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Often windturbines in the range between 300 kVA up to 3000 kVA are connected to the MV grid by a distribution transformer. Very often it is easy to measure the electricity on the LV side of the transformer, which is connected to the windturbine. Since this is not the actual point of connection with the grid, it is needed to make an estimation of the energy losses due to the transformer.
As can seen from the figure above the measured electricity at the location of the meter equals to Ewind, while the actual delivered energy to the grid equals Edelivered. The difference between them are the losses caused by the transformer (strictly the cables should also be included). To calculate the electricity delivered it is necessary to make an estimation of the losses of the transformer.
To make an estimation we do need the losses of the transformer (load and no-load losses). The measurement on the LV side, shows us the total electricity produced by the windturbine during a year. These values are not adequate to make an estimation of the losses caused by the transformer. To calculate the losses of the transformer we need to know the windspeed distribution during the year. Together with the P-v curve of the windturbine it is possible to make an estimation of the electricity profile of the windturbine as given in the figure below for a 750 kW windturbine.
Based on these figures the total annual electricity produced by the windturbine (which must be equal to the measured energy) is about 1.850.000 kWh (load factor about 28%). If a 1.000 kVA distribution transformer is used (no load losses: 1.100 W and load losses: 9.500 W), it is possible to calculate the losses due to the transformer. As long as the transformer is connected to the grid (even at low wind speeds) it will always have no load losses equal to 1.100 W * 8.760 h = 9.636 kWh. By mixing the figures given above it is possible to make a load distribution over the year as given in the figure on the next sheet.
With this distribution it is possible to calculate the total amount of load losses caused by the transformer. In this case the annual load losses are estimated at 8.279 kWh. Thus the total annual losses of the transformer equals 17.915 kWh. This is about 0,97% of the electricity produced by the wind turbine. Within this case the electricity delivered to the grid is about 1% less than measured at the LV side.
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A valid point! The extreme case is the inverter for a PV plant because the subsidies are even much higher there than for wind. There has been a discussion about poor efficiencies, while really the very best technically feasible would be the commercially best for the user. Now since said subsidies are a political issue, there are three different approaches for assessing the correct location of the meter and the evaluation of the transformer / inverter losses:
1. The plant feeds electricity into the grid, from where it is bought at market prices, so use the usual (market price based) assessment approach.
2. This energy is subsidised, so use the subsidised premium price for your assessment.
3. In Germany the subsidy does not come from the state, but government compells utilities to come up for it. So utilities should have an interest in transformer losses being as high as possible in order to let as little energy as possible escape into the grid! It only costs them their money. And the counter should be located on the other side of the transformer, otherwise creating high transformer losses is no use.
The no load, load and auxiliary losses of the transformer should be taken into account at tendering stage. This will result in the most efficient transformer for your wind power plant across its entire lifecycle. Similarly transformer loadings should be taken into consideration in your calculations.
See - IEEE Std C57.120-1991
Stefan, it is a bit controversial what you say, but the logic is there.
However despite other reasons mind the price difference of current transformers LV or MV.
The lower bound of generation is NOT zero, as implied by the graph, but is actually negative when the windmill is not generating. The core is excited from the HV side, just like in a distribution system. When there is no generation, the transformer actually pulls power from the grid. For renewables, you should be using transformers with amorphous metal cores which cut no load losses by 70 percent.