Power Quality Improvement by Joined Operation of
an UPQC and Superconducting Magnetic Energy
Storage Unit
Nuno Amaro1, João Murta Pina1, João Martins1, José Maria Ceballos2,
Universidade Nova de Lisboa (Portugal), 2Universidad de Extremadura (Spain)
nma19730@campus.fct.unl.pt, {jmmp, jf.martins}@fct.unl.pt, jmceba@unex.es
1
Abstract— Modern power grids must be highly reliable and
provide power with a high quality. Power quality issues like
voltage sags or current harmonics must be minimized, in order
to achieve high levels of reliability in the system. One possible
way to overcome such problems is through the utilization of
active power filters like a Unified Power Quality Conditioner
(UPQC). On the other hand, Superconducting Magnetic Energy
Storage (SMES) are one of the most promising superconducting
devices, considering its possible applications in power systems.
This paper contains a combination of a SMES with a UPQC for
power quality improvement in an electric grid. Through the
utilization of a SMES unit, it is possible to increase the stored
energy in the DC link of the UPQC, thus improving the system
capacity to overcome power quality issues. Voltage sags and
current harmonics are simulated and the system behavior is
demonstrated.
I. INTRODUCTION
Many of the modern electric loads are very sensitive, due to
the high dissemination of microprocessors and electronic
devices. Together with this fact, the increase of power
consumption and the need to modernize existing electric grids
raise many technological challenges. In this context, power
quality is a very important topic and faults like voltage
sags/swells, frequency oscillations and harmonic distortion,
amongst others, must be minimized or, if possible, eliminated
[1].
There are several kinds of devices that can address power
quality issues, and amongst those, active power filters play an
important role, due to their flexibility [2]. The Unified Power
Quality Conditioner (UPQC) is one of the most flexible
devices because it consists in a combination of a series and a
shunt active power filters, which allows a simultaneous
compensation of voltage and current [3]. One shunt filter and
one series filter connected through means of a DC link
compose this system. The DC link voltage is kept at a
required level using a capacitor, making this a very important
component. This is mainly due to the fact that it is only
possible to maintain a high performance, if the DC link
voltage is kept all the time at the required level [4].
Superconducting Magnetic Energy Storage (SMES)
systems are one of the devices based on the phenomena of
superconductivity that has more applications in power
systems [5][6]. Using a superconducting coil, it is possible to
store energy, which can be used when necessary. The fact that
there is not necessary any kind of energy conversion (like
electrical-chemical or electrical-mechanical) makes this a
very fast device, when compared to other energy storage
systems. The main drawback of SMES systems is that they
have a low energy density, making them impracticable for
applications like mid to long term storage. However, their
power density is very high and the device can discharge in a
few milliseconds. This characteristic makes the SMES a
power device instead of an energy device [7]. Bearing in
mind these aspects, an SMES device is considered as a valid
candidate to address power quality issues. In fact, there are
already several projects in this area and SMES devices are
already used to compensate voltage sags/swells [8], mitigate
frequency oscillations [9] and operate as UPS [10, 11].
The UPQC can be considered as a FACTS (Flexible AC
Transmission System) device. The integration of SMES and
such devices has already been discussed and have several
advantages [12]. The soundest advantage is the fact that using
an SMES it is possible to add energy storage capabilities to
the FACTS device, which will increase performance and
applicability of the hybrid system [13].
Usually the SMES is connected to an AC grid, which raises
the need to have an AC/DC bidirectional power converter. In
this specific case, since the UPQC already contain a DC link,
the overall cost of the SMES unit becomes lower because its
Power Converter System (PCS) becomes simpler. In fact, the
PCS becomes only a chopper converter connected to the DC
link. Some studies also indicate that through the addition of
an SMES unit, it is possible to decrease the power rating of
the FACTS device, which also decreases its costs [14].
Considering the various advantages in combining a FACTS
device like an UPQC with an SMES, such hybrid system is
presented in this work. The system was designed to overcome
two different power quality issues: voltage sags and harmonic
distortion. Matlab/Simulink simulations are presented to
mitigate the aforementioned power quality issues.
An easy way to comply with the symposium paper
formatting requirements is to use this document as a template
and simply type your text into it.
II. SYSTEM DESCRIPTION
The simulated system is schematized in Fig 1. To achieve a
better controllability of the DC link voltage the series active
filter is connected close to the source and the shunt filter close
to the load [15]. Other configurations are possible, but this
was chosen due to this characteristic. This aspect is very
important in a hybrid system like this one, because the SMES
is connected to the DC link, which means that the DC voltage
must be kept at the same value and with minimum ripple. A
high ripple can originate AC losses in the superconducting
coil, which will decrease the system overall performance and
increase its price (because it is necessary to increase the
cryogenic power to extract the extra generated heat).
130
Power
Source
Load
UPQC
CDC
+
~
~
SMES
Unit
Fig. 1. Implemented system.
Fig. 3. SMES system constitution.
A. UPQC
The UPQC its one of the existing technologies with more
possible applications, regarding power quality issues. This is
due to the fact that it combines a series and a shunt active
filters, allowing control of both: current and voltage. The two
filters are connected through a DC link that contains a
capacitor. The DC voltage in this link must be maintained at a
certain level in order to allow full controllability of the
system, and to charge the superconducting coil of the SMES
unit. The implemented UPQC and its controller are shown in
Fig. 2.
In this specific case, because the SMES is connected to a
DC link, the PCS becomes only a DC/DC converter (a
chopper converter). The PCS used configuration is depicted
in Fig. 4. The HTS coil is considered as an ideal coil, for
simulation purposes. Other more complex models can be used,
however that is not considered as the goal of this work.
Vin+
S1
HTS Coil
Power
Source
Load
IDC
Vf
VinPWM
Gener.
Vref
ISource
S2
If
VDC
PWM
Gener.
VSource
~
~
CDC
Fig. 4. SMES unit.
Iref
UPQC
Controller
ILoad
Fig. 2. Implemented UPQC.
The two power electronics converters (in the series and
shunt filters) are controlled using PWM signals, generated
according to voltage and current reference signals,
respectively. The reference signal for the current in the shunt
active power filter is generated following a Synchronous
Reference Frame method [4]. The reference signal (used to
generate the PWM signal) of the voltage in the series power
filter is generated by a “feedforward” control method,
comparing the voltage of the filter to a well-defined reference
value.
B. SMES
An SMES system is composed of three main sub-systems: a
superconducting coil, where the energy is stored, a Power
Converter System (PCS) responsible for the exchange of
energy between the coil and the grid to where it is connected,
and a Control System (CS) to manage energy exchanges with
the grid. Fig. 3 depicts the usual system configuration.
Since the SMES system used has a simple DC/DC converter
as PCS, then the control system must be changed according to
that. In such a system, it is only necessary to control the two
power electronic switches (in this case IGBTs) that form the
chopper converter. The implemented control strategy three
different functioning modes: charge, discharge and persistent.
Charge and discharge modes are self-explained and the
persistent mode corresponds to a situation where the coil is
already fully charged and it is then necessary to keep the
current flowing in a continuous mode. The operating mode is
selected according to the state of the two switches (S1 and S2)
in this way:
• S1 and S2 closed: charging mode;
• S1 open and S2 closed: persistent mode;
• S1 and S2 open: discharging mode.
In a situation where the SMES is operating alone, the
control of the chopper is very straightforward. The system
enters a charging mode till the current in the coil reaches the
desired value and then the persistent mode is activated. The
discharge mode is used when there is a fault in the grid.
However, in this particular case, because there is a joint
operation with the UPQC, other situations must be taken care.
The DC link voltage cannot decrease under a certain value,
131
otherwise the shunt filter enters a non-controllable situation
[16]. This means that the charging process of the SMES must
consider the DC voltage value, i.e., it is only possible to
charge the SMES when the DC link voltage is at a required
value. The persistent and discharging modes operate in the
same way as in a regular SMES system.
The main characteristics from the SMES unit simulated in
this work are presented in table 1. Such characteristics were
obtained following the method presented in [17].
TABLE I
CHARACTERISTICS OF THE SIMULATED SMES UNIT.
Characteristic
Number of pancake coils
Total inductance (H)
Nominal current value (A)
Critical current of SC tape considered
(A)
Total length of SC tape necessary to
implement this SMES (m)
Value
4
0.28
70
120
The DC link voltage, which is used to charge the SMES
coil, can be seen in Fig. 6 (above). The SMES is charged in a
way that the voltage does not decrease below 700 V. This
value was chosen in order to have always a DC voltage above
the minimum value to have full controllability of the power
filters, which in this case was calculated as 648 V, following
the formulation presented in [16]. The consecutive voltage
drops shown in the figure correspond to the charging process
of the SMES. During the charging process, current flows to
the superconducting coil, decreasing the amount of power at
the DC link, thus forcing the DC link voltage to go down. Fig.
6 (below) also shows the evolution of the current in the
SMES system, which decreases around 30 A (from 70 A to
40 A), in order to compensate the voltage sag (the SMES is in
discharge mode during this time). For simplicity purposes, the
simulation started with a current of 70 A already flowing in
the SC coil. Considering the simulations parameters, it would
take 3.5 minutes to fully charge the SC coil.
800
III. SIMULATION RESULTS
To evaluate the performance of the hybrid system,
Matlab/Simulink simulations were performed for mitigation
of two different power quality problems: voltage sags and
current harmonics (3rd and 5th).
A. Voltage Sags Compensation
Voltage sags of different levels were simulated using a
three phase programmable voltage source in Simulink. Fig. 5
shows source and load voltages for a voltage sag of 50%
(which was the worst case considered), during five cycles
(100 ms). It is possible to see that the system successfully
compensated the voltage sag, allowing the load to receive the
same voltage during the whole time of the fault, with a THD
of 3.8%.
Fig. 6. Voltage in the DC link and current at the SMES coil during a
voltage sag.
B. Current Harmonics Mitigation
Harmonic distortion is one of the worst faults that can
occur in a power grid. Also using the programmable three
phase voltage source from Simulink, a harmonic content of
0.25 p.u of the 3rd harmonic and 0.1 p.u. of the 5th harmonic
were added to the voltage signal. This corresponds to a THD
of 16%. Voltages at source and load for this scenario can be
seen in Fig. 7. The system can compensate the harmonic
distortion and the load is not affected by it. The THD at the
source is 4.87%.
Fig. 5. Voltage sag compensation.
132
REFERENCES
Fig. 7. Compensation of harmonic distortion in the system.
IV. CONCLUSION
A hybrid system consisting of an UPQC and a SMES unit for
compensation of power quality issues was presented.
Simulation results indicate that the system can be used to
overcome typical power quality problems. The addition of
and SMES unit to the DC link in an UPQC has several
advantages including: addition of stored energy in the DC
link, which increases the application range of the FACTS
device; the overall cost of the SMES becomes lower, because
its PCS is simpler than that when the SMES is connected to
an AC grid; the power rating of the UPQC decreases,
decreasing its costs; finally, the addition of the SMES does
not increase too much the complexity of the system, because
the control scheme of the full system is very similar to those
of the two systems operating alone, with only the additional
need to take care of the DC voltage when charging the SMES
coil.
Two different power quality faults were simulated: voltage
sags and harmonic distortion including harmonics of the 3rd
and 5th orders. Obtained results indicate that the system can
mitigate such faults, without any noticeable change of voltage
and current at the load side. Given the robustness of such a
hybrid system, it is expected that other major power quality
issues can be mitigated. In conclusion, this system can
improve power quality in electric grids and has a very large
spectrum of applications, without a great cost increase
comparing to that of the two individual components.
1. EURELECTRIC: Power Quality in European Electricity Supply
Networks. , Brussels (2003).
2. Akagi, H.: New trends in active filters for power conditioning.
IEEE Trans. Ind. Appl. 32, 1312–1322 (1996).
3. Akagi, H., Watanabe, E.H., Aredes, M.: Instantaneous Power
Theory and Applications to Power Conditioning. John Wiley &
Sons, Inc., Hoboken, NJ, USA (2007).
4. Rashid, M.H. ed: Power Electronics Handbook. Elsevier (2011).
5. Amaro, N., Murta Pina, J., Martins, J., Ceballos, J.M.:
SUPERCONDUCTING MAGNETIC ENERGY STORAGE - A
Technological
Contribute
to
Smart
Grid
Concept
Implementation. Proceedings of the 1st International Conference
on Smart Grids and Green IT Systems. pp. 113–120. SciTePress
- Science and and Technology Publications (2012).
6. Xiao, L., Dai, S., Lin, L., Zhang, J., Guo, W., Zhang, D., Gao,
Z., Song, N., Teng, Y., Zhu, Z., Zhang, Z., Zhang, G., Zhang, F.,
Xu, X., Zhou, W., Qiu, Q., Li, H.: Development of the World’s
First HTS Power Substation. IEEE Trans. Appl. Supercond. 22,
5000104–5000104 (2012).
7. Tixador, P., Deleglise, M., Badel, a., Berger, K., Bellin, B.,
Vallier, J.C., Allais, a., Bruzek, C.E.: First Tests of a 800 kJ
HTS SMES. IEEE Trans. Appl. Supercond. 18, 774–778 (2008).
8. Kim, H.J., Seong, K.C., Cho, J.W., Bae, J.H., Sim, K.D., Kim,
S., Lee, E.Y., Ryu, K., Kim, S.H.: 3 MJ/750 kVA SMES System
for Improving Power Quality. IEEE Trans. Appl. Supercond. 16,
574–577 (2006).
9. Xian, W., Yuan, W., Yan, Y., Coombs, T. a.: Minimize
frequency fluctuations of isolated power system with wind farm
by using superconducting magnetic energy storage. 2009
International Conference on Power Electronics and Drive
Systems (PEDS). pp. 1329–1332. IEEE (2009).
10. Aware, M., Sutanto, D.: SMES for Protection of Distributed
Critical Loads. IEEE Trans. Power Deliv. 19, 1267–1275 (2004).
11. Tixador, P., Bellin, B., Deleglise, M., Vallier, J.C., Bruzek, C.E.,
Allais, A.., Saugrain, J.M.: Design and First Tests of a 800 kJ
HTS SMES. IEEE Trans. Appl. Supercond. 17, 1967–1972
(2007).
12. Torre, W., Eckroad, S.: Improving power delivery through the
application of superconducting magnetic energy storage
(SMES). 2001 IEEE Power Engineering Society Winter
Meeting. Conference Proceedings (Cat. No.01CH37194). pp.
81–87. IEEE (2001).
13. Chen, L., Liu, Y., Arsoy, A.B., Ribeiro, P.F., Steurer, M.,
Iravani, M.R.: Detailed Modeling of Superconducting Magnetic
Energy Storage (SMES) System. IEEE Trans. Power Deliv. 21,
699–710 (2006).
14. EPRI: West Coast Utility Transmission Benefits Of
Superconducting Magnetic Energy Storage. (1996).
15. Nielsen, J.G., Blaabjerg, F.: A Detailed Comparison of System
Topologies for Dynamic Voltage Restorers. IEEE Trans. Ind.
Appl. 41, 1272–1280 (2005).
16. Teke, A.: Unified Power Quality Conditioner: Design,
Simulation and Experimental Analysis., (2011).
17. Amaro, N., Pina, J.M., Martins, J., Ceballos, J.M., Alvarez, A.:
A Fast Algorithm for Initial Design of HTS Coils for SMES
Applications. IEEE Trans. Appl. Supercond. 23, 4900104–
4900104 (2013).
133