Control Strategy of Three Phase Shunt Active Power Filter for Power Quality Improvement

Control Strategy of Three Phase Shunt

Active Power

Filter for Power Quality Improvement

Angit Kumar.G1 Ramesh Babu.U2 1

MRITS, Secunderabad, India, gak.mrits@gmail.com 2

NBKR, VidyaNagar, India, uramesh25@gmail.com 

ABSTRACT: The increasing use of power electronics-based loads (adjustable speed drives, switch mode power supplies, etc.) to improve system efficiency and controllability is increasing the concern for harmonic distortion levels in end use facilities and overall power system. The application of passive tuned filters creates new system resonances which are dependent on specific system conditions. In addition, passive filters often need to be significantly over-rated to account for possible harmonic absorption from the power system. Passive filter ratings must be coordinated with reactive power requirements of the loads. Parallel (or shunt) active filters have been recognized as a valid solution to current harmonic and reactive power compensation of non-linear loads. The basic principle of Shunt Active Power filter is that it generates a current equal and opposite in polarity to the harmonic current drawn by the load and injects it to the point of coupling thereby forcing the source current to be pure sinusoidal. The control strategy is Synchronous Detection Algorithm. This technique relies in the fact that the three phase currents are balanced. The average power is calculated and divided equally between the three phases. The signal is then synchronized relative to the mains voltage for each phase. Then the desired reference current is evaluated.

1. Introduction: Methods for limitation and elimination of disturbances and harmonic pollution in the power system have been widely investigated. This problem rapidly intensifies with the increasing amount of electronic equipment (Computers, radio set, printers, TV sets etc.). This equipment, a nonlinear load, is a source of current harmonics, which produce increase of reactive power and power losses in transmission lines. The harmonics also cause electromagnetic interference and, sometimes, dangerous resonances. They have negative influence on the control and automatic equipment, protection systems, and other electrical loads, resulting in reduced reliability and availability. Moreover, nonlinear loads and non-sinusoidal currents produce non-sinusoidal voltage drops across the network impedance’s, so that non-sinusoidal voltages appear at several points of the mains. It brings out overheating of line, transformers and generators due to the iron losses.

Reduction of harmonic content in line current to a few percent allows avoiding most of the mentioned problems. Restrictions on current and voltage harmonics maintained in many countries through IEEE 519-1992 in the USA and IEC 61000-3-2/IEC 61000-3-4 in Europe standards, are associated with the popular idea of clean power.

Many of harmonic reduction method exist. These techniques based on passive components, mixing single and three-phase diode rectifiers, and power electronics techniques as: multi pulse rectifiers, active filters and PWM rectifiers are shown in Figure 1. They can be generally divided as:

A) Harmonic reduction of already installed non-linear load.

B) Harmonic reduction through linear power electronics load installation. 2. Active Power Filters:

Fig 1. Most popular three-phase harmonic reduction techniques of current A) Harmonic reduction of already installed non-linear load.

B) Harmonic reduction through linear power electronics load installation.

3. Synchronous Detection Algorithm:

The Synchronous Detection Algorithm relies in the fact that the three phase currents are balanced. The average power is calculated and divided equally between the three phases. The signal is then synchronized relative to the mains voltage for each phase. Then the desired reference current is evaluated in this algorithm, the three phase mains current are assumed to be balanced after compensating. Thus,

Imu = Imv = Imw (1)

Where Imu, Imv, Imw are the amplitudes of the three phase mains currents after compensating, respectively. The

real power consumed by the load can be represented as,

P=ሾ݁௨ ݁௩ ݁௪ሿ ൥ ݅௅௨ ݅௅௩ ݅௅௪

൩ (2)

The real power is sent to a low pass filter to obtain its average value Pdc. The real power is then split into the

three phases of the mains supply:

Pu = (PdcEu)/ Etot (3)

Pv = (PdcEv)/ Etot (4)

Pw = (PdcEw)/ Etot (5)

Where Eu, Ev and Ew are the amplitudes of the mains voltages, and Etot is the sum of the Eu, Ev and Ew. The

desired mains currents can be calculated as,

Imu=2eu Pu /Eu ² (6) Imv=2ev Pv /Ev² (7)

Imw=2ew Pw/Ew² (8)

Harmonic

Reduction

Technique

FILTERS 

MIXING SINGLE & 

THREE PHASE DIODE 

RECTIFIERS 

PWM 

RECTIFIERS 

MULTI‐PULSE 

RECTIFIERS  PASSIVE  FILTER  ACTIVE  PWM  FILTER HYBRID  FILTER  BUCK  RECTIFIER  BOOST  RECTIFIER 

2‐LEVEL 3‐LEVEL 

i*cu = iLu - imu (9)

i*cv = iLv - imv (10)

i*cw = iLw - imw (11)

Fig 2. Block diagram for implementing synchronous detection algorithm

Fig 3. SIMULINK BLOCK DIAGRAM OF SYNCHRONOUS DETECTION ALGORITHM

ε

e

e

i

e

i

LPF  Power  Distributor

2e

/E

²

2e

/E

²

2e

/E

²

i

i

i

i

(a)

(b)

(c)

(d)

(e)

Fig 4. Simulation results for the ideal mains voltage with diode rectifier load. (a) Three phase mains voltages (b) load current in u-phase (c) compensating current for the u-phase (d) source current in the u-phase (e) the dc capacitor voltage

0 0.05 0.1 0.15 0.2

-500 0 500

0 0.05 0.1 0.15 0.2

-50 0 50

0 0.05 0.1 0.15 0.2

-40 -20 0 20 40

0 0.05 0.1 0.15 0.2

-100 -50 0 50 100

0

0 05

0 1

0 15

0 2

(a)

(b)

Fig.5 spectral analysis of the (a) load current, magnitude Vs frequency in Hz. (b) source current, magnitude Vs frequency in Hz, for the ideal mains voltage with diode rectifier load

4. Control of the dc capacitor:

The dc capacitor voltage regulation is achieved and the instantaneous active power consumed in the power converter losses are compensated by an active component drawn from the supply. Thus the dc voltage regulation of the capacitor is achieved. This is achieved in one cycle of the supply voltage waveform.

Conclusion:

REFERENCES

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