Note: Descriptions are shown in the official language in which they were submitted.
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"System and Method for Stabilising a Power System"
Field of the Invention
This invention relates to a system and method for stabilising a power system,
where predictable and/or unpredictable power fluctuations arise in the power
system, caused by the loading of, or the energy generated by, the power
system.
The invention has particular, although not, exclusive, utility in remote
locations
using renewable energy sources such as wind or sun for generating the power
supplied by such systems and which feed into a utility grid that may have
varying
load demands placed thereon. The invention also has utility in power systems
that
may have no renewable energy sources to supplement them and rely solely on
conventional power generation sources from fossil fuels such as gas and/or
diesel
driven generation sets.
Throughout the specification, unless the context requires otherwise, the word
"comprise" or variations such as "comprises" or "comprising", will be
understood to
imply the inclusion of a stated integer or group of integers but not the
exclusion of
any other integer or group of integers.
Background Art
The following discussion of the background to the invention is intended to
facilitate
an understanding of the present invention. However, it should be appreciated
that
the discussion is not an acknowledgement or admission that any of the material
referred to was published, known or part of the common general knowledge of
the
person skilled in the art as at the priority date of the application.
The quality of the power delivered, as well as the efficiency of such
delivery, is an
ongoing consideration in the design and construction of power generator plants
or
systems and power systems to be connected to utility grid systems or
individual or
groups of customers. This is true regardless of whether the power system
utilises
conventional energy sources (such as gas, diesel or a mains connected utility)
or
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renewable energy sources (such as solar, wind, bio-mass, micro-hydro energy,
tidal energy, wave energy or geo-thermal energy), although the problem is more
pronounced in the case of power systems utilising renewable energy sources.
To illustrate, renewable energy power generation systems ("RPGSs"), such as
those that utilise wind and/or solar energy, generate power having inherent
fluctuations resulting from the prevailing environmental condition, i.e. a
sudden
gust of wind or cloud cover obscuring the sun.
This problem can be overcome by combining the RPGSs with conventional power
generation systems ("CPGSs") with sufficient spinning reserve to cover these
fluctuations, but this means that at least one CPGS has to be on-line all of
the
time and the CPGSs are not necessarily loaded to their optimum efficient
operating point. It may also mean that more CPGSs are on-line than are
required
with the extra CPGS or CPGSs providing the spinning reserve.
In such situations, since some CPGSs have a minimum loading requirement, it
may mean that the amount of energy that could be generated by the RPGSs is
not fully utilised.
Furthermore, to accommodate sudden decreases in the load on CPGSs, either by
sudden increases in the available renewable energy supply or sudden decreases
in the total system load, the CPGSs must be loaded such that they can decrease
their output power quickly. This extra constraint provides additional
restriction on
the operating region of the CPGSs and has led to the development of systems
designed for stabilising power generated by RPGSs or combined RPGSs/CPGSs
rather than supplementing it at times of need.
Fig 1A illustrates the basics of a power stabilising system. The power
stabilising
system comprises a main grid line 37' to which various power sources 39a',
39b'
are connected for supplying power to the grid. Various loads (not shown) are
connected in and out over time, and a power system stabiliser 41' is connected
to
smooth out fluctuations from the power sources 39a', 39b' or for peak lopping
of
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the consumer load. The utility 39a' is indicative of RPGSs, while 39b' is
indicative
of CPGSs.
The power system stabiliser 41' commonly consists of a battery/inverter system
and this system is described in more detail below.
As shown in Fig. 1 B, a battery/inverter system 43' is connected to the
utility grid
and load 45' in conjunction with a synchronous compensator 47' (also known as
a
synchronous condenser). Essentially, a synchronous compensator comprises a
synchronous alternator connected to a three-phase power system to provide
voltage support.
The battery/inverter system 43' provides the frequency control, and the
synchronous compensator 47' supplies fault current and provides voltage
control.
Thus power generation is provided with continuous stabilising of power
fluctuations as well as backfeeding of power into the utility grid.
In this arrangement, the fluctuations from the RPGS are smoothed by
controlling
the power going into the battery/inverter system 43'; absorbing energy when
the
RPGS is generating more power, and supplying energy when the RPGS is
generating less power. Furthermore, due to the large amount of energy stored
within such a system, this system can operate on a second-by-second basis, on
a
sub-second basis and even on a minute-by-minute basis.
The problem with this arrangement, however, is that the battery/inverter
system
43' is only designed for a limited number of charge/discharge cycles before it
loses its ability to hold charge, recharging is generally slower than
discharging,
and recharging times can be significant. This is unacceptable in a RPGS where
it
is vital to be able to recharge the battery as quickly as it has been
discharged in
order to have its full capacity available again in the shortest possible time.
This
can be compensated for by oversizing the batteries to cope with fast
recharging,
but this leads to higher capital and maintenance costs and does not solve the
problem of the battery having a finite number of charge/discharge cycles.
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Furthermore, operating the synchronous compensator 47' in addition to the
battery/inverter system 43' adds additional standby losses to the system as
well
as capital cost.
A further example of a power stabilising system is shown at Fig. 1 C, where
the
battery/inverter is replaced with a flywheel system 49'. The flywheel system
49'
comprises a flywheel 51' connected to the rotor of a motor/generator 53',
which in
turn is connected to a bi-directional converter 55' and then to a bi-
directional
inverter 57' to provide for frequency regulation with changes in flywheel
speed.
Thus, power generation may be provided with continuous stabilising of power
fluctuations and backfeeding of power into the utility grid. However, in such
arrangements, the synchronous compensator 47' is still required in order to
provide fault current and, in some systems, voltage control.
As with the battery/inverter system, the fluctuations from the RPGS in this
arrangement are smoothed by controlling the power going into the flywheel
system 49'; absorbing energy when the RPGS is generating much power, and
supplying energy when the RPGS is generating less power. However, unlike the
battery/inverter system, this system can run only on a second-by-second basis,
or
on a sub-second basis, but not on a minute-by-minute basis.
The problem with this arrangement is that, in order to provide sufficient
fault
current to the RPGS, a synchronous compensator 47' still has to be used,
adding
to the additional standby losses and the capital cost of the system.
Additionally,
the inverter 57' needs to have a rating equal to the rated power output of the
flywheel/inverter system.
Yet another method of effecting power system stabilisation for renewable
energy
sources is to use dynamic dumping resistors. Dynamic dumping resistors work on
the basis that there is always excess renewable energy available and by
dumping
the excess energy dynamically, the frequency can be controlled. To elaborate,
the fluctuations from a renewable energy source are smoothed by controlling
the
power used by the dump load; absorbing more energy when the renewable
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energy source generator is generating more power, and absorbing less energy
when the renewable energy source generator is generating less power.
Dynamic dumping resistor systems can run on a second-by-second basis, on a
sub-second basis, or on a minute-by-minute basis.
However, dynamic dumping resistor systems can never supply energy since they
store no energy. The energy such systems "dump" is considered as waste energy
and is not the primary product of the system. Sometimes the "dumped" energy is
put to a useful purpose, such as space heating or water desalination.
Furthermore, dynamic load dumping systems only work when there is a constant
(not average) oversupply from the RPGS. For example, in the case of wind
turbines, the wind speed has to be constantly high or a large number of high
capacity wind turbines need to be installed. This limits this type of
controlling
method to locations with very good wind resources or where extremely high
energy costs exist such that installation of sufficient wind turbines to
achieve the
requisite capacity is justified. In any event, it is often necessary to
operate the
system in conjunction with a synchronous compensator, thus adding to the
capital
costs of the plant.
Disclosure of the Invention
It is therefore an object of the present invention to provide an improved
power
stabilising system that eliminates, or alleviates; some or all of the problems
mentioned above.
In accordance with one aspect of the present invention, there is provided a
power
system stabiliser for stabilising a power system supplying a load where power
fluctuations may arise as a consequence of variations in the power system or
the
load, comprising:
sensing means for sensing a property of the power system;
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power system interface means for electrically connecting with the power
system to allow flow of electrical energy between the power system and the
power system stabiliser; and
control means for controlling the flow of electrical energy between the
power system and the power system stabiliser;
wherein the control means is responsive to the sensing means to control the
flow
of electrical energy between the power system and the power system stabiliser
to
maintain the property of the power system at a predetermined value to
stabilise
the power system.
Preferably, the power system stabiliser further comprises load interface means
for
electrically connecting with a stabilising load to allow flow of electrical
energy
between the power system stabiliser and the stabilising load, the load
interface
means being electrically connected with the power system interface means to
allow flow of electrical energy therebetween, and the control means
controlling the
flow of electrical energy between the power system and the power system
interface means and the load interface means, wherein the control means is
responsive to the sensing means to control the flow of electrical energy
between
the power system and the power system interface means and the load interface
means to maintain the property of the power system at the predetermined.value
to
stabilise the power system.
Preferably, the property of the power system sensed by the sensing means is a
grid frequency of the power system.
Preferably, when the property is the grid frequency of the power system, the
flow
of electrical energy is controlled by the control means so that input real
power to
the power system stabiliser is generated to maintain the grid frequency at the
predetermined value.
Preferably, the property of the power system sensed by the sensing means is a
grid voltage of the power system.
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Preferably, when the property is the grid voltage of the power system, the
flow of
electrical energy is controlled by the control means so that input reactive
power to
the power system stabiliser is generated to maintain the grid voltage at the
predetermined value.
Preferably, the sensing means is integrated with the control means.
Preferably, the control means further controls the flow of electrical energy
between the power system and the power system interface means and between
the load interface means and the stabilising load.
Preferably, the control means further controls the reactive power between the
power system and the power system interface means.
Preferably, the control means comprises first control means to control flow of
electrical energy between the power system and the power system interface
means, and second control means to control flow of electrical energy between
the
load interface means and the stabilising load.
Preferably, the first control means includes a switching power supply
providing a
direct current power supply to the second control means; the sensing means
senses a voltage of the direct current power supply; and the second control
means controls flow of electrical energy in response to the voltage sensed by
the
sensing means to control the voltage level of the direct current power supply.
Preferably, the first control means receives three phase power from the power
system and the first control means controls flow of electrical energy between
the
power system and the power system interface means in response to the sensing
means.
In accordance with a further aspect of the present invention, there is
provided a
method for stabilising a power system supplying a load where power
fluctuations
may arise as a consequence of variations in the power system or the load, the
method comprising:
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sensing a property of the power system; and
controlling the flow of electrical energy from the power system to maintain
the property of the power system at a predetermined value to stabilise the
power system.
Preferably, the method comprises sensing a grid frequency of the power system.
Preferably, the method comprises controlling the flow of electrical energy to
generate an input real power to the power system stabiliser to maintain the
grid
frequency at the predetermined value.
Preferably, the method comprises sensing a grid voltage of the power system.
Preferably, the method comprises controlling the flow of electrical energy to
generate an input reactive power to the power system stabiliser to maintain
the
grid voltage at the predetermined value.
Brief Description of the Drawings
As previously described, the following drawings refer to various prior art
arrangements, wherein:
Figure 1A is a schematic block diagram of a typical utility grid power system
using
renewable energy and conventional power generation sources with a power
system stabiliser;
Figure 1 B is a schematic block diagram of a power system stabiliser using
conventional battery energy storage; and
Figure 1 C is a schematic block diagram of a power system stabiliser using
conventional flywheel energy storage.
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With respect to the following description of preferred embodiments of the
invention, the description is made with reference to the following drawings,
wherein:
Figure 2 is a schematic block diagram showing a power system stabiliser
according to a first embodiment of the invention connected into a typical grid
power system and a first load;
Figure 3A is a graph of AC Real Power against Grid Frequency exemplifying the
use of the power system stabiliser with a nominal 100% load at nominal grid
frequency using soft under frequency protection with 100% pseudo spinning
reserve;
Figure 3B is a further graph of AC Real Power against Grid Frequency
exemplifying the use of the power system stabiliser with a nominal 50% load at
nominal grid frequency using full active frequency control with 50% pseudo
spinning reserve;
Figure 4 is a schematic block diagram showing the power system stabiliser
according to the first embodiment of the invention connected to a second load;
Figure 5 is a schematic block diagram showing the power system stabiliser
according to the first embodiment of the invention connected to a third load;
Figure 6 is a schematic block diagram showing a power system stabiliser
according to a second embodiment of the invention connected to a first load;
Figure 7 is a schematic block diagram showing the power system stabiliser
according to the second embodiment of the invention connected to a second
load;
Figure 8 is a schematic block diagram showing the power system stabiliser
according to the second embodiment of the invention connected to a third load;
Figure 9 is a schematic block diagram showing a power system stabiliser
according to a third embodiment of the invention connected to a load;
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Figure 10 is a schematic block diagram showing a power system stabiliser
according to a fourth embodiment of the invention connected to a load;
Figure 10A is a graph of AC Real Power against Grid Frequency exemplifying the
use of the power system stabiliser with a nominal 0% load at nominal grid
frequency using full active frequency control with 100% spinning reserve,
according to the fourth embodiment;
Figure 11 is a schematic block diagram showing a power system stabiliser
according to the first embodiment of the invention connected to two loads.
Best Models) for Carrying Out the Invention
Several preferred embodiments of the invention will now be described by way of
example only.
In a first preferred embodiment of the present invention there is provided a
power
system stabiliser 19 for stabilising a power generation system 9 having a grid
11
supplying a load 17. Power fluctuations may arise as a consequence of
variations
in the power generation system 9 or the load 17. The power system stabiliser
19
is shown schematically in Figure 2.
The power generation system 9 is controlled by a power system controller (not
shown).
The power generation system 9 consists of a renewable energy generator 13
utilising a renewable energy source and a conventional energy generator 15
utilising a conventional energy source. The renewable energy source utilised
may
include, but is not limited to, wind, solar, biomass, micro-hydro, tidal,
wave, or
geo-thermal energy sources. The conventional energy source may include, but is
not limited to, gas, diesel, or a mains connected utility energy source.
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In this arrangement, the grid 11 is a three-phase ac wired power system that
draws power from the power generation system 9. The grid 11 supplies power to
numerous loads 17 variably connected to the grid 11.
The power system stabiliser 19 generally comprises sensors (not shown) for
sensing a property of the power generation system 9,- power system interface
means in the form of an AC grid interface 21 for electrically connecting with
the
power generation system 9 to allow for the flow of electrical energy between
the
power system stabiliser 19 and the power generation system 9, and load
interface
means in the form of an AC load interface 25 for electrically connecting with
a
stabilising load 20.
In alternative exemplary embodiments, the power system interface means is
integrated with the load interface means.
In alternative exemplary embodiments, multiple stabilising loads may be
connected to the power system stabiliser 19. Figure 11 shows such an
arrangement where the power system stabiliser is electrically connected with
two
stabilising loads 20, 20'.
Returning to Figure 2, the power system stabiliser 19 also comprises a link 29
for
electrically connecting the grid interface 21 and the load interface 25 to
allow for
the flow of electrical energy therebetween.
Control means for controlling the flow of electrical energy between the grid
interface 21 and the load interface 25 is also provided. The control means is
responsive to the sensors to control the flow of electrical energy between the
grid
interface 21 and the load interface 25 to maintain the property of the power
generation system 9 at a predetermined value to stabilise the power generation
system 9.
In the several preferred embodiments of the invention described herein, the
sensors are integrated with the control means. This is not an essential
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requirement, however, and in other embodiments of the invention the sensors
may not be integrated with the control means.
The control means comprises first control means in the form of a grid
interface
control system 23 to control the flow of electrical energy between the power
generation system 9 and the grid interface 21, and second control means in the
form of a load interface control system 27 to control the flow of electrical
energy
between the load interface 25 and the stabilising load 20.
The grid 11 supplies input real power and input reactive power to the grid
interface 21. The input real power, minus losses, flows along the link 29 to
the
load interface 25 at a dc-link voltage.
The grid interface 21 and the grid interface control system 23 enable dynamic
control of the input real power and input reactive power supplied to the power
system stabiliser 19.
The grid interface 21 has high-speed switching transistors (not shown), input
filter
inductors (not shown) and capacitors (not shown). The combination of
transistors,
filter inductors and capacitors enables a three-phase ac connection with the
grid
11. The high-speed switching transistors may be MOSFETs, IGBTs, GTOs,
IGCTs, Thyristors or similar devices known to those skilled in the art.
By using high-speed switching transistors, such as those mentioned above, in
the
grid interface 21 the current going into the grid interface 21 is controlled.
Alternatively, the operation of the high-speed switching transistors is
controlled
such that the back electromotive force ("EMF") or pseudo-back EMF of the grid
interface 21 is controlled. In this manner both the input real power and the
input
reactive power supplied to the power system stabiliser 19 can be controlled.
In one exemplary embodiment of this, the control of power is effected by
sensing
the grid voltage by the sensors and controlling the high-speed switching
transistors such that the grid-side current of the ac interface is sinusoidal
and has
some phase relationship with the grid voltage sensed. This allows the
magnitude
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of the input real power and input reactive power to be varied by varying the
magnitude of the grid-side current and phase angle of the grid side current.
In alternative exemplary embodiments, the grid interface 21 and the grid
interface
control system 23 are set up so as to act as a grid connected inverter and
emulate
a synchronous generator output with dynamic control of the sinusoidal or
pseudo-
sinusoidal internally generated back EMF or pseudo-back EMF and the power
angle. For example, the grid connected inverter is effected by controlling the
high-speed switching transistors such that a sinusoidal or other waveform back
EMF or pseudo-back EMF is generated inside the grid interface 21 with
controllable amplitude, frequency and phase. This back EMF interacts with the
grid voltage and grid characteristics such that real and reactive power flows
from
the grid 11 into the grid interface 21. .
The grid interface 21 further comprises filters to reduce/eliminate radio
interference and switching frequency harmonics injected into the power
generation system 9 and grid 11.
The grid interface 21 and the grid interface control system 23 are configured
to
operate in one of the following ways:
(a) Fully-Independent (Stand Alone): - as an independent power
system component, with no control from other components in the
power generation system 9, such as the power system controller;
(b) Semi-Independent: - as a component that communicates with an
upper-level control system (not shown) of the power system
controller of the power generation system 9; or
(c) Non-Independent: - as a component integrated in a power
generation subsystem (not shown) of the power generation system
9, namely as part of an inner kernel of the power system controller,
for example, operating with tight integration of the conventional
energy generator 15.
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Each configuration will now be described in further detail where like numerals
reference like parts.
Fully-Independent Configuration
In this configuration, the sensors for sensing a property of the power
generation
system 9 are integrated in the grid interface control system 23.
The properties of the power generation system 9 sensed by the sensors are a
frequency of the grid 11 and a voltage of the grid 11. The flow of electrical
energy
is controlled by the control means so that input real power to the power
system
stabiliser 19 is generated to maintain the grid frequency at a predetermined
grid
frequency, and input reactive power to the power system stabiliser 19 is
generated to maintain the grid voltage at a predetermined grid voltage.
This operation will now be described in further detail.
The control of input real power drawn from the grid 11 on the ac side of the
power
system stabiliser 19 is governed by the frequency of the grid 11. Control of
input
reactive power drawn from the grid 11 on the ac side of the power system
stabiliser 19 is governed by the voltage of the grid 11.
In controlling the input real power drawn from the grid 11 on the ac side of
the
power system stabiliser 19, the grid interface control system 23 (via the
sensors)
determines the frequency of the grid 11 and sets the real power according to
the
determined frequency.
To determine the frequency of the grid 11, the grid interface control system
23
measures 3-phase voltages of the grid 11 to generate a set of measured values.
The set of measured values from the 3-phase system is then transformed to
equate to a set of measured values for a 2-phase system. From this set of
measured values for a 2-phase system a rotating voltage vector is obtained.
The
grid interface control system 23 then measures the radian frequency of the
rotating voltage vector to determine the frequency of the grid 11.
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It should be noted that this type of measuring method is not prone to
measurement noise, as is the case with zero-voltage-crossing measuring
methods. However, zero-voltage-crossing methods are used in alternative
embodiments.
Furthermore, this type of frequency measuring method can determine changes in
the grid frequency very dynamically, and certainly in less than one voltage
cycle.
Once the frequency of the grid 11 has been determined, the grid interface
control
system 23 uses the frequency of the grid 11 as a variable in an algorithm
processed by the control means to determine the amount of real power to be
drawn from the grid. In the embodiment described, the algorithm is programmed
into the grid interface control system 23 to determine when the grid frequency
falls
below a predetermined level and then commences reducing the magnitude of the
input real power in a linear arrangement commensurate with the difference
between the grid frequency and the predetermined level. To elaborate, Figure
3A
shows a graph of an AC Real Power level against a Grid Frequency value
exemplifying the use of the algorithm.
Normally, when a load 17 is added to a power generation system 9 greater than
the spinning reserve on that power generation system, the frequency of the
grid
11 will decrease. If the frequency of the grid 11 decreases too much the power
generation system 9 will cease to operate and the result will be a blackout.
By
using the abovementioned algorithm with the correlation of AC Real Power to
Grid
Frequency shown in Figure 3A, the power system stabiliser 19 will reduce the
input real power drawn from the grid 11 and thus reduce the total load on the
grid
11. The input real power to the power system stabiliser 19 will be 100% of the
value of the stabilising load 20 until the grid frequency decreases to 49.5hz.
At
this grid frequency the load reduction will start and the input real power
drawn
from the grid will reduce linearly to 0% at 49.OHz. If, with this reduction in
total
load on the grid 11, there is now sufficient spinning reserve on the grid to
supply
power to the added load 17, that is, the spinning reserve is greater than zero
including the addition of the load 17, the grid frequency will reduce no
further, and
no blackout will occur.
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For example, the supply of input real power to the power stabilising system 19
may be 100kW for a 50Hz power system. If the frequency of the grid 11 falls
below 49.5Hz, the supply of input real power may be set to linearly decrease
such
that the supply of input real power will be 100kW at 49.5Hz and OkW at 49.OHz.
In this manner, an under frequency problem can be limited by reducing the load
the grid sees at the input to the power system stabiliser 19. This is referred
to as
soft-under-frequency protection.
In an alternative embodiment, the results of another linear algorithm using
the
correlation of AC Real Power to Grid Frequency shown graphically at Figure 3B
is
processed by the grid interface control system 23. The graph in Figure 3B
illustrates the use of the power system stabiliser 19 as a full active
frequency
controller actively controlling the grid frequency. Such a power system
stabiliser
19 dampens power system frequency transients and gives protection for under
frequencies due to insufficient spinning reserve in the power system 9, and
protection for over frequencies due to excessive generation, which may be
caused by a sudden reduction in load 17. When controlled in this way, the
power
system stabiliser 19 gives 50% psuedo-spinning reserve capability, meaning
that
the amount of spinning reserve necessary on the power system may be reduced
by 50% of the value of the stabilising load 20.
The grid interface control system 23 determines a voltage of the grid 11. Once
the voltage of the grid 11 has been determined, the grid interface control
system
23 uses the voltage of the grid 11 as a variable in the algorithm programmed
into
the grid interface control system 23 to determine the supply of input reactive
power.
Semi-Independent Configuration
In this configuration, the upper-level control system of the power system 9
allocates predetermined set-point values for the supply of input real power
and
input reactive power to the power system stabiliser 19.
The grid interface control system 23 then controls the operation of the high-
speed
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switching transistors in the grid interface 27 in such a way that the actual
real
power and the actual reactive power drawn from the grid 11 is the same as the
desired input real power and the desired input reactive power drawn from the
grid
11 respectively.
Additionally, the grid interface control system 23 measures the frequency of
the
grid 11 and the voltage of the grid 11 in the same manner as described for a
Fully
Independent Configuration. The frequency of the grid 11 is used as a variable
in a
first algorithm programmed into the grid interface control system 23 to
calculate
the desired supply of actual input real power to the power system stabiliser
19 and
the voltage of the grid 11 is used as a variable in a second algorithm
programmed
into the grid interface control system 23 to calculate the desired supply of
the
actual input reactive power to the power system stabiliser 19. This
configuration
uses the predetermined set-point value of input real power unless the grid
frequency drops below a predetermined value, in which case it uses the value
calculated by the first algorithm. By doing this the predetermined set-point
is
predominantly used, but the pseudo-spinning reserve capability and soft under
frequency protection are preserved.
Furthermore, this configuration uses the predetermined set-point value of
input
reactive power unless the grid voltage deviates outside some predetermined
limits, in which case it uses the value calculated by the second algorithm. By
doing this the predetermined input reactive power set-point is predominantly
used,
but the voltage control capability is preserved.
For example, the desired supply of input real power to the power stabilising
system 19 may be set to 50kW by the upper level control system for a 50Hz
power system. If the frequency of the grid 11 falls below 49.5Hz, the supply
of
input real power may be set to linearly decrease such that the supply of input
real
power will be 50kW at 49.5Hz and OkW at 49.OHz. In this manner, an under
frequency problem can be limited by reducing the total load on the grid 11.
This is
referred to as soft-under-frequency protection.
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Non-Independent Configuration
In this configuration, the grid interface 21 and the grid interface control
system 23
comprise part of an inner kernel of a power system generation control means.
For
example, the grid interface 21 and the grid interface control system 23 may be
integrated with the conventional energy generator 15 so that the control of
supply
of the real power and reactive power attained by the grid interface control
system
23 is operable against the conventional energy generator 15.
The frequency control algorithms and voltage control algorithms of the grid
interface control system 23 may be adaptive. For example, the grid interface
control system 23 may measure how many conventional energy generators 15
are present on the grid 11 by measuring the inertia of the grid 11 and change
the
supply of real input power and/or reactive input power accordingly.
With reference to Figure 4 (illustrating an ac motor load as the stabilising
load
20a) and Figure 5 (illustrating other types of ac loads as the stabilising
load 20b),
the load interface 25 and the load interface control system 27 act to
transform the
dc-link voltage into an output voltage of the load interface 25 suitable for
powering
the ac load. Further, in such a situation, the load interface 25 comprises an
inverter which changes the dc-link voltage into a 3-phase ac voltage for
powering
the stabilising load 20.
The amount of power flowing to the stabilising load 20 is dependent on the dc-
link
voltage. In this configuration, . the control of supply of the input real and
input
reactive power provided by the grid interface control system 23 is independent
of
the load interface control system 27 controlling the supply of power to the
stabilising load 20.
For very fast dynamic response, and to avoid dc-link voltage overshoots, the
power system stabiliser 19 comprises a feed-forward control line 37 between
the
grid interface control system 23 and the load interface control system 27.
This feed-forward control line 37 from the grid interface control system 23
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transmits the input .real power being drawn from the grid to the load
interface
control system 27. This enables the load interface control system 27 to
control
the load interface 25 so that it draws an equivalent amount of power from the
do
link 29. This gives superior transient dynamics and reduces the amount of do
link
variation when compared to the method where the load interface control system
27 waits until the do link voltage has risen before it increases the power
drawn by
the load interface 25.
In accordance with a second preferred embodiment of the present invention as
illustrated in Figure 6, where like numerals reference like parts, the
stabilising load
20 is a do load 31. Accordingly, load interface 25 and load interface control
system 27 act to transform the dc-link voltage into an output voltage suitable
for
powering the do load 31.
The output voltage from the load interface 25 can vary to effect a change in
power
used by the do load 31.
In the second embodiment, the sensors sense the dc-link voltage of the direct
current power supply to the load interface 25 and load interface control
system 27,
and the load interface control system 27 controls the flow of electrical
energy in
response to the dc-link voltage sensed by the sensors to control the dc-link
voltage.
When the do load 31 is resistive, the load interface control system 25 (via
the
sensors) measures the dc-link voltage and varies the duty cycle, and hence the
output voltage, of the load interface 25 such that the input real power into
the link
29 from the grid interface 21 (minus losses) flows to the do load 31. To
implement
this in the embodiment being described, the load interface 25 comprises a
chopper type dc/dc converter controlled by hysteresis band current control
provided by the load interface control system 27.
If the dc-link voltage rises above a first predetermined threshold value, for
example 800V, then the chopper switch turns on, and power flows to the do load
31, thereby reducing the dc-link voltage. Conversely, if the dc-link voltage
drops
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below a second predetermined threshold value, for example 700V, then the
chopper switch turns off and power stops flowing to the do load 31 and thereby
causing the dc-link voltage to rise. In this way, the duty cycle of the
chopper is
controlled to keep the dc-link voltage within the limits set by the first and
second
predetermined threshold values and the power flowing from the grid interface
21
to the do load 31.
Furthermore, in this embodiment, the grid interface control system 23 operates
independently of the load interface control system 27.
The load interface 25 could also have filters (not shown) to reduce/eliminate
radio
interference and switching frequency harmonics.
The do load is not limited to being resistive, however, and other types of do
loads
may be used. In this regard, Figure 7 shows a do motor 33a as the do load,
while
Figure 8 shows other do loads 33b which could include, but are not limited to,
electrolysis apparatus etc.
Some loads can not take all the power required all the time. For example, when
the load is a motor, it takes a finite amount of time for the motor to
accelerate to
operating speed, and while running at a lower speed the amount of power that
can be fed to the motor may be limited. Thus, to allow the power system
stabiliser
19 to operate with very fast dynamics it may be necessary to add a dump load
to
the link 29 so that the supply of input real power can be independently
controlled
even if the load can not take all the power that is supplied. In this case the
dump
load dissipates the transient energy.
Accordingly, such an arrangement is the subject of a third preferred
embodiment
of the invention, as shown in Figure 9, where like numerals reference like
parts.
This embodiment, as illustrated, includes an ac motor load 20. However, it
should
be appreciated that any type of ac load can be applied if an ac load interface
25 is
used, and any type of do load can be applied if a do load interface is used.
The
variation between this embodiment and other embodiments is the inclusion of a
dump load 34 as described in the previous paragraph to ac and do systems. The
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dump load 34 is connected into the dc-link 29 between the grid interface 21
and
the load interface 25, via a dc-do dump load interface 36 and its associated
dump
load control 38.
In a fourth embodiment, again where like numerals reference like parts and as
illustrated in Figure 10 of the drawings, the power system stabiliser 19 is
able to
operate in a bi-directional manner by adding a power source 40 to link 29.
Figure 10 shows one instance of this where the power source 40 comprises the
output of a diesel generator 41 connected to link 29 via an autotransformer 43
and
a rectifier 45. In this instance, power flowing from the grid 11 into the
power
system stabiliser 19 and to the load 17 would be the same as described
previously. However, the power system stabiliser 19 could supply power from
the
diesel generator 41 to the grid 11 via link 29.
Furthermore, in this embodiment, the power system stabiliser 19 is used to
hold
down the frequency of the grid 11 by increasing the load on the power
generation
system 9. Additionally, the power system stabiliser 19 is used to hold up the
frequency of the power generation system 9 by supplying power to the power
generation system 9.
Figure 10A shows a graph of AC Real Power levels against a Grid Frequency
value exemplifying the use of the power system stabiliser 19 of the fourth
embodiment as a full active frequency controller. This type of control also
dampens power system frequency transients, gives protection for under
frequencies due to insufficient spinning reserve in the power generation
system 9,
and protection for over frequencies due to excessive generation which may be
caused by a sudden reduction in load 17 on the power generation system 9. Such
a power system stabiliser 19 gives 100% real spinning reserve capability
meaning
that the power source 40 provides 100% of a power system stabiliser 19 rating
in
real spinning reserve. Additionally, it provides 100% extra loading capability
meaning that it can absorb 100% of the power system stabiliser 19 rating
should
there be an over supply of generating capacity.
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In summary, the power system stabiliser 19 as described in the aforementioned
embodiments, can:-
(a) provide pseudo-spinning reserve in a power system;
(b) provide frequency control in a power system;
(c) provide voltage control in a power system;
(d) provide reactive power support; and
(e) reduce harmonics in the power system.
The power system stabiliser 19 achieves these effects by the following means.
(a) Pseudo-spinning reserve in a power system.
When the power system stabiliser is operating above zero real power the
input real power drawn from the power system can be reduced very quickly
(dynamically) and controllably at any point in time. This reduction in the
required input real power is commonly known as "load-shedding," however
the power system stabiliser 19 described herein can do this in a continuous
(non discrete steps) and dynamically controllable manner. This may be
called "dynamic load changing" to distinguish it from load-shedding.
This ability to dynamically change the input real power to the power system
stabiliser means that the amount of spinning reserve required for the power
system can be reduced. For example, if a power system would normally
require 100kW of spinning reserve to accommodate a sudden increase in
load of 100kW, a power system stabiliser system could be used to reduce
the required spinning reserve. If a power system stabiliser was able to
reduce its input real power by 50kW then only 50kW of real spinning
reserve would be required. If the power system stabiliser could reduce its
power real input by 100kW, then zero real spinning reserve would be
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required. This shows that the power system stabiliser acts as pseudo-
spinning reserve.
One way for the power system stabiliser 19 to achieve dynamic load
changing is by measuring power system frequency and adjusting the input
real power according to an algorithm programmed into the power system
stabiliser 19 as previously described. In this manner, the power system
stabiliser 19 has power system frequency measuring techniques built into
it.
Another way for the power system stabiliser to achieve dynamic load
changing is by being able to set the instantaneous input real power value
according to an input from some other control system.
(b) Frequency control in a power system.
The power system stabiliser can achieve frequency control by changing its
input real power according to the measured frequency of the power
system. This way it increases its input real power (absorbs excess power)
when there is an over supply of power to the power system, thereby
holding the power system frequency down; and decreases its input real
power when there is an under supply of power to the power system,
thereby holding the power system frequency up. Some simple or
complicated control algorithm can be used to achieve this.
(c) Voltage control in a power system.
The power system stabiliser can achieve voltage control of the power
system by changing its input reactive power according to the measured
power system voltage. This way it absorbs excess reactive power when
there is an over supply of reactive power to the power system, thereby
holding the power system voltage down; and decreases its input reactive
power when there is an under supply of reactive power to the power
system, thereby holding up the power system voltage. Some simple or
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complicated control algorithm can be used to achieve this. The input
reactive power can also be negative or positive, and can exist with or
without a real load, i.e. the real and reactive input power can be controlled
independently.
(d) Reactive power support.
The grid interface of the power system stabiliser can supply or consume
reactive power, thus supporting the reactive power of the power system.
This reactive power can flow with or without a real load, i.e. the real and
reactive input power can be controlled independently.
(e) Harmonics in the power system.
Power Quality and the ability to actively cancel out harmonics are
becoming an increasingly important issue in power systems. The power
system stabiliser 19 is able to provide this function of cancelling out
external harmonics that are present in the power system.
It should be appreciated that the scope of the present invention is not
limited to
the particular embodiments described herein. Accordingly, variations to
certain
components of the system as dictated by conventional engineering practice or
the
practical application of the invention to a particular site and which form
part of the
common general knowledge of the field of the invention, but which do not
depart
from the general spirit and principles of the present invention, are envisaged
to fall
within the scope of the invention and not detract from it. In particular,
while the
present invention is most suitable for use with renewable energy power
generation systems because the level of penetration of the renewable energy
can
be increased, and thus assist in earlier repaying the additional capital costs
associated with such generation systems, it should not be considered as
limited to
use with such systems. For example, the present invention can be used with
conventional power generation systems to provide greater fuel efficiency by
carrying the spinning reserve in the power system stabiliser 19.
