Shortage of tools for assessing the capacity of MV networks to accept distributed generation

1. The probabilistic study of voltage problems in lightly loaded medium voltage power system connected with small CHP generatorsMarian Sobierajski, Wilhelm RojewskiWroclaw University of Technology

2. Shortage of tools for assessing the capacity of MV networks to accept distributed generation • Bus load demands in MV networks are not known precisely but only as a range of powers. • The uncertainty of load demand in planning distributed generation should be taken into considerations. • Neglecting the demand uncertainty can produce the bottle neck in sending the whole generation to the MV network. • Probabilistic load flow study seems to be an efficient tool for solving such problems.

3. The studied power system

4. Voltage regulation at MV bus was achieved using on-load tap changers having 19 taps in a range of +/-10%. • CHP generators worked with constant power factor of 1. • A dilemma arose between the required active output of CHP and the need to maintain voltage within +5%/-10%. • Only 50% of nominal generation was produced without MV bus voltage violation.

5. PLF - Probabilistic Load Flow • The bus powers are treated as rectangularly distributed random variables in the range Pmin < P < Pmax Qmin < Q < Q max • Load flow equations are linearized and treated as the function of random variables. • The calculated bus voltages are treated as the random variables with the normal probability distribution.

6. Minimal and maximal value of bus voltages from PLF • According to the 3-sigma rule the maximal value of bus voltage equals Umin = EU - 3 Umax = EU + 3  where EU – the expected value of bus voltage  - the standard deviation of bus voltage

7. The probability of voltage limit violation from PLF • Knowing expected value EU and standard deviation  the probability of the violation of upper limit can be calculated p = P(U>1.05Un) using the Gauss function

8. Maximal and minimal bus voltages obtained by PLF study

9. Probability of the violation of 1.05Un voltage obtained by PLF study

10. Main conclusion from PLF study • During sending the whole generation from CHP to MV network one should take into account that the probability of the violation of the 1.05Un is as follows: • 0.27 - at the SL bus • 0.43 - at the CHP bus • 0.12 - at MV bus

11. Experiment • On 6th Sept. 2002, from 11.30 till 12.00, CHP generators were forced to send the whole nominal active power with power factor of 1. • The receivers at all MV load buses were informed about the possibility of the violation of 1.05Un voltage limit.

12. Bus voltages had violated 1.05Un limit during experiment

13. Conclusion from PLF study and Experiment • Maximal and minimal bus voltages, and the probability of the upper limit violation obtained by PLF study were more pessimistic than obtained by Experiment. Hence, the power system operation and other decisions during Experiment had to be careful. • PLF study is quick and gives concise results. Experiment is expensive, time consuming and dangerous for energy receivers.

14. FINAL CONCLUSIONS • New requirements and constraints in voltage regulation in the vicinity of dispersed generation imply using new methods of power network analysis. • Especially, the uncertainty of bus load demand should be taken into consideration. • The probabilistic load flow study is proposed to be used for steady state analysis of small local generation connected to the MV network.

15. FINAL CONLUSIONS • The solution of probabilistic load flow gives the maximal and minimal value of bus voltage, and the probability of the violation of voltage limits. • The probabilistic results of bus voltage violation can be a good basis for creating an efficient strategy for Automatic Voltage Control of tap changer.