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Voltage Regulation and Overcurrent Protection Issues in Distribution Feeders

Voltage Regulation and Overcurrent Protection Issues in Distribution Feeders
with Distributed Generation A Case Study

Yahia Baghzouz
Department of Electrical & Computer Engineering
University of Nevada, Las Vegas
eebag@ee.unlv.edu


Abstract

The installation of distributed generation (DG) on
distribution feeders is known to have an impact on
voltage regulation. A DG can provide voltage support in
some cases, but can also cause an overvoltage or an
undervoltage, depending on the several variables
including relative DG size and location, distribution line
and load characteristics, and method of voltage
regulation. This paper shed some light the impact of DGs
on voltage regulation and on overcurrent protection.
These impacts are investigated on a feasibility study
concerning the installation of a 5 MW grid-tied PV system
to a local distribution system.

1. Introduction

Distributed generation is defined as energy sources
(ranging in size from few kilowatts to approximately 10
MW) connected to distribution systems. The main
advantage of DG units is their close proximity to the load
that they serve.
Various technologies are being used in DG
applications with a variable degree of success. Many DG
technologies utilize renewable energy due to growing
attention to air pollution and greenhouse effects. With the
assistance of government subsidiary programs, grid-
connected photovoltaic (PV) is a prospering market in
some European countries, and the same expected in the
US as many electric utilities encourage such systems
through significant rebates. For example, the local utility
company provides $5 per installed peak PV watt in rebate
literally paying for the cost of the PV panels and the
customer is responsible only for balance-of-system and
installation cost. Wind power is even more competitive
where wind resources are abundant, but wind farms are
often connected to sub-transmission or transmission
systems, rather than distribution systems.
Others DGs, such as reciprocating engines, and fuel
cells, utilize a continuous source of fuel thus making them
dispatcheable power sources. Internal combustion engine-
generators are often used for increased reliability (i.e.,
back-up power source) and peak shaving applications
during periods of high energy demand and/or high energy
cost. These engine-generator sets, however, are noisy, use
expensive fuel, and have high polluting emissions. Fuel
cells, on the other hand, are quiet and pollution-free, but
have a very limited level of penetration at present due to
thei high cost and availability of hydrogen.
Widespread proliferation of DG can have positive
effects such as reliability improvement through backup
generation, reduce equipment loading and enabling load
transfer from adjacent feeders experiencing outages [1].
On the other hand, DG can increase the complexity of
controlling, protecting and maintaining distribution
systems [2]-[8]. Thus, it is critical that such impacts be
assessed to avoid degradation of service.
This paper focuses on impact of DG on voltage
regulation, and ovecurrent protection issues. A feeder
with uniformly distributed load and a discrete number of
DGs is analyzed in an effort to shed light on when an
undervoltage or an overvoltage can occur. Then the main
concerns associated with the impact on overcurrent
protection are briefly reviewed. This is followed by a case
study of a 5 MW PV system that is being considered for
installation on the rooftop of a local large customer.

2. Impact on voltage regulation


Voltage regulation practice on distribution systems is
based on radial power flow from the substation to the
load. This is often achieved through Load-Tap-Changing
(LTC) transformers at the substation. Voltage regulation
is also enhanced by switched capacitor banks (and in
some cases voltage regulators) placed along the feeder.
Line Drop Compensators (LDC) are seldom used except
in rural areas where load density is low, or in substations
where each feeder is fitted with an

independent voltage
regulator.


To illustrate the impact of DG on voltage regulation,
consider a feeder of length l with a uniformly distributed
load as shown in Fig. 1(a). Three distributed generators
DG
1
, DG
2
, DG
3
are added at locations d
1
, d
2
, d
3
,
respectively. Like conventional large power system
generators, distributed generators using synchronous
machines can generate as well as absorb reactive power
0-7695-2268-8/05/$20.00 (C) 2005 IEEE
Proceedings of the 38th Hawaii International Conference on System Sciences - 2005
1 (i.e., operate at both leading and lagging power factor).
On the other hand, DGs utilizing induction generators can
only absorb reactive power, and inverter-based DGs are
normally designed to operate near unit power factor. In
any case, the standard requirement is that all DGs are
expected to operate at a power factor of at least 85% (lead
or lag) [9].
For diversity purposes, let DG
1
, DG
2
, DG
3
operate at a
lagging power factor, unity power factor, and leading
power factor, respectively. Their corresponding real and
reactive power production are (P
G1
, -Q
G1
), (P
G2
, 0), (P
G3
,
Q
G3
). Note that Q
G1
denotes the magnitude of reactive
power consumed by DG
1
since a negative sign is placed
in front of it. The resulting real and reactive power flow
along the feeder are shown in Figures 1(b) and 1(c),
respectively. Herein, P
s
and Q
s
represent the total feeder
active and reactive power loading without the DGs.

(a)

(b)

(c)
Fig. 1. (a) Feeder with uniformly distributed load and 3
DGs, (b) active power profile, (c) reactive power profile.


The voltage drop (or rise) along the feeder can be
computer at each line segment defined by the DG
locations, assuming that the voltage at the secondary
substation transformer V
o
is known. Starting with the first
segment between the substation and DG
1
, i.e., 0 < d d
1
,
the voltage drop (rise) can be decomposed in two
components: The component due to the load downstream
is given by
where P
d1
and Q
d1
are the active and reactive power flows
right before d
1
, i.e.,
)
2
(
)
1
(
3
2
1
1
1
G
G
G
s
d
P
P
P
d
P
P
=
and
)
3
(
)
1
(
3
1
1
1
G
G
s
d
Q
Q
d
Q
Q + =
The second component is due to the lumped-sum load
upstream located at half the distance,
The total voltage drop (rise) is then computed by
summing (1) and (4),

Solving for the voltage at d
1
yields
Where
)
7
(
)}
(
)
(
{
2
1
1
1
1
d
s
d
s
d
Q
Q
X
P
P
R
d
a
+
+
+
=
Knowing the voltage at d
1
, the above procedure can
then be extended to determine the voltage drop across the
line segments downstream by simple substitutions. For
instance, to compute the voltage drop in the middle
segment d
1
< d d
2
,

replace (P
d1
, Q
d1
) by (P
d2
, Q
d2
) in
(1) and (4), (P
s
, Q
s
) by (P
d1
, Q
d1
) in (4) and (7),and V
0

by V
d1
in (5) and (6). Here, (P
d2
, Q
d2
) are computed as in
(2) and (3) while ignoring the powers generated by DG
1

and replacing d
1
with d
2
.
Note that the introduction of DG reduces the amount
of power that must be supplied from substation, hence
reduces the voltage drop across the feeder. In the even
that the DG produces more power than the local demand,
the net power will flow upstream (towards the substation).
If this reverse flow is sufficiently large, its will overcome
the voltage drop caused by the reactive power flow (i.e.,
the XQ part in (1) and (4)) and may result in an
overvoltage. On the other hand, if sufficient DG is
installed close to the substation, it can lead the LTC to
operate at a lower secondary voltage, and this can result
in an undervoltage towards the feeder end.
There are a number of methods by which over- or
under-voltages caused by DG can be mitigated [10]-[14].
Some of these include adjusting the sending end voltage,
)
4
(
)
(
)
(
2
1
1
1
1
1
d
d
s
d
s
b
V
Q
Q
X
P
P
R
d
VD + =
)
1
(
1
1
1
1
1
d
d
d
a
V
XQ
RP
d
VD
+
=
)
5
(
1
0
1
1
1
d
b
a
d
V
V
VD
VD
VD =
+
=
)
6
(
}
)
4
(
{
2
1
2
/
1
1
2
0
0
1
d
d
a
V
V
V +
=
0-7695-2268-8/05/$20.00 (C) 2005 IEEE
Proceedings of the 38th Hawaii International Conference on System Sciences - 2005
2 installing voltage regulators on the line, upgrading the
network, and constraining the DG. But each method has a
cost implication and many jeopardize the financial
viability of DG projects.

3. Impact on overcurrent protection

Overcurrent protection schemes for radial distribution
systems are designed based on the available short circuit
ratios, maximum load currents, system voltage and
insulation levels. The addition of generation on the feeder
results in altered currents flowing in various parts of the
feeder for faults at different points on the feeder. The
primary concerns for DG interconnection are typically
sympathetic tripping issues, failure of fuse-saving
schemes, and reduction of reach - potentially resulting in
undet