Control system for active power filters

Details Control system for active power filters Control system for active power filters

2001-10-01

2003-12-02

Toatley, Jr., Gregory J. (Department: 2836)

Electrical transmission or interconnection systems

With harmonic filter or neutralizer

C363S089000, C323S207000

Reexamination Certificate

active

06657322

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The present invention concerns electrical filters for eliminating transients and distortion in an alternating current (AC) utility grid and more specifically to active filters for such use.
Ideally, a utility grid for providing three phase AC power to factories and offices (i.e., industrial environments) includes three AC power conductors or lines, each line providing a pure sine wave of current and voltage, the sine waves having equal and constant amplitude and frequency, and each separated from the others by 120°. The utility lines are linked to facility coupling lines at a point of common coupling (PCC) (i.e., a utility-customer connection point) which in turn provide power to facility equipment. As well known in the power industry, all power electronic equipment can potentially act as non-linear loads creating utility line disturbances and distorting utility waveforms by injecting harmonic currents into the utility grid.
To illustrate the effects of distorting currents on a utility power grid, consider
FIG. 1
wherein a utility source
10
is shown connected at a point of common coupling (PCC) to a load
12
(e.g., a first utility customer) and other loads (e.g., other utility customers) represented collectively by numeral
14
. The lines that link the PCC to loads
12
,
14
are referred to herein as coupling lines. The utility source
10
includes a finite internal impedance Ls. Due to impedance Ls, when load
12
draws a non-sinusoidal current from source
10
, the waveform through the coupling lines and at the PCC becomes distorted with harmonic coupling line currents that can cause machinery and equipment connected at the other loads
14
to malfunction.
In addition to voltage waveform distortion at the PCC, other problems related to harmonic currents include additional heating and possibly over voltages in utility distribution and transmission equipment, errors in metering and malfunctioning of utility relays, interference with communication and control signals and equipment damage from voltage spikes created by high frequency resonance's.
Unfortunately, harmonic or non-linear loads comprise an ever increasing portion of the total load for a typical industrial plant. In fact, by 1992, harmonic loads had become such a pervasive problem that the Institute of Electrical and Electronic Engineers (IEEE) recommended stringent harmonics standards, including strict utilities limitations, in a document referred to in the industry as IEEE Standard 519 which has generally been accepted in North America. Standard 519 was written with the general understanding that harmonics should be within a reasonable limit at the PCC and therefore puts limits on individual load and total (i.e., distortion from all loads connected at a PCC) harmonic distortion.
One potential source of utility grid distortion includes power electronics required to modify utility voltages for driving motors. Generally, power electronic systems for receiving and converting utility voltages into AC voltages suitable for driving an AC motor include two converter stages, the first converter stage being a rectifier stage and the second converter stage being an inverter stage. The rectifier stage receives and converts the AC utility voltages to DC voltage and provides the DC voltage across positive and negative DC buses. The inverter stage receives and converts the DC voltage to AC voltages, usually at a different frequency and amplitude than the utility voltages, and provides the converted AC voltages to motor terminals to drive a motor.
One way that has been adopted in many applications to reduce harmonic distortion at the PCC is to position passive filters between harmonic generating loads (e.g., motor drives at an industrial facility) and the PCC. Passive filters typically include inductor and capacitor configurations designed to have a series resonance at the harmonic frequencies to be mitigated. While simple in design, unfortunately such passive filters have a number of shortcomings. First, passive filters are typically bulky and expensive. Second, passive filters cannot adapt to changes in harmonic frequencies caused by shifts in the fundamental AC frequency. Third, passive filters cannot account for variations in the series impedance of the utility grid.
The disadvantages associated with passive filters may be overcome by use of active filters in which a compensating power source is connected directly to the coupling lines to provide a countervailing or compensating current that effectively cancels the distorting harmonic currents. Harmonic currents, like the fundamental line current, are sometimes positive and sometimes negative (i.e., have positive and negative segments). For this reason, in order to eliminate harmonic currents, compensating active filters must be able to operate as both a current sink during positive harmonic segments and as a current source during negative harmonic segments.
To this end many active filters include a pulse width modulating (PWM) inverter controllable to provide current/voltage to, or sink current/voltage from, a line. To sink and provide power, the PWM power source in many active filters comprises a simple power capacitor linked in parallel with a PWM inverter bridge across positive and negative DC buses. The power capacitor is charged by coupling line harmonics whenever current is sinked from the lines and is discharged whenever used to provide current to the lines.
Active filters can generally be grouped into two different categories including pure shunt active filters and hybrid shunt active filters. U.S. Pat. No. 5,063,532 (hereinafter “the '532 patent”) which issued on Nov. 5, 1991 and is entitled “Active Filter Device”, describes an exemplary pure shunt active filter. The '532 patent filter senses coupling line currents, identifies harmonic current waveforms in each line current, compares compensating currents to the harmonic waveforms, adjusts pulse width modulating (PWM) firing signals as a function of the difference between the compensating and harmonic currents and controls a PWM inverter via the firing signals. PWM inverter output lines are linked to the three coupling lines to provide the compensating currents directly thereto thereby eliminating or substantially mitigating coupling line harmonics.
U.S. Pat. No. 5,567,994 (hereinafter “the '994 patent) which issued on Oct. 22, 1996 and is entitled “Active Harmonic Filter With Time Domain Analysis” describes an exemplary hybrid shunt active filter that, like the pure shunt filter, senses line currents on coupling lines and identifies harmonic current waveforms in each line current. Unlike the pure shunt filters, the hybrid filter does not include a feedback loop that compares the compensating and harmonic currents. Instead, hybrid filters simply generate PWM firing pulses calculated to generate compensating voltages that should cancel the harmonic currents and then applies compensating voltages to the lines via transformers and passive filters.
While each of the pure and hybrid shunt filters have several advantages, each also has several shortcomings. For example, it has been determined through experimentation that the power capacitor employed in the filters may not alone be able to maintain sufficient charge or may become overcharged during the compensating process. To this end, it should be appreciated that the power capacitor cannot cause a desired compensating current on a linked coupling line unless the capacitor charge exceeds the coupling line voltage level. Where harmonic currents are relatively more negative than positive (i.e., provide a negative DC offset), the DC bus capacitor charge is quickly drained and the capacitor ceases to operate as a compensating source. Similarly, where harmonic currents are relatively more positive than negative (i.e., provide a positive DC offset), the DC bus capacitor may quickly become ex

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