and 110kV transmission systems is 60 electrical degrees; in such cases, a three-phase, three-winding power transformer with a 110/35/10kV voltage ratio and YN, yn0, y10 connection configuration may be used, provided it utilizes a three-phase, three-limb core structure.
4). However, note the following: when single-phase transformers are connected to form a three-phase bank, the YNy0 connection configuration cannot be used. Similarly, three-phase shell-type transformers cannot utilize the YNy0 connection configuration.
When a three-phase, five-limb core transformer employs the YN, yn0, yn0 connection configuration, it must include an internal fourth winding connected in a delta (triangle) configuration, with its terminals not brought out externally (excluding terminals required for structural electrical testing).
5). When transformers with different connection groups operate in parallel, the general rule is that their connection group designations must be identical.
6). For distribution transformers used in lightning-prone areas, the Yzn11 connection configuration may be employed. When the zigzag (z) connection is used, the calculation for impedance voltage differs from that of the Yyn0 configuration, and the winding requires more copper. Distribution transformers with the Yzn11 connection configuration offer superior lightning protection performance.
7). Three-phase transformers utilizing four wound-core frames must not employ the YNy0 connection configuration.
8). The connection configurations mentioned above apply to transformers intended for the domestic market; for export units, the appropriate connection configuration and vector group designation must be supplied in accordance with specific requirements.
9). High-voltage windings typically feature taps connected to a tap changer. Therefore, when selecting a tap changer (whether on-load or off-circuit), it is essential to ensure compatibility between the transformer connection and the tap changer specifications (including connection type, test voltage, rated current, voltage step, and regulation range). For on-load tap changers used with YN-connected transformers, it is also necessary to ensure that the neutral point can be brought out.
