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The ip-iq detection method based on instantaneous inactive power theory has been applied widely in active power filter because of its good real-time. But it needs large computation, and three-phase currents are processed as integrity, thus calculation accuracy can't be ensured. Based on adaptive interference canceling theory, this paper presents a new ad- aptive detection method for harmonic current, it is a continuously regulated closed-loop system, and its operating characteristics are almost independent of the parameter variations of the elements, thus it performs better than that based on traditional theory. At last this paper provides the simulation of active power filter including the detecting cir-cuit which proved the design is feasible and correct.

Because of the use of more nonlinear loads, especially more power electronic equipments, a large number of harmonic and reactive currents have been introduced into power grid, resulting in some problems such as voltage flicker, frequency variation, imbalance of three-phase problem, etc [

This paper presents a new adaptive closed-loop detection method based on adaptive interference canceling theory, and the simulation results show that the filter based on this new method performs better than that based on the three-phase instantaneous inactive power theory, and with higher accuracy [

An active power filter is a new power electronic device of dynamic harmonic suppression. _{Lh}, in the load current i_{L}, and takes the opposite value as command signal. The principle can be expressed by the following formula

where i_{S}, i_{L} are currents of the supply and a nonlinear load, respectively, and i_{c} is the compensation current. i_{L}_{f}, i_{Lh} are the fundamental active and harmonic reactive components of the load current, respectively.

The adaptive interference canceling technique has been widely used in recent years [_{0} and reference input n_{1}. And s is unrelated with n_{0} and n_{1}, while n_{0} and n_{1} are related. The reference input signal n_{1} is filtered by an adaptive filter to produce an output signal, which is an approximate replica of n_{0}. This output is subtracted from the original input signal s+n_{0} to produce, the system output signal.

In the system shown in _{0} continuously and adjust the system to minimize the error signal e. It can be proved that is the best least-squares estimate of n_{0}, when the filter is adjusted to make the error signal power minimum.

Based on the principle of adaptive noise canceling theory, adaptive harmonic current detecting circuit is shown in ^{0} phase-shifter [_{1}(t) is the fundamental current, i_{h}(t) is the sum of all harmonic components, and i_{p}(t), i_{q}(t) are the active component and the reactive component of i_{1}(t), respectively in _{1}(t) are the AC source voltage and its fundamental component, respectively. R_{1}(t) and R_{2}(t) are two reference inputs orthogonal to each other, and i_{0}(t) is the system output.

As shown in _{0}(t) is multiplied by, while other components produce AC signals after the same procession. The DC component can be integrated to get the average value of fundamental reactive current I_{Fp}, while the AC component will be zero after the same calculation. Thus, we can get the instantaneous fundamental reactive current i_{fq}(t) by

multiply I_{Fp} with R_{1}(t). Similarly, using R_{2}(t), we can get the instantaneous fundamental active current i_{fp}(t). At last, by adding the reverse of i_{fp}(t)+i_{fq}(t) to i(t), the output current i_{0}(t)=i_{h}(t) is produced. If only the current i_{0}(t)=i_{h}(t)+i_{fq}(t) is needed, what we should do is remove the R_{1}(t) branch.

We can also explain the principle in the phase space. Assume the reference inputs which processed by the BPF are:

,

.

Then the output of the multiplier M1 can be expressed as:

Taking the Laplace transform of (1), we have

where, I_{0}(s) is the Laplace transform of i_{0}(t). After processed by the integrator, whose transform is (here G is the integration gain), the transform of the feedback signal can be expressed as:

The output of the multiplier is simply the feedback signal of the lower branch, which mean. Its transform is:

Similarly, the transform F_{2}(s) of the feedback signal f_{2}(t) for the upper feedback branch can be expressed as:

The total feedback signal is:

Its transform is:

Thus the feedback coefficient of the whole system is:

Then the transfer function H(s) of the system is:

From (8), when, , which means a zero point exists in the system corresponding to the fundamental frequency Consequently the fundametal signal will be greatly attenuated. It is obvious that the system shown in

Ideally, what an active filter compensates is the non-active power; that is to say, it neither absorbs active power from the power supply nor outputs to it, so the DC side voltage of an active filter is constant. However, due to the loss of the active filter, energy in the capacitor on

the DC side will reduce, making the voltage on the capacitor drop.

In order to maintain the voltage on the capacitor, the feedback method has usually been adopted, whose purpose is to obtain some active power from the source to compensate the corresponding loss.

As shown in _{cr , }U_{cf} are the reference and feedback values of U_{c}, respectively. The difference between U_{cr} and U_{cf} is regulated by PI to get the signal.

Since contains the fundamental active component , i_{c}, which comes from, also contains such a component. Therefore, when i_{c} is introduced into the power system, APF can exchange the active energy between AC and DC sides, which keeps U_{c} constant.

In this section, computer simulation is carried out to verify the design of the adaptive shunt active filter. A three-phase distribution system is built using Matlab as shown in

Since the APF adopts traditional hysteresis current control method, the tracking ability of APF is limited, resulting in some ripples in the current when it changes suddenly, as shown in

The DC capacitor voltage is shown in

takes about 0.05s to reach at the desired value of 1000V and stabilize rapidly.

Compared

It shows from

Overall, it shows that the proposed adaptive shunt active filter can compensate nonlinear load current, adapt itself to compensate the variations in nonlinear load currents and correct the power factor of the supply side nearly to unity.

In this paper, a novel adaptive detection method for harmonic and reactive current is proposed. This method is analyzed systematically and verified by Matlab simulation. It is a continuously regulated closed-loop system, and the operating characteristics are nearly independent of the parameter variations of the elements, and bandwidth behaving as one of a second-order notch filter can be regulated easily by controlling the amplitude of the reference input and the gain of the integrator. Furthermore, this paper also introduces DC side voltage control method, which is simple and effective. Finally, simulation result is given to conform the feasibility of the design.