A distribution transformer, commonly known as a "distribution," is a static electrical device that transfers AC energy via electromagnetic induction to adjust voltage and current levels. In certain areas, transformers with voltage ratings below 35 kV (most commonly 10 kV or lower) are termed "distribution transformers" or simply "distributions." These transformers are typically installed either on poles or on open floors within substations. Details about their installation methods, precautions, supply and distribution strategies, capacity selection, operational maintenance, and more are outlined extensively.
### Detailed Explanation on the Distribution and Capacity Selection of Distribution Transformers
#### (1) Distribution Transformers and Their Configurations
The 10 kV high-voltage power grid operates using a three-phase three-wire system with an ungrounded neutral point. Many user transformers employ a D/yn11 connection mode with direct neutral grounding, enabling a three-phase four-wire power supply system.
#### (2) Capacity Selection of Distribution Transformers
In the operational context of distribution transformers, instances of oversized capacities leading to underloading or overheating and equipment damage due to overloading or overcurrent operation are common. Improper device capacity selection can compromise the reliability and economic efficiency of the power supply system.
Transformer capacity is usually chosen based on load statistics. Given the unpredictability of load predictions, the anticipated maximum load is generally used for selection. This approach often results in excessively large capacity settings, negatively impacting power system operations. Opting for economic operation involves deriving the maximum economic load rate, balancing copper losses and iron losses against the transformer's rated capacity and maximum load ratio. However, since the actual operating load may not match the calculated maximum load and varies randomly, achieving optimal economic operation remains challenging.
The replacement of high-energy transformers with new low-loss models is currently being implemented in power distribution networks, reducing single-unit iron losses by approximately 40%. Considering the vast number of distribution transformers and significant load fluctuations, the economic advantages are substantial.
To ensure the transformer's normal service life while maximizing its set capacity, capacity selection should primarily focus on these considerations. A recommended approach is to select the distribution transformer capacity based on the maximum expected load (Smax) and the typical daily load curve, adhering to the International Electrotechnical Commission (IEC) standard (1972), which includes an oil-immersed transformer load guide. This method has been adopted in China. Its advantage lies in considering the normal overload capacity of the transformer, fully utilizing its capacity without shortening its lifespan. This reduces investment costs, improves distribution network conditions, and yields significant economic benefits.
A computer program has been developed based on this method, generating a transformer capacity selection table corresponding to six typical daily load curves. These curves represent different types of loads:
- **I:** Watering, wheat fields;
- **II:** Village commercial activities; lighting, yard usage;
- **III:** Payment-related activities; lighting, watering, and yard usage;
- **IV:** Industrial use in towns and counties;
- **V:** Combined village and industrial loads;
- **VI:** Urban industrial comprehensive load.
The scheduling method is as follows:
1. Identify the load type and select a typical daily load curve.
2. Determine the equivalent air temperature (θδ). The IEC standard uses equivalent air temperature rather than the average environmental temperature. This means maintaining a specific temperature during the load period will cause insulation degradation equivalent to natural temperature fluctuations. For convenience, it is suggested to use 22°C for Jiangnan, 20°C for Jiangbei, and 16°C or 18°C for the northwest and northeast regions.
Based on the expected maximum load value (in kVA), refer to the table to determine the nominal capacity (Sn) of the selected transformer.
For instance, for load curve I, with an annual equivalent air temperature of 9°C and a maximum load of 1000 kVA, an 800 kVA distribution transformer should be chosen.
3. According to environmental conditions and the type of load, determine the normal overload capacity of the working transformer.
For example, for load curve VI, with an annual equivalent air temperature of 22°C, a front-angle capacity of 315 kVA, and a maximum load of 340 kVA, the working transformer can handle this load.
It is important to note that transformer capacity should be selected following the aforementioned method. During actual operation, the maximum load continuous operation time should be considered to ensure safety. Exceeding this time could pose a risk of transformer burnout. The allowable time can be obtained using the maximum temperature calculation formula for natural circulation oil-immersed transformer windings. For convenience, tables have calculated the maximum load limit running time (τmaxe). The calculation condition is: ambient temperature is 35°C, and the hotspot temperature of the winding does not exceed 140°C. For example, for load curve IV, with an annual equivalent air temperature of 20°C, the penalty load limit running time is 17 hours. When the load-to-rated-capacity ratio is 3dl, the maximum load is extremely limited, i.e., there is no danger of overheating when the ratio is below 1.17.
This method ensures optimal utilization of the transformer's capacity while maintaining operational safety and long-term reliability.
Input/Output Connector
Input/Output Connector
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