Regenerative loading system 2

International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 4, July-August (2013), © IAEME
213
REGENERATIVE LOADING SYSTEM
Navonita Sharma1
, Abanishwar Chakrabarti2
, Prabir Ranjan Kasari3
, Bikram Das4
1
Electrical engineering M.Tech National Institute of Technology, Agartala
2
Department of Electrical Engineering, National Institute of Technology, Agartala
3
Electrical Engineering, National Institute of Technology, Agartala
4
Department of Electrical Engineering, National Institute of Technology, Agartala
ABSTRACT
In this paper, the concept of a” REGENRATIVE LOAD”, is proposed. In this proposal a
converter system is designed to make a regenerative load set, that can emulate various active and
reactive power load and their various combinations, and also capable of regenerating the consumed
power by the load back to the grid. The vector control approach is used for independent control of
active and reactive power. The proposed scheme is simulated in MATLAB simulink and gives
satisfactory result.
I. INTRODUCTION
Testing of machine is limited by the size of load and associated cost of energy. Since it is not
feasible or economical to test machine on full load, different methodologies are developed to
evaluate the performance characteristics of machine. These test methods such as no load test, short
circuit test, back to back test, Z.P.F test etc gives approximate result. The converter proposed in this
paper is capable of loading a machine up to the ratings of the converter. The power consumed by the
converter is feedback to the grid as a result the net power consumed and hence energy cost is very
low. Such a loading arrangement may find wide spread application in laboratories where there is
diversified requirement of load.
INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &
TECHNOLOGY (IJEET)
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 4, Issue 4, July-August (2013), pp. 213-224
© IAEME: www.iaeme.com/ijeet.asp
Journal Impact Factor (2013): 5.5028 (Calculated by GISI)
www.jifactor.com
IJEET
© I A E M E

214
Fig1: Basic Block Diagram of Project model.
The variable regenerative load proposed here consists of a rectifier and an inverter. The
rectifier acts as the load to the machine and the power consumed by the load is controlled by
controlling Iq (Quadrature axis current) signal. The inverter acts as the regenerating part, and
regenerates the consumed power back to the source/grid. The concept of this loading arrangement is
derived from the concept of U.P.F.C (Unified Power Flow Controller) which feed reactive power to
the grid. UPFC is primarily used for independent control of real and reactive power in transmission
lines for a flexible, reliable and economic operation and loading of power system [1]. The
independent control of active and reactive power is done using the vector control approach in dq
reference frame [2]. To convert a 3 phase quantity to synchronously rotating d-q reference frame,
and vice versa, Park’s transform and Inverse Park’s transform is used [3]. For converting the 3 phase
parameters in dq frame a unit vector (sinθ, cosθ) is generated. The unit vector rotates at the
synchronous speed along with the dq reference frame. Various methods are there to generate unit
vector, the one which is used in the paper is using LPF (Low pass filter) [2] and PLL (Phase Locked
Loop) [4]. Both current and voltage are converted into dq reference frame and d-axis and q-axis
current controls the reactive and active power respectively.
The paper is articulated as follows: Section (II) deals with the theory of the proposed idea.
Section (III) gives the introduction to synchronously rotating frame. Section (IV) deals with unit
vector generation. Design of controller is discussed in Section (V). Section (VI) deals with converter
schematic and feed forward terms. Section (VII) shows current and voltage controller design
procedure. Simulation results and analysis is given in Section (VIII). Conclusion and Index is given
in Section (IX) and Section (X) respectively.
II. THEORY
Power flow between any two electrical sources is governed by the equations [5]:
s i ni j
i j
V V
P
X
δ
= (1)
2
c o si ji
ij
V VV
Q
X X
δ
= − (2)
( )i jV Vδ = ∠ − (3)

215
From these equations it is clear that, for power flow between two sources, there must be a
voltage difference or a phase angle difference between them. It is well known that the active power
flows from the leading voltage to the lagging voltage and the reactive power flows from the higher
voltage to the lower voltage. Therefore, both active power and reactive power can be controlled by
controlling the phase and magnitude of the converter voltage fundamental component with respect to
the grid voltage. But phase angle and voltage is present in both equation of active and reactive
power, thus there exist a coupling between them, thus to control active and reactive power
independently, the above equation cannot be followed since varying voltage or phase angle will vary
both active and reactive power. In order to control active and reactive power independently, vector
control approach has been taken into account.
Vector control is a popular method for speed control of three-phase induction motors, by
converting the 3 phase parameters to dq reference frame. The basic idea of vector control scheme is
to control the flux producing component (direct axis current) and the torque-producing component
(quadrature axis current) in a decoupled manner. Keeping analogy with the above convention, in this
paper, the current component is divided into its direct axis and quadrature axis component. The
control of quadrature axis current will control the active power and control of direct axis component
will control the reactive power. The equations of active and reactive power using direct and
quadrature axis components are follow [2]:
2
3
s q s qP v i= (4)
2
3
s q s dQ v i= (5)
III. SYNCHRONOUS ROTATING FRAME (D-Q AXIS)
The DQ transformation is a transformation of coordinates from the three-phase stationary
coordinate system to the dq rotating coordinate system.
Let a three phase voltage is represented by these following equations:
2 c o sa sV V tω= (6)
2 cos( 120 )o
b sV V tω= − (7)
2 cos( 240 )o
c sV V tω= − (8)
The transformation from three phases to dq is made in two steps [3]:
1). A transformation from the three-phase stationary coordinate system to the two-phase, so-called
ab/αβ, stationary coordinate system:
3 3
2 c o s
2 2
a sV V V tα ω= = (9)
3 3
( ) 2 sin
2 2
b c sV V V V tβ ω= − = (10)
2) A transformation from the ab/αβ stationary coordinate system to the dq rotating coordinate
system:
( c o s sin )dV V Vα βθ θ= + (11)
( sin cos )qV V Vα βθ θ= − + (12)
,
( )d
w h e r e
V V αθ = ∠ −
(13)

216
Fig2 [2]: Stationary and synchronously revolving frames of reference.
The inverse transformation equations for dq to ab/αβ to abc transformation is given by [3]:
( cos sin )d qV V Vα θ θ= − (14)
( sin cos )d qV V Vβ θ θ= + (15)
aV V α= (16)
1
( 3 )
2
bV V Vβ α= − (17)
1
( 3 )
2
cV V Vβ α= − + (18)
IV. UNIT VECTOR GENERATION
In order to transform any vector from stationary reference frame into d–q reference frame, the
quantities sin (θ) and cos(θ), which are the components of a revolving unit vector are required. These
quantities should have the same frequency as that of the system voltage.
A. Using LPF Filter
The voltage vector Vs is at an angle ωt with respect to the a-axis. A low-pass filter, whose
corner frequency equals the mains frequency, delays vsa and vsb by 45◦
, two such filters in cascade
delay vsa and vsb by 90◦
. Such filtering and normalization yields cosθ and sinθ, where θ is the angle
between a-axis and d-axis required for the transformation.
Fig3: Block diagram for unit vector generation using LPF

217
Equations to generate unit vector [2]:
3 3
2 cos( ) 2 cos
4 2 4
s sx V t V
π
ω θ= − = (19)
3 3
2 sin( ) 2 sin
4 2 4
s sy V t V
π
ω θ= − = (20)
2 2
2 2
cos
sin
x
x y
NORMALISATION
y
x y
θ
θ
 
= 
+ 
 
 =
 + 
(21)
Simulation results using LPF to obtain cosθ and sinθ:
Fig4: Generation of cosθ and sinθ using LPF.
B. Using PLL
PLL techniques cause one signal to track another one. It keeps an output signal synchronized
with a reference input signal in frequency and phase. In three phase grid connected system, PLL can
be implemented using the d-q transformation and with a proper design of loop filter.
Fig.5 shows the block diagram of three phase PLL, where Vabc is the sensed grid voltage which
is transformed in to DC components using coordinate transformation abc-dq and the PLL gets locked
by setting Vd* to zero. The loop filter PI is a low pass filter. It is used to suppress high frequency
component and provide DC controlled signal to voltage controlled oscillator (VCO) which acts as an
integrator. The output of the PI controller is the grid frequency that is integrated to obtain phase
angle of the converter θ. When the difference between grid phase angle and converter phase angle is
reduced to zero PLL becomes active which results in synchronously rotating voltages Vd = 0 and Vq
gives magnitude of grid voltage.
Fig5: General structure of three phase d-q PLL

218
Simulation results using LPF to obtain cosθ and sinθ:
Fig6: Generation of cosθ and sinθ, using PLL.
ANALYSIS
From Fig4 and Fig6, it is clear that generation of unit vector using PLL gives a much better
response compare to LPF filter, as LPF filter response contains transients from 0 to 0.01 sec
simulation time. In this paper, we have used PLL to generate cosθ and sinθ.
V. DESIGN OF CONTROLLER
The overall block diagram of a vector controlled converter is shown in figure7. The grid
voltages and the line currents are transformed into d–q reference frame, and are used as feedback
variables for the controller as shown in the figure7. The controllers are designed in d–q reference
frame.
Fig7: Block diagram of a vector controlled front-end converter.
As shown in figure 8, the controller has an outer voltage loop to control Vdc. The voltage
controller sets the reference to the inner q-axis current controller. The q-axis current loop controls the
flow of real power P. There is an independent loop for the control of isd, which controls reactive
power as per equations (4) and (5).

219
Fig8: Voltage and current controllers in a vector controlled front-end converter.
VI. CONVERTER SCHEMATIC AND FEEDFORWARD TERMS [2]
Fig9: Schematic of converter.
The voltage equations of the converter in the d–q reference frame are given by (22) and (23),
where Rs and Ls are the resistance and inductance, respectively, of the line inductor.
0sd
s sd s s sq id
di
R i L L i V
dt
ω+ − + = (22)
0sq
s sq s s sd iq
di
R i L L i V
dt
ω+ + + = (23)
A cross coupling exists between the d-axis and the q-axis quantities as seen from the above equations
To ensure decoupled control of isd and isq , feed forward terms vdff and vqff respectively, are added to
the outputs of the d-axis controller and the q-axis controller as shown in figure8, given by the
equations:
* '' '' s sq
d id dff id
L i
v v v v
G
ω
= − + = − + (24)
* '' '' sq s sd
q iq qff iq
v L i
v v v v
G G
ω
= − + = − + − (25)
2
dc
c
V
G
V
= (26)

220
VII. CURRENT AND VOLTAGE CONTROLLER DESIGN
The converter is modeled using its gain G and the delay time Td. The delay Td is equal to half
the time period of the carrier signal. The Bode Plot of the linear transfer function of the model is
plotted and the gain of the PI controller is calculated depending upon the desired gain and phase
margin [6]. The gain margin, phase margin and gains of PI controller of the controlled system are:
Gain Margin: infinite db
Phase Margin: 72.2o
Proportional Gain: 10
Integral Gain: 2
Fig10: Design of inner loop current controller
Similarly with the current controller parameters, the outer loop voltage controller is designed,
using the liner transfer function bode plot of the following block diagram:
Fig11: Design of outer loop voltage controller
2
3
sq
dc sq sq
dc
v
I i K i
V
= = (27)
The Gains of the outer loop PI controller for voltage control are:
Proportional Gain: 100
Integral Gain: 1.3

221
VIII. SIMULATION RESULT AND ANALYSIS
A 2 KVA, 400V machine is loaded using the proposed converter (Simulated in MATLAB simulink).
The system parameters are shown in the table:
Table I: Specification of system parameter
Sl. No. Parameters Value
1. Source Voltage 400V
2. Inductor 0.2 H
3. Dc Link Capacitor 50 uF
4. Converter Gain (G) 326
5. Carrier Frequency Of Sine PWM
Generator (Fsw)
5 KHz
6. Delay Time Td 100us
OBSERVATIONS
A. For a given reference value of Id = 2A and Iq = 6A, we have obtain the following graph of Id and Iq,
which shows that, the reference and output value is almost equal, and both Id and Iq can be controlled
independently.
Fig12: Control of Id and Iq.

222
B. The power flow equation in d-q reference frame I given by equations (4) and (5), thus, with
Vq = 450V, Id = 2A and Iq = 6A, calculated active power P = 1800 watt, and reactive power Q = 600
watt. The following graph shows that the calculated and measured values are almost equal:
Fig13: Active and reactive power flowing from machine to load, controlled using vector control.
C. The inverter regenerates the power consumed by the load and feedback it to the grid. Keeping
id=0, (i.e. the reactive power consumed by the load is fully delivered by the inverter without loading
the grid), and controlling iq with Vdc=600V, we observe that some of the active power is consumed,
by the power electronic devices but the inverter can feedback almost equal active power to the grid,
and the reactive power is zero, as per given reference. The negative value of the power shows that, it
is being feedback.
Fig14: Active and reactive power being feedback from load grid.

223
OBSERVATIONS TABLE
Table II: Control of Id and Iq from machine to load side
REFERENCE OBSERVED
Id (A) Iq (A) Id (A) Iq (A)
2 6 2 6
Table III: Power flow from machine to load side
Calculated Power
(W)
Measured
Power (W)
Deviation (%)
P Q P Q P Q
1800 600 1740 605 3.33 0.8
Table IV: Power flow from load to grid side
Desired Power To
Be Feedback (W)
Actual Feedback
Power (W)
Deviation (%)
P Q P Q P Q
-1800 0 -1710 0 5.0 0
IX. CONCLUSION
A variable regenerative load, using a power electronic converter is designed, and controlled
using vector control approach. The design of controllers is based on the specification of the system
parameters given in table I. PI controllers gain has been calculated using BODE PLOT method. The
vector control method is used for independent control of active and reactive power from machine to
load and load to source. The controller is designed to give high accuracy that can be seen from the
output. The machine is load using various reference values of Id and Iq and the converter give
satisfactory output under all conditions employed in the simulation.
X. INDEX
Vi = voltage of ith
source.
Vj = voltage of jth
source.
Pij = Active power
Qij = Reactive power
X = reactance
vsq = quadrature axis voltage component.
isq = quadrature axis current component.
isd = direct axis current component.
Rs = Resistance of Front-End converter.
Ls = Inductance of Front-End converter.
G= Gain of converter.
Vs = source voltage
Td = converter time delay
C0= dc link capacitor
fsw= carrier frequency of sine PWM generator.
Vc = the peak of the triangular carrier used in PWM.
vdff = d- axis feed forward term.
vqff = q-axis feed forward term.

224
XI. REFERENCES
[1]. Higorani, N.G., Gyugyi,L., Understanding FACTS Devices, IEEE Press 2000.
[2]. J S Siva Prasad, Tushar Bhavsar, Rajesh Ghosh and G Narayanan “Vector control of three-
phase AC/DC front-end converter”. Sadhana Vol.33, Part 5, October, 2008.
[3]. Park, Robert (1929). "Two Reaction Theory of Synchronous Machines". Trans. of the
AIEE 48: 716–730.
[4]. Sangita R Nandurkar and Mini Rajeev, “Design and Simulation of three phase Inverter for grid
connected Photovoltaic systems”. Proceedings of Third Biennial National Conference,
NCNTE- 2012, Feb 24-25.
[5]. Kundur .P “Power System Stability and control”, McGraw Hill, Inc, 1994.
[6]. Norman S. N., “Control System Engineering”. John Wiley & Sons, Inc, 2011.
[7]. Ivan Jadric “Modeling and Control of a Synchronous generator With Electronic Load”. Thesis
submitted to the Faculty of Virginia Polytechnic Institute and State University, 1998.
[8]. JiahuGuo, Luhua Zhang, Fujing Deng, “Decoupled Control of the Active and Reactive Power
in Three-phase PWM Converter Based on Inverse System Theory”. International Conference
on Automation and Logistics August 18 - 21, 2007, Jinan, China.
[9]. SIMULINK, “Model-based and system-based design using Simulink”, Mathworks Inc, Natick,
MA, 2000.
[10]. Ogata, K., “Modern Control Engineering”, Englewood Cliffs, NJ: Prentice Hall, 2001.
[11]. Mohan N. “Power Electronics”, John Wiley and Sons, 1989.
[12]. B.Kiran Kumar, Y.V.Sivareddy and M.Vijayakumar, “Comparative Analysis of Sine Triangle
and Space Vector PWM for Cascaded Multilevel Inverters”, International Journal of Electrical
Engineering & Technology (IJEET), Volume 4, Issue 2, 2013, pp. 155 - 164, ISSN Print:
0976-6545, ISSN Online: 0976-6553.
[13]. Manoj Kumar, Dr. F. Ansari, Dr. A. K. Jha, “Analysis and Design of Grid Connected
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Volume 3, Issue 2, 2012, pp. 69 - 75, ISSN Print : 0976-6545, ISSN Online: 0976-6553.
[14]. Bikram Das, Prabir Rn. Kasari, Abanishwar Chakraborti, Prasul Jain, Sanjay Raghuwanshi and
Pawan Kr. Navin, “Battery Energy Storage and Power Electronics Based Voltage and
Frequency Controller for WECS Connected to Grid”, International Journal of Electrical
Engineering & Technology (IJEET), Volume 4, Issue 2, 2013, pp. 283 - 292, ISSN Print:
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