In grid-connected photovoltaic (PV) power generation systems, the step-up transformer is one of the critical components. Optimizing transformer selection to reduce inherent losses and improve efficiency is essential for enhancing the overall system performance. This article analyzes various aspects to guide the proper selection of step-up transformers in PV systems.
1. Transformer Capacity Selection
Basis: The capacity of the transformer required can be calculated using the formula: Apparent Power = Active Power / Power Factor. The power factor requirements vary by region, but typically, the power factor is 0.85 for construction and small industrial loads and 0.9 for large industrial loads. For example, the appropriate transformer size for a 550 kW construction load is calculated as 550 kW / 0.85 = 647 kVA. Therefore, a 630 kVA transformer should be selected. The total load power should not exceed 80% of the transformer's rated capacity.
2. Transformer Voltage Selection
The primary winding voltage of the transformer should be selected based on the power source's line voltage, while the secondary winding voltage should match the electrical equipment's requirements. For low-voltage three-phase four-wire power distribution, it is important to choose the appropriate voltage level, whether 10 kV, 35 kV, or 110 kV, depending on the primary side requirements.
3. Transformer Phase Selection
The number of phases for the transformer (single-phase or three-phase) should be chosen based on the power source and load requirements.
4. Transformer Winding Connection Group Selection
Three-phase transformer windings can be connected in star (Y), delta (D), or zigzag (Z) configurations. The most commonly adopted configuration worldwide for distribution transformers is the Dyn11 connection. The advantages of Dyn11 over Yyn0 connection include:
1). Harmonic Suppression: The D connection effectively mitigates the adverse effects of higher-order harmonics.
2). Harmonic Circulation: In the D-connected winding, third harmonic currents circulate, creating a balancing magnetic force that cancels out the third harmonic flux from the low-voltage winding.
3). Harmonic Current Containment: The third harmonic electromotive force (EMF) in the high-voltage winding circulates within the D-connected loop, preventing it from being injected into the public high-voltage grid.
4). Reduced Zero-Sequence Impedance: Dyn11 transformers have much lower zero-sequence impedance than Yyn0 transformers, facilitating the clearing of low-voltage single-phase ground faults.
5). Neutral Current Handling: Dyn11 transformers can handle neutral currents exceeding 75% of the phase current, making them better suited for unbalanced loads compared to Yyn0 transformers.
6). Phase Loss Continuity: If one phase of the high-voltage side loses its fuse, the remaining two phases can continue operating with a Dyn11 transformer, unlike a Yyn0 configuration.
Thus, selecting a Dyn11-connected transformer is highly advisable.
5. Transformer Load Loss, No-Load Loss, and Impedance Voltage
Given the characteristics of PV power generation, particularly daytime operation, the transformer will incur no-load losses whenever it is connected to the system, regardless of power output. Minimizing load losses in the transformer is crucial; if the transformer operates at night, no-load losses should also be kept low.
This selection strategy ensures that the transformer operates efficiently within the PV system, reducing overall system losses and improving power generation performance.