Influence of protection on transient stability of medium voltage grids including distributed generation

Due to the increase of distributed generation (DG) in the future it can become important to keep DG connected to the grid in order to maintain the balance between consumed and generated electrical power. Keeping DG-units connected to the grid during a disturbance, the dynamic behavior of the DG-units becomes important. Protection schemes based on definite and inverse over-current are simulated and transient stability of DG-units is investigated. The simulations have shown that there is no major benefit to apply inverse over-current relays in MV-grids with short feeder length and high short circuit power. With conventional grading of the protection in most cases the DG-units become unstable. The paper shows that reduction of the time grading is possible however, this reduction is not sufficient to guarantee stable operation. The best option is fast fault clearing of the complete feeder. It is shown that this is possible without loss of selectivity and with stable operation of the remaining DG-units