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Enhancement of Wind Farm Electrical System
With a Superconducting Dynamic Synchronous Condenser
Floriane Fesquet, Pierre Bousseau,
Dr. Swarn S. Kalsi, Michael Ross Jean-Yves Roger, Dr. Régine Belhomme
American Superconductor Corporation Electricité de France
Advanced technology Department EDF R&D, TESE Department
121 Flanders Road 1 Avenue du Général de Gaulle
Westborough, MA 01581, USA 92141 Clamart Cedex, France
skalsi@amsuper.com jean-yves.roger@edf.fr
Tel: +1 508 621 4269, Fax: + 1 508 621 4321 Tel: + 33 1 47 65 3536, FAX: +33 1 47 65 3251
SUMMARY
As a result of increasing environmental concerns, wind power generation is growing worldwide. As wind energy
penetration levels increase, issues related to integration, transient effects, and voltage impacts become increasingly
important. The possible applications of the SuperVAR®, a Superconducting Dynamic Synchronous Condenser
(SDSC) for wind farms are discussed in this paper.
During grid voltage disturbances, the SDSC supplies VARs that keep the voltage close to the nominal value,
minimizing the risk of disconnection of the wind farm from the grid. For this reason, the incorporation of the SDSC
for providing voltage regulation and for improving the low voltage ride-through (LVRT) capability of wind farms is
considered.
The paper presents a simulation case study where a wind farm connected to a typical French distribution network is
operated with an SDSC. The results show that with an SDSC connected at the wind farm interface bus, the system
helps the wind farm withstand faults of longer duration. By increasing the voltage during a dip, the SDSC can:
• Improve stability by increasing the maximum power that can be provided to the network
• Sustain loads close to the wind farm (if any), which enables to limit the rotor speed increase of the wind
turbine
• Increase the voltage, thus decreasing the risk of disconnection by the triggering of the low voltage protection.
Thus, the SDSC helps keep the wind farm connected to the grid and enhances its LVRT capability. The SDSC can
also be utilized for voltage control during normal operation.
1 INTRODUCTION
Since the beginning of the 1990s, many countries have experienced a substantial growth of wind power generation
within their power systems. For instance, wind power generation capacity in the world increased from 2.9 GW in
1993 to 57.8 GW in 2005 (with countries such as Germany, Spain and Denmark leading this increase) [1]. Depending
on their size, wind farms can be connected to transmission or distribution networks. As wind energy becomes a larger
part of the total system generation and penetration levels increase, issues related to integration, transient effects, and
voltage impacts increase in importance. In particular, the connection of wind farms to a grid generally affects the
network voltage profile and may cause voltage fluctuations because the power generated is a function of the wind
speed.
Furthermore, there is a risk that the wind turbines will be disconnected if there is a voltage dip on the grid. This
situation mainly applies to classical induction generators, but may also be applicable to other types of wind turbine
generators. Several causes may lead to this grid disconnection:
• A loss of stability of the wind turbines
• The tripping of an over-speed protection
• The tripping of the low voltage protection (for instance with a relay threshold at 85%).
This sudden disconnection leads to a corresponding generation loss for several minutes. When this generation loss is
significant, the situation of the grid (which is already weakened by a voltage dip) may be worsened and its stability
may be further endangered.
As wind energy cannot be considered as marginal anymore, its low voltage ride-through (LVRT) issue has to be
resolved; otherwise it will limit wind power development. Note, however, that the new wind turbines (based on
doubly-fed generators or generators with full-power electronics converters at the stator) generally have a built-in
LVRT capability at an incremental cost of zero to a few percentage points of the base turbine cost [2]. For classical
1
induction generators and for the other wind turbines that do not have built-in LVRT capabilities, solutions should be
found in terms of additional external devices installed on wind farms installations.
The Superconducting Dynamic Synchronous Condenser (SDSC) is an external device that can be installed in a wind
farm at the interface bus with the grid to provide the wind farm a LVRT capability. In the event of a low impedance
fault in the system causing a deep voltage drop at the interface bus, the SDSC instantly supplies MVARs that can
boost the bus voltage, and enhances, with appropriate rating, the LVRT capability, helping to keep the wind farm
connected to the grid. In normal operation, the SDSC can be utilized for voltage control by supplying or absorbing
VARs.
The SDSC is a synchronous condenser featuring a rotor wound with high-temperature superconductor (HTS) wire.
The SDSC machine is capable of running with a very high field current (up to 2.0 pu.) for tens of seconds. This
capability allows the machine to deliver up to three-times rated output during a transient low-voltage event. An
8 MVAR, 13.8 kV SDSC unit was tested for about a year in a Tennessee Valley Authority (TVA) grid, supporting an
arc furnace. The results of this test and its potential applications in a wind farm are summarized elsewhere [3, 4].
A description of the SDSC and its ability to help a wind farm to cope with faults on the grid and to comply with fault
ride-through requirements of system operators are included in this paper. Simulation results are presented for a case
study where a wind farm was connected to a typical French distribution network and operated with an SDSC. In
addition, a decoupling inductor, in series with the line between the wind farm and the connection point, is also
considered. The decoupling inductor increases the electrical distance between the wind farm and the network such
that during faults the wind farm voltage remains high because of the short-circuit current contributions from both the
wind farm generators and the SDSC. The following four configurations are considered:
• With and without the SDSC
• With and without the decoupling inductor.
2 SUPERCONDUCTING DYNAMIC SYNCHRONOUS CONDENSERS
Conventional synchronous condensers properly sized for specific applications have been widely used by utilities over
long time with moderate level of maintenance. Their useful lifetime is often limited by field-winding insulation
degradation caused by field current heating during cyclic operation [5]. As shown in Figure 1, the field current of a
conventional machine must be increased by three times between no-load and full-load, which causes significant field
winding heating and adversely impacts its life. The design of the synchronous condensers based on HTS wire in the
rotor field winding requires only a small change in field current between no-load and full-load, and it always operates
at a constant temperature. Because of the reduced losses from the HTS wire, the SDSC is estimated to be about 1%
more efficient than copper-based conventional machines. The SDSC retains its high efficiency down to partial loads
of 25%, whereas the efficiency of conventional machines drops more rapidly due to its field winding losses, which
are caused by the high field current. Thus, the net impact on system losses and operating costs of SDSCs is very
favorable.
SuperVAR® Machine
Absorbing Generating
VARS VARS
1
VARS
(PU)
Conventional Synchronous
Machine
1 2 3
Field Current (Per Unit)
Figure 1 V-curves for Conventional Synchronous and SDSC (SUPERVAR) Machines
Because of its compact size and low-cost design, the new SDSC will also be an economic option for providing peak
and dynamic reactive compensation to a power system. It has a small foot print and is readily transportable in either a
trailer-mounted or skid-mounted format, making it easily placed in distribution substations. It has been shown to be
inherently stable to transient faults, and can provide up to twice its nominal rating for about one minute (peak rating)
during depressed voltage events. Because the large temperature excursions characteristic of copper-field windings are
eliminated, and because large VAR output is possible with very small variations in excitation current, the SDSC is
expected to be more reliable.
To demonstrate these attractive features of the SDSC, the first ± 8 MVAR prototype unit was operated for about a
year in the TVA grid near an arc furnace [3]. At this site the testing was intended to show not only that the SDSC is a
new, preferred option for handling arc furnace flicker, but that it also handles thousands of transients per day. The as-
2
delivered picture of the ± 8 MVAR SDSC machine (trailer-mounted), with key components highlighted, is shown in
Figure 2.
Start-up Synchronous Cryocooler
Motor Condenser Cryocoolers Compressors
Figure 2 +/- 8 MVAR SDSC with Key Characteristics Highlighted (Prototype)
The specifications for the prototype and production versions of the HTS dynamic synchronous condenser are
provided in Table 1. The price of a 12 MVAR, 13.8 kV skid mounted unit is expected to be US$1.2-1.5M, plus
US$10,000 annually to operate (in addition to operating losses). The TVA has placed orders for two commercial 12
MVAR machines, which are under construction for delivery in December 2006.
Table 1: SDSC Specifications
Parameter Value
Rating ± 12 MVAR (production unit)
Voltage 13.8 kV line-to-line
Ambient Temperature -30° C to +50°C
Losses 1.5% rating at 12 MVA, including 50 kW 480 V auxiliary power
3 DESCRIPTION OF THE STUDIED SYSTEM
The use of the SDSC to enhance the dynamic behavior of wind farm installations on the grid is discussed in this
paper.
A simulation study was performed to assess the enhancement of a wind farm electrical system with an SDSC and its
ability to assist a wind farm comply with the grid code requirements regarding LVRT capability (i.e. the ability of a
wind farm to stay connected during and following a specified voltage dip on the network). To do this, dynamic
simulations were completed, using EMTP and PSCAD software, for a typical medium-voltage system consisting of
the following:
• A classical 20 kV French distribution network
• A 10 MW wind farm composed of classical induction generators connected to a non dedicated 20 kV feeder
• A MV/HV transformer
• A simplified modelling of the HV network
• The SDSC.
Different MV and HV three-phase faults were then applied to this system.
3.1 General Characteristics of the System Studied
The layout of the system selected for simulations is shown in Figure 3 as a one-line diagram. The 20 kV MV network
is composed of seven feeders. However in the study, only the feeder to which the wind farm is connected is
represented in detail. The other feeders are modelled by equivalent loads connected to nodes NHTA and N20. The
NHTA-N20 feeder is a 12 km long feeder. The load of this feeder has been aggregated at node N20. The detailed
feeder where the wind farm is connected is 11.5 km long. The total load on the MV network is 20.4 MW,
8.16 MVAR. It is represented by constant impedances at the different locations, indicated by arrows in Figure 3. The
3
grounding of the 20 kV network is performed at the HV/MV substation with a reactance of 40 Ω and an X/R ratio
of 4.
The MV network is connected to the HV network (63 kV) through a 30 MVA 63kV/20kV transformer (between
nodes NHTA and NHTB) with the following characteristics (base Nominal Rating, Nominal Voltage):
• A 13% short-circuit impedance
• A 1% magnetizing current
• 0.57% Cu losses
• 0.1% iron losses.
This transformer is equipped with a tap-changer with 17 taps (±12%). The short-circuit power at the HV level of this
transformer (node NHTB) is 500 MVA when the wind farm is not connected. The HV network is modelled by two
lines and an infinite bus. The impedances of HV lines are chosen in order to obtain the appropriate short-circuit
power at bus NHTB.
The wind farm is connected to the MV network at node N11 (6.5 km from the bus bar of the HV/MV substation)
through a 20 kV line (between nodes NWF and N11).
The 12 MVA, 13.8 kV SDSC is connected to the network at node NWF through a 13.8kV/20kV transformer.
Simulations were performed with and without a decoupling inductor in series with the NWF-N11 20 kV line, with
and without the SDSC.
NINF
SCC 500 MVA
NHTB 63 kV
NHTA 20 kV
Grounding
N10 N20
N11
Decoupling reactance,
when considered
NWF N12
DSC
NWT N13
FC1 FC0 WTG
Figure 3 One-line Diagram of the Studied System
3.2 Wind Farm Including SuperVAR Superconducting Dynamic Synchronous Condenser
The wind farm is composed of 15 classical 660 kW induction generators and the SDSC. The SDSC can be modeled
in a grid simulation study with commercially available codes, such as PSS/E, EUROTAG, EMTP, PSCAD,
MATLAB, etc. The parameters for the 12 MVAR production model (which were also used in the simulation studies)
are summarized in Table 2. Just like the 8 MVAR prototype, the 12 MVAR machine is not expected to exhibit any
saturation effect [3] over the operating range (1.6 pu open-circuit voltage).
Squirrel cage induction generators were considered because the goal of the study was to assess possible enhancement
of LVRT capability of a wind farm having such generators. Even if this type of generator is not now frequently
installed, it is still relevant to consider them for wind farms with <1 MW unit generators and/or for weak grids
(including island grids).
To represent the wind farm, an aggregated equivalent 690V/20kV step-up transformer and an aggregated equivalent
squirrel cage induction generator were assumed with apparent power equal to the sum of apparent powers of the wind
turbine transformers and wind turbine generators, respectively. The equivalent transformer was a 12 MVA
4
0.69kV/20kV transformer with a short-circuit impedance of 6% and a magnetizing current of 1%. The data of the
equivalent wind turbine induction generator are given in Table 2.
Table 2: Parameters for the 12 MVAR SDSC and Equivalent Wind Turbine Induction Generator
Parameters SDSC WF equivalent generator Units Value
Synchronous reactance (xd), pu 0.63 Sn (apparent power) MVA 11.25
(15*0.75)
Transient reactance (xd’), pu 0.37 Un (nominal voltage) kV 0.69
Sub-transient reactance (xd”), pu 0.23 ωo (base frequency) rad/s 314.16
Sub-transient reactance (xq”), pu 0.23 Xs (stator reactance) pu (Un, Sn, W0) 0.107
Armature short-circuit time constant (τsc), s 0.048 Rs (stator resistance) pu (Un, Sn, W0) 0.0076
D-axis Transient O/C time constant (τdo’), s 968 Rr (rotor resistance) pu (Un, Sn, W0) 0.0063
D-axis Sub-transient O/C time constant (τdo”), s 0.14 Xr (rotor reactance) pu (Un, Sn, W0) 0.141
Q-axis Sub-transient O/C time constant (τqo”), s 0.28 Xm (magnetizing rea.) pu (Un, Sn, W0) 4.43
Armature resistance (ra), pu 0.005 H (inertia constant) MW/MVA*s 4
Inertia constant, s 1.1 Ri (negative sequence res.) pu (Un, Sn, W0) 0.0046
Xi (negative sequence rea.) pu (Un, Sn, W0) 0.2437
A constant mechanical torque was assumed on the generator shaft for this study. Furthermore, for the reactive power
compensation, a fixed 3 MVAR capacitor bank was assumed at wind turbine NWT node (for no load compensation
15*200 KVAR) along with a 1.5 MVAR additional capacitor bank (for load compensation 15*100 KVAR).
Parts of the simulations were performed with and without a 5mH decoupling inductor in series with the line NWF-
N11. This inductor increases the electrical distance between the wind farm and the network such that during faults the
wind farm bus (NWF) voltage remains high due to the short-circuit current contributions from both the wind farm
generators and the SDSC.
4 SIMULATION RESULTS
4.1 Description of the Simulation Cases
Simulations were performed for the following two types of faults:
• A three-phase fault on the distribution network, located on the line NHTA-N20 close to the node NHTA - This
fault is cleared in 450 ms by opening the feeder circuit breaker. This duration of the fault is close to a standard
requirement on the French MV network constrained by the protection scheme. No fault on the feeder of the
wind farm is considered because for such a fault leading to the opening of the circuit breaker of the feeder, the
disconnection of the wind farm is required by the system operator.
• A 400 ms three-phase fault on the transmission network - Such a fault on the transmission network can be seen
(depending on the network specificities) on a large part of the network and can impact numerous distributed
generation units. This fault represents a short-circuit on the HV side of the transformer and clears after 400 ms.
No line opening is simulated for this grid because it is assumed that the opening of the affected line will not
significantly modify the short-circuit power at the transformer.
In order to assess the contribution of the SDSC, these faults were simulated first without the SDSC, and then with it.
In addition, cases with a series inductor were considered for reducing the depth of the voltage dip at the wind farm
bus. More precisely, a 5 mH inductor was added between the wind farm and its connection point to the MV feeder.
This inductor increases the electrical distance between the fault and the wind farm. It has the following different
effects:
1. It minimizes wind-farm voltage reduction during the fault due to the short-circuit current from the wind
turbines and the SDSC. This can prevent disconnection due to low voltage protection relay tripping.
2. It decreases the power that can be transmitted to the network. Depending on the network specificities, it
could increase the risk of loss of stability of the wind farm.
3. Depending on the location of the decoupling inductor and the load downstream, sustained high voltage
could support the load and increase the active power the wind farm will provide during the fault. This can
limit the rotor speed increase and prevent disconnection from the grid due to rotor speed protection.
The size of the decoupling inductor has to be selected with judicial balance between voltage depth during the fault
and stability of the wind farm.
The results presented below consider eight cases, resulting from the possible combinations of two faults with two
topologies (with/without 5mH inductor) and the SDSC in or out.
5
4.2 Results Without “Decoupling Inductor”
Simulations were made as follows:
• With and without SDSC at the wind farm bus
• With SDSC but without wind turbine capacitors.
NWF voltage with a fault at 10% distance along the NHTA-N20 line is shown in Figure 4. During the fault simulated
on the network, because the voltage is low, the active power injected into the network by the wind farm is reduced.
This power becomes, at least during the fault, lower than the mechanical power available, thus the rotor speed of the
WTG increases. However, during the fault, the SDSC sustains the voltage by providing short-circuit current. This
situation enables the wind farm to provide more active power during the fault with the SDSC than without it. Thus,
the increase of the rotor speed of the wind turbines is reduced.
After the fault clearing, with SDSC, the slip is lower and the voltage recovery is quicker, so that the wind farm does
not loose stability. Without SDSC, a loss of stability is observed. The system recovers following a 0.45 s fault with
SDSC at NWF bus. Without SDSC, NWF voltage drops to about 0.2 pu and the system does not recover. However,
when the SDSC is present, the voltage drop is only about 0.3 pu, with or without NWT capacitors.
During the fault, the SDSC supplies reactive power to the system and maintains NWF voltage at a higher level. NWT
capacitors provide little help for maintaining NWF voltage at a higher level. After the fault is cleared, the voltage
recovers to its pre-fault value in 1.0 s with capacitors and in 2 s without capacitors. It can, therefore, be concluded for
the network considered that it is possible to eliminate NWT capacitors, if a SDSC is present at NWF bus.
During the fault, the VARs contribution of the SDSC is about the same irrespective of the presence of capacitors at
NWT bus, as shown in Figure 5. During the post-fault period, the SDSC contributes less VARs for a shorter period of
time with capacitors than without them. The SDSC contributes >2.5 pu current during the transient, as shown in
Figure 5. Current contribution of the SDSC is larger and for a longer duration without the capacitors than with them.
The SDSC can also be used to correct the system power factor during normal operation. The transient response of the
SDSC remains the same irrespective of the VARs it supplies during the normal operation preceding the fault.
1.2
With SDSC & NWT Caps
1.0
With SDSC but
0.8 Without Caps
Voltage (PU)
0.6
Without SDSC
0.4
0.2
0.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0
Time (s)
Figure 4 NWF Voltage with No SDSC, with SDSC, and with No Capacitors but with SDSC due to a
Fault on NHTA-N20 line – without Decoupling Inductor
20 4 With SDSC &
NWT Caps
3 With SDSC but
15 Without Caps
Reactive Power (MVAR)
2
SDSC Current (PU)
10 1
With SDSC & With SDSC but
NWT Caps 0
Without Caps
5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
-1
-2
0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 -3
-5 -4
Time (S) Time (s)
Figure 5 SDSC Reactive Power and Current During Transient with and without NWT Bus Capacitors
due to a Fault on NHTA-N20 Line – without Decoupling Inductor
6
Similar simulations were performed for faults on the NHTB bus. NWF Voltage with and without SDSC at NWF bus
are shown in Figure 6. At the end of a 0.4 s fault, NWF voltage drops to 0.05 pu without the SDSC and to 0.2 pu with
the SDSC. The system does not recover from this fault in the absence of the SDSC. Thus, the SDSC is very helpful in
letting the system survive this low voltage event. The VARs and current supplied by the SDSC are shown in Figure 7.
The first peak of VARs supplied by the SDSC is about 2-times its rating. Later the VARs contribution of the SDSC
drops to about 0.5- to1-time its rating for the remainder of the fault period and less than 0.5-time its rating following
the fault to keep the system connected to the grid through this low voltage event. Initially the SDSC current is >4 pu,
but later it settles to about 3 pu for the remainder of the fault duration. This short-circuit current contribution is
important for maintaining NWF voltage at a higher level than is possible without the SDSC. The higher voltage
enables the wind turbine to generate more power and to limit its speed increase.
The voltage drop to 0.2-0.3 pu at NWF bus is unacceptable if conventional protection schemes are employed, i.e. the
protection system might disconnect the wind farm if the voltage drops below 0.85 pu. In such a case, a series
decoupling inductor may be helpful, and/or a different rating of the SDSC should be selected. Nevertheless, it appears
necessary to modify the protection strategy to keep a wind farm connected long enough so that it could ride through a
low voltage event.
1.2
With SDSC & NWT Caps
1.0
0.8
Voltage (PU)
Without SDSC
0.6
0.4
0.2
0.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0
Time (s)
Figure 6 NWF Voltage with and without SDSC, Fault on NHTA-N20 Line – without Decoupling
Inductor
30.0 6
25.0
With SDSC & NWT Caps
4
Reactive Power (MVAR)
SDSC Current (PU)
20.0
15.0
2
10.0
0
5.0
0.0 1.0 2.0 3.0 4.0
0.0 -2
0.0 1.0 2.0 3.0 4.0 5.0 6.0
-5.0
-4
-10.0
Time (S) Time (s)
Figure 7 SDSC Reactive Power and Current During Transient, Fault on NHTB Bus – without
Decoupling Inductor
4.3 Results with a Decoupling Inductor
A series inductor (5 mH) is assumed in series with the N11-NWF line for isolating the wind farm from the system.
Simulations were again made with and without the SDSC at NWF bus, and with the SDSC but without NWT
capacitors. NWF voltage with a fault at 10% distance along the NHTA-N20 line is shown in Figure 8. The system
recovers from a 0.45 s fault with SDSC. Without SDSC, NWF voltage drops to about 0.15 pu but it does not recover.
However, when SDSC is present, the voltage drops to about 0.4 pu, with or without NWT capacitors. During the
fault, the SDSC supplies reactive power to the system and maintains NWF voltage at a higher level. NWT capacitors
help maintain NWF voltage at a higher level. The system voltage recovers to its pre-fault value within a couple of
seconds after the fault is cleared with the SDSC. It can, therefore, be concluded that NWT capacitors could be
eliminated if the SDSC is present at NWF bus.
The VARs contribution of the SDSC is higher and for a longer duration without capacitors at NWT bus than with
them, as shown in Figure 9. The presence of NWT capacitors reduces VARs support from the SDSC. Nevertheless,
the SDSC allows the system to handle this transient event without the support of capacitors. Current contribution of
the SDSC is shown in Figure 9 during the transient event. The SDSC contributes >2.5 pu current during the fault, and
contributes it for a longer period of time during the post fault period in the absence of capacitors.
7
1.2
With SDSC & NWT Caps
1.0
With SDSC but
0.8 Without Caps
Voltage (PU)
0.6 Without SDSC
0.4
0.2
0.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0
Time (s)
Figure 8 NWF Voltage, Capacitors and No SDSC, Capacitors and SDSC, SDSC – with Decoupling
Inductor
20 4
With SDSC &
18
3 NWT Caps
16 With SDSC but
Reactive Power (MVAR)
14
Without Caps
2
SDSC Current (PU)
12
1
10 With SDSC but
8 Without Caps
0
6 With SDSC & 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
4 NWT Caps -1
2
-2
0
-2 0.0 1.0 2.0 3.0 4.0 5.0 6.0 -3
Time (S) Time (s)
Figure 9 SDSC Reactive Power and Current during Transient with and without NWT Capacitors due
to a Fault on NHTA-N20 Line – with Decoupling Inductor
Similar simulations were performed with 0.4 s faults on the NHTB bus. The voltage at NWF bus, with and without
the SDSC at that bus, is shown in Figure 10. At the end of a 0.4 s fault, NWF voltage drops to 0.1 pu without the
SDSC and to 0.3 pu with the SDSC. The system does not recover from this fault in the absence of the SDSC. Thus,
the SDSC is very helpful in letting the system survive this low voltage event. The VARs and current supplied by the
SDSC are shown in Figure 11. The SDSC initially supplies 2-times its rating. Later the VARs contribution of the
SDSC drops to 0.5- to 1-time its rating for the remainder of the fault duration and less than 0.5-time its rating
following the fault to keep system operating through this low voltage event.
Even with the 5 mH decoupling series inductor, the voltage drops to 0.20-0.3pu at NWF bus. This drop may still be
unacceptable for the reasons discussed in Section 4.2.
1.2
1.0
With SDSC & NWT Caps
0.8
Voltage (PU)
0.6
Without SDSC
0.4
0.2
0.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0
Time (s)
Figure 10 NWF Voltage with and without SDSC – with Decoupling Inductor
8
30.0 5
25.0 4
With SDSC & NWT Caps
3
SDSC Current (PU)
Reactive Power (MVAR)
20.0
15.0
2
1
10.0
0
5.0
-1 0.0 1.0 2.0 3.0 4.0
0.0 -2
0.0 1.0 2.0 3.0 4.0 5.0 6.0
-5.0 -3
-10.0
-4
Time (S) Time (s)
Figure 11 SDSC Reactive Power and Current during Transient, Fault on NHTB Bus – with Decoupling
Inductor
4.4 Synthesis of the Results
The SDSC enhances the ability of the wind farm to withstand voltage dips without disconnection:
• During the fault, the SDSC brings additional short-circuit power that increases the voltage to a higher value
than without the SDSC present. Depending on the location of the load on the network, a higher voltage will
result in a higher load, thus, the wind farm can provide more active power to the network. This situation helps
to limit the turbine speed increase during the fault and, therefore, limits the risk of losing stability and of
disconnection due to the tripping of the over-speed protection. Furthermore, depending on the settings of the
protection system of the wind turbines, the higher voltage can prevent a disconnection due to tripping of
under-voltage protections.
• After the fault, the voltage recovery is quicker with the SDSC than without it. This feature reduces the risk of
losing stability.
The presence of the SDSC enhances wind farm bus voltage with and without the series inductor in the line
N11-NWF. Even with the SDSC, however, if NWF voltage drops below the 0.85 pu threshold voltage, the protection
system might automatically disconnect the wind farm from the grid. Depending on network and wind turbine
protections specificities, an appropriate sizing of the decoupling inductor and of the SDSC rating might help to
sustain the voltage above the protection threshold. However it might be required to amend the current protection
system to keep a wind farm connected to the grid to let it survive LVRT events.
This study also shows that wind turbine capacitors do not provide much help to the system during a transient event.
The system can survive LVRT events without these capacitors, provided an SDSC with an appropriate rating is
connected to the wind farm bus. The SDSC can provide the needed VARS during pre- and post-fault periods without
impacting its beneficial transient response during a fault.
5 DISCUSSION
The wind farm considered in this study consisted of squirrel-cage induction wind turbines. This choice was guided by
the following aspects:
• Currently, this technology is mostly used in the existing wind farms.
• Nowadays, the new wind turbines (based on doubly-fed generators or generators with full-power electronics
converters at the stator) generally have a built-in LVRT capability as well as reactive power (or even voltage)
control capabilities.
• For classical induction generators and for the other wind turbines that do not have built-in LVRT capabilities,
solutions should be found in terms of additional external devices installed on wind farms installations.
In many cases it might not be possible or economical to design the individual turbines in such a manner that the wind
farm meets the interconnection requirements based on the reactive capabilities of the wind turbines alone. In these
cases, the best approach could be to install ancillary reactive/voltage control equipment at the wind farm collector bus
to assist the wind farm weather the LVRT events.
SDSC machine technology is well suited for this purpose. With a combination of steady voltage regulating output and
fast reacting transient capability, the SDSC machine can address a number of additional common interconnection
issues and can assist the wind farm in meeting the requirements of the interconnection agreement.
A technical solution to provide a wind farm with fault ride through capability using ancillary reactive/voltage control
equipment is highly dependent on the specificities of the network and the wind farm considered. Thus, the final
choice of ancillary equipment (including the SDSC) requires a thorough system analysis that takes into account these
specificities.
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6 CONCLUSION
The study performed shows that the SDSC can efficiently help a wind farm to comply with the fault ride through
requirements of system operators. However, this is very specific to the wind farm and the network considered so that
the choice of the SDSC for this application requires a thorough analysis.
An SDSC provides VARs support to a wind farm to let it survive LVRT event without being disconnected from the
grid. This improves reliability of the wind farm system as well that of the grid to which it is connected. These
machines can also support distribution and transmission systems to regulate voltages during normal operation and to
provide LVRT capability. Moreover, site tests have shown that they can be used for mitigating flicker caused by arc
furnaces.
7 REFERENCES
1. EurObserv’ER. Wind energy barometer – 40,000 MW European target smashed. Systèmes Solaires, No.
171, February 2006, pp. 49-68.
2. SP Transmission Ltd Scottish Hydro-Electric Transmission Ltd. Scottish grid code review panel,
Consultation SA/2004 - Consultation on Technical Requirements for Windfarms. Available on SPT’s
website at http://gso.scottishpower.com/publicdocs/
3. S. S. Kalsi, D. Madura, T. MacDonald, M. Ingram, and I. Grant. Operating Experience of a
Superconductor Dynamic Synchronous Condenser. To be presented at 2005/2006 IEEE PES
Transmission and Distribution Conference and Exposition, May 21-24, 2006, in Dallas, USA.
4. S. S. Kalsi and M. Ross. Applications of Superconducting Synchronous Condensers in Wind Power
Integration. To be presented at 2005/2006 IEEE PES Transmission and Distribution Conference and
Exposition, May 21-24, 2006, in Dallas, USA.
5. S. S. Kalsi, K. Weeber, H. Takesue, C. Lewis, H-W. Neumueller and R. D. Blaugher. Development
Status of Rotating Machines Employing Superconducting Field Windings. Proceedings of the IEEE,
No. 10, October 2004, pp. 1688-1704
6. S. S. Kalsi, D. Madura, M. Ross, R. Belhomme, P. Bousseau and J-Y. Roger. Operating Experience of a
Superconductor Dynamic Synchronous Compensator. To be presented at the 2006 CIGRE Session, 27
August-1September, 2006, Paris, France.
7. D. Bradshaw. “Super” Reactive Power for the Power System through SuperVAR(TM) High Temperature
Superconductor Dynamic Synchronous Condensers. 2004 IEEE Power Engineering Society General
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