impact of renewable energy resources on the dynamics...

V 10 2016 •1Impact of Renewable Energy on Quality and Dynamics of Power

Impact of Renewable Energy Resources on the Quality of

power of the Electrical Power System

M. E. El-Hawary, Dalhousie University, Halifax, Nova Scotia

PES DLP LectureTuesday December 20, 2016

Bangkok, Thailand

V 10 2016 •2

Agenda1. The Scene in Thailand2. Power Quality Phenomena.3. Renewable Energy: Wind.4. Renewable Energy : PV. 5. Effects of Harmonics 6. Harmonics: Mitigation7. Harmonics: Effects of Capacitors.8. FACTS

Impact of Renewable Energy on Quality and Dynamics of Power

V 10 2016 •3

The Scene in Thailand

• Alternative Energy Development Plan (AEDP)

• Increase consumption of renewables by 25% by 2021 (relative to 2015)



Defining Power Quality • “… the degree to which both the

utilization and delivery of electric power affects the performance of electrical equipment.”

• Transient Disturbances, and• Steady State Disturbances


Conducted low-frequency phenomena

Harmonics, inter-harmonicsSignal systems (power line carrier)Voltage fluctuations

Voltage dips and interruptionsVoltage imbalance

Power-frequency variationsInduced low-frequency voltagesDC in ac networks


Radiated low-frequency phenomena

Magnetic Fields

Electric Fields

Conducted high-frequency phenomena

Induced continuous wave voltages or currentsUnidirectional transientsOscillatory transients


Radiated high-frequency phenomena

Magnetic FieldsElectric FieldsElectromagnetic FieldsContinuous wavesTransients

Electrostatic discharge phenomena

Nuclear electromagnetic pulse


Attributes of non-steady state phenomena

• Rate of rise• Amplitude• Duration• Spectrum• Frequency• Rate of occurrence• Energy potential• Source impedance


Attributes of steady-state phenomena

• Amplitude• Frequency• Spectrum• Modulation• Source impedance• Notch depth• Notch area


Transient Disturbances• Generally refers to a change (increase or

decrease) in voltage of less than one half-cycle.

• A short duration disturbance of from less than 1 s to several milliseconds superimposed on the normal voltage wave.

• Often used loosely to describe any disturbance that is transitory.


Figure (1) Typical transient disturbances


Wave-shape Faults• Sample wave-shape fault. • Oscillatory transients can also be

classified as wave-shape faults.


• These are damped high frequency oscillations (from a few hundred Hz to 500 kHz), that decay to zero within a few milliseconds.

• Arise from switching of lines, transformer and capacitor banks.

• In each case, a major factor is the amount of energy stored in the fields.


• Switching is common interrupt faults, restore service, or to isolate equipment for maintenance.

• Capacitor banks are often switched daily to keep the system at nominal voltage levels.


Voltage Concerns• Voltage regulation is an important

consideration in the quality of service. • Fairly definite limits between which the

voltage must be maintained at the point of delivery for proper operation of utilization equipment.


• Maintaining close limits than necessary will result in higher system cost.

• Automatic voltage regulation must be carefully sized and located appropriately to assure economic system design.


• Utilization equipment is also cost sensitive to voltage limits.

• Standardization resulted in adopting uniform limits of supplied voltages.

• In addition to noise and radio frequency interference concerns, the following are important voltage issues.

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• Harmonics• Interharmonics.• Voltage unbalance.


V 10 2016 •20Power Quality

Harmonics• Harmonics are sinusoidal voltages or

currents whose frequencies are integer multiples of the designed supply frequency (termed the fundamental frequency; usually 50 Hz or 60 Hz)


• Harmonics combine with the fundamental voltage or current, and produce waveform distortion.

• Harmonic distortion is caused by the nonlinear characteristics of devices and loads on the power system.


• These devices can usually be modeled as current sources that inject harmonic currents into the power system.

• Voltage distortion results as these currents cause nonlinear voltage drops across the system impedance.


• Harmonic distortion is a concern for many customers and for the overall power system due to:

• 1- Renewable energy generation.• 2- power electronics equipment.• Harmonic distortion levels can be

characterized by the complete harmonic spectrum with magnitudes and phase angles of each individual harmonic component.


• It is also common to use a single quantity, the total harmonic distortion, as a measure of the magnitude of harmonic distortion.

• Harmonic currents result from the normal operation of nonlinear devices on the power system.


• Current distortion levels can be characterized by a total harmonic distortion, but this can often be misleading.

• For instance, many adjustable speed drives will exhibit high total harmonic distortion values for the input current when they are operating at very light loads.

• This is not a significant concern because the magnitude of harmonic current is low, even though its relative distortion is high.


• To handle this concern for characterizing harmonic currents in a consistent fashion, IEEE Std 519-1992 defines another term, the total demand distortion.

• This term is the same as the total harmonic distortion except that the distortion is expressed as a percent of some rated load current rather than as a percent of the fundamental current magnitude.


Interharmonics

• Can be found in networks of all voltage classes.

• Can appear as discrete frequencies or as a wide-band spectrum.

• The main sources are static frequency converters, cyclo-converters, induction motors, and arcing devices.

• Power-line carrier signals can also be considered as interharmonics.


• The effects of interharmonics are not well known, but have been shown to affect power line carrier signaling, and induce visual flicker in display devices.

V 10 2016 •31

Sources of Harmonics

Impact of Renewable Energy on Quality of Power


Voltage Unbalance• Utility supply voltage is maintained at

a relatively low level of phase unbalance (typically maintained at less than 1%, although 2% is not uncommon.)

• Unbalance > 2% should be reduced,, by balancing single-phase loads.


• Overheating of motors and transformers can readily occur if the imbalance is not corrected. Phase current imbalance to three-phase induction motors varies almost as the cube of the voltage imbalance applied to the motor terminals.


• Voltage Unbalance greater than 5% can be caused by single-phasing conditions, during which one phase of a three-phase circuit is missing or de-energized.


• When unbalanced phase voltages are applied to three-phase motors, the phase-voltage unbalance causes additional negative-sequence currents to circulate in the motor, increasing the heat losses primarily in the rotor.

• The most severe condition occurs when one phase is opened and the motor runs on single-phase power.


• All motors are sensitive to phase-voltage unbalance, but sealed compressor motors used in air conditioners are most susceptible to this condition.

• These motors operate with higher current densities in the windings because of the added cooling effect of the refrigerant.


• Some electronic equipment such as computers may also be affected by phase voltage unbalance of more than 2 or 2.5%.

• The equipment manufacture can supply the necessary information.


• In general, single-phase loads should not be connected to three-phase circuits supplying equipment sensitive to phase-voltage unbalance.

• A separate circuit should be used to supply this equipment.


Long duration variations• Variations in supply voltage lasting

longer than 1 min can cause equipment problems.

• Overvoltage and undervoltage problems are less likely to occur on utility feeders, as most utilities strive to maintain ±5% voltage regulation.


• Overvoltage and undervoltage problems can occur, however, due to overloaded feeders, incorrect tap settings on transformers, blown fuses on capacitor banks, and capacitor banks in service during light load conditions.


Undervoltages• Undervoltages in excess of 1 min can

also cause equipment to malfunction. • Motor controllers can drop out during

undervoltage conditions. • The dropout voltage of motor

controllers is typically 70Ð80% of nominal voltage.


• Long duration undervoltages cause an increased heating loss in induction motors due to increased motor current. Speed changes are possible for induction machinery during undervoltage conditions.

• Electronic devices such as computers and electronic controllers may stop operating during this condition.


• Undervoltage conditions on capacitor banks result in a reduction of output of the bank, since var output is proportional to the square of the applied voltage.

• Generally, undervoltage conditions on transformers, cable, bus, switchgear, CTs, PTs, metering devices, and transducers do not cause problems for the equipment.


Overvoltages• Overvoltages may cause equipment

failure. • Electronic devices may experience

immediate failure during the overvoltage conditions; however, transformers, cable, bus, switchgear, CTs, PTs, and rotating machinery do not generally show immediate failure.


• Sustained overvoltage on transformers, cable, bus, switchgear, CTs, PTs and rotating machinery can result in loss of equipment life.

• An overvoltage condition on some protective relays may result in unwanted operations while others will not be affected.


• A sign of frequent overvoltage conditions on a capacitor bank is the bulge of individual cans.

• The var output of a capacitor will increase with the square of the voltage during an overvoltage condition.

• The visible light output from some lighting devices may be increased during overvoltage conditions.

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Semiconductor device technology• Improvements in the Performance and

reliability of power-electronic variable frequency drives for wind- turbine applications have been directly related to the availability of power semiconductor devices with better electrical characteristics and lower prices because the device performance determines the size, weight, and cost of the entire power electronics used as interfaces in wind turbines.


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• Recently, the integrated gated control thyristor (IGCT) has been developed as a mechanical integration of a GTO plus a delicate hard drive circuit that transforms the GTO into a modern high-performance component with a large safe operation area (SOA), lower switching losses, and a short storage time.


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• IGBTs have higher switching frequency than IGCTs, so they introduce less distortion in the grid.

• There is also cooling problem in IGCTs as they are made like disk devices.

• They have to be cooled with a cooling plate by electrical contact on the high-voltage side.


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• This is a problem because high electromagnetic emission will occur.

• The main advantage of IGCTs over IGBTs is that they have a less ON-state voltage drop, which is about 3.0 V for a 4500-V device.


Renewable Energy:Wind

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Variable-Speed concept• The generator is completely

decoupled from the grid . The majority of wind turbines are equipped with a multipole synchronous generator, although it is quite possible (but rather rare) to use an induction generator and a gearbox.


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There are many advantages for removing the gearbox: lower losses, lower costs due to the elimination of this expensive component, and increased reliability due to the elimination of rotating mechanical components.• .


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• IGBTs have higher switching frequency than IGCTs, so they introduce less distortion in the grid.

• There is also cooling problem in IGCTs as they are made like disk devices.

• They have to be cooled with a cooling plate by electrical contact on the high-voltage side.


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• Recently, the integrated gated control thyristor (IGCT) has been developed as a mechanical integration of a GTO plus a delicate hard drive circuit that transforms the GTO into a modern high-performance component with a large safe operation area (SOA), lower switching losses, and a short storage time.


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Single Doubly Fed Induction Machine with Two Fully Controlled AC–DC Power Converters


This converter decouples mechanical and electrical frequencies and thus makes variable-speed operation possible.

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Single Doubly Fed Induction Machine Controlled with Slip Power

Dissipation in an Internal Resistor


The variable-speed is achieved by dissipating the energy within a resistor placed in the rotor

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Double Three-phase VSI


The energy from the generator is rectified to a dc link and after is converted to suitable ac energy for the grid.

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Step-up Converter in the Rectifier Circuit and Full Power Inverter

Topology Used in Wind-turbine Applications


Synchronous generator is used instead of an induction one. Three-phase converter (connected to the generator) replaced by a three-phase diode rectifier and a chopper. Such choice is based on the low cost as compared to an induction generator connected to a voltage-source inverter (VSI) used as a rectifier.

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Control Block Diagram of PMSG based WECS


PMSG Wind energy conversion system (WECS) with two stages as optimization and electrical controllers.

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Renewable Energy: PV


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Unified Diagram of PV System


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• The photovoltaic (PV) energy is the most promising source of energy since it is pollution free and abundantly available everywhere in the world.

• PV energy is especially beneficial in remote sites like deserts or rural zones where the difficulties to transport fuel and the lack of energy grid lines make the use of conventional resources impossible.


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MPPT• An MPPT, or maximum power point

tracker is an electronic DC to DC converter that optimizes the match between the solar array (PV panels), and the battery bank or utility grid.

• They convert a higher voltage DC output from solar panels (and a few wind generators) down to the lower voltage needed to charge batteries.


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• Many MPPT methods have been presented, such as the hill climbing, incremental conductance and the P&O.

• These algorithms consist of introducing a crisp values positive or negative (decrease or increase) all around the actual photovoltaic generator (PVG) operating point.


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• These algorithms consist of introducing a crisp values positive or negative (decrease or increase) all around the actual photovoltaic generator (PVG) operating point.


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• From the previous power point position, the trajectory of the new one helps the algorithm to decide on the command output value.

• This algorithm may fail to act as an accurate MPPT because of the used crisp value (step size) that is mainly fixed by trial and tests running.


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• From the previous power point position, the trajectory of the new one helps the algorithm to decide on the command output value.

• This algorithm may fail to act as an accurate MPPT because of the used crisp value (step size) that is mainly fixed by trial and tests running.


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• One scheme reduces harmonic current for grid- connected PV generation system was developed/

• Experiments using a prototype of the power conditioning system (PCS) showed its validity.


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• 400 kW PCSs with the control scheme have been installed and have been in service since the end of

• 2009. • Three control methods were

developed such as generation power control for fault ride through harmonic current reduction control scheme and grid voltage stabilization using optimal reactive power control.


V 10 2016 •71

Effects of Harmonics



• Power system operating equipment is not immune to harmonic interference.

• Potential for possible problems can only be evaluated and appropriate countermeasures can be taken if harmonic analysis can be performed hence the need for advanced harmonic analysis software.


Effect on Motors

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• Hysteresis losses are proportional to frequency, and eddy current losses vary as the square of the frequency.

• Higher frequency voltage components produce additional losses in the core of AC motors.


Heating & Temperature Rise

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• Iron losses in the frame of the motor increase.

• This increases the operating temperature of the core and the surrounding windings.

• The copper losses in the motor windings vary as the square of the rms current.

• Due to skin effect, actual losses would be slightly higher than calculated values.


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• Stray motor losses, which include winding eddy current losses, high frequency rotor and stator surface losses, and tooth pulsation losses, also increase due to harmonic voltages and currents.

• The designer bases the motors' heating characteristics and cooling methods on power supplied at fixed voltage and frequency.


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• Torsional oscillation of the motor shaft due to harmonics is often disregarded.

• Torque is produced by interaction between the air gap magnetic field and the rotor-induced currents.

• The air gap magnetic fields and the rotor currents contain harmonics


Torsional Oscillations

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• The harmonics are grouped into positive (+), negative (-) and zero (0) sequence components.

• Positive sequence harmonics (harmonic numbers 1,7,13, etc.) produce magnetic fields and currents rotating in the same direction as the fundamental frequency harmonic.


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• Negative sequence harmonics (harmonic numbers 5,11,17, etc.) develop magnetic fields and currents that rotate in a direction opposite to the positive frequency set.

• Zero sequence harmonics (harmonic numbers 3,9,15,21, etc.) do not develop usable torque, but produce additional losses in the machine.


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• The interaction between the positive and negative sequence magnetic fields and currents produces torsional oscillations of the motor shaft.

• These oscillations result in shaft vibrations.


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• If the frequency of oscillations coincides with the natural mechanical frequency of the shaft, the vibrations are amplified and severe damage to the motor shaft may occur.

• It is important that for large VFD motor installations, harmonic analyses be performed to determine the levels of harmonic distortions and assess their impact on the motor.


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• Higher frequency voltage components induce currents in the motor’s bearings.

• This cause performance degradation and decrease life expectancy of the bearings


Induced Currents in the bearings

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• Harmonics increase the motor's acoustical noise level.

• The non-sinusoidal (current and voltage) waveforms produce vibration in the motor's laminations.

• Motor noise may not present a problem depending on motor location and the amount of noise produced by other equipment.


Acoustical Noise

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• The harmful effects of harmonics on transformer performance often go unnoticed until an actual failure occurs.

• Transformers that have operated satisfactorily for long periods have failed in a relatively short time when plant loads were changed or a facility's electrical system was reconfigured.


Effects on Transformers

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• Changes could include installation of variable frequency drives, electronic ballasts, power factor improvement capacitors, arc furnaces, and the addition or removal of large motors.

• Harmonics increase iron losses in the magnetic core of the transformer.


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• Eddy current concentrations are higher at the ends of the transformer windings due to the crowding effect of the leakage magnetic fields at the coil extremities.

• The eddy current losses increase as the square of the current in the conductor and the square of its frequency.

• This has a significant effect on the operating temperature of the transformer.


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• Transformers supplying power to nonlinear loads must be derated based on the percentages of harmonic components in the load current and the rated winding eddy current loss.

• One method of determining the capability of transformers to handle harmonic loads is by k factor ratings.

• The k factor is equal to the sum of the square of the harmonic currents multiplied by the square of the frequencies.


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• By providing additional capacity (larger-size or multiple winding conductors), k factor rated transformers can safely withstand additional winding eddy current losses equal to k times the rated eddy current loss.

• Also, due to the additive nature of triplenharmonic (3, 9, 15, etc.) currents flowing in the neutral conductor, k rated transformers are provided with a neutral terminal that is sized at least twice as large as the phase conductors.


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• Power factor correction capacitor banks are commonly applied.

• Capacitors are designed to operate at a maximum of 110% of rated voltage and at 135% of their kvar ratings

• These limits are exceeded because of large voltage or current harmonics which may result in capacitor bank failure

• . Impact of Renewable Energy on Quality and Dynamics of Power

Effects on Capacitors

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• Capacitive reactance is inversely proportional to frequency.

• Unfiltered harmonic currents find their way into capacitor banks.

• Banks act like a sink, attracting harmonic currents, becoming overloaded.

• Resonant conditions are created when the inductive and capacitive reactances are equal.


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• Capacitor banks do not generate harmonics but affect the system response and can cause resonant conditions which magnify the distortion problem.

• Series resonance produces voltage amplification.

• Parallel resonance causes current amplification.

• Considerable damage to capacitor banks and other electrical equipment would result.


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• The flow of current in a cable produces copper losses and current distortion introduces additional losses.

• Also, the effective resistance of the cable increases with frequency due to skin effect, where unequal flux linkages across the cross section of the cable causes the AC current to flow on the outer periphery of the conductor.


Effect on Cables

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• The higher the frequency the greater this tendency.

• It is important to make sure a cable is rated for the proper current flow.

• A set of calculations should be carried out to determine a cable's ampacity level.

• A more serious condition, with potential for substantial damage, occurs as a result of harmonic resonance.


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• During resonant conditions, if the amplitude of the offending frequency is large, considerable damage to capacitor banks would result.

• And, there is a high probability that other electrical equipment on the system would also be damaged.


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Symptoms• The actual manifestations of harmonic

problems will vary, depending on the types and number of installed harmonic producing loads.

• Most commercial buildings can withstand nonlinear loads of up to 15% of the total electrical system capacity without concern, but, when the nonlinear loads exceed 15% some problems may arise.


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• Blinking of Incandescent Lights -Transformer Saturation

• Capacitor Failure - Harmonic Resonance

• Circuit Breakers Tripping - Inductive Heating and Overload

• Computer Malfunction or Lockup -Voltage Distortion

• Conductor Failure - Inductive Heating .


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• Electronic Equipment Shutting down - Voltage Distortion

• Flickering of Fluorescent Lights - Transformer Saturation

• Fuses Blowing for No Apparent Reason - Inductive Heating and Overload

• Motor Failures (overheating) - Voltage Drop • Neutral Conductor and Terminal Failures -

Additive Triplen Currents .


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• Electromagnetic Load Failures -Inductive Heating

• Overheating of Metal Enclosures -Inductive Heating

• Power Interference on Voice Communication - Harmonic Noise

• Transformer Failures - Inductive Heating


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Measurents• Harmonic measurement meters are

used to determine the extent to which harmonics exist on the line. The following instrumentation is commonly employed.


V 10 2016 •101Impact of Renewable Energy on Quality of Power


Fluke 43B Power Quality Analyzer combines the most useful capabilities of a power quality analyzer, scope and multimeter in a single easy-to-use instrument.


Fluke 430 Series Three-Phase Power Quality Analyzers locate, predict, prevent and troubleshoot problems in power distribution systems.

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True-rms meters• The root mean square (rms) value is

the true measure of electrical parameters.

• Conventional analog style meters do not accurately measure the rms values of nonsinusoidal voltages and currents due to deficiencies in their response to higher frequency components.


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• In a harmonic-rich environment earlier forms of digital meters that measure the average or peak values and use multiplication factors to derive rms values are not appropriate.

• True-rms meters, by a process that involves high rate of signal sampling, recreate the waveform, and use frequency transformation techniques to obtain the true-rms values.


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• True-rms meters may indicate that harmonics are present in an electrical system but may not provide a breakdown of the significant harmonics


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Harmonic analyzers• Harmonic analyzers are effective for

determining the waveshapes of voltage and current and measuring the respective frequency spectrum.

• The simplest type of harmonic measuring instruments measure single-phase harmonic voltage and current, and provide information on the harmonic spectrums.


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• Power factor and phase angle information are also measured by the harmonic analyzer used. Three-phase harmonic analyzers measure the harmonic characteristics of the three phases and the neutral simultaneously.

• Some of the 3-phase analyzers provide graphs of the current and voltagedistortion variations with time.


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• Some analyzers are capable of measuring power, power factor, and transient disturbance data to help assess power quality within the power system.

• Harmonic analyzers calculate the total harmonic distortion (THD) of the waveform.


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• Normally, these instrument transformers are designed for optimum performance up to a cutoff frequency, beyond which their accuracies drop off considerably, introducing errors in the measurement.


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• While using a harmonic analyzer, it's important that you verify that voltage and current transformers (PTs and CTs) used with the analyzer have satisfactory higher frequency response characteristics.


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• While using a harmonic analyzer, it's important that you verify that voltage and current transformers (PTs and CTs) used with the analyzer have satisfactory higher frequency response characteristics.


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Using harmonic analyzers• Harmonic analyzers are provided with

voltage probes and current sensors. • Some analyzers have seven to nine

channels for simultaneous measurement of 3-phase voltages and currents and the neutral (each channel reading one parameter).


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• For high voltage > 600V and high current installations, PTs and CTs should be used.

• The location where an analyzer is installed depends on the type of data required.


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• If you suspect that a certain piece of equipment is generating harmonics, the analyzer must be located in the lines feeding the equipment at a location close to the equipment


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Voltage and current resonance

• Resonant conditions are created when the inductive and capacitive reactances become equal in an electrical system.

• Resonance in a power system may be classified as series or parallel resonance, depending on the configuration of the resonance circuit.


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• Series resonance produces voltage amplification.

• Parallel resonance causes current multiplication within an electrical system.

• In a harmonic rich environment, both types of resonance are present.


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Diagnosing Harmonics


impact of renewable energy resources on the dynamics...

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