MITIGATION OF POWER SYSTEM HARMONICS.pptx

MITIGATION OF POWER SYSTEM HARMONICS- power quality

1. MITIGATION OF POWER SYSTEM HARMONICS

2. Objective The reduction or suppression of power system harmonics. Precautionary (Preventive) solutions: Phase cancellation or harmonic control in power converters. Developing procedures and methods to control, reduce or eliminate harmonics in power system equipment; mainly capacitors, transformers and generators. Corrective (Remedial) solutions : The use of filters. Circuit detuning which involves the reconfiguration of feeders or relocation of capacitor banks to overcome resonance.

3. Harmonic Sources Transformer Magnetisation Nonlinearities Symmetrical Over excitation Inrush Current Harmonics D.C. Magnetisation Rotating Machine Harmonics M.M.F. Distribution of A.C. Windings Three-Phase Winding Slot Harmonics Voltage Harmonics Produced by Synchronous Machines Rotor Saliency Effects Voltage Harmonics Produced by Induction Motors Distortion Caused by Arcing Devices Electric Arc Furnaces Discharge-Type Lighting Single-Phase Rectification D. C. Power Supplies Line-Commutated Railway Rectifiers Three-Phase Current-Source Conversion Basic (Six-Pulse) Configuration Effect of Transformer Connection Twelve-Pulse Related Harmonics Higher-Pulse Configurations Effect of Transformer and System Impedance Direct Voltage Harmonics Imperfect D.C. Voltage Smoothing Half-Controlled Rectification Uncharacteristic Harmonic and Inter-Harmonic Generation Frequency Cross-Modulation in Line-Commutated Converter Systems Three-Phase Voltage-Source Conversion Multi-Level VSC Configurations Inverter-Fed A.C. Drives Modulated Phase Control The Switching Function Approach A.C. Regulators Single-Phase Full-Wave Controller

4. Effects of Harmonic Distortion Resonances Parallel Resonance Series Resonance Effects of Resonance on System Behaviour Complementary and Composite Resonances Poor Damping Effects of Harmonics on Rotating Machines Harmonic Losses Harmonic Torques Effect of Harmonics on Static Power Plant Transmission System Transformers Capacitor Banks Power Assessment with Distorted Waveforms Power Factor Under Harmonic Distortion Effect of Harmonics on Measuring Instruments Harmonic Interference with Ripple Control Systems Harmonic Interference with Power System Protection Harmonic Problems During Fault Conditions Harmonic Problems Outside Fault Conditions Effect of Harmonics on Consumer Equipment Interference with Communications Simple Model of a Telephone Circuit Factors Influencing Interference Coupling to Communication Circuits Effect on Communication Circuits ( Susceptiveness ) Telephone Circuit Balance to Earth Audible Noise from Electric Motors

5. Effects of Harmonics The main effects of voltage and current harmonics within the power system are: The possibility of amplification of harmonic levels resulting from series and parallel resonances. A reduction in the efficiency of the generation, transmission and utilisation of electric energy. Ageing of the insulation of electrical plant components with consequent shortening of their useful life. Malfunctioning of system or plant components

6. Definitions and Terms THD Total Harmonic Distortion (or Distortion Factor) of voltage or current is the ratio of the rms value of harmonics above fundamental, divided by the rms value of the fundamental . PCC . Point of Common Coupling is a point of metering , or any point as long as both the utility and the customer can either access the point for direct measurements of the harmonic indices . Within an industrial load, the point of common coupling is the point between the nonlinear load and other loads. ISC . Maximum short circuit current at the PCC. IL . Maximum demand load current (fundamental frequency component) at the PCC, calculated as the average current of the maximum demands for each of the preceeding twelve months. For new customers, this value must be estimated. TDD . Total demand distortion, which is the THD of current (using a 15 or 30 minute averaging measurement period) normalized to the maximum demand load current IL.

7. Measures of Harmonic Distortion

8. Voltage and Current Distortion Factors

9. Contd..

10. Complex Waveforms due to Harmonics

11. Mitigation : Harmonic Filters Series-Tuned Filters Double Band-Pass Filters Damped Filters Detuned (Anti-Resonant) Filters Active Filters

12. Series-Tuned Filters A series-tuned filter consists of a series combination of a capacitor and a reactor and is tuned to low harmonic frequencies. At the tuned harmonic, the capacitor and the reactor have equal reactances and the filter has a purely resistive impedance. The filter's impedance is capacitive for lower harmonics and inductive for higher harmonics , a consequence of which is aggravating the impedance below the lowest tuned frequency.

13. Contd. Q=X0/R Filter pass band (PB): frequencies at which the filter reactance equals its resistance, i.e. the impedance angle is 45◦ and the impedance module √2R. The quality factor and pass band are related by the expression Q= ωn /PB

14. Contd.. A single tuned filter is a series RLC circuit (as shown in Figure 6.1) tuned to the frequency of one harmonic (generally a lower characteristic harmonic ). Its impedance is given by Z1 = R+j ( ωL − 1 / ωC ) There are two basic design parameters to be considered: the quality factor (Q), relative frequency deviation (δ), Filters are not usually designed to give minimum harmonic voltage and normally a higher Q is selected in order to reduce losses. A condition that also has to be considered in the design of filters, and which can restrict the operation of the converters, is an outage of one or more filter branches. The remaining filter branches may then be over-stressed as they have to take the total harmonic current generated by the converter

15. Harmonic Filter Design Tuning a capacitor to a certain harmonic, alternatively, designing the capacitor to trap (filter) a certain harmonic, requires the addition of a reactor. At the tuned harmonic

16. Being sensitive to peak voltages, the capacitor needs to be able to with stand the total peak voltage across it. That is, it needs to have a voltage rating equal to the algebraic sum of the fundamental and tuned harmonic voltages.

17. Design steps: series-tuned filter tuned to the hn harmonic

18. Problem: A filter is tuned to the 13th harmonic. Given Xc = 507ohms, calculate the filter elements and plots its impedance.

19. Double Band-Pass Filters A double band-pass filter is a series combination of a main capacitor, a main reactor and a tuning device which consists of a tuning capacitor and a tuning reactor connected in parallel. The impedance of such a filter is low at two tuned frequencies.

20. Damped Filters Damped filters can be 1st, 2nd or 3rd_order (commonly used is the 2nd-order). A 2nd-order damped filter consists of a capacitor in series with a parallel combination of a reactor and a resistor. It provides a low impedance for a moderately wide range of frequencies. When used to eliminate high order har monics (17th and above), a damped filter is referred to as a high-pass filter , providing a low impedance for high frequencies but entry restricted for low frequencies . Damped filters have a low quality factor, 0.5 < Q < 5.

21. Design steps: For a second-order damped filter tuned to the hn harmonic:

22. Advantages: Its performance and loading are less sensitive to temperature variation, frequency deviation , component manufacturing tolerances , loss of capacitor elements, etc . It provides a low impedance for a wide spectrum of harmonics without the need for subdivision of parallel branches, which increases switching and maintenance problems . The use of tuned filters often results in parallel resonance between the filter and system admittances at a harmonic order below the lower tuned filter frequency, or in between tuned filter frequencies. In such cases the use of one or more damped filters is a more acceptable alternative .

23. Disadvantages : To achieve a similar level of filtering performance the damped filter needs to be designed for higher fundamental VA ratings , though in most cases a good performance can be met within the limits required for power factor correction . The losses in the resistor and reactor are generally higher.

24. Problem: A second-order damped filter is tuned to hn > 17. Knowing Xc = 1.734 D, calculate the filter elements and plot its impedance.

25. Detuned (Anti-Resonant) Filters A detuned filter is tuned below a characteristic harmonic (usually tuned to the fourth harmonic). It absorbs some of the harmonic but not as much as a higher tuned one. It is a reliable and time-tested method to improve the power factor and also mitigating the risk of resonance. This is achieved by shifting the resonance frequency to lower levels, thereby ensuring that no harmonic currents are present

26. Active Filters Active filters are being developed to overcome the disadvantages of conventional passive filters: The filtering characteristics being dependent on the source impedance. Aggravating the impedance below the lowest tuned harmonic. Being inadequate for filtering non-characteristic harmonics (different from the filter's tuned frequency), such as those produced by cycloconverters .

27. POWER CONVERTERS

28. Impedance Plots for Filter Banks The plots for the impedance of three different filter banks, namely: A three-branch 33 kV filter; 7th and 11th tuned plus a high pass (second order damped) branch for all harmonics from the 17th and above. A four-branch 20 kV filter; 5th, 7th, nth and 13th tuned . A five-branch 690 v filter; sth ' 7th, 11th and 13th tuned plus a high pass (second-order damped) branch for all harmonics from the 17th and above.

29. Impedance Plots for a Three-Branch 33kV Filter

30. Impedance plots for the three-branch filter