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FACTORS
CAUSING INSTABILITY OF WIND POWER

As
due to nonlinear and delicate in nature of these loads are majorly pretentious
by the power worth problematic .There are dual sorts of turbines namely fixed
speed and variable speed turbine, in fixed speed turbine variation in haste offer
to large voltage changing on network where as in variable speed wind turbine’s
situation, where we use power electronic devices, in this can we need to take
into attention about harmonic distortion. 

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A:  Voltage
Variations 

 

On the native level, voltage variances are the core issue related
with wind power. This can be the limiting factor on the quantity of wind power
which can be installed. In ordinary operational condition the impact of attaching
a wind farm on the gird voltage is directly related to the short circuit power
level.

 

The short circuit power level at a specified point in the
electrical system indicates the strength of a system. If the voltage at a
distant point is represented by Us which is a constant voltage and the short
circuit power level (commonly represented by SSC )is demarcated as Us2 /  Zk where Zk represents total impedance between
the points concerned. The system voltage at the supposed inestimable busbar and
the voltage at the Point of Common Coupling (PCC)
are Us and Ug, correspondingly. The output power and reactive power of the
generation entity are Pg and Qg, which corresponds to a current Ig. It is clear
that the variations in the produced power will cause variations in the voltage
at PCC. If the impedance Zk is
small then the voltage deviations will be small. Similarly, if   Zk is large, then the voltage deviations
will be large .

 B:  Steady-state voltage

Operation of airstream turbines may disturb the
voltage in the concomitant network. If essential, the suitable methods should
be taken guarantee that the wind turbine installation does not fetch the amount
of the voltage outside the compulsory restrictions. It is suggested that
load-flow analyses be accompanied to evaluate this effect.

 Several
wind turbines are fortified with induction generators which ingest reactive
power. At no load the reactive power intake is around 35-40% of the rated active
power, and rises  around 60% at rated
power. Reactive power is one of the main reasons of voltage uncertainty in the system
due to the associated voltage drops in the transmission lines, reactive current
also subsidizes to system losses. 

C:  Voltage Fluctuation

Rise
and fall in the system voltage may be the source of observable light flicker
depending on the amount and frequency of the variation. This kind of disorder
is called voltage flicker.

 The flicker extent is based on the
measurements of three sudden phase voltages and currents shadowed by means of  “flicker algorithm” to compute the Pst and
Plt. where Pst is the short term flicker severity factor and computed over 10
minutes, and the long term flicker severity factor Plt is distinct for two hour
periods. The flicker calculations can also be accompanied with simulation
procedure.

Turbulences
just noticeable are supposed to have a flicker severity factor of Pst = 1 The
flicker emanations, Pst and Plt may also be predictable with the coefficient plus
factors, cf(?k, va ) and kf(?k) attained from the  measurements, which are generally given by
wind turbine manufacturers. 

It
is suggested that Plt?0.50 in 10-20 kV networks and Plt?0.35 in 50-60 kV
networks are considered satisfactory. However, unlike utilities may have different
flicker emission limits.

D:  Harmonics

Harmonic disorders are a phenomenon associated by means of
the disturbance of the fundamental sine wave and are shaped by
non-linearity of electrical apparatus. Harmonics causes rise in currents, power
losses and probable disparaging over-heating in the instrument.  Harmonics may as well increase difficulties in
communiqué circuits.

 

Entire harmonic distortion, or THD can be  related to either current harmonics or
voltage harmonics and it is distinct as the fraction of total harmonics to the
amount at fundamental frequency multiplies by 100%

THDv = (V2^2+V3^2+V4^2+…. + Vn^2) ^. 5   * 100%

                                  V1

THDI = (I2^2+I3^2+I4^2+…. + In^2) ^ .5   * 100%

                                  I1

 

The  Pulse 
Width  Modulation  (PWM) 
switching frequency,  with  a  characteristic  switching 
frequency  of  a  few  thousand Hz, alternates the harmonics to upper
 frequencies where the harmonics can be simply
detached by smaller filters. Popular general harmonic standards can be encountered
by means of contemporary wind turbines. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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