Behavior of small hydroelectric power plant generators during the fault in distribution network (digital simulation study)

In one mountain valley in Slovenia, over 50 small hydroelectric power plants (SHPP) with mostly asynchronous generators cover more than 45% of the power demand of the area (4500 MW). Generators in these small power plants are normally connected directly to low (400 V) or medium (20 kV) voltage overhead distribution networks. In such networks various faults, such as one-phase or three-phase short circuits, occur quite frequently. With the help of digital simulations, the behavior of the generators during faults was investigated. During a three-phase short circuit fault in the network, voltages are lowered so the generators cannot send as much power to the network as they did before the fault. The surplus of mechanical power over electrical power causes the generators to operate abnormally. The speed of asynchronous generators increases far beyond their nominal speed, while the rotor angle of synchronous generators increases, which can cause the generator to operate unstably. The results of the simulations show that short circuit faults in a medium voltage network can cause serious problems to the generators in SHPP if they are not cleared fast enough. In the worst case, these problems may include the unstable operation of synchronous generators and a great increase in the speed of asynchronous generators even at normal (for MV networks) fault clearing times of 400 ms. The results also show that the generators contribute to the size of the short-circuit current, thus increasing the short-circuit powers in the network. This paper presents findings of particular interest relating to: stability limits of synchronous generators, the increase in speed of asynchronous generators at different fault clearing times, the influence of the fault residual voltage on the behavior of generators and the influence of the generators on fault currents.