Continuously controllable apparatus
This group of apparatus will normally include a voltage source converter, controlled by various control strategies. The connection to the network grid is usually done by use of transformers. New power electronic devices such as Insulated Gate Bipolar Transistors (IGBT), Insulated Gate Commutated Thyristors (IGCT), and MOS controlled Thyristors (MCT) are used in the converters. Depending on the topology used for the converters they are split up into two groups:
1) Shunt converters.
2) Series converters.
The shunt converter injects current into the network at its coupling point. This current injection may then be used to compensate for different disturbances. Typical shunt connected apparatus may be the following: Static Var Compensator SVC, Static Synchronous Compensator (STATCOM) and for harmonic compensation Active Harmonic Filters (AHF).
The series converter injects a voltage with a certain phase lag or lead to the line between the supply and load. The resulting power flow in the circuit where the voltage is injected will be changed, dependent on the resulting voltage and phase-shift across the load. The most frequently used series apparatus is the Dynamic Voltage Restorer (DVR).
Shunt connected compensators are useful for cancelling out disturbances in the network current, whereas the series compensators are useful for cancelling out voltage disturbances at the load side.
The two kinds of apparatus may also be combined. The apparatus is then called a Unified apparatus, for instance a Unified Power Quality Conditioner (UPQC).
Finally, the active compensator may be combined with passive filter elements. Then the apparatus is called hybrid apparatus. Some of the compensators also have an energy storage device connected, to be able to deliver active power, and not only reactive power. In this way also voltage dips and voltage fluctuations may be compensated. Depending on the amount of energy stored, different sizes and duration of the dips can be compensated. On some occasions a shunt apparatus in connection with a series apparatus can replace the energy storage device. A Unified Voltage Controller (UVC) is an example of such an apparatus.
An overview of the different kind of apparatus used at the distribution level (CUPS) and at the transmission level (FACTS) is shown in Table 10.
Table 10. Different CUPS and FACTS apparatus (Thomsen, 1999).
Generation group 1 is the first generation converters and is based on thyristors and current source line- commutated converters
|
Generation -Group |
CUPS |
FACTS |
|
1 - shunt |
SVC Static Var Compensator | |
|
1 - series |
TCSC Thyristor Controlled Series Capacitor | |
|
On/off apparatus | ||
|
1-2 - on/off |
SSB Solid State circuit Breaker SSTS Solid State Transfer Switch or STS Static TS BTS Bus Transfer Switch | |
|
1-2 - on/off |
FCL Fault Current Limiter | |
|
Stepwise controllable apparatus | ||
|
1-2 |
SVR Static Voltage Regulator SSLTC Solid State Load Tap Changer | |
|
1-2 |
ASVC Advanced Static Var Compensator AVC Adaptive Var Compensator | |
|
Continuous controllable apparatus | ||
|
2 - shunt |
STATCOM STATic COMpensator or STATCON STATic CONditioner AHF Active Harmonic Filter |
STATCON STATic CONdenser or ASVC Advanced SVC |
|
2 - series |
DVR Dynamic Voltage Restorer SABF Series Active Blocking Filter |
SSSC Static Synchronous Series Compensator PAR Phase Angle Regulator or PAC Phase Angle Controller |
|
2 - Combined |
UPQC Unified Power Quality Conditioner APLC Active Power Line Conditioner UVC Unified Voltage Controller PQM PQ-Manager, Hybrid |
UPFC Unified Power Flow Controller |
Oscillating transients generated by coupling of capacitors must be eliminated at the generation point by use of coupling resistors or techniques, such as synchronised coupling or passive filters have to be used because of the high frequency (Thomsen, 1999).
Table 11. Compensation apparatus for different disturbances (Thomsen, 1999).
Distortions
Apparatus
Effect
Voltage dip and sags
StatCom.
UPQC
Moderate
Good
Good
Voltage swell
StatCom
UPQC
Moderate
Good
Good
Harmonic distortion
AHF SABF
APLC, UPQC
Filtering, good Does not block alone Good, efficient
Voltage-fluctuations
StatCom
UPQC
Good questionable Best
Short duration interruptions
StatCom
UPQC
UPS -function
Hardly, but possible with UPS-function UPS - function
Unbalance
StatCom DVR
UPQC, UVC
Moderate, problematic Good
Good, difficult
4.2.6 Measures and indicators
Different types of disturbances have been described in this section. Measurements and indicators are needed to identify the nature of these phenomena. In connection with wind turbines, methods for measurement and assessment of the influence of wind turbines on the power quality are specified in (IEC 614OO-21, 2OOO). This standard includes measures for maximum power, reactive power, voltage fluctuations and harmonics.
The following three measures related to voltage fluctuations and flicker are used:
• Flicker coefficient cf(yk,va) - is used to characterise the flicker emission from the wind turbine in continuous operation e.g. in the PCC (Point of common coupling) without generator couplings. The flicker emission for continuous operation is defined to be:
Pst = Plt = cf (Wk,va ) Sk where yk is the grid short circuit angle and va is the annual mean wind speed, while Sn denotes the nominal apparent power of the wind turbine and Sk is the apparent short circuit power e.g. in the PCC. According to the standard, cf(yk,va) is given in a table with four values of yk and va respectively, and the value in an actual case is obtained by interpolation of yk and va.
• Flicker step factor kf(yk) - is used to characterise the flicker emission from the wind turbine arising from generator switching operations. The short term flicker emission is defined to be:
Sk where N10 is the number of switching operations occurring during ten minutes. The long-term emission is defined to be:
Sk where N120 is the number of switching operations occurring during 120 minutes.
• Voltage change factor ku(yk) - is used to characterise the voltage changes, which occur on switching. The relative voltage change in %, d, is defined as:
The above formulas for assessment of the influence of a wind turbine on the power quality are valid for a single wind turbine. (IEC 61400-21, 2000) also specifies methods for summation of the effect of wind turbines in a cluster, or a wind farm, taking into account the smoothing effect of the contributions from the individual wind turbines.
Another basic characteristic measure, which has been used in earlier standards, is the cut-in current factor ki:
k _ Imax r where Imax denotes the maximum current during one half period and Ir denotes reference current of the wind turbine/farm (both are rms. values). The cut-in current factor is usually used to specify an upper limit for the current at cut-in of a load (passive as well as active). The cut-in current factor ki has been used to predict the maximum voltage change substituting ku(yk) in the above equation. However, this method is conservative, because it does not take into account the influence of the impedance angle of the grid. This is particularly relevant for the cut-in of induction generators on grids with high values of short circuit impedance angle, because typically Imax is due to reactive power, and therefore ki> ku(Vk)
A widely used technique in signal analysis is to analyse a signal by its Fourier transform. This technique is especially relevant in connection to wind turbines provided with power electronic, due to the harmonics produced by the switching patterns of the applied power electronic device.
The ideal case is a sinusoidal voltage and a sinusoidal current, with a frequency of 50 Hz. In order to be able to describe deviations from the ideal case three different measures have been defined as presented below (Arrillaga et al., 2000):
Total harmonic voltage distortion is given by:
where Uh denotes the amplitude of the h'th harmonic. A small THDU value indicates a signal close to the ideal case. Thus, THDU is a normalised measure of the harmonic content due to over harmonics up to and including the 40th harmonic.
Total voltage distortion factor is given by:
This includes all kinds of distortion. A small TDFU value indicates a signal approaching the ideal case. TDFU can be seen as a normalised measure of the energy of the sinusoidal signal content at frequencies other than 50 Hz.
The weighted distortion factor is given by:
The weighted distortion factor is dedicated to systems employing shunt capacitors, as it weights the higher frequencies in a manner corresponding to the frequency dependence of the current in a shunt capacitor.
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