Fluke 43B Application Note 2103535_HarmonicResonance
2015-09-09
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5th Harmonic resonance Power Quality Case Study Measuring tools: Fluke 43B Power Quality Analyzer Operator: Water plant electrical engineer Application Note Problem description The power to this motor comes via a 30-mile utility line A small city gets its water from originating in the city. A power a mountain lake 30 miles away. factor correction capacitor near A pumping system at the lake the backwash pump improves brings water up a short incline power factor and reduces voltage and into a long gravity pipeline loss at the end of that 30-mile that feeds the city’s water line. For a long time, this distribution system. configuration worked with no Before entering the pipeline, problems. the water passes through a After a small earthquake in filter that removes debris. When the nearby mountains, questions the differential pressure across arose regarding the continuation the filter gets too high, a large of the water supply in the event backwash pump reverses the of a power interruption. How flow momentarily to clear the would the city get its water if filter. The backwash flow the utility connection were carries debris to a settling pond. lost due to an earthquake? Powering this backwash The answer was a generator. pump is a 650 hp, 3-phase Subsequently, the city added motor equipped with a soft a diesel-powered engine start. A “negative sequence generator and transfer switch detector” helps protect this to the system (see Fig. 1). important motor by shutting it off upon the loss of a phase, or when voltage distortion exceeds a predetermined level. Features used: Voltage, harmonic spectrum Shop Generator Office Utility Transfer Switch P.F. Correction Capacitors Main Pump Backwash Pump Fig. 1 One-line diagram of the water plant From the Fluke Digital Library @ www.fluke.com/library System testing under generator power revealed a problem. The backwash pump would suddenly stop part way through the start sequence. And it was stopping because the negative sequence detector was putting out a stop signal to the motor controls. What electrical anomaly was the negative sequence detector protecting the motor from? Was it a phase loss? Excessive voltage distortion? What were the characteristics and source of this anomaly? Unfortunately, the city engineer was unable to answer these questions with his existing test equipment. After some research to identify a device that would allow him to see what was going on, he purchased a Fluke 43B. Measurements The engineer suspected voltage distortion, so he connected the Fluke 43B phase-to-phase across the input to the soft start and selected “Harmonics.” During the start sequence, the Fluke 43B displayed 5th harmonic distortion rising to 80 % of the fundamental prior to the stop signal. Problems can occur if the circuit has a source of harmonics near the resonant frequency. In the case of the backwash pump, the source of harmonics was the semiconductor switchTheory and analysis ing operation of the soft start. The ideal displacement power Why did the circuit work fine factor (DPF) is 1.0. This occurs when powered by the utility, when the current and voltage yet not work when powered by are “in-phase.” Inductive motor the generator? The difference is loads cause the current to lag, the source impedance. The low thereby lowering the DPF. This impedance of the utility means usually results in a penalty it can absorb harmonic currents charge from the utility, so many without causing severe voltage users install a capacitor to distortion. The impedance of provide DPF correction. the generator is much higher However, the combination of than that of the utility. inductance and capacitance will Harmonic currents flowing into form a resonant circuit that may the generator caused enough result in high circulating current voltage distortion to trigger the at the resonant frequency. It’s negative sequence protection standard practice to size a circuit to produce a stop signal. capacitor at a value large enough to correct the DPF to Solution at least 0.9, but not so large that it makes the voltage lag the The city engineer solved the current. The resulting configura- problem by moving the capacition yields a resonant frequency tor to the utility side of the between the 5th and 7th transfer switch. With this harmonic. configuration, the generator never sees the capacitor and the resonance condition does not exist when the generator powers the circuit. Fluke. Keeping your world up and running. Fluke Corporation PO Box 9090, Everett, WA USA 98206 Fluke Europe B.V. PO Box 1186, 5602 BD Eindhoven, The Netherlands For more information call: In the U.S.A. (800) 443-5853 or Fax (425) 446-5116 In Europe/M-East/Africa (31 40) 2 675 200 or Fax (31 40) 2 675 222 In Canada (800)-36-FLUKE or Fax (905) 890-6866 From other countries +1 (425) 446-5500 or Fax +1 (425) 446-5116 Web access: http://www.fluke.com/ 2 Fluke Corporation Power Quality Case Study: 5th Harmonic resonance ©2003 Fluke Corporation. All rights reserved. Printed in U.S.A. 8/2003 2103535 A-ENG-N Rev A
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