Currently one of the biggest difficulties is the connection of renewable sources to the power grid without disrupting it. To ensure a stable and reliable electricity supply there are many requirements for solar and wind farms. This case study depicts the connection of a new photovoltaic system to a low voltage industrial power grid.
Firstly, requirements for connecting a photovoltaic (PV) system will be overviewed. To connect a PV system to the grid, it is necessary to evaluate certain parameters. It is always necessary to calculate power flow, short circuit currents and voltage level as well as its difference from nominal grid voltage. There are also additional conditions depending on how powerful the connected energy source is. These requirements are stricter for more powerful energy generation sources. In this case study, one medium power level (450 kW) PV system’s connection to low voltage grid is analysed. Medium power level systems (0.25-4.99 MW) have more requirements. In addition to previously mentioned conditions, voltage surge, short circuit phase angles and short circuit power at the point of connection to the grid must be calculated. It is also necessary to carry out a harmonic analysis. All these calculations will be carried out using the EA-PSM program. Please keep in mind that these requirements may be different in other countries. The scheme used in this case study is provided in Figure 1 and the analysed solar farm can be found on lower left side.
There are four main requirements that must be considered when connecting a renewable energy source to low voltage grid. They are as follow:
To ensure voltage quality in the electric network, certain standards must be maintained. For low voltage power grid its voltage limits must be within +10%/-10% (0.36-0.44 kV). According to the requirements voltage level and its difference must be evaluated when the energy source generation is at the highest capacity and when no loads are connected to the grid. Power flow test results with voltage level evaluations are shown in Figure 2. Disconnected loads are indicated with colour red.
It is also necessary to calculate power flows when there is a fault in the power grid or repairs are taking place. Figure 3 and Figure 4 show such situations when only one transformer is working, no loads are connected, and PV systems are generating the highest amount of energy.
According to the requirements it is necessary to evaluate three phase minimum and maximum short circuit currents as well as two phase minimum short circuit current. This was performed at the PV system’s connection point and near the inverter. Short circuit currents were calculated with no load condition, but these tests could also be carried out with loads by just clicking a few buttons if needed. Minimum short circuit current analysis near the inverter can be seen in Figure 5 and the maximum can be seen in Figure 6 The test at connection point can be found in Figure 7 (minimum) and Figure 8 (maximum).
Harmonic current distortions are another parameter that must be calculated when connecting a medium power level PV system. It must be calculated at the connection point and it cannot exceed 6.5% total harmonic distortion (THD). The analysis results can be seen in Figure 9 and Figure 10.
Figure 9 shows THD at different parts of the grid, current distortion can be seen on the lines and voltage distortion is visible near buses. It is required to calculate current THD at connection point, this can be found in Figure 10. If THD would exceed 6.5% a filter is needed to minimize the PV system’s created distortion.
Finally, voltage surge, short circuit phase angles and short circuit power at the point of connection to the grid must be calculated. Currently EA-PSM only counts short circuit phase angles, that is why the other two parameters will be calculated using appropriate formulas. Table 1 displays short circuit phase angle analysis results.
Analysed at | Minimum or maximum short circuit current | Short circuit current phase angle, ∠δ, ° |
PV system’s connection point to the grid | Maximum | -25.9 |
Minimum | -27.9 |
Short circuit power at connection point is calculated by equation:
Voltage surge at no load conditions is calculated by equation:
Solar power plant numbers are growing, and it is important that PV systems do not disrupt the existing power grid. Voltage level must be within ±10% limits, total harmonic distortion for current at the connection point cannot exceed 6.5% and power flow calculation, short circuit current analysis, voltage surge, short circuit phase angles and short circuit power at the point of connection must be carried out when connecting a new medium power PV system to the grid. It is important to keep in mind that these requirements may be different in different countries.
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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