Introduction to Working Principle of Power Transformer
Introduction to working principle of power transformerThe high-voltage winding used in domestic transformers is generally connected into Y connection method, and the connection method of medium voltage winding and low-voltage winding shall be determined according to the system situation. The so-called system situation refers to the relationship between the voltage phasor of high voltage transmission system and the voltage phasor of medium voltage or low voltage transmission system. If it is a low-voltage distribution system, it can be determined according to the standard.High voltage windings are often connected into Y connection because the phase voltage can be equal to 57.7% of the line voltage and the voltage per turn can be lower.
1). The voltage phasors of domestic 500, 330, 220 and 110kV transmission systems are in the same phase. Therefore, for three-phase three winding or three-phase autotransformer with the following voltage ratio, star connection method shall be used for high-voltage and medium voltage windings. When the three-phase three iron core column iron core structure is adopted, the low-voltage winding can also adopt star connection method or angle connection method, which depends on that the voltage phasor of the low-voltage transmission system is in phase with the voltage phasor of the medium voltage and high-voltage transmission system or lags by 30 electrical angle.
500 / 220 / lvkv YN, yn0, yn0 or YN, yn0, D11220 / 110 / lvkv YN, yn0, yn0 or YN, yn0, D11330 / 220 / lvkv YN, yn0, yn0 or YN, yn0, D11
330 / 110 / lvkv YN, yn0, yn0 or YN, yn0, D112). There are two different phase angles for domestic 60 and 35kV Transmission system voltages.For example, YNd11 connection method is adopted for 220 / 60kV transformer and YN, yn0 and D11 connection method is adopted for 220 / 69 / 10kV transformer. The difference between the two 60kV transmission systems is 30 electrical angle.
When the 220 / 110 / 35kV transformer adopts YN, yn0 and D11 connection method and the 110 / 35 / 10kV transformer adopts YN, yn0 and D11 connection method, the voltage phasor of the above two 35kV Transmission systems is also different by 30 electrical angle.Therefore, caution should be taken when determining the connection method of 60 and 35kV windings, which must meet the requirements of voltage phasor of transmission system. The connection method of 60 and 35kV winding is determined according to the relative relationship of voltage phasor. Otherwise, even if the capacity and voltage ratio are right, the transformer cannot be used, the connection method is wrong, and the transformer cannot be connected to the transmission system.3). The phasors of domestic 10, 6, 3 and 0.4kV transmission and distribution systems also have two phases. In Shanghai, there is an electrical angle with a voltage phasor difference of 60 between 10kV and 110kV transmission systems. At this time, the three-phase three winding power transformer with 110 / 35 / 10KV voltage ratio and YN, yn0 and Y10 connection method can be used, but the three-phase three core column core is limited.
4). However, it should be noted that when the single-phase transformer is connected into three-phase group connection method, the three-phase group of yny0 connection method cannot be used. Yny0 connection method cannot be adopted for three-phase shell transformer.When YN, yn0 and yn0 connection method must be adopted for three-phase five column iron core transformer, there shall be a fourth winding connected into angular connection method in the transformer, and its outlet shall not be led out (the outlet led out during electrical test in structure is not in this example).5). When transformers of different connection groups operate in parallel, the general rule is that the connection group label must be the same.
6). When the distribution transformer is used in thunder prone areas, yzn11 connection method can be adopted. When Z connection method is adopted, the impedance voltage algorithm is different from Yyn0 connection method, and the copper consumption of Z connection winding is more. Yzn11 connected distribution transformer has good lightning protection performance.7). Yny0 connection method cannot be adopted when four wound core frames are adopted for three-phase transformer.8). The above connection methods are used for domestic transformers. For export, appropriate connection methods and connection group labels shall be provided as required.
9). Generally, there are taps in the high-voltage winding connected with the tap changer. Therefore, when selecting tap changer (including on load voltage regulating tap changer and non excitation voltage regulating tap changer), attention must be paid to the coordination of transformer connection method and tap changer connection method (including connection method, test voltage, rated current, voltage of each stage, voltage regulation range, etc.). For on load tap changer used in on load tap changer of YN connected on load tap changer, it should be noted that the midpoint must be able to be led out
