Download Wind turbine integration on island weak grids _3

Wind turbine integration on island weak grids
Guillaume Pinçon, Eric Andrieu, Pierre Pesnel
Vergnet SA 1 Rue des Châtaigniers 45140 ORMES , France - +33 (0)2 38 52 35 60
E-mail: [email protected], [email protected], [email protected]
Abstract –.
To answer to a wind power expanding market in oildependent and developing islands and countries, the Vergnet
company has design several generations of suited wind turbines from 5kW to 1MW and connected more than 700 of
them in these areas where local conditions make electrical
grids fragile and weak : the Farwind® areas.
This weak nature of grids can present specificities as high
frequency deviations, fluctuating voltage levels, high load
variations, exposed electric lines, manual managements, nonconnecting dispatching or load shedding policies. Contrary to
continental grids, load variations and wind power/short circuit
power ratio become at these rates critical for the stability and
consequently for safety of supply.
All these specificities are stated and directly integrated in
Vergnet’s project studies and wind farm regulation studies.
But Farwind® wind farms need an important and dedicated
machine flexibility, that is why weak grids found an adapted
response in a full-scale drive technology specially design for
these conditions.
INTRODUCTION
The ratio Scc/Swindfarm defines the strength in terms
of power quality : harmonics emitted at the grid
connection point, flicker values, voltage drops
propagation and support, energizing of transformers will be directly impacted by this value.
Usual criteria values
Interconnected
Weak
Swind /Sbase consumption
< 30 %
> 30%
> 20
4 < r < 15
Voltage deviations
continuous
Un ± 5%
Un ± 6 to 10%
Voltage long-term
deviations
90%<Un<110%
90%<Un<110%
Frequency deviations
±0,5 to 1% Fn
± 1,5 to 2%Fn
Nb of voltage dips
(<300ms)
≈70
≥ 1 per day
Nb of short-term outages (<3mn)
≈30
≥ 1 per week
Nb of long-term outages (>3mn)
≈6
≥ 1 per month
Frequency of load
shedding
Rare
≥ 1 per week
Remote – ½ automatic
Manual
cc
II. AN ACTIVE APPROACH TO UTILITIES
This section details the different issues to interface an
impacting wind power plant on an existing island grid in
Farwind® areas.
windfarm
Nature of Dispatching
Which grid is called weak ?
Is called “weak grid” an electrical transmission or distribution system with high rates of electrical deviations.
The critical value to estimate the strength of a grid compared to a wind power installation to connect are :
-
S /S
French wind power market has known a remarkable
growth since the year 2000 with 3404 MW and 2350 wind
turbines connected today, but associated issues regarding
grid integration has not only begun on continental interconnected grid but above all in Caribbean islands with Vergnet
SA first wind power plants and first connections in the
middle of the 90’s.
There, both the utility and the wind turbine manufacturer
were facing most of the issues relative to high wind power
rate integration as load fluctuations, power quality and stability impacts that should have been considered in the own
design of the wind turbines.
A.
The ratio S
/S
wind to install base consumption , or wind power
penetration. It defines the load fluctuation limits
and its the thresholds can be adjusted according to
existing or forecasted power regulation systems
performances.
Hereunder you will find some typical values or criteria
used to estimate the global nature of a island grid and risks
associated.
Index Terms – island grids, weak grids, Vergnet,
I.
-
Table 1 – Interconnected and island grids typical values
B.
Mutual technical knowledge
In terms of energy and electrical supply, geographical
isolation of islands historically made them deeply dependent from oil generation and costs supply associated. Wind
power is generally an important source of independence
gain but often the first wind power experience as well.
Among the grid utility experience, important exchanges
shall be done.
The wind turbines supplier has to understand the local
environment and rules, how the grid is operating, which is
consumption nature and variation, which are the electrical
criteria to respect (the local grid code).
The grid utility has to understand how a wind power
source is operating (as it often does not know about it),
what measures must be taken regarding its new fluctuating
source, how to adapt its grid at the connection point, which
are the operating limits.
The manufacturer provides generally, actually as for
every wind power plant project, all electrical data needed
plus the IEC 61400-21 report concerning the wind turbine
to connect. Experience shows that it is generally not sufficient in developing island countries and more detailed support shall be provided by the wind turbine supplier whereas
interconnected grid utility will not be involved in the wind
turbine operation more than data sheets provided.
When necessary, the wind turbine supplier support and its
explanation purpose are critical for the grid safety and a
correct interpretation of the wind power source management.
1.
Telecommunication adjustments allow reliable and suited
remote connections with the wind farm. Radio, GSM, satellite, PSTN shall be indifferently compatible with systems
and SCADA performances shall not be impacted.
2.
Island markets lead Vergnet’s company to adopt real
abilities in wind farm connection modeling. Indeed, as the
knowledge and the modeling of the interconnected grid is
the normally property of the utility, most of the impacts and
contributions are generally done by him.
In Farwind areas, the wind turbine supplier shall be able
to propose his allowances to non equipped or incipient utilities, and particularly concerning:
-
-
Be adjustable
The cultural and political multiplicity encountered in
Farwind countries and islands is also visible in electrical
grid numerous kinds.
To meet the requirements of each of them, high adjustment abilities shall be proposed.
Power control at the connection point is the primary response to critical load fluctuating operations and is specially used where consumption can unbalance the conventional power/wind power/consumption equilibrium point.
Voltage control at the connection point by reactive
power adjustment, used on grids where power injection are
too much important at some discrete times. On Vergnet’s
GEV HP wind turbine, reactive current can reach about
40% of the rated current.
This is easily explained. Voltage evolution follows the formula:
∆U/U=(P.R+Q.X)/U²
[1]
With
P : active power produced
Q : reactive power produced
R : Resistance
X : Inductance
Power ramp adjustment of the wind farm allows smooth
and coherent behaviours of electrical values
Lead studies
Leading wind farm contribution studies and loadflow simulations
Leading electrical quality studies to estimate harmonics and flicker emission according to IEC
standards
Leading energizing and transient studies
Determining optimized sequential tripping after
black out
Leading protection logic studies
Participating to setting values of the grid side electrical components
When he proposes such services, the wind turbine supplier is face to new responsibility limits as he is not a single
supplier anymore. All parts shall define clearly these limits,
knowing that the safety of the grid and the power plant
availability are the first objectives.
III. DESIGN ANSWERS OF THE WTG
A.
Integration of Grid codes
In its wind turbine generators design, all electrical components liable to be impacted by grid electrical characteristics are sized complying most of the strictest interconnected
grid code.
This allows disposing of several reference criteria, corresponding to different grid connection policies including
main issues encountered wherever connections have to be
done.
- E-On – German grid code
- AEMC – Australian grid code
- REE – Spanish grid code
- EDF – French grid code
But Farwind areas do not fully comply with the quantitative criteria of these grid codes. To reach the majority of the
requirements of island utilities, an high flexibility has been
introduced in Vergnet’s wind turbine.
B.
Introducing flexibility
Recurrent electrical impacts from the grid (voltage drops,
reactive needs, etc...) and the need to enlarge abilities of the
wind turbine without impacting users (generator decoupling) led Vergnet to chose full scale drive technology.
Figure 2 – Capability curve of GEV HP wind turbine – 1MW
C.
Store energy by DC bus
Figure 1 – Power conversion chain
The Direct Torque Control used is a Power Width Modulation technology providing a characteristic modulated signal.
Generator 690VAC current is levelled out to DC current ,
rated at 1000VDC, and then re-inverted to AC current.
The inverter sized on the whole power range of the generator uncouples the grid from any mechanical impacts :
- Blades rotation disturbances
- Wind short term disturbances
- Generator outputs
Inverter is totally settable in terms of Low Voltage Ride
Through functions and thresholds and can separately:
- Operate without disconnection during the drop
- Support the voltage during the drop by reactive
current injection
As described above, the full scale inverter is equipped
with a DC bus regulating around its rated voltage of
1000VDC.
- An absorption of energy by the DC bus allows a
dedicated control in case of drop voltage. In that
case, DC bus voltage can increase until its safety
limit (1200VDC) and inverter regulation decreases
immediately generator torque, causing a light
overspeed which will be corrected or adusted by
the pitch system.
-
An injection of energy by the DC bus is also possible to support power during transient lack of
wind until he reaches its safety limit ( 412V). See
Figure 3 – example of power injection - GEV HP – 1MW
Figure 2 – Voltage drop template
IV. CONCLUSION
-
Provide continuous reactive power to maintain
voltage without degrading active power
About 3000MW of wind power could be integrated on
weak grids around the world. But this kind of connections, as in other domains as logistics, civil works or project management require adapted methods and products deeply defined by a sharp industrial approach
dedicated to this market.