To connect a Photovoltaic system (PV system) to a low voltage (LV) grid a feasibility study was carried out following IEC standards. The study evaluates the connection of a 250 kW solar power plant to LV Section 2. The report example and the network’s single line diagram can be found in the downloadable pdf file down below. All the calculations were performed using EA-PSM Electric software. Learn more about the program. To learn how to conduct such studies in more detail join our hosted online webinar.
Short circuit currents calculations are based on IEC 60909 standard. Network data is depicted below:
Distribution network voltage is controlled with 110/10 kV autotransformers, they ensure that the voltage level is about 10.5 kV at the transformer terminals.
The main characteristics of transformers and low voltage cables are provided in the tables below.
Firstly, in this study low voltage, AC cables for solar power plant inverters are selected. When connecting a PV system to the grid it is important to choose the cables according to the maximum output power of the inverter. The permitted currents for the cables are calculated as stated in IEC 60364-5-52 standard.
Maximum long term current from inverter: 250 kW -> 360 A
For 250 kW inverter 2(4×120) AL cables line is selected. Cable ampacity calculation results are depicted on the right.
Derated ampacity for AL 1(4×120) cable is 212 A. Therefore, 2 parallel cables must be used to ensure cable line capability in case of maximum long-term current. Two parallel cables will ensure 424 A line permitted current and about 17% safety margin. One step smaller cables line 2(4×95) would only allow 364 A maximum current, therefore, is not recommended due to the small safety margin.
Voltage drop calculation results in the selected cable line are depicted below:
The maximum voltage drop is equal to 2,493%. When the voltage in LV Section 2 bus is less than 107%, the voltage at solar inverter terminals will not exceed the permitted 10% overvoltage limit. Voltage calculation results with different transformer tap positions:
TAP = -1
TAP = 0
Calculation results show, that setting transformer tap changer position TAP = -1 or TAP = -2 should not be allowed.
We will use the energy yield coefficient for inverter power derating to calculate annual power losses in the selected cable line. According to historical solar irradiation data, the energy yield coefficient is equal to 30%. Power losses calculation results with this energy yield coefficient are depicted below:
Calculated average energy losses are equal to 500 W. Annual energy losses in cable lines are equal to 4380 kWh. The average electricity price is assumed to be 0.07 Eur/kWh. Annual financial losses due to power losses in the cable are equal to 306 Eur.
One step bigger cable line AL 2(4×150) would allow saving 876 kWh (61 Eur) per year, therefore, it is not economically beneficial to increase the cross-section area of the cable.
After selecting proper cables the grid connection feasibility study can be performed. In this study such calculations will be carried out:
The ratio between network short circuit power and solar plant power:
Because r < 0.1% power plant impact to voltage level and power quality in the distribution network is minimal and IEC 61000-3-7 standard does not limit the number of voltage changes.
After determining short circuit power the voltage is evaluated. Let us estimate minimum loading conditions (all distribution network loads will consume 0% of their nominal power). When the solar power plant is working at full power, the voltage level at medium voltage Section 2 bus is equal to 10.503 kV (105.033%). Due to generation loss from the solar power plant, the voltage would go down to 10.502 kV (105.023%). As a result, the voltage difference is only 0.01%, therefore, the autotransformer will not change tap position.
Not only does the no-load operation mode must be determined but also the maximum loading conditions have to be evaluated (all distribution network loads will consume 100% from their nominal power). When the solar power plant is working at full power, the voltage at medium voltage Section 2 bus is equal to 10.548 kV (105.478%). Due to generation loss from the solar power plant, the voltage would go down to 10.547 kV (105.466%). As a result, the voltage difference is only 0.012%, therefore, the autotransformer will not change tap position.
In conclusion, power losses from the solar power plant’s effect on voltage levels in the distribution network are negligible.
Next voltage fluctuation will be analyzed. Voltage fluctuation coefficient is calculated according to IEC 61000-3-7 standard using the following formula:
The maximum value of Pst2 (equal to 5.6) will be assumed in this calculation. Voltage difference d is calculated in the previous section (voltage level). The maximum voltage fluctuation coefficient is equal to:
According to IEC 61000-3-7 standard allowable Pst value is < 1. Therefore, the solar power plant will not cause voltage fluctuations.
Let’s evaluate harmonic distortions next. Calculated current harmonics in TR2 10 are depicted in the graph below:
According to calculation results, we can see that the 5th and the 7th harmonics exceed IEEE 519 standard allowable limit. For this reason, we select the detuned passive harmonics filter. Reactor parameters for the filter are depicted below:
Harmonic results with the detuned reactor are depicted below:
After adding the filter, all harmonics are within permissible limits. The detuned filter is the cheapest option to reduce higher harmonics. Other possible alternatives are active filters and tuned passive filters.
In addition to previously carried out, calculations load flows have to also be assessed. Results for the maximum load and generation conditions can be found in the tables below:
The blue color indicates that the current exceeds recommended economical limit (1 mm² = 1 A).
Orange and yellow indicate a higher than recommended voltage drop in the cable.
Finally, short circuit analysis is conducted. The result can be found in the table below. Types of short circuits:
To learn how to carry out such a feasibility study in more detail join our hosted online webinar.
Before each new release of EA-PSM Electric, validation cases are prepared, thus ensuring new features are working properly. At this validation case, arc flash scenarios for three-phase medium voltage and low voltage systems according to the IEEE 1584-2018 standard are analysed.
Scheme link element is dedicated to dividing the scheme into logical blocks thus reducing graphics lag and allowing to calculate equivalents
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