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Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)
ISSN 1843-6188
CONSIDERATIONS ON THE ELECTRICAL GRIDS
OF WIND GENERATOR INTERCONNECTIONS
Gheorghe COMĂNESCU, Sorina COSTINAŞ
Power Engineering Faculty, University “Politehnica” of Bucharest, Romania
E-mail: [email protected]
Abstract: Wind power centers are composed of a relatively large
number of small power generators. All these generators are
connected to an internal network which serves to discharge power
and energy produced. The work is analyzed in terms of technical,
economic and safety aspects in operating versions of schemes for
this network connection, in the context of their possible use of wind
power that will be built in Dobrogea (Romania). The influence of
the number and power of each unit power generators is
highlighted, as well as the way their field placement in determining
optimal solutions both in terms of configuration schemes and in
terms of voltage level of this network.
Mechanically, the turbine must be protected from
rotational speeds above some value that could lead to
catastrophic failure.
Keywords: wind power plant, electrical network, energy
where:  is the installation yield; Cp itself is not a
constant for a given airfoil, but rather is dependent on a
parameter, called the tip-speed ratio, which is the ratio of
the speed of the type of the blade to the speed of the
moving air stream [3].
Wind energy technology has become not only a mature
renewable energy technology, but also a mature
electricity generation technology. A lot of different
concepts have been developed and tested ([1], [2], [5],
[6]). These developments have led to conceive many
wind energy conversion systems schemes based on many
criteria such: project size, WT models and age of
turbines, fixed speed or variable speed wind turbine,
environmental conditions and implementation site
(onshore or offshore), the rate of produced power (small
or large wind turbine), islanded or grid connected WT.
Whatever type of generators and power electronics used
can all be regarded as equivalent generators producing,
finally, three-phase alternating current at 50Hz at
voltages from low voltage field and unitary power
between hundreds kilowatts and 4 MW.
1.
An important issue for wind turbines is the efficiency. In
fairly steady conditions, the power extracted from the air
stream by the turbine blades can be characterized by the
Betz’s formula:
Pel  c p   Pwind ,
INTRODUCTION
In Romania, especially since the liberalization of the
energy market, the electrical companies are forced to
identify and coordinate a lot of processes and promote
competition. The necessity to increase the use of the
renewable energy sources in the energy balance of
Romania will most probably lead to such legislative
changes which would make. Wind Power Plants (WPP)
is the fastest growing energy sector.
A Wind Power Plant, among which are as the one being
being deployed in Dobrogea, is composed of a relatively
large number of electric generators powered by wind
turbines. A wind turbine (WT) is a rotating machine which
converts the kinetic energy from wind (Pwind) into mechanical
energy (Pmec). If the mechanical energy is then converted to
electrical energy (Pel), the machine is called wind power
converter (Figure No. 1). The kinetic energy stored is
proportional to the rotational inertia and the square of rotor
speed. Thus, increasing the speed to twice the original value
means that the kinetic energy increment is four times higher.
2.
[kW]
(1)
ELECTRICAL GRIDS OF WIND
GENERATOR INTERCONNECTIONS
Collection of energy generation products through a low
voltage network whose voltage (Un) equals the terminal
voltage generators (Ung), although apparently attractive,
has many drawbacks: high sections and voltage drop,
low transport capacity, etc. As a result, the vast majority
of WPP use a higher voltage network, typically medium
voltage. This involves fitting each equivalent generator
(G) through a transformer (T) whose energy charge
collection in the internal network.
In essence, a generator driven by a wind turbine will
have a simplified scheme in Figure No. 2.
Figure1. Wind Power Conversion [6].
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Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)
ISSN 1843-6188
The WPP will connect to the power system at a point of
interconnection (POI) found normally in the network of high
or very high voltage to it, since the expected completion of
power plants whose total power (Pt) could exceed 100MW.
problems of achieving internal network to collect
electricity. The amount of energy that can be transited
through a network strongly depends on the mains voltage
and the type of lines used: overhead or underground.
Thus, tension is greater, the transport capacity is higher,
energy losses are lower. Moreover, the cost of preparing
the network will increase with increasing voltage and
what is important to increase substantially if you switch
from one voltage domain to another, for example, from
medium voltage (6, 10, 20kV) at high voltage (110kV).
Regarding the networking, underground lines are more
expensive than the overhead once but have a higher
transit capacity. That is why the vast majority of
solutions used in WPP uses medium voltage for the
internal network to collect and frequently used
underground power lines.
Since the cost differences in medium voltage networks
for 6kV, 10kV and 20kV are small steps is preferable to
use higher voltage. Medium voltage range is between
6kV and 35kV. In our country, voltage average value
most commonly used is 20kV.
Transformer
Wind turbine
generator equivalent
To collector
internal network
Transformer
Switchgear
Medium
voltage
Low
voltage
Figure 2. Scheme Of An Equivalent Generator.
In most cases the internal network for the collection of
energy is achieved at a lower voltage and therefore, the
WPP will have to contain one or more substations in
high or very high voltage transmission lines and energy
to the point of interconnection the power system [4].
Schematically, a WPP internal network topology with a
single transformer substation is shown in Figure No. 3.
4.
POI – point of
interconnection to the
national grid
High voltage
transmission
lines
Using an internal network of medium voltage power
collection inevitably leads to limitation of power that can
be released by the network and therefore the number of
generators that can be connected to that network. An
item that will limit the total power generators that can be
connected to a transformer station is the maximum
allowable short circuit at this substation and network
transmission lines connected to it.
Short-circuit current in medium voltage network depends on:
- the transformer strength 110kV/20kV;
- generators type driven by wind turbines;
- the number of the generators and their nominal
power.
Contribution to short circuit a transformer can be found
through a relationship like:
High voltage
switchgear
Transformer
substation
Collector network
Medium voltage
switchgear
Wind
turbine
generators
Wind
turbine
generator
s
I scc 
S nT
3 U n  u sc%
100
(2)
that were used notations: SnT = nominal power
transformer, in kVA; Un = nominal voltage network, in
kV; Usc = transformer short-circuit voltage, in %.
Turbine driven generators contribution depends on their
type. For example, high frequency synchronous
generators that charged energy at 50Hz through a
rectifier-inverter have a contribution not exceeding the
rated current of the generator. Such a power plant of
100MVA, intake is the order of 3kA. In case of
synchronous generators without power electronics,
intake may be higher and may exceed 10kA.
If we consider a threshold of short-circuit current of
40kA that in 110kV/20kV station can be installed more
than 100MVA transformer (to take into account a shortcircuit voltage in transformer circuit to 10%).
Figure 3. Internal Network Of An WPP.
The following will address some aspects of the internal
network for the collection of energy produced by wind
turbine driven generators in the prospect of WPP in
Dobrogea.
3.
TRANSIT CAPACITY OF THE INTERNAL
NETWORK WITH A SINGLE
TRANSFORMER
THE VOLTAGE OF INTERNAL NETWORK
COLLECTING THE ELECTRICAL POWER
The large number of generators and large surface on
which they stand raises technical and economic
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Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)
Under these conditions a single transformer power can
not be more than 100MVA.
To dispose higher power, the substation may be equipped
with several transformers, with a corresponding
interconnection pattern on the medium voltage (sliced bus
bars etc.). And perhaps use a voltage range of very high
voltage (for example, 220kV) for interconnection with
the power system.
Internal network or collection of energy produced by the
WPP will be composed of several networks of stations
attached to each transformer. In the following we will
refer to the case of WPP single transformer station.
5.
STRUCTURES
NETWORK
OF
MEDIUM
ISSN 1843-6188
More generators are connected by switching terminal
apparatus, a medium voltage line, either directly on its
axis or the derivatives on this line.
Number of generators that can be connected to a
departure from the station medium voltage is limited by
the maximum section may be used in medium voltage
line type used.
For example, in a 20 kV cable line with section 150mm2,
the total power that can be connected does not exceed
15MVA, while on an overhead line the same maximum
load voltage can not exceed 6 MVA.
For a line power plant of 100MVA number of departures
from 20kV station will be around 10 to 15 departures.
VOLTAGE
Medium voltage
switchgear
Medium voltage network of WPP may have different
structures inspired by existing electricity distribution
networks in urban or rural. We address below only three
types of such networks.
5.1. Individual Network For Each Generator
Such a network is schematically represented in Figure
No. 4. Each generator is connected by a line of medium
voltage direct its own station medium voltage.
Medium voltage
switchgear
Wind turbine
generators
Figure 5. Internal Radial Tree Network.
The advantage of such a network is represented by the
fact that the volume of the network is lowered and the
number of line cells from the station is likely reliable.
The main obstacle is reliability: any defect on a medium
voltage line or the cell will leave the station out of
service more than one generator. Also equipped with
switching apparatus out of the transformer from the
generator terminals is more complex and therefore more
expensive. It is one of the solutions commonly used in
view of its advantages.
Wind turbine generators
Figure 4. Individual Network For Each Generator.
5.3. Loop Network
A loop network (Figure No. 6) involves connecting a
number of generators, the entry-exit system at a medium
voltage line connected to both ends of the medium
voltage station. It is commonly used system in our
country in urban areas.
The maximum number of generators connected to such a
line is limited, as in the tree network, such as section
lines and maximum use. Network volume, respectively,
its cost is higher than for tree network but the network is
to better internal electrical fault lines or in cells medium
voltage substation.
The only advantage of this scheme is that there is a
certain independence of each generator to the network
and possible faults in medium voltage lines affecting
only one generator.
Instead the volume of medium voltage network is very
high and medium voltage station should have an equal
number of cells at the line of generators. These elements
will drastically limit the number of generators that can
be connected to medium voltage station. Is an acceptable
solution only for WPP with a small number of turbines.
5.2. Radial Tree Network
Radial tree network is represented schematically in
Figure No. 5.
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ISSN 1843-6188
Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)
at stations with a small number of generators, usually
under 20.
For plants with high number of generators are
recommended tree networks radial or looped networks.
Looped networks, although more expensive, offer
greater safety in operation.
Medium voltage
switchgear
7.
Journals:
[1] Amirat,Y., Benbouzid,M.E.H. Bensaker,B., Wamkeue,R.,
Generators for Wind Energy Conversion Systems: State of
the Art and Coming Attractions, J.Electrical Systems 31(2007): 26-28.
[2] Shikha, Bhatt,T.S., Kothari,DP, Evolution of Wind Power
Technology A Review. E(I) Journal–ID, Vol. 84, May
2003.
Books:
[3] Burton,T., Sharpe,D., Jenkins,N. and Bossanyi,E., Wind
Energy Handbook. John Wiley & Sons, New-York, 2001.
[4] Western Electricity Coordinating Council. Modeling and
Validation Work , Group WECC Wind Power Plant.
Power Flow Modeling Guide, 2008.
Conference Proceedings:
[5] Chen Wang, Liming Wang, Libao Shi, Yixin Ni, A
Survey on Wind Power Technologies in Power Systems,
Power Engineering Society General Meeting, 2007. IEEE
Volume Issue, 24-28 June 2007, Page(s):1-6.
[6] Costinaş, Sorina, Diaconescu,I., Fagarasanu,I. New Wind
Power Plant Condition Monitoring Technology,
Proceeding of the 3rd WSEAS International Conference
on Energy Planning, Energy Saving, Environmental
Education, Paper 624-174, La Laguna University,
Tenerife, Spain, 2009, pg.71-76.
Wind turbine
generators
Figure 6. Internal Loop Network.
6.
REFERENCES
CONCLUSIONS
With our country, it is preferable that the internal
networks to collect energy produced by wind turbines of
power is achieved at a voltage of 20 kV, transformer
stations of 20kV/110kV.
Wind farms with a network attached to a single
transformer station can power up to 100 - 150MW.
The substation may be equipped with transformers with
maximum rated capacity of 80 to 100MVA in order not
to exceed the limit of 40kA short circuit. An internal
network of collection carried out by connecting the
individual generators in the substation may be used only
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