Because the solar photovoltaic isolation transformer has no oil, there is no fire, explosion, pollution or other problems, so the electrical specifications and regulations do not require the dry-type transformer to be placed in a separate room. The safe operation and service life of solar photovoltaic transformers largely depend on the safety and reliability of the solar photovoltaic transformer winding insulation. The winding temperature exceeds the insulation withstand temperature and the insulation is damaged, which is one of the main reasons for the failure of the solar photovoltaic transformer to work normally. Therefore, the monitoring of the operating temperature of the transformer and its alarm control are very important.
The three-phase photovoltaic transformer is a special transformer for photovoltaic grid-connected inverters. The coil adopts foil winding technology, and the iron core sheet is made of low-loss oriented silicon steel sheet, which has the advantages of low loss, high efficiency, and low temperature rise; and it is also a transformer and an inductor. A two-in-one integrated product, this transformer is suitable for circuits with AC 50Hz/60Hz and rated voltage below 690V. In the photovoltaic power generation system, our isolation transformer is used in many applications and is one of the most critical equipment. In addition to changing the voltage and transferring energy, it also has the following functions: the role of the isolation transformer, the capacity design of the step-up transformer, etc.
First, the role of photovoltaic isolation transformers
1. Electrical isolation: The primary and secondary of the isolation transformer rely on the magnetic circuit to transfer energy, and the components are electrically isolated from the power grid, which can prevent the DC component and leakage current from entering the power grid and protect the equipment and the power grid.
2. In the solution of suppressing the PID of the component, an isolation transformer is connected behind the inverter, and then the potential of the N pole to the ground is increased, and the potential of the negative pole of the component is indirectly increased to achieve the purpose of suppressing the PID of the component.
3. Matching voltage: The grid voltage in some countries is different from that in my country. For example, in the United States, single-phase 110V and three-phase 220V can be added. A transformer can be added behind the inverter to match the voltage of the access country.
4. Elimination of the third harmonic: Using a transformer connected with Dyn11, the third harmonic circulating current in the D-connected winding can generate the third harmonic magnetomotive force in the transformer, which is balanced with the third harmonic magnetomotive force of the low-voltage winding. .
5. The power grid adaptability is stronger: in some factories with large motors or electric welding machines, the power grid voltage fluctuates greatly, the harmonic current is large, and the three-phase voltage is extremely unbalanced. The inductance of the primary side and the secondary side of the isolation transformer can prevent the sudden change of current. The Dyn11 connection transformer allows the neutral line current to reach more than 75% of the phase current, and can withstand three-phase unbalance.
Second, Capacity design of photovoltaic step-up transformer
There are two options to use the public transformer and photovoltaic special transformer. If the surrounding load cannot be used up, it must be sent to a higher voltage level through the transformer, and the original public step-down transformer will be used to connect to the Internet. According to the requirements of 4.3.1 of "Technical Regulations for Solar Power Stations of State Grid Corporation of China": The total capacity of small photovoltaic power stations should not exceed 25% of the maximum load in the power supply area of the upper-level transformer, which is mainly considered from the perspective of power grid security. Affected by the weather and the environment, the output power of photovoltaics is unstable, and the grid needs to provide a strong balance of energy, and these energy needs to be provided by the electromagnetic exchange between the primary side and the secondary side of the transformer. The ratio of 25% is the empirical value obtained after many experiments.
When using a photovoltaic special transformer to connect to the Internet, the transformer has no other load. The main factor to consider is that the maximum output power of the inverter cannot exceed the capacity of the transformer, and the maximum output power of the inverter and the capacity, installation azimuth and inclination of photovoltaic modules, and Weather conditions, inverter installation site and other factors are related. The maximum output power of photovoltaic inverters is generally about 0.9 for components, and the power factor of transformers is generally about 0.9. Therefore, photovoltaic modules and transformers are generally required to be configured at a ratio of 1:1. Or the transformer capacity is slightly larger than the component capacity.
Third, Dry-type and oil-immersed photovoltaic isolation transformers
If the working place of the transformer is clean, the dust-free capacity is not too large, the dry-type transformer can be selected, otherwise, the oil-immersed transformer should be selected. The oil-immersed transformer will naturally decide for you whether it is natural cooling or air cooling according to the size of the capacity.
Features of dry-type transformers
1. Safe, fireproof, non-polluting, can run directly in the load center;
2. Adopt domestic advanced technology, high mechanical strength, strong short-circuit resistance, small partial discharge, good thermal stability, high reliability and long service life;
3. Low loss, low noise, obvious energy saving effect, maintenance free;
4. Good heat dissipation performance, strong overload capacity, can increase capacity operation when forced air cooling;
5. Good moisture-proof performance, suitable for operation in high humidity and other harsh environments;
6. Dry-type transformer can be equipped with a complete temperature detection and protection system. Using intelligent signal temperature control system, it can automatically detect and display the respective working temperatures of the three-phase windings, automatically start and stop the fan, and have functions such as alarm and trip settings;
7. Small size, light weight, small footprint and low installation cost.
From the comparison of comprehensive technical and economic performances such as low noise, energy saving, fire prevention, saving civil construction cost, operation and maintenance management cost, and 30-year lifespan, dry-type transformers show their obvious advantages.
Fourth, how to reduce the no-load loss of photovoltaic transformers
Considering the particularity of photovoltaic power generation, that is, when there is sun during the day, power is generated, but no power is generated at night or in cloudy and rainy days. Regardless of whether the power generation device outputs power or not, as long as the transformer is connected to the system, the transformer will always produce no-load loss. A 1000kVA transformer, if it works for 10 hours during the day and does not work for 14 hours, will produce 30-40 degrees of no-load loss every day, about 1000 kWh of electricity a month. Shenzhen Hongda Photovoltaic Engineering Co., Ltd. invented a Patented technology, according to the change of light conditions, when there is sun in the morning, the transformer is automatically started slightly ahead of the inverter, and when there is no sun at night, the transformer is automatically turned off slightly behind the inverter, which can reduce the no-load loss of the transformer by 99%. The life of the transformer can be improved.
