STATCOM Controller Operation: Voltage and Reactive Power Control

This report provides a detailed analysis of STATCOM controller operation, focusing on automatic voltage and reactive power control within power systems. The document outlines the functionality of two voltage governor loops and a 3-phase PWM rectifier/inverter controller, including active and reactive current control loops. It explains the AC and DC voltage control loops, detailing how they maintain voltage stability through reactive and active current adjustments. The report also covers automatic reactive power control, explaining how STATCOMs manage reactive power flow to maintain a power factor of 0 Var. The analysis includes d-q transformations for current analysis and the roles of PI inverting amplifiers in error correction. Finally, the report describes the voltage control loop for the DC bus, illustrating how it sustains bus capacitor voltage by adjusting active current commands. The report also discusses FACTS devices and HVDC systems.

STATCOM CONTROLLER OPERATION
Automatic voltage control.
The system consists of two
voltage governor loops and a 3-
phase PWM rectifier inverter
controller that has two control
loops namely; active and reactive
current control loops. Of the
voltage control loops, one loop
controls ac voltage of the
system from the ac side of
STATCOM interfaced with step-
down transformer by modifying
the reactive current command Iqref of the 3-phase PWM rectifier/inverter controller. The other voltage
control dc voltage EDC of the STATCOM by adjusting active current Idref command of the 3-phase-PWM
rectifier/inverter controller.
Voltage control loop of AC bus line.
It stabilizes voltage of the STATCOM section interlinked to the 3-step-down-transformer as described by
ac bus line voltage command figure thereby maintaining ac power line voltage. To achieve this, it follows
stages described below.
I. Vectorial addition of the active component (E¿¿ d)¿ and with reactive component,
corresponding to the line voltage of the STATCOM to the section linked with step-down
transformer, is determined using magnitude calculator.
II. Vectorial summation is deducted from the voltage command EAC ref of the ac bus on the
STATCOM and the resultant error relayed to the PI inverting amplifier.
III. Polarity of the error in inverted by PI inverting amplifier, then amplified so as to acquire
reactive current (Iq ¿¿ ref ) ¿ command of the PWM rectifier/inverter controller. The
reactive power is supplied by 3-phase PWM inverter/rectifier when STATCOM’s line ac
voltage is lower than ac bus line voltage command EAC ref . Resultant effect increases line
voltage of the ac power system thus correcting the error and vice versa.
DC bus voltage control loop
Bus capacitor voltage on the DC side of the STATCOM is maintained by the following process;
IV. DC voltage across the STATCOM is determined by the voltage sensor and relayed to
STATCOM controller.
V. The error is determined by the difference between dc voltage EDC and DC bus voltage
command EDC ref and the resultant error in transferred to the PI inverting amplifier.
VI. In the PI, error is inverted and amplified in search of active current ( Idref ) command of
the 3-phase PWM rectifier/amplifier controller. The active current command Idref is
negative when determined voltage EDC is lesser than dc bus voltage command and hence

3-phase-PWM rectifier/inverter consumes active power coming from the ac power system
to charge Cbus connected to the STATCOM and thus correcting the error and vice versa.
Automatic reactive power control
It is used for active power factor improvement for the process with huge unsystematic peaks of reactive
power demand. Exchange of reactive power is maintained at 0 Var by STATCOM monitoring the reactive
current component I q of the ac line currents to the network and ac power input to the STATCOM.
Reactive power control loop
It maintains reactive power of the
ac power network to 0 var by
ensuring reactive ac line current is
0A using following steps.
I. Two ac line current
I L 1
and
I L 2
is
measured using two current sensors while I L 3=−(I L1 + I L2 ).
II. A d-q transformation is then applied to the 3-phase line currents converting them into fixed values
I d
¿ and I q
¿ representing active and reactive components respectively. A 30o phase introduced by
wye-delta transformer is compensated by 30o in d-q transformation.
III. To determine the error in reactive component, I q
¿ is subtracted from the reactive current
command Iqref (0 A ). Reactive power is consumed from the ac network when polarity of the
reactive component is negative meaning the reactive component of line current is positive and vice
versa.
IV. Polarity of the error in reactive component is inverted by PI inverting amplifier and amplifies to
acquire reactive current command Iqref of the 3-phase PWM rectifier/inverter controller. Positive
polarity of the error results to reactive power drained from the ac power network since reactive
current command Iqref . Polarity is negative. Compensation is achieved by 3-phase PWM
rectifier/inverter supplying reactive power and minimizing reactive power drawn from the network
to 0Var and vice versa.
Voltage control loop of dc Bus.
It is used to sustain bus capacitor voltage by the process as shown below
V. Voltage across the dc side, EDC, measured by voltage sensor is relayed to the STATCOM controller.
VI. EDC is then deducted from the dc bus voltage command EDCref to determine error in the voltage.
VII. The error is inverted and amplified by the PI inverting amplifier in order to obtain active current
command I dRef of the 3-phase PWM rectifier/inverter controller. I dRef has negative polarity when
EDC <EDCref and thus 3-phase PWM rectifier/inverter draws active power from the STATCOM
scheme to charge bus capacitor and correcting the error and vice versa.