A Review of Transient Effects of Different Types of Distributed Generation Units on Overcurrent Protection System

Increasing the integration of distributed generation (DG), especially renewable energy based, in the distribution network and close to the consumption, provides several advantages like lower CO2 emission, but also some severe difficulties such as voltage and frequency instability, voltage distortion and protection problems.
Indeed, the added DGs to the grid could contribute to the fault current and increase the short circuit current in case of downstream fault. In addition to the fault current increase, DG reduces the grid fault current contribution and increases the risk of blinding of protection. Moreover, the presence of DG could change the direction of power flows and fault currents. Therefore, in the case of neighbouring feeder faults, it can force the relay to make an inappropriate tripping command.
The intensity of these problems depends on the penetration level of DG and its location. Therefore finding a comprehensive solution for protection issues needs accurate studies, analysis and simulation.
Firstly, the effects of DGs should be identified. Lots of studies and researches have been done in this field and results are published in different papers. In most of the publications, DG is modelled as a simple voltage source and its dynamics have not been considered. Using ideal model makes the simulation fast and simple but with a low accuracy in the results. Computations based on ideal source representations can, indeed, produce extreme short circuit current depending on the fault impedance while the short circuit power of DGs, especially renewable energy based, is limited and do not effectively contribute to fault current like an ideal source in the same conditions. Practically, the duration of fault current and its transient behavior depends, in fact, on the type of generator, on its interface with grid and also depends on the control strategy which is used. The mentioned problems could be more tangible in case of power electronic based DGs. The produced fault current is usually not greater than 2 or 2.5 times of converter rated current. Finally it must also be taken into account that DGs inject high order harmonics to the grid which could affect the protection performance. In this paper we compare fault current generated by ideal models of DG with those calculated with exact representations of 'renewable energy based' generators. PSCAD is used for the transient simulations. Investigations in this paper are focused on medium voltage distribution network. Firstly, the three phase short circuit (without the presence of DG) is simulated in different nodes of the considered test grid. Then, an ideal model of DG is added to the distribution grid and the short circuit current is measured at the same nodes. In the next step, the exact model of DGs based on wind power conversion, such as DFIG (partial size converter), PSMG (full size converter) and IG (directly connected to grid) including all power electronics and control systems, are added to the grid. Faults are again applied to the different nodes of the network and the related fault currents are compared to the 'ideal' ones. Finally, the difference between transient fault currents produced by dynamic models of DGs and the ones produced by simple models are discussed.