APPLICATION OF SMES UNIT TO IMPROVE THE VOLTAGE PROFILE OF THE SYSTEM WITH DFIG DURING GRID DIP AND SWELL

International Journal of Advances in Engineering & Technology, Nov 2011.
(c)IJAET ISSN: 2231-1963
APPLICATION OF SMES UNIT TO IMPROVE THE VOLTAGE
PROFILE OF THE SYSTEM WITH DFIG DURING GRID DIP
AND SWELL
A. M. Shiddiq Yunus1, 2, A. Abu-Siada2 and M. A. S. Masoum2
1Department of Mechanical Engineering, Energy Conversion Study Program,
State Polytechnic of Ujung Pandang, Makassar, Indonesia
2Departement of Electrical and Computer Engineering, Curtin University, Perth, Australia






ABSTRACT
One of the most important parameters of the system where wind turbine generators (WTGs) are connected is
voltage profile at the point of common coupling (PCC). In the earlier stage, WTGs were possible to be
disconnected from the system to avoid the damage of WTGs. Following the rapid injection of WTGs to the
existing network during last decades, the transmission line operators (TSOs) require WTGs to stay connected in
certain level of fault to continue support the grid. This new requirements have been compiled in new
international grid codes. In this paper, superconducting magnetic energy storage (SMES) is applied to improve
the voltage profile of PCC bus where WTGs equipped with doubly fed induction generator (DFIG) is connected
to meet the used gird codes of Spain and German during grid dip and swell. The voltage dip at the grid side is
examined to comply with the low voltage ride through (LVRT) while the voltage swell at the grid side is
examined to comply with the high voltage ride through (HVRT) of both Spain and German voltage ride through
(VRT).
KEYWORDS: Voltage Ride through (VRT), SMES, DFIG, Voltage Dip & Voltage Swell.
I. INTRODUCTION
The effect of pollution from conventional energy to the environment and the implementation of
carbon tax have become a trigger of the increase of renewable energy utilization in the world. In
addition, conventional energy is very limited and would soon be finished if exploited on a large scale
because of oil, gas or coal is a material created in the process of millions of years. The limited amount
and high demand for energy resources will affect the rise in oil prices from time to time. Therefore,
attention is directed now onto the renewable energies which are clean and abundantly available in the
nature [1]. The first wind turbines for electricity generation had already been developed at the
beginning of the twentieth century. The technology was improved step by step from the early 1970s.
By the end of the 1990s, wind energy has re-emerged as one of the most important sustainable energy
resources. During the last decade of the twentieth century, worldwide wind capacity doubled
approximately every three years [2]. The global installed capacity worldwide increased from just less
than 2000 MW at the end of 1990 to 94000 MW by the end of 2007. In 2008, wind power already
provides a little over 1% of global electricity generation and by about 2020, it is expected that wind
power to be providing about 10% of global electricity [3]. Moreover, the total 121 GW installed
capacity of wind turbine in 2008 has produced 260 TWh of electricity and has saved about 158
million tons of CO2. In addition, the predication of total installed capacity of wind turbines will be
573 GW in 2030 [4]. Power quality issue is the common consideration for new construction or
connection of power generation system including WTGs installation and their connection to the
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International Journal of Advances in Engineering & Technology, Nov 2011.
(c)IJAET ISSN: 2231-1963
existing power system. In this paper, voltage dip (sag) and swell will be considered as the conditions
of the fault ride through capability of WTG equipped with DFIG. Voltage dip (sag) and swell are two
common types of power quality issue. Voltage dip is a decrease to between 0.1 and 0.9 pu in rms
voltage or current at the power frequency for durations of 0.5 cycles to 1 minute. Voltage dips are
usually associated with system faults but can also be caused by switching of heavy loads or starting of
large motors. A swell is defined as an increase in rms voltage or current at the power frequency for
durations from 0.5 cycles to 1 minute. Typical magnitudes are between 1.1 and 1.8 pu. As with dips,
swells are usually associated with system fault conditions, but they are much less common than
voltage dips. A swell can occur due to a single line-to-ground fault on the system resulting in a
temporary voltage rise on the unfaulted phases. Swells can also be caused by switching off a large
load or switching on a large capacitor bank [5, 6]. Since voltage dip is a common power quality
problem in power systems, most of studies are focused on the performance of WTGs during voltage
dip [7-14]. Although it is a less power quality problem, voltage swell may also lead to the
disconnection of WTGs from the grid. In this paper, voltage dip and swell will applied on the grid
side to investigate their effects on PCC which could affect the continuation of WTGs connection if
complying with the used grid codes in this paper as explained below with and without SMES
connected.
II. SPAIN AND GERMAN GRID CODE
In the earlier stage, WTGs are possible to be disconnected from the system to avoid the damage of
WTGs. Following the rapid injection of WTGs to the existing network during last decades, the
transmission line operators (TSOs) require WTGs to stay connected in certain level of fault to
continue support the grid. This new requirements have been compiled in new grid codes. However,
most of grid codes are only providing low voltage ride through (LVRT) in their codes without any
restriction information regarding the high voltage ride through (HVRT) which might be can lead
instability in the PCC. The following figures are the international grid codes of Spain and German
which used in this study. Figure 1a and 1b show the voltage ride through (VRT) of Spain and German
respectively. The selection of these grid codes is based on their strictness in low voltage ride through
(LVRT), meanwhile providing complete voltage ride through (VRT) with their HVRT.


(a)
(b)
Figure 1. (a) FRT of Spain grid code and (b) FRT of German grid code [15]
In Figure 1 (a), the FRT of Spain is divided by three main blocks. "A" block is representing the high
voltage ride through (HVRT) of Spain grid code. The maximum allowable high voltage in the vicinity
of PCC is 130% lasts for 0.5 s. After that the maximum high voltage is reduced to 120% until next
0.5 s. All high voltage profiles above "A" block will lead the disconnection of WTGs from the
system. The normal condition of this grid code is laid on "B" block. All voltage profiles within this
block range are classified as a normal condition (90% to 110%). The low voltage ride through
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International Journal of Advances in Engineering & Technology, Nov 2011.
(c)IJAET ISSN: 2231-1963
(LVRT) is limited in "C" block. The minimum voltage drop allows in this grid code is 50% lasts for
0.15 s and increased to 60% until 0.25s. The low voltage restriction then ramp to 80% at 1 s and
reaching the normal condition in 15 s since the fault occurs. The HVRT of German grid code (shown
in Figure 1(b)) is much strict then Spain. The maximum allowable HVRT is 120% for 0.1 s (shown in
"A" block). The normal condition that is shown in "B" block is the same with Spain grid code.
However, the LVRT is allowed to reach 45% lasts for 0.15 s and should be at least 70% until 0.7 s.
After that the voltage margin ramps to 85% at 1.5 s.
III. SYSTEM UNDER STUDY
There are two major classifications of wind turbine generator, fixed-speed turbine and variable-speed
turbines. One of the most popular variable speed wind turbine is doubly fed induction generator
(DFIG). About 46.8 % of this type has been installed in 2002 [2]. A doubly fed induction generator
(DFIG) using a medium scale power converter. Slip rings are making the electrical connection to the
rotor. If the generator is running super-synchronously, electrical power is delivered to the grid through
both the rotor and the stator. If the generator is running sub- synchronously, electrical power is
delivered into the rotor from the grid. A speed variation of + 30% around synchronous speed can be
obtained by the use of a power converter of 30% of nominal power. The stator winding of the
generator is coupled to the grid, and the rotor winding to a power electronic converter, nowadays
usually a back-to-back voltage source converter with current control loops. In this way, the electrical
and mechanical rotor frequencies are decoupled, because the power electronic converter compensates
the different between mechanical and electrical frequency by injecting a rotor current with variable
frequency. Variable speed operation thus became possible. The typical of generic model of DFIG is
shown in Figure 1.

Figure 2. Typical configuration of WTG equipped with DFIG
The system under study shown in Figure 3 consists of six-1.5 MW DFIG connected to the AC grid at
PCC via Y/ step up transformer. The grid is represented by an ideal 3-phase voltage source of
constant frequency and is connected to the wind turbines via 30 km transmission line. The reactive
power produced by the wind turbine is regulated at 0 Mvar at normal operating conditions. For an
average wind speed of 15 m/s which is used in this study, the turbine output power is 1 pu and the
generator speed is 1 pu. SMES Unit is connected to the 25 KV (PCC) bus and is assumed to be fully
charged at its maximum capacity of 2 MJ.


Figure 3. System under study

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International Journal of Advances in Engineering & Technology, Nov 2011.
(c)IJAET ISSN: 2231-1963
IV. SMES CONFIGURATION AND CONTROL SYSTEM
The selection of SMES Unit in this paper is based on its advantages over other energy storage
technologies. Compared to other energy storage options, the SMES unit is ranked first in terms of
highest efficiency which is 90-99% [16-18]. The high efficiency of the SMES unit is achieved by its
lower power loss because electric currents in the coil encounter almost no resistance and there are no
moving parts, which means no friction losses. SMES stores energy within a magnetic field created by
the flow of direct current in a coil of superconducting material. Typically, the coil is maintained in its
superconducting state through immersion in liquid helium at 4.2 K within a vacuum - insulated
cryostat. A power electronic converter interfaces the SMES to the grid and controls the energy flow
bidirectionally. With the recent development of materials that exhibit superconductivity closer to
room temperatures this technology may become economically viable [1]. The stored energy in the
SMES coil can be calculated as:
1
2
E =
I
L
(1)
SM
SM
2
Where E is the SMES energy; ISM is the SMES Current and LSM is the SMES inductor coil.
The SMES unit configuration used in this paper consists of voltage source converter (VSC) and
DC-DC chopper which are connected through a DC shunt capacitor. The VSC is controlled by a
hysteresis current controller (HCC) while the DC-DC chopper is controlled by fuzzy logic controller
(FLC) as shown in Figure 4.


Figure 4. SMES configuration
DC-DC Chopper along with FLC is used to control charging and discharging process of the SMES
coil energy. The generator active power and the current in the superconductor coil are used as inputs
to the fuzzy logic controller to determine the value of the DC chopper duty cycle, active power of
DFIG and SMES coil current are used as inputs of the fuzzy logic controller. The duty cycle (D) is
compared with 1000 Hz saw-tooth signal to produce signal for the DC-DC chopper as can be seen in
Figure 5.

Figure 5. Control algorithm of DC-DC chopper
Compared with pulse width modulation (PWM) technique, the hysteresis band current control has the
advantages of ease implementation, fast response, and it is not dependent on load parameters [19].
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International Journal of Advances in Engineering & Technology, Nov 2011.
(c)IJAET ISSN: 2231-1963
Hysteresis current control (HCC) is used to control the power flow exchange between the grid and the
SMES unit. HCC is comparing the 3-phase line currents with the reference currents (Id* and Iq*). The
value of Id* and Iq* are generated through the conventional PIs controller both from the deviation of
the capacitor voltage Vdc and system voltage Vs. To minimize the effect of phases interference while
maintaining the advantages of the hysteresis methods, phase-locked loop (PLL) technique is applied
to limit the converter switching at a fixed predetermined frequency [20]. The proposed control
algorithm in this paper is much simpler and closer to realistic application compared with the controller
used in [21], where four PIs controller were used which complicate the process of finding the optimal
parameters of the PIs, moreover, only Pg was used as the control parameter of the DC-DC chopper
and it ignored the energy capacity of the SMES coil. The detailed VSC control scheme used in this
paper is shown in Figure 6. The rules of duty cycles D and the corresponding SMES action are shown
in Table I. When D is equal to 0.5, SMES unit is in idle condition and there is no power exchange
between the SMES unit and the system. When there is any voltage drop because of fault, the
controller generates a duty cycle in the range of 0 to 0.5 according to the value of the inputs and
power will be transferred from SMES coil to the system. The charging action (corresponding to the
duty cycle higher than 0.5) will take place when SMES coil capacity is dropped and power will be
transferred from the grid to the SMES unit.

Figure 6. Control algorithm of VSC

Table 1. Rules of duty cycle
Duty cycle (D)
SMES coil action
D = 0.5
standby condition
0 D < 0.5
discharging condition
0.5 < D 1
charging condition

The variation range in SMES current and DFIG output power and the corresponding duty cycle are used to
develop a set of fuzzy logic rules in the form of (IF-AND-THEN) statements to relate the input variables to
the output. The duty cycle for any set of input date (Pg and ISM) can be evaluated from the surface graph
shown in Figure 7.
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International Journal of Advances in Engineering & Technology, Nov 2011.
(c)IJAET ISSN: 2231-1963

Figure 7. Surface graph- Duty cycle
V. SIMULATION RESULTS
In this paper, two grid disturbances will be applied. The first disturbance would be a voltage dip of
20% and the second is a voltage swell of 135%. Both of disturbances are applied at 0.5 s and last for 5
cycles.
5.1. Voltage Dip

Figure 8. Complying voltage profile at PCC with Spain VRT during grid dip

Figure 9. Complying voltage profile at PCC with German VRT during grid dip
As can be seen in Figures 8 and 9, during voltage dip at the grid side, voltage profile at the PCC will
be dropped about 0.35 pu without SMES connected. This value is beyond the LVRT of both Spain
and German, therefore in this case, the DFIGs have to be disconnected from the system. However,
when SMES is connected voltage drop at the PCC can be significantly corrected to about 0.8 pu far
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International Journal of Advances in Engineering & Technology, Nov 2011.
(c)IJAET ISSN: 2231-1963
from the lowest limit of LVRT of both Spain and German. When fault is cleared, it is naturally that
there is a spark which forces the overshoot voltage, however, the overshoot is still under the safety
margin of both Spain and German HVRT.

Figure 10. Shaft speed during grid dip
During voltage dip, the speed of shaft will increase at the time when the grip dip occurs to compensate
the power drop due to the voltage drop at the PCC as shown in Figure 10. In some severe grid dip
cases the extreme oscillation on shaft speed will lead to instability of the system. With SMES
connected to the PCC, the oscillation, settling time and the overshoot of the shaft speed are
significantly reduced if compared with the system without SMES.

Figure 11. Current behaviour of SMES coil during grid dip

Figure 12. Stored energy behaviour of SMES coil during grid dip
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International Journal of Advances in Engineering & Technology, Nov 2011.
(c)IJAET ISSN: 2231-1963

Figure 13. Voltage behaviour across the SMES coil during grid dip

Figure 14. Duty cycle of DC-DC chopper during grid dip
The behavior of the SMES coil during the fault can be investigated through Fig 11 to Fig.13 which
respectively show the SMES coil current, SMES stored energy and the voltage across the coil. The
SMES coil energy is 2 MJ during normal operating conditions, when voltage dip occurs, SMES coil
instantly discharges its energy into the grid as shown in Figure 11. The characteristic of SMES
current shown in Figure 12 is similar to the energy stored in the coil. The charging and discharging
process of SMES coil can also be examined from the voltage across SMES coil (VSM) shown in
Figure 13. During normal operating conditions, VSM is equal to zero, it goes to negative value during
discharging process and will return back to zero level after the fault is cleared. As mentioned before,
the duty cycle of DC-DC chopper play important role to determine the charging and discharging
process of SMES coil energy. As shown in Figure 14, when voltage dip occur, power produced by
DFIG will also reduced, hence the FLC will see this reduction and act according to the membership
function rules shown in Figure 7, the duty cycle will in the range between 0 to 0.5 at this stage and
once the fault is cleared, the control system will act to charging the SMES coil. In this stage, duty
cycle will be in the range of 0.5 to 1 and will be back to its idle value of 0.5 once the SMES coil
energy reach its rated capacity.
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International Journal of Advances in Engineering & Technology, Nov 2011.
(c)IJAET ISSN: 2231-1963
5.2. Voltage Swell

Figure 15. Complying voltage profile at PCC with Spain and German HVRT during grid swell
The grid swell is started at 0.5 s and lasts for 5 cycles. As can be observed in Figure 15, without
SMES unit connected, during grid swell, voltage profile at the PCC will rise above 130 % and in this
condition, DFIGs that connected at the PCC have to be disconnected from the grid if complying with
both HVRT of Spain and German, however when fault is cleared out, the voltage profile can be soon
recovered and remains in the safety margin of both LVRT of Spain and German. When SMES unit is
connected, the voltage at the PCC is corrected to the safety margin of both HVRT of the grid codes of
Spain and German, hence avoid the disconnection of DFIGs from the grid.

Figure 16. Shaft speed during grid swell
Voltage swell at the grid side will force the voltage at the PCC will increase accordingly depends on
the percentage level of the swell. Hence, the power will be forced to level above the pre determined
rated, the speed control in this condition will limit the speed to avoid over-speeding of the shaft,
however in certain level of swell, the over speed protection may work and lead the generator to be
shut down. As described in Figure 16, with SMES connected to the PCC, the settling time and
oscillation of the shaft speed can be considerably reduced compared with the system without SMES.
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International Journal of Advances in Engineering & Technology, Nov 2011.
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Figure 17. Current behaviour of SMES coil during grid swell

Figure 18. Stored energy behaviour of SMES coil during grid swell

Figure 19. Voltage behaviour across the SMES coil during grid swell
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