AD6-P5c18-电网与电力电子系统的接口优化

Chapter 18:Optimizing the Utility Interface with Power Electronic Systems 18-1 Introduction

18-6 Improved Single-Phase Utility Interface18-7 Improved Three-Phase Utility Interface

18-1 IntroductionPower quality has become an important issue

1 Generation of Current HarmonicsDiode-Rectifier Bridge

Diode-Rectifier Bridge waveform

Diode-Rectifier Bridge waveform

Typical Harmonics in the Input Current

Single-phase diode-rectifier bridge5

2 Deleterious effects of harmonic current

Due to harmonic currents injected into the utility grid, the voltage waveform at the point of common coupling will become distortion.

PCC is the point of common coupling

Deleterious effects of harmonic distortion and a poor Power Factor Increasing power loss in transmission lines, transformers, generators. Overloading the shunt capacitors used by utilities for voltage support. may causing resonance conditions between the capacitive reactance of the shunt capacitors and the inductive reactance of the transmission lines. The utility voltage waveform will also become distorted, adversely affecting other linear loads. Resulting in errors in metering, malfunction of utility relays, interference with communication and control signals, and so on.

3 Harmonic Guidelines: IEEE 519

Commonly used for specifying limits on the input current distortion8

3 Harmonic Guidelines: IEEE 519

Limits on distortion in the input voltage supplied by the utility9

18-6 Improved single-Phase Utility Interface18-6-1 passive filters

The added inductor results in a higher effective value of the acside inductance Ls, which improve the power factor and reduces harmonics.

The effect of Ls on diode rectifier with a capacitor: ☆ The power factor is improved from very poor to somewhat acceptable. Harmonics are decreased. The output voltage Vd is lower compared with the noinductance case. The overall energy efficiency remains essentially the same; there are additional losses in the inductor, but the conduction losses in the diodes are lower.

18-6-2 Power-Factor-Correction (PFC) CircuitBetween the utility supply and dc-bus capacitor, a Boost dc-dc converter is introduced. Use a boost dc-dc converter to shape the rectified current.

1 Operation principle of PFC By pulse-width-modulating the transistor at a constant switching frequency, the current iL through the Ld is shaped to have the fullwave-rectified waveform.

sin t Similar to vs (t )The input current is is sinusoidal, and in phase with the supply voltage vs.14

sin t I sin t pin (t ) V s s Vs I s Vs I s cos 2 t (18 7)

Because of a fairly large Cd, vd(t)=Vd. Therefore,pd (t ) Vd id (t ) id (t ) I load ic (t ) (18 8) (18 9)

If Ld is very small, then pin(t)=pd (t). Therefore,

id (t ) I load ic (t ) Vs I s Vs I s cos2 t Vd Vd I d I d cos2

t (18 10)

Output current consists of dc component Id and second harmonic component.16

vd ,ripple (t )

1 iC dt Cd (18 13)

Id sin 2 t 2 Cd

to minimize the ripple of vd an appropriate value of Cd must be chosen. A series-tuned LC filter tuned for twice the ac frequency may be put in parallel with Cd.

Note: Vd must be greater than the peak of the supply voltage.

Vd V s

2 Control of PFC (1) the main control objectives ① to draw a sinusoidal current, in phase with the utility voltage; ② to maintain Vd at the reference value. is iLoad vs i L L D v s RFI VdC1 T + C

iL

vsMultipiler

i* L

PWM controlCA RA A +

KL

Vd V*d19

PFC control loop

is

vs

RFI

vs i LC1

L T

D

iLoad+ C

Vd

The control objectives lead to two control loops: The inner current loop ensures the form of i*L(t) based on the utility voltage is(t). The outer voltage loop determines the amplitude of i*L(t) based on the voltage feedback and regulates the voltage of the PFC to the preselected dc voltage.

iL

vsMultipile r

i* L

PWM contro lCA RA A +

KL

PFC control loop

Vd V*d

Note: The inner current loop is required to have a very high bandwidth compared the outer voltage loop. hence, each loop can be designed separately.

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