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Interrelationships among the network code, the evolution of wind power generation, and
reactive power compensation. Special subjects contained in a network code.
Rafael Guerrero C. Inelap-Arteche
Abstract— As it appears, the evolution of the
generation of wind power has been oriented
towards satisfaction of the requirements,
increasingly within more narrow limits
imposed by network codes. Among all of the
determining factors, the administration of
reactive power has stood out. In the era of
fixed-speed induction generators, the
requirements for network access were lax,
considering the lack, for all practical
purposes, of capability curves.
The advent of DFIGs and the evolution of
electronic switches (IGBTs), have allowed
wind farms (WFs), for the first time, to be
able to administer reactive power through
the availability of a capability curve and in
this way to comply with more stringent
network codes.
This updating has taken place very recently,
with the start-up of a WF integrated with
DFIGs and full-capacity converters (full
converters), which has a capability curve
similar to that of a conventional plant.
Looking towards the future, we can perhaps
predict the elimination of the gearbox.
Currently, wind turbine generators operate
better at high speeds, and they therefore
require gear boxes to reduce the speed.
Considering that these gear trains are
expensive, subject to vibrations, noise, and
fatigue, and require lubrication and
maintenance, elimination of the gearbox
would have great advantages.
Coincidentally, recent versions of network
codes authorize the operator to present the
WF as if it were a conventional plant, using
the capacity for reactive power within a wide
segment of the capability curve. This already
involves not only a narrowing of the bands
within which the voltage must move as well
as the power factor, but now the WF must
conform with a polygon of critical points of
operation, which are impossible to satisfy
with first-generation generators. Under this
new operating policy, proper administration
of reactive power involves a combination of
the supply from the machines, from switched
capacitor banks, and from dynamic reactive
compensation achieved through devices that
are based upon the application of electronic
switches.
The network code, tacitly, gives the owner of
the WF responsibility for solving
extraordinary problems, or problems outside
of the typical catalog, known as subsynchronic resonance or torsional effects.
The purpose of this presentation is to provide
some idea of the linkages among all of these
factors.
Keywords—Wind farm (WF), network code,
point of common coupling point (PCC)
induction machines, synchronous generator,
reactive control, sub-synchronous resonance.
I.
INTRODUCTION
Those who have observed the continuing
development of wind power generation can
understand our perspective. It is more than a
prediction. Instead, it is the point of view of someone
interested in the evolution of this engineering
problem.
Electrical companies think that the reliability of
electrical power systems could be exposed to great
risk if the variable sources of wind energy continue to
increase in the near future. These concerns have
recently been expressed in the contents of the
network codes.
As the degree of penetration of wind generation
increases day by day, the codes used to regulate
interconnections contain correspondingly strict and
demanding requirements. These can be summarized
as follows:
A. Requirements of the network codes
• The operator of the system can opt for two modes of
control: by voltage or by power factor (pf), with a
particular hierarchy. The metering point will be that
of the Point of Common Coupling, or PCC.
• Controls by voltage or by pf must function
correctly, either within or bordering on the limits of
pf=0.95
(forward/reverse), and within or bordering on the
voltage band that runs from 0.95 to 1.05 p.u. In
graphic form, these conditions represent a polygon of
operation.
• Independently of the limits imposed, any mode of
control must function correctly whenever the WF is
generating, even when it is generating at maximum
rated power.
• Section 9.6.2 in reference [1] confirms the
applicability of this section for all of the generation
installations, including wind farms. Section 9.6.1
indicates that the pf of 0.95
(forward/reverse) is not necessarily applicable
automatically to wind generation installations.
However, studies of impact on the system (System
Impact Study, or SIS) have determined that such is
the range required. Consequentially, the range of 0.95
(forward/reverse) for the power factor has been
accepted as a general rule.
Voltage gaps (Low Voltage Ride Through, or LVRT).
Figure 1
(voltage–time) is self-explanatory. In the blue area,
the tripping of the installation is not permitted. In the
green area, a reduction of the installation's generation
is permitted.
Finally, for any pair of voltage-time coordinates in
the white area, wind generation sources can be
tripped.
for external disturbances in the network. The
generator owner must ensure that any problem related
to resonance in the system will be covered by the
design of the WF's own protection and control
schemes.
B. Historical review
Wind Energy Conversion Systems (WECS) transform
the energy of wind into electrical energy. Wind is a
highly variable resource that cannot be stored, and
wind energy conversion systems must be operated as
such.
A brief review of these systems is included next.
• Wind energy is transformed into mechanical energy
by means of a wind turbine that has one or more
blades (three is the most common number). The
graphic in figure 3 shows the behavior of a wind
generator.
Figure 1. Requirements for voltage gaps.
• The WF can generate reactive power in forward or
reverse, according to the request of the system
dispatcher.
• From a system management point of view, it is
highly desirable to view a WF as if it were a
conventional generation plant. This is equivalent to
saying that, with very few exceptions, it must comply
with the dispatch policies for real and reactive power.
• Strict compliance with the conditions of the
polygon of operation as shown in figure 2 could be
interpreted as a full integration of the wind generation
in the same manner as a conventional generation
plant, without exceptions.
Figure 2. Minimum requirements for reactive power
• In order to preserve the reliability of the system, the
WF must operate with rolling reserve.
• Network codes include constrictive conditions, such
as those specified in the NERC in reference [2].
This text suggests the restriction (including the
disconnection) of variable generation as soon as the
conventional sources see their output reduced to a
value near the minimum possible.
• Resonance. The installation of wind generators and
their interconnections must be designed so as avoid
provoking resonances in the external system, as well
as to avoid unnecessary outputs from the WF itself,
Figure 3. Behavior of a wind generator.
• The turbine is coupled to the generator by means of
a mechanical traction train.
• This usually includes a gearbox that is used to
synchronize the low speed of the turbine with the
higher speed of the generator.
• One recent set of design criteria for wind turbines
uses multiple-pole machines and low generator
speeds, usually synchronous with field winding or
exciting using permanent magnets, in order to
eliminate the gearbox.
• Some turbines include control of the wind's
incidence angle (pitch regulated), in order to control
the amount of power that will be transformed.
Turbines that lack this type of control are known as
Stall.
• The electrical generator transforms the mechanical
energy into electrical energy. The generator can be
either synchronous or asynchronous. If the first case,
a more or less elaborate system for exciting will be
required, or else a configuration with permanent
magnets.
• Variable-speed systems require the presence of an
electronic power interface that can be configured in a
variety of ways.
• Compensated units can include pf correction
equipment (active or passive) and filtering devices.
• Switched equipment can be designed to develop a
smooth connection, and this is usually a requirement
in the standards.
The standards also specify the minimum number of
protection schemes that, as a minimum, must
accompany the generator units.
• Finally, the control systems can have varying
degrees of complexity.
prevented from taking an excess of power from the
wind by means of an aerodynamic effect.
C. Energy conversion systems
In regard to their turning speed, turbines can be
divided into two types: fixed-speed units and
variable-speed units. In fixed-speed machines, the
generator is connected directly to the supply network.
The frequency of the network determines the turning
speed of the generator, and therefore the rotor. The
low turning speed of the turbine rotor is coupled to
the turning speed of the generator through a gearbox
with a particular transmission relationship. The speed
of the generator depends upon the number of pairs of
poles and the frequency of the network.
Asynchronous generators should have equipment for
control of reactive power.
The advantages of fixed-speed systems are their
simplicity and low cost, but the problem is that they
do not participate in the reactive power problems in
the system. See figure 4
Variable-speed systems feature better energy
production, less mechanical stress, and less
dependable production of power due to wind
variations and the oscillations of the system. In most
of these systems, the gearbox can be eliminated.
Some of this equipment can require harmonic
compensation as a result of the presence of the
electronic converters.
The scheme that is most commonly applied consists
of asynchronous generators connected directly to the
network. This is used in a great number of turbines,
from 50 to 2500 kW. Those with the highest capacity
use a pitch control, while others have some other type
of control.
The direct connection of the synchronous generators
to the network has few applications and is used only
with a small number of low-capacity turbines, mainly
in isolated systems (Figure 6).
Figure 4. Fixed-speed system.
• This is essentially an induction motor with
mechanical parity applied at deflection.
• The stator winding is tri-phasal, and the machine
tends to have 4, 6, or 8 poles.
• A cage rotor is typically used.
• It may have a higher-than-usual rotor resistance in
order to improve stability and global cushioning.
• There is no control over the reactive power (Q) and
the fault current is not sustained.
In variable-speed machines, the generator is
connected to the network either directly or through an
inverter system. The system for exciting is supplied
through an inverter (see figure 5). The generator's
turning speed, and therefore that of the rotor also, is
not linked to the frequency of the network, and the
rotor can operate at a speed adjusted to the actual
situation in terms of wind speed.
Figure 5. Variable-speed system.
All of the equipment can have pitch or stall
regulation. Systems with pitch regulation rotate
around their longitudinal axes through a regulation
mechanism. With this mechanism, mechanical power
can be reduced, in accordance with the characteristics
of the turbine. Systems with stall regulation do not
have such mechanisms, but when the wind exceeds
the levels of rated operation, these systems are
Figure 6. Full converter system.
Variable-speed systems have the most applications,
with a progressive increase in the size of the wind
turbines. The main objective of operation with
variable speed is to optimize efficiency, for example,
in order to maximize the capture of the available
wind energy.
The three basic types of turbines mentioned above
can be reconfigured or expanded. The figure
presented by CIGRE [3] is a good summary that
suggests and documents these changes.
II. DEVELOPMENT
A. Sources of reactive power – capacity for reactive
power Premises:
1. It seems to be a general criterion that, in a stable
state, the voltages should remain within the range of
0.95 to 1.05 p.u. Following a simple contingency
(output of an element), the voltages of the
transmission bars in a stable state should remain
between 0.9 and 1.1 p.u.
2. Most companies, if not all, expect that the reactive
power compensation should be capable of sustaining
a pf of ±0.95 in the PCC, at full generation and with
voltages in the range of 0.95 to 1.05 p.u.
3. The reactive power generation resources are
defined or restricted by the capability curves that are
shown, progressively, later in this article.
4. The load tap changer (LTC) has a range of
regulation of ±10%, generally in 32 steps.
5. This is a convenient quantity for banks of switched
capacitors.
6. Some manufacturers use the converter line-side in
the rotor circuit to supply or consume reactive power.
This allows regulation of voltage/pf that exploits the
reactive power capacities of the individual wind
turbines.
A primitive capability curve is shown in figure 7.
Figure 9. Characteristics of a DFIG wind turbine.
With the support of reactive power from the line-side
converter and selection of the appropriate generator,
the capacity for reactive power can expand up to a pf
of 0.9 forward/reverse.
Figure 7. Primitive capability curve.
The figure shows the switching sequences, inside and
outside, of the capacitor bank. When the smooth
start-up finishes, the turbine itself generates 200 kW.
Then the condensers (1×170 and 1×120 kVAR) enter
as soon as the induction machine begins to generate
any amount of real power. These two capacitors
remain on-line at all levels of generation. The 50
kVAR capacitor is switched inside when the
generation reaches 100%, and is switched outside
when generation falls below 50%.
The following capability curves are given in p.u. The
curve shown in figure 8 uses power electronics to
delimit the authorized zone. Wind turbines can
operate permanently at a point within the zone
delimited by the black lines (triangular area). The
zone itself is the limit for having a pf of ±0.95.
In fact, the manufacturer's guarantee is only effective
if the machine operates within the triangular area.
In regard to the voltage gaps (LVRT), wind turbines
usually go out of service when the voltage drops
below 0.9 p.u. There is an option that allows the wind
turbines to continue operating during and after faults
in the transmission system, resulting in a severe drop
in voltage in the WF. Other available options could
be:
• 0.3 p.u. of voltage per 100 mseg in the PCC.
• 0.15 p.u. of voltage for 625 mseg in the PCC, for
mono-phasal and tri-phasal faults.
• 0 p.u. of voltage for 200 mseg in the PCC, for
mono-phasal and tri-phasal faults.
Finally, for a wind turbine with full converter, the use
of a line power converter to supply or absorb
reactive power results in a family of curves that
depend on the voltage present at the generator's
terminals. Figure 10 is self-explanatory. However, a
few comments may be appropriate. At higher voltage,
less capacity for reactive supply. At a voltage of 1.1
p.u., the capacity of reactive power generated in
reverse is almost zero. To a certain level of voltage,
the capacity of the pf in forward is substantially high.
This is the closest approximation to the capability
curve of a synchronous generator and it satisfies, on
its own, the verticies of the operating conditions for
the polygon shown in figure 2.
Currently, the act of using the converter line-side to
reach an extended capability curve can be viewed as
if each wind unit had a VAR controller (Dynamic
source of reactive power).
Figure 8. Characteristics of a DFIG wind turbine.
A capability curve that is a little more advanced uses
the converter line-side in the rotor circuit, in order to
supply or consume reactive power, and in order to
open up the wind turbine's reactive capacity more
widely (see figure 9). The limits seen in red are
optional.
The owner of the WF and the selected turbine
manufacturer must determine whether the multiresonance mechanical system is susceptible to the
interactions and the consequential damage caused by
its proximity to the compensated transmission line.
This information needed for this type of study is very
specialized. The cooperation of the turbine
manufacturer is indispensable.
Figure 10. Characteristics of a full converter wind
turbine.
B. More about the network code - Requirements
• The WF (the generation system) and its
interconnections must be designed to avoid the
introduction of disturbance in the external network
and to avoid unnecessary output of the wind farm
caused by disturbances occurring in the network.
• The capacity for reactive power must be sufficient
to ensure stability in a stable state and in a transitory
state during and after a disturbance.
• The interconnection studies will determine the
appropriate quantity of reactive power and must be
performed for each particular case.
• The supply of reactive power must be available
across the full range of operating conditions.
• The use of mechanically switched, static VAR
compensators or similar equipment can represent
acceptable alternatives for providing all or part of the
reactive supply, and must be corroborated by an
interconnection study.
C. First documented case – Sub-Synchronous
Resonance (SSR)
In this case, the WF is interconnected with a systems
whose topology includes a line with serial
compensation. There is a potential risk that, for some
reason, the WF will remain connected radially
through the compensated line.
Connected in this manner, the serial capacitors would
be able to resonate with the inductance of the
network, inducing the phenomenon of subsynchronous resonance, with high possibilities of
interaction with the WF's generators. In agreement
with the network code, the external electrical
company has an enormous interest in knowing the
magnitude and the effects of the interaction that
would occur between the converter's controls and the
transmission system in the presence of SSR.
It is also important that the owner of the WF
understands the severity of the phenomenon viewed
in terms of his own interest. The administration of the
WF must perform the analysis and must take the
necessary measures to protect its equipment. It must
be emphasized that it is responsible for the damage
that could occur on its own side of the
interconnection. On the other hand, it must be
recognized that the interconnection between the
external transmission system and the serial
compensated line is not a typical interconnection.
The crowbar:
The crowbar, located in the rotor circuit, is used to
protect the power converter from the excessive
overcurrents that are presented as a natural response
when some type of fault occurs. It allows safe
demagnetization of the machine through the
intercalation of resistances in the rotor circuit.
Figure 11. Diagram of a DFIG with a crowbar.
As soon as the presence of high currents is detected,
the crowbar is inserted in the rotor circuit that
remains, temporarily, in short circuit through a DC
resistance (25 Ω), by the action of a rapid-action
switch (Insulated Gate Bipolar Transistor, or IGBT).
The crowbar is automatically activated when a
specific level of overcurrent is exceeded in the rotor
(2 p.u.). The crowbar redirects the current of the
Doubly-Fed Induction Generator, or DFIG, which
remains temporarily disconnected.
The advantage of the crowbar becomes evident after
the following qualitative comparison: a simple
induction machine, with the three phases of the rotor
winding in short circuit. With a particular value of
real power generated and a constant speed, it will
respond to a fault in the following terms: the direct
current in the rotor is greater than the AC, indicating
that the rotor's transitory time constant is large. The
quasi-component of DC in the stator current drops in
a time span of approximately 25 msec, while the
quasi-component of DC in the rotor falls at around 40
mseg. The value of the rotor's peak current will be a
little under 5 p.u.
By comparison, the induction machine is configured
as DFIG, with a crowbar that significantly increases
the resistance of the rotor circuit. As a consequence,
this causes the rotor's time constant to be much
shorter (10 msec). We can affirm that the high
resistance of the rotor will cause the rotor's flow to
drop, when high, in about 10 msec.
The oscillogram shown in figure 12 was recorded
after a severe disturbance, in which the WF remained
connected in series with a compensated line. The
crowbar's control was not capable of decoding a
high-magnitude current with a frequency of 24
cycles, and as a consequence, simultaneous failure of
35 crowbars was produced.
In the case of figure 14, the reactive power supplied
by the capacitor bank flows towards the system
(blue), but only if the system's voltage in p.u. is lower
than the voltage in the WF (red). On the other hand,
the reactive power is absorbed by each of the WF's
units, which is absolutely not recommendable. If the
system voltage, in reality, has such low values, it
could be concluded that the network must be severely
perturbed, which would signify that the bypass
capacitors would be necessary for satisfaction of only
one of the operational conditions for the polygon in
figure 2. The description in this paragraph is valid for
a scheme using full converter wind turbines and
control by voltage.
Figure 12. Oscillogram of an actual disturbance.
D. Second documented case – Supply of reactive
power
In this case, the reactive power supplied by the
capacitor bank, shown in red in figure 13, reestablishes the natural power of the 230 kV line (135
MW). Without the injection of the bypass capacitors,
the line itself requires 53.7 MVAR, while
transporting 2.2 times the natural power.
Figure 14.
III. CONCLUSIONS
• The continuing integration of variable wind
energy into electrical systems raises concerns in
terms of reliability. All of these concerns can be
seen reflected in the clauses of the network codes.
• The magnitude of the integration of variable
resources will require significant changes to the
traditional operational and planning criteria.
• Large-scale wind farm developments are typically
located in remote areas that lack sufficient
infrastructure. The criteria for planning must
guarantee the availability of sufficient transmission
and distribution resources, as well as resource
flexibility, for the unblocking of energy resources, as
well as for the management of their inherent
variability.
• Variable sources can be disconnected if the
generation coming from conventional sources is close
to the safe minimum. Disconnection of the power
generated by steam units is a cause for concern;
reliable short-term operation would be seen as
compromised, since these would not be capable of
being synchronized when the wind stops being
available. This implies a great risk of seeing the
global reliability of the system reduced.
Figure 13.
BIOGRAPHY
Rafael Guerrero C. Electrical Engineer, National
Autonomous University of Mexico (UNAM by its
Spanish acronym), 1963. He has spent most of his
career working for Mexico's Federal Electricity
Commission (CFE by its Spanish acronym), as the
head of the central office of protective engineering,
the department of network analysis, and the
specialized engineering unit. He spent 2 years in
Spain with PTI (Power Technologies Inc). He was
coordinator and full-time head of the power systems
engineering program at the UNAM until 2008. He is
the author of computer programs for calculation of
faults, power flows, transitory analysis, harmonic
analysis, and dynamic analysis. He is a member of
the Inelap-Arteche team as a power systems engineer.