In the power distribution network, the parallel operation of three - phase distribution transformers is a common practice. As a supplier of three - phase distribution transformers, I am well - versed in the technical details and requirements of this operation. This blog post will delve into the conditions necessary for the parallel operation of three - phase distribution transformers.
1. Same Voltage Ratio
The first and foremost condition for the parallel operation of three - phase distribution transformers is that they must have the same voltage ratio. The voltage ratio is defined as the ratio of the primary voltage to the secondary voltage of a transformer. If two or more transformers with different voltage ratios are connected in parallel, a circulating current will flow between them even when there is no load on the secondary side.
Let's assume we have two transformers (T_1) and (T_2). Transformer (T_1) has a voltage ratio (V_{1p}/V_{1s}) and (T_2) has a voltage ratio (V_{2p}/V_{2s}). If (V_{1p}/V_{1s}\neq V_{2p}/V_{2s}), a potential difference will exist between the secondary windings of the two transformers. According to Ohm's law (I = V/R), this potential difference will drive a circulating current through the low - impedance secondary windings. This circulating current not only increases the copper losses in the transformers but also reduces their efficiency and may even lead to overheating.
For example, if one transformer has a voltage ratio of 10kV/400V and another has a voltage ratio of 10.5kV/400V, when connected in parallel, the secondary voltages will not match exactly, and a circulating current will be generated. As a supplier, we ensure that our 50 - 2500kVA/20(10)kV Low - Loss Oil Immersed Transformer（hermetically Sealed Oil Filled Transformer） series has precise voltage ratios to meet the requirements of parallel operation.
2. Same Percentage Impedance
The percentage impedance of a transformer is an important parameter that represents the impedance of the transformer in percentage terms with respect to its rated voltage and rated power. When transformers are connected in parallel, they should have the same percentage impedance.
The current shared by each transformer in a parallel operation is inversely proportional to its impedance. If transformers have different percentage impedances, they will not share the load in proportion to their ratings. A transformer with a lower percentage impedance will carry more load than its rated capacity, while a transformer with a higher percentage impedance will be under - loaded.
Mathematically, if we have two transformers (T_1) and (T_2) with percentage impedances (Z_{1}%) and (Z_{2}%) respectively, and the total load current is (I_{total}), the current carried by (T_1), (I_1) and (T_2), (I_2) are given by:
(I_1=\frac{Z_{2}%}{Z_{1}% + Z_{2}%}I_{total}) and (I_2=\frac{Z_{1}%}{Z_{1}%+Z_{2}%}I_{total})
If (Z_{1}%) and (Z_{2}%) differ significantly, one transformer may be overloaded while the other is not fully utilized. Our 30 - 2500kVA/10kV Three Phase Oil Immersed Transformer series is designed with carefully controlled percentage impedances to ensure proper load sharing in parallel operation.
3. Same Connection Group
The connection group of a three - phase transformer indicates how the primary and secondary windings are connected (e.g., star - star, star - delta, delta - star, delta - delta) and the phase relationship between the primary and secondary voltages. Transformers connected in parallel must have the same connection group.
If transformers with different connection groups are connected in parallel, there will be a large phase difference between the secondary voltages of the transformers. This phase difference will result in a large circulating current, which can cause severe damage to the transformers.
For instance, a star - star connected transformer and a star - delta connected transformer cannot be connected in parallel because the phase relationship between their secondary voltages is different. The phase shift between the secondary voltages of a star - delta transformer is 30 degrees compared to a star - star transformer. As a supplier, we ensure that all our transformers, including the 80 - 31500kVA/35kV Double - winding On - load Voltage Regulating Oil - immersed Power Transformer, are clearly labeled with their connection groups to avoid any connection errors.
4. Same Polarity
The polarity of a transformer refers to the relative direction of the induced voltages in the primary and secondary windings. For parallel operation, transformers must have the same polarity.
If the polarities of two transformers are not the same, when they are connected in parallel, the secondary voltages will oppose each other. This will result in a short - circuit condition, and a very large current will flow through the transformers, which can damage the windings and other components of the transformers.
In practice, the polarity of a transformer is determined during the manufacturing process. We perform strict polarity tests on all our three - phase distribution transformers to ensure that they can be safely connected in parallel.
5. Same Phase Sequence
In a three - phase system, the phase sequence is the order in which the voltages of the three phases reach their maximum values. Transformers connected in parallel must have the same phase sequence.
If the phase sequences of two transformers are different, the voltages of the corresponding phases will not be in phase, and a large circulating current will flow between the transformers. This circulating current can cause overheating, damage to the insulation, and even lead to the failure of the transformers.
We use advanced testing equipment to verify the phase sequence of our transformers before they are shipped to customers. This ensures that our customers can easily connect our transformers in parallel without any phase - sequence - related issues.
Importance of Meeting These Conditions
Meeting the above conditions for the parallel operation of three - phase distribution transformers is crucial for the reliable and efficient operation of the power distribution system. When these conditions are met, the transformers can share the load evenly, which improves the overall capacity of the power distribution system, reduces losses, and enhances the reliability of power supply.
On the other hand, if these conditions are not met, it can lead to various problems such as overheating, increased losses, reduced efficiency, and even damage to the transformers. This can result in power outages, increased maintenance costs, and potential safety hazards.
Conclusion
As a supplier of three - phase distribution transformers, we understand the importance of these conditions for parallel operation. Our products are designed and manufactured to meet these strict requirements, ensuring that our customers can operate multiple transformers in parallel safely and efficiently.
If you are in need of three - phase distribution transformers for parallel operation or have any questions about our products, we encourage you to contact us for procurement and further technical discussions. We are committed to providing high - quality transformers and professional technical support to meet your power distribution needs.
References
- Electric Power Distribution Handbook, by Dugan, McGranaghan, and Beaty.
- Transformer Engineering: Design, Technology, and Diagnostics, by G. K. Dubey.