Direct Power Control for three-phase PWM rectifier

with active filtering function

ABSTRACT:

A novel Virtual Flux based Direct Power Control Space Vector Modulated

(DPC-SVM) for 3-phase PWM rectifier with compensation of higher harmonics function is

presented. The active and reactive power is used as a control variables for the PWM rectifier and

active filtering operation. As a result several coordinate transformations are eliminated.

Simulated and experimental steady state and dynamic performance for PWM rectifier and active

filtering operation are presented. Among the main advantages of DPC-SVM are: simple

algorithm, good dynamic and operation at constant switching frequency. Additionally a line

voltage sensors were replaced by the virtual flux estimator which also help to achieve sinusoidal

line current in case of distorted line voltage.

I. INTRODUCTION

Harmonics-related problems in utility are a result of usage equipments like a

diode or thyristor rectifiers, which takes non-sinusoidal currents from the grid. Several solutions

to eliminate pollution problems have been developed. They are an answer for more and more

restricted norms which intend to limit the harmonic current of power electronic converters.

Active power filters and PWM rectifiers are two typical examples of these methods. Both of

them have basically the same circuit configuration and can operate based on the same control

principle.

Active filters are able to compensate not only current harmonics, but also a reactive power and

unbalance of load.

Design and control have been investigated in many papers [9]- [12] where useful of active filters

were proofed.

PWM Rectifiers [1], [4] as a non-polluting equipment with sinusoidal input currents are going to

be more popular because of several advantages described as:

- Precise regulation of output DC voltage,

- Low harmonic distortion of line currents,

- Near sinusoidal current waveforms,

- Regulation of input power factor to unity,

- Bi-directional power flow.

This paper explores another task of PWM rectifier –

Active filtering function, which intend to connect advantages of active filters and PWM

rectifiers. So, the PWM rectifiers supply its load and at the same time compensate AC line

current (Fig. 1). This concept was previously introduced by some authors [6] - [8]. However, in

this paper a new approach for this problem is presented. The Virtual Flux based Direct Power

Control (DPC) [1] - [3] with Space Vector Modulator (SVM) [13] is applied to control of a

PWM rectifier. Thanks to DPC-SVM, AC line voltage sensor-less operation and sinusoidal line

current in case of distorted line voltage is obtained. Moreover, thanks to SVM algorithm, PWM

rectifier operates at constant switching frequency.

Block scheme of PWM Rectifier with active filtering function

II. BASIC PRINCIPLES OF VIRTUAL FLUX BASED ACTIVE AND

REACTIVE POWER ESTIMATION

It is economically motivated to replace the AC-line voltage sensors with a virtual

flux (VF) estimator The principle of VF is based on assumption that the voltages imposed by the

line power in combination with the AC side inductors can be quantities related to a virtual AC

motor (see Fig. 2). Where R and L represent the stator resistance and leakage inductance of the

virtual motor. Phase-to-phase line voltages: Uab, Ubc, Uca can be considered as induced by a

virtual flux. Hence the integration of the voltages leads to a virtual flux vector ΨL , in stationary

α-β coordinates as follows:

Figure 2. Three-phase PWM rectifier system with AC-side presented as virtual AC motor

Where

Operation of PWM rectifier is based on assumption, that input current ic is controlled

by the voltage drop across the inductor L interconnecting line and converter voltage sources. It

means that the inductance voltage uI equals the difference between the line voltage uL and the

converter voltage uS

And similarly a virtual flux equation can be presented as:

Figure 3. Reference coordinates and vectors: ΨL – virtual line flux vector, ΨS – virtual flux

vector of converter, ΨL – virtual flux vector of inductor, uS – converter voltage vector, uL -

line voltage vector, uI – inductance voltage vector, iL – line current vector

Based on the measured DC link voltage Udc and the duty cycles of SVM

modulator DA, DB, DC the virtual flux ΨL components are calculated in stationary coordinates

system as follows:

The measured input converter currents ica, icb and the estimated virtual flux

components ΨLα ,ΨLβ are used for the power estimation. The voltage equation (using (2)) can

be written as:

In practice, R can be neglected, giving

Using complex notation, the instantaneous power can be calculated as follows:

Where * denotes the conjugate line current vector. The line voltage can be expressed by the

virtual flux as:

Where denotes the space vector and its amplitude.

That gives

And

For sinusoidal and balanced line voltage the derivatives of the flux amplitudes are zero. The

instantaneous active and reactive powers can be computed as:

Fig. 4. Presents the block scheme of virtual flux and power estimators including active filtering

function

Figure 4. Block scheme of estimators

III. CONTROL ALGORITHM OF PWM RECTIFIER WITH ACTIVE

FILTERING FUNCTION

Control algorithm presented in Fig. 5. is divided into two parts: active rectifier and

active filtering operation described below.

A. PWM Rectifier operation

The commanded (delivered from the outer PI DC voltage controller) active power

pref and reactive power qref (set to zero for unity power factor) values are compared with the

estimated instantaneous p and q values, respectively. The errors are delivered to PI controllers,

where the variables are DC quantities and steady state error were eliminated. The output signals

from PI controllers after transformation described as:

Figure 5. Block scheme of control strategy

are used as a reference signals for Space Vector Modulator. Simulations and experimental results

for PWM rectifier under DPC-SVM control are presented on Fig. 6. and Fig. 7.

B. Active filtering operation

In 1983, Akagi has proposed the "The Generalized Theory of the

Instantaneous Reactive Power in Three-Phase Circuits", also known as instantaneous power

theory , or p-q theory. It was used to calculate the reference compensation currents in the α -β

coordinates [5], [9]. This paper presents modified algorithm based on virtual flux, which operates

directly on instantaneous active and reactive power components. The instantaneous active and

reactive powers are estimated using currents intended to compensate (diode rectifier input

currents see Fig. 1.) and virtual flux according to Eqs (11a and b) as:

The calculated active power (pA) and reactive power (qA) are delivered to the

high pass filter to obtain the variable value of the instantaneous active power ( p A) and reactive

power ( %q A) which finally are used as a compensating components. Enclosure of active

filtering function will cause suitable distortion of input PWM rectifier current. This will assure

almost sinusoidal line current. It permits to use non polluting equipment what is PWM rectifier

as a current harmonics eliminating device. Simulations and experimental results for active

filtering operation are presented in Fig. 8, 9 and 10

Fig. 8b. Presents the experimental results for PWM rectifier having active filtering

function. The notches visible on the line voltage waveform are generated by highly distorted

currents drawed by the diode rectifier and PWM rectifier. The active filtering operation startup

and the transient of the step change of the diode rectifier load are presented in Fig. 9. and Fig. 10

respectively. Those figures demonstrate stability of the system during dynamic conditions. It is

shown that line current during active filtering operation is almost sinusoidal

The main electrical parameters of the power circuit and control data are given in the Table I.

V. CONCLUSIONS

The line voltage sensor less Virtual Flux based Direct Power Control Space Vector Modulated

(DPC-SVM) for 3-phase PWM rectifier with active filtering function is presented. Based on

simulation study carried-out in SABER simulation package as well as experimental results

measured in the laboratory setup of Fig. 11. the main features and advantages of the system can

be summarized as:

1. No line voltage sensors are required,

2. Simple control algorithm without several coordinate transformation,

3. No current control loops, the system operates directly on instantaneous active and

reactive powers,

4. Good dynamics and no coupling between active and reactive power,

5. Sinusoidal line currents for ideal and distorted line voltage, thanks to the natural low-pass

filter behavior of the integrators used in flux estimator (Eqs. 4a and 4b),

6. Constant switching frequency thanks to SVM,

7. Proposed system can operate as a PWM rectifier, Shunt Active Filter or it can take the

role of PWM rectifier

8. Having active filtering function. This extends tasks of PWM rectifier on eliminating of

higher harmonics in line current. In this case PWM rectifier supply its load and at the

same time compensate for harmonics AC line current,

9. Thanks to active filtering function it is possible to use non polluting equipment what is

PWM rectifier as a current harmonics eliminating device, it is also possible to add this

function to currently working PWM rectifiers,

10. The system has been verified by the simulation and experimental study,

11. Compared to standard PWM rectifier, it have to be dimensioned for a bigger power ratio.