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Assessment of Medium Voltage Distribution Feeders under High Penetration of PV Generation

Fig. 1. One line diagram of the distribution system.
Due to environmental problems caused by fossil fuels and sustainable development challenges, worldwide installation of photovoltaic (PV) systems is increasing exponentially. According to solar energy industries association (SEIA), U.S PV installations are increasing more than 50% annually from 2009 to 2013. A dominant part of these installations is the PV farm, but commercial (>10kW) and residential (2-10kW) PV generators are also increasing rapidly, making their total installations even comparable to PV farms. The distribution power system is therefore seeing the potential for high penetrations of PV systems as well as their transmission level.

Such a significant energy supply change may have a remarkable effect on the static and dynamic performance of the distribution system. Impacts of PV injection include voltage profile changes due to solar generator output intermittency, and a power loss increase caused by varied power flow. A lot of research has been done to assess these static problems in distribution systems, but most papers assume that no reactive power is produced by PV inverters. However, according to the revised IEEE 1547 standard, distribution resources can actively participate to regulate the voltage by changes of real and reactive power. This paper develops a static model considering three different reactive power operation modes of PV generators and applies the model in a medium voltage distribution feeder. According to the Jacobian matrix of power flow equations, sensitivity indices of voltage or power loss increase over solar power input are calculated, to quantify the impact of PV injection at different locations, different capacities and different reactive power operation modes.

A steady state model is proposed for the PV generator working at different reactive power operation modes, and the sensitivity matrix for voltage increase over solar active power is defined using a Jacobian matrix deducted from the power flow equations, depending on which sensitivity index for system power loss over solar active power is built. The sensitivity matrix and index for a 12kV radial distribution system case are calculated by MATLAB, and proven to be accurate by simulation results in PSS/E. The sensitivity matrix and index can be used to analyze the impact of PV injection on bus voltages within the whole system and system power loss. For the selected distribution system, more active power can be injected when the PV generator is working on mode 3 and PV generators working on mode 2 will increase power system loss more rapidly.

Fig. 2. System voltage contour after 1.36MW injected at bus 19
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