Harmonic control devices. ECE 528 Understanding Power Quality

Transcription

1 ECE 528 Understanding Power Quality Paul Ortmann (voice) Lecture 12 1 Today Harmonic control devices In-line reactors (chokes) Zigzag transformers Passive filters Active filters Designing a harmonic filter Lecture

2 In-line reactors (chokes) Simply a series inductance Presents a series impedance that is directly proportional to frequency Forces DC bus capacitor to charge more slowly Additional benefit: Reduces DC bus overvoltages due to capacitor switching transients reduced nuisance tripping Lecture 12 3 In-line reactors (chokes) Sizing the in-line reactor Line reactors are typically described as a 3% reactor, 3% to 5% are common Size is based on the VA base of the drive Using 750kVA/hp for VA base is typical 2 V base X L_5% 0.05 VA base Inductance in Henrys is based on X L at the fundamental frequency Lecture

3 Zigzag transformers Used for zero-sequence currents Commercial facilities single-phase non-linear loads Provides a path for zerosequence currents between the phase and neutral conductors Useful in existing facilities I 3 Lecture 12 5 Passive filters Capacitors and inductors can be arranged to produce high or low impedances at certain frequencies Resistors can be added to provide damping Shunt passive filters provide a low-impedance alternate path Series passive filters increase the series impedance for certain frequencies Lecture

4 Passive filters Shunt passive filters Notch filter is the most popular May employ delta or wye connected capacitors connected to the line or neutral through inductors Lecture 12 7 Passive filters Series passive filters Provide a high impedance to the target harmonic Must carry full load current Not practical for multiple harmonics Useful in single-phase applications Lecture

5 Low-pass broadband filter Combines shunt and series elements Low impedance for low frequencies Hi impedance for high frequencies (See PSQ p.287) Several basic building blocks of the low-pass filter can be placed in series to produce a steeper slope in the frequency response Lecture 12 9 General approach with passive filters Start at the lowest harmonic of concern Tune filters slightly lower than the target harmonic Check for resonant points creating high impedances If system impedance changes, re-evaluate filter Lecture

6 Active filters Use power-electronics to inject the missing current in the non-linear load s current waveform Results in minimal distortion on the source side No resonance concerns May also correct power factor and flicker Lecture Filter design procedure Pick tuned frequency Calculate VAR requirements Calculate reactor size Determine filter duty requirements Fundamental Harmonic RMS current and peak voltage Check capacitor ratings Calculate filter frequency response check for resonance at other harmonics Lecture

7 Interharmonics: Definitions (PSQ pg. 250) Harmonic: Integer multiple of the fundamental frequency Interharmonic: Non-integer multiple of the fundamental frequency Sub-harmonic: frequency less than the fundamental frequency Lecture Interharmonics examples: 1 V( t) t 1 V( t) Lecture t

8 Sources of interharmonics Power-line-carrier signals Frequency converters Drives Cycloconverters Induction furnaces Arcing loads Load variations Lecture Interharmonics issues Analysis Power quality instruments may not accurately measure them Impacts Same heating and distortion issues as regular harmonics Oscillations in mechanical systems Interference with power-line-carrier systems Tuned filters may not work because of the variability of the harmonic spectrum Lecture

9 Interharmonics data analysis issue, 1-60Hz cycle Lecture Interharmonics data analysis issue Lecture

10 Interharmonics addressing issues Not all power quality instruments can measure interharmonics check documentation Broadband low-pass filters or active filters are both effective at mitigating interharmonic distortion Lecture Why have harmonic (and other) standards? Compatibility between the power system and end-user equipment For utilities Provides measurable limits that can be used as the basis for system design For equipment manufacturers Describes the electrical environment the equipment may be expected to operate in Helps manufacturers design equipment to operate acceptably Lecture

11 Harmonic standards IEEE Recommended Practice and Requirements for Harmonic Control in Electrical Power Systems Requirements Limits harmonic current produced by loads Limits voltage distortion on the utility system Reference material Describes harmonic sources and impacts Provides a list of other useful references Describes interharmonic limits to to help control lamp flicker Lecture Harmonic standards IEEE /1992 Practices Covers analysis methods for System frequency response System modeling Telephone interference And more Covers measurements Describes a methodology for evaluating new harmonic sources Includes examples Lecture

12 The IEC standards (See PSQ pg. 313+) IEC International Electrotechnical Commission Main organization developing power quality standards for the international community Standards may be adopted in individual countries Lecture General IEC standard overview Six parts: 1. General definitions, terms 2. Environment description and characteristics 3. Limits Allowable disturbances caused by equipment 4. Testing and measurement techniques guidelines for measurement equipment and test procedures 5. Installation and mitigation guidelines application of filters, surge suppressors, etc. 6. Generic and product standards define equipment immunity levels Lecture

13 Other standards and standards-related work EN Voltage Characteristics Of Electricity Supplied By Public Distribution Systems Voltage at the PCC only Addresses harmonics, sags, swells, etc. Incorporated in newer power quality recorders Describes 95% conditions Operation at minimum requirements is not likely to be acceptable to customers. Lecture Standards Standards are becoming more international several IEEE standards reference IEC standards. IEEE Power Engineering Society Power Quality Subcommittee Works to coordinate international power quality standards including harmonic standards In some cases identical standards are issued from multiple organizations now ANSI/ISA is practically identical to IEC , CSA C22.2 No , and UL Lecture

14 Next time Start long-duration voltage variations Lecture