Download International Electrical Engineering Journal (IEEJ) Vol. 5 (2014) No.11, pp. 1619-1625

International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.11, pp. 1619-1625
ISSN 2078-2365
http://www.ieejournal.com/
Energy Capture Maximization from
Distributed Generators in Rural
Distribution Networks
S. Sunisith 1, N. Sridhar Reddy2, K S Mann3
[email protected], [email protected], [email protected]

Abstract—Voltage control problems are reported as one of the
main obstacles against installation of large amounts of
distributed generation. This research work is aimed to provide
a voltage within the tolerable limits at all the nodes in the system
when the system is connected with the distributed generation.
The modern approach using consumer load control is discussed
and compared with the existing methods using a simulation case
study. Several methods like reducing primary substation
voltage, constraining the generator operation are discussed. The
voltage rise is more onerous when there is no demand on the
system. In the proposed work, an algorithm for load flow study
of radial distribution system is considered. Using the
forward/backward sweep method, for the given typical radially
tapered network voltage magnitude and phase angle at each bus
are determined. This research discusses about the factors
influencing voltage rise in a system like the connection point of
the generator, rating of distributed generator, and network
characteristic.
1.1. IMPACTS OF DG ON POWER SYSTEM
In recent years there has been a considerable
increase in the amount of generation connected to
distribution networks. This has largely arisen due to
environmental legislation encouraging the development of
renewable generation based on wind, hydro or waste fuels,
and a growth in industrial Combined Heat and Power (CHP).
Modern distribution systems are designed to accept bulk
power at the bulk supply transformers and to distribute it to
customers. Thus the flow of both real power (P) and reactive
power (Q) was always from the higher to lower voltage level
and even with interconnected distribution system, the
behavior of the network is well understood and the
procedures for both design and operation long established.
Index Terms—Distribution Network, Load flow methods,
Distributed generator.
I.
INTRODUCTION
Electrical distribution networks are designed to
present the users with a reasonably constant voltage. Load
current passing through the system causes increasing voltage
drop with increasing load. This drop is compensated by
transformers which can boost their output voltage by
changing their transformation ratio in response to the load.
This method, using on-load tap changers, compensate for any
drop in the input voltage to the transformer. Grid and primary
transformers employ the method. At lower voltage levels,
typically where the output voltage is 11 kV, the output
voltage is also boosted as the load increases to compensate
for voltage drops between the transformer and the customer.
The degree of boost depends on an assumption or estimate of
the system impedance between the two, and implicitly
assumes no significant amount of embedded generation.
11kV/400V transformers may have manual off-line
tap changers, usually set at installation, to a system design
which makes certain assumptions, a radial system and
unidirectional power flow.
Fig. 1.1 Distributed Systems with Embedded Generation
However, with significant penetration of embedded
generation the power flows may become reversed and the
distribution network is no longer a passive circuit supplying
loads but an active system with power flows and voltages
determined by the generation as well as the loads. This is
shown schematically in Fig. 1.1, for example the combined
Heat and Power (CHP) scheme with the Synchronous
generator (S) will export real power when the electrical load
of the premises falls below the output of the generator but
may absorb or export reactive power depending on the setting
of the excitation system of the generator. The wind turbine
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Sriramoju Sunisith
et. al.,
Energy capture maximization from distributed generators in rural distribution networks
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.11, pp. 1619-1625
ISSN 2078-2365
http://www.ieejournal.com/
will export real power but is likely to absorb reactive power
as induction (some times known as asynchronous) generator
(A) requires a source of reactive power to operate. The
voltage source converter of the photovoltaic (PV) system will
allow export of real power at a set power factor but may
introduce harmonic currents, as indicated in Fig. 1.1 thus the
power flows through the circuits may be in either direction
depending on the relative magnitudes of the real and reactive
network loads compared to the generator outputs and any
losses in the network.
There are many technical issues that must be
considered when connecting a generating scheme to the
distribution system, such as:
i.
Steady-state voltage rise
ii.
Thermal rating of equipment
iii. System fault levels
iv.
Stability
v.
Reverse power flow capability of tap-changers
vi.
Line-drop compensation
vii.
Losses
viii. Power quality (such as flicker, harmonics)
ix. Protection.
The Electricity Supply Regulations stipulate that,
unless otherwise agreed, the steady-state voltage of systems
between 1000V and 132kV should be maintained within ±6%
of the nominal voltage. For systems above 50V and below
1000V, variations of between +l0% and -6% of nominal
voltage are permitted. However, at the planning stage, the
11kV system is often designed to maintain voltages within
±3% of nominal, so that the voltage variations seen by the LV
connected customers remain within the permitted +l0% and
-6% limits.
1.2. EFFECT OF CONNECTING GENERATION TO
DISTRIBUTION SYSTEMS
Connecting a generator to the distribution system
will affect the flow of power and the voltage profiles. To
export its power, a generator is likely to have to operate at a
higher voltage than the primary substation, unless it is able to
absorb a significant amount of reactive power. This is
explained using Fig. 1.2.
Fig. 1.2 Distribution systems with embedded generation
where,
VPS is the primary substation voltage,
Vgen is the voltage at the generator point,
R, X are the resistance and reactance of the line,
P, Q are the active power and reactive power transmitted
from the generator to the over head line.
V  Vgen  VC 
RP  XQ
Vgen
(1.2)
As the X/R ratio of the 11kV line is small, neither
RP nor XQ is negligible. The XQ term may be positive or
negative, depending on whether the generator is exporting or
importing reactive power. However, as the magnitude of the
reactive power will be small compared to that of the power
(unless some form of compensation is used), the RP+XQ
term will tend to be positive. Thus, the voltage at the point of
connection of the generator to the 11kV system will rise
above that of the primary substation. The voltage rise is more
onerous when there is no demand on the system, as all the
generation is exported back to the primary substation. When
connecting a generator to the distribution system, a DNO
must consider whether the power may be exported back
through the primary substation and must ensure that the
transformer's tap-changers are capable of operating with a
reverse power flow. And this voltage rise depends on the
many factors such as
i.
Location of the generator from the substation
ii.
Line parameter values(X/R ratio of the line).
iii.
Network characteristics.
iv.
Size of generator.
v.
Type of the source used for generation.
II.
DISTRIBUTION LOAD FLOW
2.1. DISTRIBUTION LOAD FLOW METHOD
Load-flow studies are probably the most common of all
power system analysis calculations. They are used in
planning studies to determine if and when specific elements
will become overloaded. Major investment decisions begin
with reinforcement strategies based on load-flow analysis. In
operating studies, load-flow analysis is used to ensure that
each generator runs at the optimum operating point; demand
will be met without overloading facilities; and maintenance
plans can proceed without undermining the security of the
system.
The objective of any load-flow program is to produce the
following information:
 Voltage magnitude and phase angle at each bus.
 Real and reactive power flowing in each element.
 Reactive power loading on each generator.
2.2 FORWARD /BACKWARD SWEEP METHOD
Different techniques have been developed to solve
radial distribution network. One of such technique is
forward/backward sweep method. In forward /backward
sweep algorithm, regardless of its original topology, the
distribution network is first converted to a radial network.
Hence, an efficient algorithm for the solution of radial
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Sriramoju Sunisith
et. al.,
Energy capture maximization from distributed generators in rural distribution networks
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.11, pp. 1619-1625
ISSN 2078-2365
http://www.ieejournal.com/
networks is crucial to the viability of the overall solution
method. The solution method used for radial distribution
networks is based on the direct application of the KVL and
KCL. For our implementation, A branch oriented approach
using an efficient branch numbering scheme is developed to
enhance the numerical performance of the solution
method.
2.3
BRANCH NUMBERING
In contrast to all classical power flow techniques which use
nodal solution methods for the network, this algorithm is
branch-oriented. Figure 2.1 shows a typical radial
distribution network with ‘n’ nodes, ‘b’ (=n-1) branches and
a single voltage source at the root node. In this tree structure,
the node of a branch L closest to the root note is denoted by
L1 and the other node by L2. Number the branches in layers
away from the root node as shown in Figure 2.2. The
numbering of branches in one layer starts only after all the
branches in the previous layer have been numbered. This
numbering scheme is very simple and straight forward and
has been implemented in power flow program.
2.3 ALGORITHM
SWEEP METHOD
FOR FORWARD/BACKWARD
Given the voltage at the root node and assuming a flat
profile for the initial voltages at all other nodes, the
iterative solution algorithm consists of three steps:
1. Nodal current calculation: At iteration k, the nodal
current injection, I i(k) , at network node i is
calculated as,
Ii(k)= (Si/Vi(k-1))* - YiVi(k-1) ,
i= 1,2,3…,n
(2.1)
where Vi(k-l) is the voltage at node i calculated during
the (k-l)th iteration and Si is the specified power
injection at node i. Yi is the sum of all the shunt
elements at the node i.
2. Backward sweep: At iteration k, starting from the
branches in the last layer and moving towards the
branches connected to the root node the current in
branch L, JL , is calculated as:
JL(k)= -IL2(k) + Σ( currents in branches emanating from
node L2),
L=b,b-1,…,1
(2.2)
3. Forward sweep: Nodal voltages are updated in a
forward sweep starting from branches in the first
layer toward those in the last. For each branch,
L, the voltage at node L2 is calculated using the
updated voltage at node L1 and the branch current
calculated in the preceding backward sweep:
VL2(k)= VL1(k) – ZLJL(k) ,
L=1,2,…,b
(2.3)
Where, ZL is the series impedance of branch L. This
is the direct application of the KVL. Steps 1, 2 and 3 are
repeated until convergence is achieved.
Fig. 2.1 Typical radial distribution network
2.4 CONVERGENCE CRITERION
The maximum real and reactive power mismatches at
the network nodes as convergence criterion. As described
in the solution algorithm, the nodal current injections, at
iteration k, are calculated using the scheduled nodal power
injections and node voltages from the previous iteration
equation (2.1). The node voltages at the same iteration are
then calculated using these nodal current injections equations
(2.2) and (2.3). Hence, the power injection for node i at kth
iteration, Si(k) is calculated as:
Si(k) = Vi(k) ( Ii(k) )* - Yi | Vi(k) |2
Fig.2.2 Branch numbering of the radial distribution network
(2.4)
The real and reactive power mismatches at bus i are then
calculated as:
ΔPi(k) = Re[ Si(k) –Si]
ΔQi(k)=Im[ Si(k) –Si], i= 1,2,…,n (2.5)
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Sriramoju Sunisith
et. al.,
Energy capture maximization from distributed generators in rural distribution networks
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.11, pp. 1619-1625
ISSN 2078-2365
http://www.ieejournal.com/
III. HYBRID METHOD OF CONTROL
3.1 HYBRID METHOD OF CONTROL
Since in a machine the magnetizing current lags the
supply voltage by 900 .In view of this, the excess flux can be
counter balanced only if the armature supplies a magnetizing
current or lagging reactive VA to the system. Consequently
the synchronous machine (generator) operates at a leading
power factor. It can be inferred from the above discussion
that a synchronous machine (generator) operates at a leading
power factor when under excited and at lagging power factor
when over excited. The extents of power factor lag or power
factor lead, depend upon the degree of over excitation or
under excitation.
3.2 AUTOMATIC VOLTAGE - POWER FACTOR
CONTROL (AVPFC)
Considering a basic 2-busbar system such as the one
shown in Fig. 3.1, the operating constraints that the network
operator imposes on the DG (i.e. ranges of permissible
voltage and power factor values) are shown in the voltage
vector diagram of Fig. 3.2.
out on its constant power factor line) until the bus bar voltage
exceeded a threshold voltage within the statutory limits. At
that point APFC could be replaced (smoothly) with AVC to
vary excitation and move the operating point around the
constant voltage circle within the bus bar overvoltage limit.
Increased local load and reduction in terminal voltage could
take the system back into APFC by sensing the increased
demand for reactive power and therefore the reduction of
power factor, as could variation in network impedance. This
system would also enable voltage support at times of heavy
local load. On reaching a voltage just above the under
voltage limit the excitation system would be transferred to
AVC to maintain local voltage. Excitation under and
over-voltage and over-current protection should protect the
EG excitation and control system against prolonged forcing.
3.2.1 OPERATION OF AVPFC
The block diagram of AVPFC is shown in Fig.3.3
wherein separate auto-voltage and auto-PF controllers
operate alternately dependent on the voltage at the generator
terminals. The output of either the voltage or power factor
controller is connected by V/PF switching logic to the
raise/lower controls of the motorized voltage setting
potentiometer and sets the reference voltage for the
Automatic Voltage Regulator (AVR). This is an established
practice. Alternatively in some applications the power factor
controller adjusts Vref by injecting a current through the
whole potentiometer winding, which provides faster
response.
Fig.3.1 Basic 2-bus bar system
In Fig.3.2 Vmin and Vmax are the minimum and
maximum acceptable voltages and PFmin and PFmax are the
lines of constant minimum and maximum power factor,
respectively. The allowable area of operation is the shadowed
region between these boundaries. The block diagram of a
possible system that restricts operation of the synchronous
generator within the shadowed area is shown in Fig.3.3.
Fig. 3.3 Block Diagram of AVPFC
3.2.2 DETERMINATION OF AVPFC MODE
SELECTION RULE SET
The control block in Fig. 3.3 is responsible for the
control mode decision making. The system will attempt to
regulate the DG operation around one of three possible set
points: the power factor setting PFref, and the high-or
low-voltage setting Vh and Vl, respectively. Table 3.1 shows
the heuristics of the control algorithm, the reference value
which is applied to the controller for every combination of
voltage and power factor conditions. In order to prevent
hunting, a dead band exists around the decision thresholds.
Fig. 3.2 Voltage vector diagram for the two-bus bar system
After synchronizing the DG could be brought
towards capacity in constant power factor mode (travelling
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Sriramoju Sunisith
et. al.,
Energy capture maximization from distributed generators in rural distribution networks
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.11, pp. 1619-1625
ISSN 2078-2365
http://www.ieejournal.com/
Table 3.1: AVPFC mode selection rule set
3.3 FLOW CHART
Voltage dead band is annotated as VD and power
factor dead band is PFD, where:
VB = V l - VD
VT = V h + V D
PFB = PFref + PFD
and
PFT =PFref + PFD
When voltage is within the statutory limits, the DG
is in APFC mode and tries to set the power factor at PF ref.
When a voltage higher than VT is detected and at the same
time the power factor is equal to or higher than PFB, the
controller will switch to AVC and will try to adjust V2 to Vh.
However, if the power factor is lower than PFB the controller
will operate in APFC, because its attempt to raise the power
factor will result in lowering the terminal voltage (and thus
the exported reactive power).This way it is ensured that the
system will never lock into one of its states. The inverse
procedure is followed when the terminal voltage is lower than
VB.
IV. TEST RESULTS
The voltage profiles of the radially tapered network
without DG, with DG, with reduced output DG are presented
in this Chapter. In order to make assessment, the simulations
are carried out for the radially tapered network shown in Fig
4.1.
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Sriramoju Sunisith
et. al.,
Energy capture maximization from distributed generators in rural distribution networks
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.11, pp. 1619-1625
ISSN 2078-2365
http://www.ieejournal.com/
Fig. 4.3 Voltage profile of radially tapered network with DG at bus 4
Fig 4.4 Voltage Profile with reduced output DG connected at bus 4
Fig. 4.1 Typical Radial Feeder
Fig 4.5 Voltage profile of radially tapered network with DG at bus 3
Fig. 4.2 Voltage profile of radially tapered network with DG at bus 4
Fig 4.6 Exported Real Power (AVPFC)
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Sriramoju Sunisith
et. al.,
Energy capture maximization from distributed generators in rural distribution networks
International Electrical Engineering Journal (IEEJ)
Vol. 5 (2014) No.11, pp. 1619-1625
ISSN 2078-2365
http://www.ieejournal.com/
larger capacities of new generation to be connected without
the need for network upgrade. Comparison between two
controllers AVC and AVPFC can be observed by using the
graphs.
REFERENCES
Fig. 4.7 Exported Reactive Power (AVPFC)
Fig. 4.8 SG Power Factor (AVPFC)
Fig. 4.9 Terminal Voltage (AVPFC)
V.
CONCLUSION
In this, a hybrid control algorithm that combines
automatic voltage and power factor control has been
presented and compared to one of the traditional control
method, Automatic Voltage Control (AVC). A hybrid
voltage / power factor control scheme of the DG may be a
viable alternative since it was shown that can maintain
voltage within a predefined band with no extra equipment
installation required by the DNO. An existing generator
would then be allowed to stay connected for longer periods,
resulting in increased power export, and would also allow
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Energy capture maximization from distributed generators in rural distribution networks