| Frequently Asked Questions : |
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1. What is THD% ?
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| It is the measure of distortion in the waveform. It applies both the voltage and current. |
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| In this equation I is replaced with V when referring to voltage harmonics. |
2. The failure rate of capacitors in our plant is quiet high. We have tried with Heavy Duty, Extra Heavy Duty types of capacitors.
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Capacitors are most affected due to harmonic in the system. The current drawn by capacitors increases with higher frequency terminal voltage. In worst cases resonance could take place between capacitor and system impedance causing excessively high flow of current leading to permanent damage either to capacitor or other equipment in system. The ‘type’ of capacitor does not really prevent this occurrence. You may get a little extra working life of capacitor, but the very fact that capacitors help amplify harmonics will further distort the “Quality of Power”. |
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3. We do not have any harmonic generating loads in our system. We still face some typical issues that could be related to harmonics.
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| You are connected to a common grid. Non-linear loads could be working in neighboring plants. The grid acts as common connection between your plant and the non-linear load in other plant, which could cause issues related to harmonic in your plant. |
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4. We have mixed loads in our plants with some loads as linear and some as non-linear. Do we need to take care of harmonics ?
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| In general when the non-linear load is about 15-20% of total load and power factor improvement capacitors are installed, harmonics could become a serious issue. |
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5. Is it necessary to conduct harmonic analysis to design the filter systems ?
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While it is advisable conducting the analysis it is not utmost necessary. Based on the load characteristics a very good approximation can be made while project is under design stage. Fine adjustments can be made once the plant goes in full running condition. For a working system however, analysis helps arriving at precise solutions. |
6. As a consumer of electricity which harmonic limits we must follow ?
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The utility supplies pure voltage waveform. The current drawn by consumer equipment is distorted. It is the consumer’s responsibility to maintain the Current Distortion within limits. Consequently, it is the utility responsibility to maintain the voltage harmonic levels. The following table shows permissible current distortion levels for a consumer. |
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Technical Overview |
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| What is it all about ? |
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| Electrical engineers were a relatively happy lot till the time the term “Efficient Use of Electricity” was coined. The stress shifted from continuous- breakdown free operations to “Efficient Operations” with reduced energy bills. |
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| Basic minimum installation of Power Factor Improvement Capacitors would suffice the purpose of reducing demand from the utility. A little studious engineer would go the extra mile to connect capacitors across major loads to reduce the line losses. |
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| Rising costs of electricity production and transmission necessitated the shift towards “Efficient Operations”. |
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| In general a simple induction motor was replaced with more efficient VFDs and fuel guzzling furnaces were replaced with higher efficiency Medium frequency Induction Furnaces. |
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| This new generation equipment commonly based on the principal of “Frequency Conversion” came with a side-effect, “Waveform Distortion”. |
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| The good old sine-wave changed its shape. |
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| With the proliferation of new generation equipment the current drawn from source no longer remains sine wave. |
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| The current has frequency components other than fundamental frequency. Frequency components like, 150 Hz, 250 Hz, 350 Hz, etc. are superimposed on the fundamental waveform. These components are called harmonics of the fundamental and in case of 50 Hz system they are called as 3rd, 5th, 7th harmonics of the fundamental cycle. |
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 Sources of Harmonics: |
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| As a general rule any equipment working on the ‘frequency conversion’ principal is a source of harmonics. These include; |
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• Rectifiers
• Induction furnaces
• Induction heaters
• DC Drives
• AC Drives
• Welding equipment
• Energy Efficient Lighting Products
• UPS Systems
• SMPS |
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| Effects of Harmonics : |
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| A voltage is developed when current flows through any impedance. This simple law applies to harmonic currents also. While the harmonic currents flow in a system harmonic voltages are developed. |
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| This distorted voltage is fed to other equipment in the power system. This would mean that even if the equipment is not generating harmonics on its own it is still affected by the distorted voltage waveform. |
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The “Quality of Power” is deteriorated. Equipment that is supposed to work on pure sine wave supply no longer works satisfactorily.
Apart from apparent breakdowns in system following issues become dominant in case of power systems with non-linear loads. |
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• Increased system losses
• Spurious tripping of protective devices
• Failure or de-rating of capacitors
• Reduction in working life of equipment |
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