Historically, the current of the system was divided into two parts: These are; active current, which transports the net energy from source to the load, and reactive current
that is the remaining current component when the active part is subtracted from the total current.
Meanwhile, with the rapid development of large-scale integrated circuit, especially digital signal processor technology, the technology of extracting harmonic and reactive current
in APF system has gradually developed from analog to digital form, which greatly improves the speed of current extraction and the accuracy of compensation .
As shown in Figure 3, the source reference active current [i.sub.rd] is equal to the sum of load's active current [i.sup.*.sub.Ld] and DC-link voltage regulating current [i.sup.*.sub.dc]; the reference reactive current
[i.sub.rq] and zero sequence current are equal to zero.
AU = U - [U.sub.0] and [DELTA][I.sub.B] = [I.sub.B] - [I.sub.B0], where U is the present voltage during the fault, [U.sub.n] is the rated voltage, [U.sub.0] is voltage before the fault, [I.sub.B] is the reactive current
, [I.sub.n] is the rated current, and [I.sub.B0] is the reactive current
before the fault.
FACTS devices are connected at the PCC to inject or absorb reactive current
to keep voltage within the prescribed operational limits.
"An instantaneous active and reactive current
component method for active filters," IEEE Trans.
Iq component on q-axis is negative-sequence reactive current
, since it is right-angled according to Id.
Such instability problems can be solved by dynamic compensation of generator's reactive power demand by the following two methods: (i) using shunt flexible AC transmission system (FACTS) devices, such as the static VAR compensator (SVC) and the static synchronous compensator (STATCOM), to generate a set of balanced three-phased sinusoidal voltages at the fundamental frequency with rapidly controllable amplitude and phase angle to drive PWM converter with dc-link capacitor and transport necessary reactive power in the grid [5, 27, 28], and (ii) reconfiguring RSC in parallel with GSC during fault so that both feed a good amount of reactive current
, which is again intensified by transient control mode (TCM) .
In this mode, a voltage regulation droop is used to allow the terminal voltage of the HES to vary in proportion with the compensating reactive current
. The reference for terminal voltage including droop system is obtained as follows:
In the outer power control loop, according to PI regulators, the errors between active and reactive power output of the DG and their preset values are transformed into the reference values of active and reactive current
Additional benefits include the increased life span of equipment and the reduction of reactive current
on the electrical network in the plant.