International Conference on Electrical, Electronics, and Optimization Techniques (ICEEOT) - 2016
Load Balancing and Voltage Regulation for Two Leg
VSC Based Islanded DG Set Supply System
V.Rajagopal1,J.Bangarraju2
V.Sharada1 and K.Sravanthi1
EEE Department
B V Raju Institute of Technology2
Narsapur, Medak(Dist),Telangana,India2
rajsarang@gmail.com1, rajujbr@gmail.com2
EEE Department
Stanley College of Engineering and Technology for
Women, Abids, Hyderabad, Telangana, India1
vsharada@stanley.edu.in1, ksravanthi@stanley.edu.inl
Abstract— This paper discusses the unit template control
strategy for load balancing and voltage regulation of induction
generator based DG (Diesel generator) set for islanded system.
The proposed system consists of DG set along with DSTATCOM
(Distributed Static Compensator) and BESS (Battery Energy
Storage System) is used for load balancing along with harmonics
elimination and reactive power compensation. An isolated T-T
Connected transformer is used to reduce size of two-leg VSC and
also neutral current compensation. The battery on two-leg VSC
(Voltage Source Converter) of DSTATCOM with BESS is able to
supply active power when the load requirement is more than
rating of DG set and battery can store energy when load
requirement is less than DG set rating. The reference sources for
two-leg VSC are estimated using unit template control strategy
and this algorithm has quick dynamic response. The proposed
DG set system with DSTATCOM is modeled in MATLAB
R2009a along with SIMULINK and results are verified under
three phase linear/non-linear loads.
Keywords—Unit Template Control Strategy, Voltage
Regulation, Load balancing, Power Quality, DG Set Supply System.
I.
INTRODUCTION
In the recent years, due to the increasing requirement of
electrical energy and fast reduction in fossil fuels in the nation,
the necessity of isolated standalone DG systems becomes
inevitable. Power generation from wind, small hydro heads
and solar sources in the isolated areas is the other alternative
method for eco-friendly energy generation [1]-[2].
Asynchronous Generators have received much attention in
WECS (Wind Energy Conversion Systems) because of low
maintenance cost, self-excitation, high efficiency and high
power factor. These are mostly variable speed WECS and
these systems convert ac-dc-ac to achieve fixed frequency and
voltage for islanded standalone system. An attention is to be
paid on fixed speed operation in the applications of DG Sets
[3].
The DG sets are used in many electrical applications such
as remotely located sparsely populated areas, moving trains,
backup source of supply etc. The wound-field synchronous
generators are most commonly used in DG sets [4]. These
wound-field synchronous generators have a separate excitation
system for voltage control and a speed governor system for
frequency control. The PMSG in the DG set supply system is
978-1-4673-9939-5/16/$31.00 ©2016 IEEE
a good alternative for isolated generation [5]. In recent years
the technology advancement in the area of power converters
has made it possible to control voltage of DG set supply
system [6]-[7].
DE (Diesel Engine) which has constant speed prime mover
generates constant frequency. The main challenge in
asynchronous generator is to maintain terminal voltage
constant in the DG set system. In literature it is reported that
the DG sets uses ac-dc-ac converters with capacitors (as
energy storage elements) for voltage control [8]-[11].
However, a few researchers reported in the literature that the
use of DSTATCOM with BESS or without BESS is used
instead of ac-dc-ac converters in DG sets for power quality
improvements. The proposed DSTATCOM has been used for
load balancing, voltage regulation, load leveling, and load
compensation in islanded DG set supply system[12]-[13].
The switching pulses for two-leg VSC based DSTATCOM
are generated using various control strategies. In this paper
unit template control strategy is used due to its simplicity and
fast response. The main problem in DG set system is that
terminal voltage falls below rated value with the increase in
the load condition [14]. In the proposed unit template control
strategy based DSTATCOM with BESS is show effective
solution for load balancing, voltage regulation, neutral current
compensation and improves fuel efficiency of DG in the
fluctuating loads [15]. The neutral current compensation is
obtained using isolated T-T connected transformer and to
reduce size of two-leg VSC [16]-[17]. The proposed DG set
system with two-leg VSC based DSTATCOM is modeled in
MATLAB/ SIMULINK and results of islanded DG system is
verified under linear/non-linear loads.
II.
SYSTEM CONFIGURATION AND PRINICIPLE OF
OPERATION
Fig.1 shows schematic diagram of DG set system as prime
mover with two-leg VSC (Voltage Source Converter) based
DSTATCOM and BESS. The proposed DG system feeding to
different types of linear/nonlinear loads and the three
interfacing inductors (Lf) are connected between VSC and
PCC (Point of Common Coupling) to eliminate current
ripples. The reference source currents for two-leg VSC based
DSTATCOM using unit template control strategy to improve
fuel efficiency of diesel generator set. Based on loading of
DG set the reference source currents are generated using unit
template control strategy. An isolated T-T Connected
transformer is connected at PCC to reduce size of two-leg
VSC and neutral current compensation.
Fig. 2 shows T- T connected Transformer and its phasor
diagram for two-leg VSC. The isolated transformer provides a
path for harmonic current, fundamental zero sequence current
and neutral current compensation at PCC. The phasor diagram
of Fig.2 gives relations turns ratio of T-connected transformer.
The phase winding voltages (va1 and vb1) of T-connected
transformer and va is resultant voltage then
va1 = k1va ; vb1 = k2 va ;
(1)
Assuming |va| = |vb| = v and va1= vacos30o, vb1= vasin30o, from
equation (1) we get k1=0.8660 and k2 = 0.50 then the line to
line voltage vca=415V.
(2)
v = v = v = 415
= 239.6V
a
b
c
3
v a 1 = 207.4V , vb1 = 119.8V ;
(3)
vtp = ( 2 3 ) *( va2 + vb2 + vc2 )
1
Hence transformer 5kVA,240/120/120V and 5kVA, 208/208V
are selected for two single-phase transformer.
III.
A. In Phase Component of Reference source currents for DG
set
A fixed current is given as the magnitude of the active power
component of source current (IGsd*). The magnitude of inphase unit templates (upa, upb, upc) are estimated using three
phase voltages(va, vb, vc) of DG set divided by magnitude of
terminal voltage (vtp).
(4)
u pa = va vtp ; u pb = vb vtp ; u pc = vc vtp ;
The magnitude of terminal voltage (vtp) of DG set is estimated
as
vtp = ( 2 3 ) * ( va2 + vb2 + vc2 )
2
u pb = vb vtp ;
u pc = vc vtp ;
3 + u pc
1
2
(5)
The in-phase unit templates are multiplied with magnitude of
u pa = va vtp ;
wpa = −u pb
CONTROL STRATEGY
3;
wpb = 3u pa 2 +(u pb − u pc ) 2 3;
wpc = − 3u pa 2 +(u pb − u pc ) 2 3;
Fig.1 System configuration of DG set with DSTATCOM based on unit template control strategy.
active power component of DG source current (IGsd*) to
estimate in-phase component current of active power (isda*,
isdb*) are
*
*
*
*
(6)
isda
= I Gsd
u pa ; isdb
= I Gsd
u pb ;
C. Generation of Reference Source Currents for DG Set
The total reference source currents (isa*, isb*) for
DSTATCOM are the summation of quadrature component
currents (isqa*, isqb*) and in-phase component currents (isda*,
isdb*).
*
*
isa* = isqa
+ isda
; i sb* = i sq* b + i sd* b ;
(12)
The difference in reference source currents (isa*, isb*) for
DSTATCOM is compared with sensed source currents (isa isb)
for DSTATCOM are considered and these errors currents of
DSTATCOM are compared with fixed triangular wave
(frequency of 10 kHz) to generate switching pulses for two-leg
VSC.
IV.
Fig. 2 T – T Connected Transformer and its phasor diagram
B. Quadrature Component of Reference source currents for
DG set
The AC terminal voltage of DG set is regulated using
difference between magnitude of actual terminal voltage (vtp)
and reference terminal voltage (vref) is taken as AC bus
voltage error is given as AC bus PI. The voltage error (vret) is
magnitude of AC voltage at kth sample instant as
(7)
vret ( k ) = −vtp ( k ) + vref ( k )
The output of AC bus PI Controller (I*seq) decides the
magnitude of reactive current generated for DG system
I *seq ( k ) = I *seq ( k −1) + K vp {vret ( k ) − vret ( k −1) } + Kvi vret ( k )
(8)
Where Kvp and Kvi are the PI controller gains of terminal
voltage .The vret(k) and vret(k-1) are voltage errors at kth and (k1)th instants.
The magnitude of quadrature phase unit templates (wpa, wpb)
in unit template control strategy are derived from in-phase unit
templates (upa, upb, upc).
(9)
w pa = − u pb 3 + u pc 3 ;
wpb = 3u pa 2 + (u pb − u pc ) 2 3;
MODELING OF DG SET
A model of proposed asynchronous generator based DG
system with interfacing inductors, Isolated T – T connected
transformer, two-leg VSC based DSTATCOM, excitation
capacitor bank, linear/non-linear loads and unit template
control strategy are developed in MATLAB R2009a is carried
out in the discrete mode with fixed step size of 5e-6 with
ode23tb solver. The modeling and simulation of 4-pole,
7.5KW, 415V, 50Hz of DG set and a excitation capacitor bank
(4.6 kVAR rating). The proposed DG set supply system is
realized as two-leg VSC with a battery at the DC bus and
linear/non-linear loads are taken to demonstrate capabilities of
the controller.
(10)
The quadrature phase unit templates (wpa, wpb) are multiplied
with magnitude of reactive power component of DG set source
current (I*seq) to estimate quadrature-phase component current
of reactive power (isqa*, isqb*) are
*
*
*
*
(11)
isqa
= I seq
w pa ; isqb
= I seq
w pb ;
V.
RESULTS AND DISCUSSION
The performance of unit template control strategy for twoleg VSC based Islanded DG set system feeding linear/nonlinear 3-phase loads are depicted in Fig.3-Fig.5.
A. Performance of two-leg VSC based Islanded DG Set with
load balancing for linear loads.
Fig.3 shows the performance of two-leg VSC based
Islanded DG set for load balancing for linear loads. The Fig.3
shows waveforms of DG Set System of terminal voltage (vabc),
DG set source currents (isabc), excitation capacitor current
(icca), load current (iLr, iLy, iLb),
isolated T-connected
transformer currents (icon), neutral load current(iLn), battery
bank current (ib), DG Set terminal voltage magnitude(vt), DC
Capacitor voltage (‘vdc1’), DC Capacitor voltage (‘vdc2’) , DG
set Powers at source (PG), load power (PL). A 3.5kW of singlephase load is connected DG Set and battery bank supply
shortage power to the load. At t= 4.2 seconds one of the phase
is suddenly removed in the three phase linear load and DG set
makes to charge the battery bank with the extra power. At
t=4.3 seconds other phase non-linear load is also removed and
DG set makes to charge the battery bank with the excess
power. At t=4.4 seconds all the three phases are disconnected
and DG set feeding all the power to battery bank. The
proposed isolated T–T connected transformer exhibits
unbalanced currents and this transformer provide neutral
current compensation. It was observed from Fig.3 during t =
4.15 seconds to 4.45 seconds the DG source voltage (vabc) and
DG set source currents (isabc) are balanced and harmonic free. It
was also observed that DG Set terminal voltage magnitude (vt)
is maintained constant at 339.1V. Two dc bus voltages (‘vdc1’
and ‘vdc2’) are also maintained at 200V.
B. Performance of two-leg VSC based Islanded DG Set with
load balancing for non-linear loads.
Fig.4 shows the performance of two-leg VSC based Islanded
DG set for load balancing for non-linear loads. The Fig.4
shows waveforms of DG Set System of terminal voltage
(vabc), DG set source currents (isabc), excitation capacitor
current (icca), load current (iLr, iLy, iLb), isolated T-connected
transformer currents (icon), neutral load current(iLn), battery
bank current (ib), DG Set terminal voltage magnitude(vt), DC
Capacitor voltage (‘vdc1’), DC Capacitor voltage (‘vdc2’), DG
set Powers at source(PG), load power(PL). At t= 4.2 seconds
one of the phase is suddenly removed in the three phase nonlinear load and DG set makes to charge the battery bank with
the extra power. At t=4.3 seconds other phase load is also
removed and DG set makes to charge the battery bank with the
excess power. At t=4.4 seconds all the three phases are
disconnected and DG set feeds all the power to battery bank.
The proposed isolated T-connected transformer exhibits
unbalanced currents and this transformer provides neutral
current compensation. Fig.5 (a) shows the DG set source
voltage (vabc) waveform with fundamental voltage magnitude
of 340.3V and has %THD of 0.92%, Fig.5 b) shows the DG
Fig.3 Performance of Two-leg VSC based Islanded DG Set System with feeding linear loads.
set source current (isa) waveform with fundamental current
magnitude of 53.4A and has %THD of 1.70%, Fig.5 c) shows
the non-linear load current (iLa) waveform with fundamental
current magnitude of 36.66A and has %THD of 60.50%. The
above results of proposed two-leg Islanded DG system shows
%THD of voltage (vabc) and current (isa) are within 5% limit of
IEC and IEEE standards. It was observed from Fig.4 that
during t=4.15seconds to 4.45 seconds the DG source voltage
(vabc) and DG set source currents (isabc) are balanced and
harmonic free. It was also observed that DG Set terminal
voltage magnitude (vt) is maintained constant at 339.1V and
also two-dc bus voltages (‘vdc1’ and ‘vdc2’) are also maintained
at 200V.
VI.
CONCLUSION
The performance of two-leg VSC based Islanded DG set
for load balancing and voltage regulation for different load
conditions (linear/non-linear loads) are modeled and simulated
in MATLAB R2009a. It was observed that DG set gives
satisfactory results at different load conditions. A Unit
Template Control Strategy is used for DG Set to validate the
performance results. The proposed Unit Template Control
Strategy base DG set improves power quality such as load
balancing, voltage regulation, neutral current compensation
and current harmonics mitigation. The isolated T-connected
Transformer is used to mitigate neutral current. The main
Fig.4 Performance of Two-leg VSC based Islanded DG Set System with feeding non-linear loads.
advantage of two-leg VSC is it reduces cost compared to
three-leg VSC. The performance results of DG system shows
%THD of voltage (vabc) and current (isa) are within 5% limit of
IEC and IEEE standards.
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