1. University of Ljubljana, Faculty of Electrical Engineering - Slovenia
INFLUENCE OF FAULTS IN DISTRIBUTION NETWORK ON BEHAVIOR OF ASYNCHRONOUS GENERATORS IN SMALL HYDROELECTRIC POWER PLANTS
Grega BIZJAK
University of Ljubljana, Faculty of Electrical Engineering - Slovenia
Thank you mister president.
Dear coleags, ladies and gentleman!

2. Introduction
The number of small Hydroelectric power plants (SHPP) in distribution networks in Slovenia increase.
Generators in SHPP react to the disturbances in distribution network:
increase in speed
increase in short circuit power in network
Digital simulation is an appropriate tool for the investigation of such problems in networks.
In some mountainous regions in Slovenia the production of electrical energy from small hydro-electric power plants covers almost 50% of power demand. A three phase asynchronous machines are normally used as generators. In this networks there are a large number of short circuit faults and line interruptions each year. Each fault represent a disturbance to the generators.

3. Observed distribution network
20 kV distribution network with overhead lines.
More than 50 small hydroelectric power plants with mostly asynchronous generators.
Total power generation of 2000 kW or 45% of the load demand.
The observed network operates on 20 kV voltage level. In the network there are 50 small hydroelectric power stations, more than 30 of which are normally in operation. Most of them have one or two asynchronous generators. The total average active power generation is around 45% of the load demand.

4. Dynamic analysis of the network
Influence of the faults on the behavior of generators in SHPP
Increase in speed of asynchronous generators
Influence of the generators on the short-circuit current in a network
Influence on currents and voltages in network during the one-phase short-circuit
Influence on currents and voltages in network during the three-phase short-circuit
With the help of dynamic simulation a different short-circuit faults were simulated in the network and the generators responses were observed. We focused on two main subjects:
the increase in speed of generators and
the increase in the short circuit power by the generators in the network.

5. Increase in speed of asynchronous generators
The speed of asynchronous generators during the fault increases far beyond their nominal speed. According to results, the most dangerous fault is near the generator. With a normal fault clearing times, faults with a residual voltage of less than 20% will result in a speed twice as fast as synchronous speed.

6. Simulation of three-phase short-circuit
Simulations of short-circuit fault in a network with and without the consideration of generators were also made. The aim was to determine the influence of generators on the amplitude of short-circuit currents in the network. The results show that generators increase the short circuit power in the network for over 25%.

7. Conclusions
During the fault in distribution network, which is not cleared fast enough, the asynchronous generators may reach the twice of nominal speed.
Generators have very small influence on conditions in the network except on the tree-phase short-circuit currents.
After the results were obtained, the generators’ protection devices were checked, as well as the circuit breaker capacities in the substations. In the period since most of the power plants were put into operation, no serious problems have occurred in the network.