t

1s IEEE International Conference on Power Electronics, Intelligent Control and Energy Systems (ICPEICES-2016)

Multilevel Inverter based Electric Spring for

Voltage Regulation and Active Reactive
Power Control

Nair Syam Sundarl; Libin Philip2; Patel Tapasvi3; Mistry Akash4 and Dholiya Hira/
12345
, , , , Department of Electrical Engineering; Chhotubhai Gopalbhai Patel Institute of Technology;
Uka Tarsadia University; Surat; Gujarat-394350; India
I
E-mail: syamakrishnan.nair@gmail.com

Abstract-Nowadays electric power generation from the non critical loads like electronic appliances including
renewable sources of energy is growing vastly. In India; TV; music system; DVD; etc possess inbuiIt SMPS
twenty eight percent of total power is being generated using (Switched Mode Power Supply) converting 230 to 12/24
non-renewable sources like solar; wind; biogas etc. The
V; so these can bear voltage fluctuations up to some
power generated is fed to the main grid via micro grid
extent. Taking advantage of this; ELECTRIC SPRING
system. As the nature does not remain similar all the time;
(ES) is used to connect in series with the non-critical load
there is disturbance in wind velocity; intensity of solar
radiation; etc. This results in the voltage fluctuation for short and the series combination in parallel with the critical
as well as long period. Some of the loads cannot withstand load. ES is a five level cascaded H-Bridge multilevel
such variations; or to be specific; critical load. There are inverter which gives a sinusoidal signal whose R. M.S.
voltage and reactive power compensating devices such as value is decided as per command signal provided to it as
DVR; DSTATCOM; SSSC etc. to overcome such problems. feedback. Thus; this becomes a closed loop system. Also;
But it cannot be used at particular demand side due to its from the input side the reactive power is reduced and the
high cost. This paper gives an idea to mitigate the voltage of
power factor gets improved. This becomes a plus point for
critical load by varying the voltage of non-critical load to
the device.
some extent employing electric spring-MLI (5 levels).
The phase angle between the ES voltage and the non­
Keywords-Electric Spring; 5 Level Casacaded H-Bridge
MLl; Design of LC Filter; Active-reactive Power Control critical load current is maintained at 90. It is not always
necessary that this relation results in positive response. It
I. INTRODUCTION specifies some range to operate. As it is a closed loop
system; it is necessary to sense the voltage magnitude of
The topic electric spring refers to the device which
critical load; phase angle of non-critical load current and
acts as a compensator in tenns of voltage or reactive
respond to error signal generated after comparing the
power or power factor. This is a project on which the
R. M.S. value of critical load voltage and reference signal
research is carried out still. To maintain the critical load
(magnitude). This response results in the variation of pulse
voltage to a constant value some voltage variation has to
width of the gate signals fed to the 5-Level MLI and thus
be made across the non critical load. This principle is
leads to variation in the magnitude ES voltage.
based on Hooke's law for a mechanical spring which is
analogue to electric spring [1]. The voltage fluctuation is II. ELECTRIC SPRING
mainly due to the change in the nature-wind speed
A. 5-Level Cascaded H-Bridge MLi
variation; change in the intensity of solar light etc. which
are renewable source of energy. So the micro-grid system This multilevel inverter is of 5-level Cascaded H­
supplies power collected from the renewable source of Bridge type (Vdc; 2Vdc; 0; -Vdc; -2Vdc). The level shifting
energy to the main grid system [2]. There are many principle is applied over here. The importance of this
devices used to compensate such fluctuation w.r. t. voltage inverter is that it generates nearly sinusoidal waveform
and reactive power like DVR also DST ATCOM; SSSC which is essential in ES (Electric spring). The SPWM
and other such devices [3]. But this electric spring is a (sinusoidal pulse width modulation) technique is used here
new a device accommodated as a smart load system [4]. to obtain sinusoidal waveform. The reference signal is sine
This electric spring is a device which maintains a wave of 50 Hz while the carrier signal is triangular
constant voltage level across the critical load by waveform of 5 KHz. These are compared in comparator
compensating the voltage across the non-critical load. The and the pulses generated from the comparator are given to
critical loads such as medical instruments of hospital; the gate circuit of Cascaded H-Bridge inverter as per the
security system of the building; air conditioning system switching pattern. There are four comparators which are
etc. cannot bear even minor voltage fluctuations; whereas given common reference signal and individual carrier

978-1-4673-8587-9/16/$31.00 ©2016 IEEE [1)

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1s IEEE International Conference on Power Electronics, Intelligent Control and Energy Systems (ICPEICES-2016)

2
signal of different magnitudes. The first carrier generator
generates carrier signal of magnitude level 0 to 1; the
second one generates the magnitude level of 1 to 2; the
third one generates 0 to -1 and the fourth one generates the
magnitude level of -1 to -2. The magnitude level of sine

! .��lIil l Ilr�l �
wave is 0 to 2.
Thus the output of comparator which compares the

ijl l lt
reference and carrier signal of level 1 results in Vdc at the
output of cascaded H-Bridge inverter. Similarly; the
comparator output of magnitude level 1 to 2 generates the
2Vdc. Here; both the comparators generate the pulses at the
same time for level 2Vdc. Comparators can be Op-Amp in
which reference (sine) signal is given to the non-inverting o 0,(02 0.004 0.0:6 0.003 0.Q1 0,012 0,014 0.Q16 0.018 0,(12
Time (sec)
input while the carrier signal to the inverting input of Op­
Fig. 2: Comparators' Input Signal-one is Sinewave and
Amp. So during comparison; when the magnitude of sine
Others are Four Level Shifted Triangular Signals
wave is more than that of the carrier signal; pulse of logic
1 is generated and for the reverse case i. e. if carrier is Now it becomes simpler to filter out nearly sine signal
more than sine wave; logic 0 is generated. So; during the from the SPWM (Sinusoidal Pulse Width Modulated)
generation of the level 2Vdc; multi pulses are generated by output as shown in Fig. 2.
1-2 level comparator while on the other hand; only one
pulse is generated by 0-1 level comparator. The same case
B. Design ofLe Filter
is applicable for comparators doing comparison operation A mathematical expression is needed to ease the out
between negative half of the reference signal and the pass filtering process.
negative magnitude level carrier signal for obtaining -Vdc First of all it is assumed that the square wave is the
and -2Vdc. Thus after one cycle the output of cascaded H­ input and L (inductor) and C (capacitor) are the filter
Bridge inverter will be stepped sine wave predicting 5- components.
Levels. So; for positive half wave generation of H-Bridge The square wave as a function of time can be defined
inverter; two comparators will be under operation out of
OO
by Fourier series;
four and the other two comparators for negative half
f(t) - --
4xVdc ( -)
L
1 . nnt
- SIn (1)
generation. n n 2
n=1,3,5

TABLE I Applying KVL around the closed path,

Switching Pattern
S I,S4,S5,D6
Output Level
Vdc
f(t) = i(t) [d��) ] + G)
* R +L * f[i(t) * dt] (2)

S I,S4,S5,S8 2Vdc � � ---"OTO"'-

0 0 I L

S2,S3,S6,D5
S2,S3,S6; S7
-V dc 1 R
-2Vdc A squa.-e
Y Wave
The switching pattern is designed in such a manner
the switching losses are reduced compared to all other
patterns and fulfill efficient use of all the switches.
Fig. 3: Concept of LC Filter

Where,
i(t)=current flowing in the RLC circuit
Assume that the voltage across capacitor is function
of sine wave such that Vc(t)= Vm*sin(wt). Thus i(t) can be
found out from the equation of capacitor voltage Vc(t).
Substituting obtained Vc(t); i(t) and eq.(1) in eq.(2); a
equation is formed and evaluate the equation by keeping
the value of t such that the sine term present in the
equation becomes 1 i.e. wt=90.
Here at this stage; R (resistance) of RLC branch gets
deleted. And defining the value of L; V m (peak amplitude
of sinusoidal output) and Vdc (obtained after summ ation of
Fig. I: Comparator Simulation for Generating Gate Signals to 5-Level
the dc voltages of both individual H-Bridge inverters)
MLI

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1s IEEE International Conference on Power Electronics, Intelligent Control and Energy Systems (ICPEICES-2016)

value of capacitance C can be known. The resistance can Where; Vnel = Non-Critical Load Voltage; Vel =
be kept above 1 11 in the simulation. Critical Load voltage; Vin= input voltage; ES = electric

1�

.�-
F

i:
0 : : '
...... .
, ...., .. . : . ,
......
, .... ,
.
,.,�..
......
...... .,
.
; , ..
......
. .... ,
.
, . : ..
......
......
.
, ..�.,
......
, .. . . ,
.
, . ...
.....
... ...
.
, .. . .
..
. . ... , .
spring voltage or inverter voltage; Inel = non-critical load
current and Vline = line voltage drop.
Case-2
If the transmission line impedance is

� : :: : : :
10
o 0.02

:
0,04 0.00 0,03 0.1 0.12 0,14 0.16
resistive+capacitive (RC); then for under-voltage
phase angle relation between ES voltage vector and non­
the

I: �. 10
0 om 0,04 0.00 D.1lI
Time (sec)
0.1 0.12 0.14 0.16
critical load current vector should be +90° and during
over-voltage condition it should be _90°.

Vel

Vline
Fig. 4: Output Voltage before Filter and After Filter Circuit ��----��----�

The output of 5-level MLI can approx to sine wave

using the capacitor and inductor of values obtained from
the equation no.2.

C. Application ofES into the Circuit ofNon-critical

Fig. 7: Under-Voltage Condition; _900 between ES and Inel; Line
Load Impedance is of RC Characteristic

After obtaining sinusoidal waveform from the

inverter; provide a phase shifting of 90 between ES
voltage and non-critical Load current. In actual practice;
the sinusoidal output voltage after filter circuit already
possess some phase shift of -90. So there is no need of any Critical Load
Medical
additional phase-shift. Now the concept of ELECTRIC Instnllllents,
security system
SPRING comes into the picture. The phase shifting etc.
Electric Heater.
between ES voltage and non-critical load current is ±90 ElectTonic
Appliances
depending on the transmission impedance during under­
voltage or overvoltage condition. Under-voltage case
comes when the R. M.S. value of critical load voltage is Fig. 8: Block Diagram of Electrical System with ES
less than the 230 V (reference) and similarly when it is
The block diagram of the power system including
more than the reference value it is over-voltage condition.
ELECTRIC SPRING (ES) depicts source voltage and line
Case i:
impedance at the input side and critical and series
If the transmission line impedance is resistive +
combination of non-critical load and ES parallel to the
inductive (RL); then for under-voltage condition the phase critical load at the output side. The multilevel inverter
angle between ES voltage vector and non-critical load
boosts or bucks the voltage across the critical load as per
current vector is -90 and it should be +90 for over-voltage
the under-voltage or voltage condition.
condition.
The system completing closed loop configuration of
Vncl the above system is shown below.

�
Fig. 5: Under-Voltage Condition _900 between ES and 1,,,,,; Line
Vel
Vln

Impedance is of RL Characteristic

Fig. 9: Closed Loop Configuration of the System

Which Completes the ES
Fig. 6: Over-Voltage Condition; +900 between ES and Inel; Line
Impedance is of RL Type

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1s IEEE International Conference on Power Electronics, Intelligent Control and Energy Systems (ICPEICES-2016)

The R. M.S. value of critical load voltage is compared The overall active power consumption from the input
with the reference voltage as shown in the above diagram. side gets reduced. As shown in the figure.1O when ES
The error signal generated is given to PI (Proportional comes into the picture the active power to critical load
Integrator). The output of PI can be decided as per the gets increased to a determined value decreasing the active
values of Kp (proportional gain) and Ki (integral gain). power across the non-critical load.
This can be positive or negative as per the error signal
generated depending on over voltage or under-voltage
condition. In this paper the comparator is selected in such �IIXlJj
1
a manner that error signal generated will be positive for
&.� = �--�--
----------
'---'----�
under-voltage case and negative for over-voltage case. EOD
1
� F---�
Thus the output of PI can be +a or -a. .�.all _
--------��---'i
p.,

The output of the inverter is with _90° phase-shifted.

So when +a results (under-voltage); a*sin(wt+<j» is ���--�-T--T--7,--+--7--7-��"
Time (sec)
generated by the sine-wave signal generator which is
equivalent to a*sin(wt+<j>-900) as compared to output side. Fig. I I: Active Power Consumption by the Loads and the System in
Under-Voltage Condition
<I> is the angle of non-critical load current. Similarly; when
-a results (overvoltage); -a*sin(wt+<j» is generated by
sine-wave signal generator which is equivalent to
a*sin(wt+<j>+90} Thus as per the gain values set in the PI � _
Cd

the ES voltage increases due to sinusoidal pulse width

cm - ..,
modulated triggering to the gate circuit of MLI (Multilevel �� 1500 ,;.

�--�----�
Inverter). At last the voltage across the critical load gets .�� 1001

constant. The load can be RL up to some desired range for "

various PI tuned values. If case 2 (Re line impedance) is

considered then the only change that has to be made to
fulfill its condition is to reverse the polarity or to
Fig. 12: Reactive Power Consumption by the System and the Loads
interchange the position of reference value and R. M. S. during Under-Voltage Consumption
value fed to the comparator.
If transmission impedance is considered pure resistive Qin = Input Reactive Power; QeI = Reactive Power
i. e. Z= R; the there is no impact of ±90° phase-shifting consumed by critical Load and Qnel = Reactive Power
between ES voltage vector and non-critical load current consumed by non-critical load
vector on critical load voltage during overvoltage As shown in the Fig. 12 the input reactive power
condition. In both cases the critical load voltage goes on becomes negative but the power factor of the system gets
increasing by varying the non-critical load voltage vastly. improved. Similarly as the voltage across the critical load
is maintained at 230 volts the critical load recover its
III. SIMULATION RESULTS reactive power after 2 sec when ES comes into the picture.
In order maintain critical load the sacrifice of non-critical
parameters get disturbed. So the reactive power of the
non-critical load gets decreased.

aa��--�-7--+--+,--+-�--7-��
Time (scc)

Fig. 10: R.M.S. Values of Voltages during Under-Voltage Condition Vin

= Input Source Voltage; Vel = Critical Load Voltage Vnel = Non-Critical

Load Voltage; ES = MLI Output Voltage

In the Fig. 10 the ES is bypassed till 2 sec. After this Fig. 13: R.M.S. Values of Voltages during Over-Voltage Condition
period the ES comes into the picture and thus the value Vin = Input Source Voltage; Vel = Critical Load Voltage Vnel = Non­
Critical Load Voltage; ES = MLI Output Voltage
critical load voltage which was below 230 volts before 2
sec boosts up to 230 volts. Its stability period is around In overvoltage condition; the critical load voltage is
500 millisecond and completely gets stable after 1 sec. more than 230 V and due to voltage drop across the line
Pin = Input Active power; Pel = ActivePower impedance the input voltage is obviously more than that of
consumed by critical Load and Pnel = Active Power the critical load voltage. As shown in the fig. 13; before 2
consumed by non-critical load secs the voltage across the critical load is more than 230 V

[41
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1s IEEE International Conference on Power Electronics, Intelligent Control and Energy Systems (ICPEICES-2016)

and after 2 secs when the ES comes into the picture; the the same as this matter depends on the line impedance
voltage gets reduced to 230 V. Here there is some change parameter.
of voltage across the non-critical load. Thus; from the
above analysis it can be said that there is a spring effect in IV. CONCLUSION

order to maintain critical load voltage. After the detailed analysis and research on electric
The voltage variation across the non-critical load is spring as a voltage regulator; a reactive power
not more as compared to critical load condition. Also the compensator or line power factor improver and power
voltage-range above 230 V for which the voltage across quality improver it can be concluded that electric spring
the critical load can be set to reference cannot be decided. can be termed as smart load system for modem grid
system including microgrid which growing fastIy all over
--- .. the globe keeping in mind the conservation of non­
�mll renewable source of energy. The variation in the non­
�
• ""F-----------------1 critical load in order to avoid the voltage fluctuation of
& em'i----....
.�
critical load is quite more which mainly depends on the
� an ----
;. . ,. . ----1
system parameters. Apart from the power part the input
active power gets reduced without disturbing the power of
00 .
Time (sec) critical load in both under-voltage and over voltage
conditions. The input reactive power gets decreased
Fig. 14: Active Power Consumption by the System and the Loads during
Over-voltage Consumption (supplying to the input) in under-voltage condition while it
gets increased (absorbing from the input) in over-voltage
In over-voltage condition there is not much difference
condition. Thus power quality of the critical load is
in the power from the input side compared to that of improved by the device electric spring and the device can
under-voltage condition. The power across the critical­ be driven in both supplying and absorbing mode as
load gets reduced as the voltage across it is reduced by reactive power compensator. And at the end; the concept
230 V. Similarly; following the spring principle; the of phase shifting of ±90 between ES voltage and non­
power of non-critical load also gets reduced to maintain a critical load parameter can be decided from the the
constant power across the critical load. characteristic of line impedance (RL or RC).
ACKNOWLEDGMENT
� ...

�Ym
• ""f---{ The authors would like to thank their professors and
!zm colleagues at Chhotubhai Gopalbhai Patel Institute of
�=
'H 1U1F ________________ Technology for their support and for availing the facilities
"',�
and equipments at the institute.
00 .
Timc(sec)
REFERENCES
Fig. IS: Reactive Power Consumption by the System and the Loads
[I] Jayantika Soni; Krishnanand K.R. and Panda Sanjib in "Load Side
during Under-Voltage Consumption
Demand Management in Buildings Using Electric Springs" in
In Fig. 14; it can be clearly seen that after 2 secs the CREST Center for Research and Energy System Transformations
and in University of California; August 20 14,eScholarship
reactive power gets increased as the ES phase voltage is
[2] Siew Chong Tan; Xia Chen; Yuhen Hou and other IEEE crew
shifted by +90°. So the resultant current flowing through members "Mitigating Voltage and Frequency Fluctution in
the line impedance lags behind the input voltage by some Microgrids Using Electric Springs"; IEEE Transaction on Smart
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reactive power of critical load is increased decreasing the
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Smart Grid Technology"; IEEE Transaction; published in 2 0 12
TABLE 2 H. Shu Yuen; L. Chi Kwan and F. F. Wu; "Electric springs- a new
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Line Parameters R-O.I fl; L-1.22 mH S.Y. Ron Hui "Dynamic Modeling of Electric Springs"; IEEE
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for Power Quality Improvement"; IEEE Transaction; published in
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[5)