Network Reconfiguration in Balanced Distribution Systems with Variable Load Demand

1. Network Reconfiguration in
Balanced Distribution Systems
with Variable Load Demand
A. Zidan, Student Member, IEEE,
E. F. El-Saadany, Senior Member, IEEE,

2. Outline
• Introduction
• Motivations and objectives
• Problem definition
• Problem formulation
• Results and discussion
• Conclusions
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3. Introduction
Distribution system reconfiguration: is defined as altering the topological structure of
distribution feeders by changing the open/closed states of the sectionalizing switchs
(normally closed) and tie switches (normally open).
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Benefits from reconfiguration
during normal operation :
1)Minimizing the system losses.
2)Load balancing among feeders.
3) Reliability indices improvement.
4) Increase loadability/DG penetration.

4. Reconfiguration under Abnormal Operation
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Benefits from reconfiguration during
abnormal operation responding to
faults by:
1) isolating the faulted areas;
2) supplying power to the non-faulted
areas with minimum load shedding.
fault

5. Motivations
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2 4 6 8 10 12 14 16 18 20 22 24
0
10
20
30
40
50
60
70
80
90
100
Time (hour)
Load%
Residential
Commercial
Industrial
 Load profiles for distribution systems vary
due to:
– different customer types (i.e.,
residential, commercial, and industrial)
– the variable demands all over the day.
 Distributed generation (DG units) have
been inserted in distribution systems.

6. Objectives
6
 The main objective is to minimize the system losses.
 To study the effect of DG units on the reconfiguration problem, two
case studies are presented for the fixed load (i.e., one snapshot load).
- Optimal configuration without DG
- Optimal configuration with DG
 To study the effect of variable load demand on the reconfiguration
problem, two scenarios are suggested :
1) fixed configuration (one configuration for the whole planning period)
2) hourly reconfiguration.

7. Typical Reconfiguration Problem Definition
The objective is to minimize the system losses.
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Minimize i
Nb
i
i RI∑=1
2 where Nb: total number of branches,
Ii : current in branch i,
Ri: resistance of branch i
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The constraints to be satisfied are:
• Keeping the system radial
• Keeping the voltages at all buses within the limits
• Keeping all branch currents within their limits
maxII j ≤
minmax VVV i ≥≥

8. Problem Formulation
8
-8-
• Branch current is related to apparent power flow in
the branch by:
• Minimizing the apparent power flow in branches
provides lower active and reactive losses.
• In meshed networks, each loop will have best
opening switch for lower losses.
• Opening a switch in a mesh with minimum apparent
power yields radial configuration with a minimum
disturbance in the flow pattern of the network.
• Therefore, the process starts by closing all tie
switches creating meshed network which contains
many closed loops. Then, the radial topology is
retained by opening one switch in each loop.
• A proposed KVA index is used to decide which
switch is opened for each loop.
∗
= ijiij IVKVA
method

9. Problem Formulation 9
A KVA index can be defined for each
branch (n) by:
KVAn : apparent power flow in branch n,
KVAav: average branch KVA of all branches for a
chosen loop,
w: weighing factor defined as explained in [7].
)(
exp)( av
n
KVA
KVA
w
KVA n
−
=µ
branch w
b0, b1, b1’ 1/m
b2, b2’, b3, b3’ 3/m
B4, b4’,b5, b5’ 5/m

10. Results and Discussion
Two scenarios are studied and compared to the base case (network
without reconfiguration). These scenarios are :
1)Optimum network with fixed load (i.e., one snapshot load ).
a) Without DG
b) With DG
2) Optimum network with variable demand (without DG)
a) Based on fixed configuration
b) Based on hourly configuration.

11. 33 bus system without DG 1111
Case
Open
switch
Loss
% Loss
reduction
kW Kvar kW Kvar
Initial
network
(base)
203 135 - -
Proposed
method
s7, s11,
s14, s32
141 104 30.54 22.96

12. 33 bus system with DG 121212
Case Open switch
Loss
% Loss
reduction
kW Kvar kW kvar
Initial
network
with DG
180 120 11.3 11.1
Proposed
method
s7, s11, s14,
s32
126 94 37.93 30.4

13. Results with variable demand
 Two scenarios are suggested :
1) Fixed configuration (one configuration for the whole planning period)
2) Hourly reconfiguration.
 The second scenario may be more flexible in terms of following variation in demands and
in generations for renewable sources.
 However, conducting several reconfigurations increases:
– The transient disturbance due to the multiple switching operations;
– The operational cost of these switching operations (reducing life span)
 This part presents a comparison between the two scenarios in terms of the energy losses
and the required number of switching operations.
 The hourly configuration is equivalent to solving several uncoupled problems; one for each
time interval (the load is fixed during each hour).

14. Flow chart for the fixed configuration
Run AC load flow for the meshed network for each hour during the day
Calculate the average apparent power flow in each branch from all time intervals:
KVAt (n): apparent power flow in branch n during interval t, and Δt: duration of interval t.
∑
∑
∈
∈
∆
∆
=
Tt
Tt
t
t
tnKVA
nKVAAverage
*)(
)(:
Calculate the KVA switching index to select switches to be opened.
The maximum demand is used to check the constraints of selected
configuration, such that if the solution satisfies the maximum demand it
will satisfy all the loading states.

15. Civanlar system under variable demand
Base configuration Hourly configuration Fixed configuration
losses
Switching actions
losses
Switching actions
losses
MWh MVARh MWh MVARh MWh MVARh
4.957 5.747
16 switching
operation
4.431 5.184
4 switching
operation
4.442 5.219
2 4 6 8 10 12 14 16 18 20 22 24
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Time (hr)
Ploss(MWh)
base
hourly
fixed
Civanlar system was used by considering that feeder F1: residential, feeder F2 :
commercial and feeder F3 : industrial [21]. The load variation during the 24 hour for each
load type is considered as in [20].

16. Conclusions
 Based on studied systems, reconfiguration has the following benefits:
– Reduces the active and reactive power losses (kW & kvar loss).
– Improves the voltage profile at most buses.
 The topological structure of the optimum networks without DG units
may be different from those with DG units.
 Fixed configuration is more acceptable with respect to the operational
practices. It reduces the number of switching operations and thus:
– reduces the possibility of switching surges, the risk of outages, and transient
disturbances in the system due to the multiple switching.
– reduces the operational cost of these switching operations (i.e., the cost of
dispatching technicians for non-automated systems, maintenance, and
reduction in the lifetime of switches).
 The results conclude that fixed configuration is more effective than
hourly one; it realizes lower losses with lower switching operations.
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17. Thank You
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