Harmonic Performance Improvement of STATCOM
Honkanen, Jani (2018)
Honkanen, Jani
2018
Sähkötekniikka
Tieto- ja sähkötekniikan tiedekunta - Faculty of Computing and Electrical Engineering
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Hyväksymispäivämäärä
2018-06-06
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201805141677
https://urn.fi/URN:NBN:fi:tty-201805141677
Tiivistelmä
In this thesis, harmonic performance of STATCOM (Static Synchronous Compensator) was studied. Harmonic performance means the amount of harmonic current emissions and their effect on harmonic voltages at the connection point of the compensator. It has been observed that harmonic current emissions are small when grid voltage is sinusoidal but they increase when harmonics are added to grid voltage. This is a problem as the currents cause harmonic voltages in grid impedances which are added to the existing grid harmonic voltages at the connection point. Standards and transmission operators’ specifications set limits for harmonic voltages at the connection point which are exceeded in some cases with the present current emissions.
The objective of the thesis was to study reasons for the problem and decrease harmonic current emissions. The problem was studied by simulations in PSCAD. Harmonic current and voltage spectrums at the connection point of STATCOM were simulated with same grid harmonic voltages in several cases where some STATCOM attributes were modified. Especially, the effect of different control functions on harmonic performance was studied.
Two voltage feedforward options were compared. It was observed that harmonic currents of orders from 20 to 40 were smaller with fundamental component feedforward. Instantaneous value of voltage as feedforward is intended to remove harmonic currents completely. It was noticed that the delay caused by modulator deteriorates its performance especially at high-order harmonics. Low-order harmonics were still smaller than with fundamental feedforward. However, the performance of a real system will not be so good due to errors in harmonic voltage measurement. Also, performance with low-pass filtered feedforward was investigated. Harmonic currents around 30th were well mitigated. Low-order harmonics were smaller than with fundamental feedforward but not as small as with instantaneous feedforward due to filter’s phase delay.
Low-order harmonic currents were reduced to almost zero by actively controlling them. Control was implemented by adding resonant branches for these harmonics in PR-controller (proportional-resonant controller). Second harmonic current could be removed only partly by control. Part of it resulted from the controller which controls the average of DC-voltages in STATCOM branches to nominal value. Still, harmonic current control requires exact harmonic current measurement which may be challenging in a real system.
Second harmonic current was observed to increase in voltage control mode when operating point was shifted from capacitive to inductive. To mitigate it, a band-stop filter was designed for voltage d-component which is produced by synchronization.
The objective of the thesis was to study reasons for the problem and decrease harmonic current emissions. The problem was studied by simulations in PSCAD. Harmonic current and voltage spectrums at the connection point of STATCOM were simulated with same grid harmonic voltages in several cases where some STATCOM attributes were modified. Especially, the effect of different control functions on harmonic performance was studied.
Two voltage feedforward options were compared. It was observed that harmonic currents of orders from 20 to 40 were smaller with fundamental component feedforward. Instantaneous value of voltage as feedforward is intended to remove harmonic currents completely. It was noticed that the delay caused by modulator deteriorates its performance especially at high-order harmonics. Low-order harmonics were still smaller than with fundamental feedforward. However, the performance of a real system will not be so good due to errors in harmonic voltage measurement. Also, performance with low-pass filtered feedforward was investigated. Harmonic currents around 30th were well mitigated. Low-order harmonics were smaller than with fundamental feedforward but not as small as with instantaneous feedforward due to filter’s phase delay.
Low-order harmonic currents were reduced to almost zero by actively controlling them. Control was implemented by adding resonant branches for these harmonics in PR-controller (proportional-resonant controller). Second harmonic current could be removed only partly by control. Part of it resulted from the controller which controls the average of DC-voltages in STATCOM branches to nominal value. Still, harmonic current control requires exact harmonic current measurement which may be challenging in a real system.
Second harmonic current was observed to increase in voltage control mode when operating point was shifted from capacitive to inductive. To mitigate it, a band-stop filter was designed for voltage d-component which is produced by synchronization.