Transcript SOBIERAJSKI_PL_author_ALPHA4_BLOCK4.4_Question9

Barcelona 12-15 May 2003
The probabilistic study of voltage
problems in lightly loaded medium
voltage power system connected with
small CHP generators
Marian Sobierajski, Wilhelm Rojewski
Wroclaw University of Technology
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Shortage of tools for assessing the capacity of
MV networks to accept distributed generation
1. Bus load demands in MV networks are not
known precisely but only as a range of powers.
2. The uncertainty of load demand in planning
distributed generation should be taken into
considerations.
3. Neglecting the demand uncertainty can produce
the bottle neck in sending the whole generation
to the MV network.
4. Probabilistic load flow study seems to be an
efficient tool for solving such problems.
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• The studied power system
CHP
SL MV
G1
HV
G2
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• 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.
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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.
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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
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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
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Maximal and minimal bus
voltages obtained by PLF study
Random voltage magnitude
1.15
1.1
U,p.u.
1.05
1
0.95
0.9
CHP
SL
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MV
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Probability of the violation of
1.05Un voltage obtained by PLF
study
Probability of U > 1.05Un
0.5
Probability
0.4
0.3
0.2
0.1
MV
SL
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MV
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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
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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.
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Bus voltages had violated 1.05Un
limit during experiment
voltage magnitude at CHP Station
1.06
1.05
1.04
1.03
1.02
1.01
1
5
10
15
20
25
30
time, min
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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.
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FINAL CONCLUSIONS
1. New requirements and constraints in voltage
regulation in the vicinity of dispersed generation
imply using new methods of power network
analysis.
2. Especially, the uncertainty of bus load demand
should be taken into consideration.
3. The probabilistic load flow study is proposed to
be used for steady state analysis of small local
generation connected to the MV network.
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FINAL CONLUSIONS
4. The solution of probabilistic load flow
gives the maximal and minimal value of
bus voltage, and the probability of the
violation of voltage limits.
5. The probabilistic results of bus voltage
violation can be a good basis for creating
an efficient strategy for Automatic Voltage
Control of tap changer.
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