Metering For Harmonic Environment
SHAN BALAKRISHNAN SARASWATHI
Assistant Engineer at Kerala State Electricity Board
Metering For Harmonic Environment
Prepared by SHAN.B.S, AE,KSEB January 2019
Abstract
??????????? Nowadays harmonics are generated even by domestic consumers, as majority of the domestic appliances are using electronic components and other programmable micro controllers or even SCR like power electronic components as parts of variable controllers for power drives. These devices draw current which is distorted in nature and this may sometimes distorts the voltage waveform. There are consumers who doesn’t produce harmonics. But when this good consumers are fed with distorted supply voltage , their linear loads will be forced to draw current which will essentially be distorted in nature. For those consumers who produces distortion shall have negative harmonics power components while for those good consumers,? their power harmonics shall be positive in nature. When the presently used energy meters calculate the energy consumption, the total energy consumption for the good consumers shall be much more than that of the consumers who produces harmonics. This paper tries to discuss more about this issue and proposes a solution through new smart meter technology.
Synopsis
??????????? Harmonics generated by consumer equipments are making havoc in the electric supply system as it is causing distortion in the supply voltage waveform from pure sinusoidal to distorted sinusoidal. This distorted voltage when supplied to consumers using linear/non harmonic loads , draws current in the same distorted form leading to increased power consumption , malfunction of equipments and in some cases damage to equipments and wiring system. At the same time generated harmonics currents flows back to the system and causes heating in the lines, transformers and capacitors. The resonance due to harmonics is quite common which leads to excessive RMS current and more loading than actual useful power required or delivered. Due to the likely hood of occurrences of resonance with capacitors,? used for power factor improvement and voltage regulations, leading to heavy current flow through it, which frequently results in the blasting of capacitors.
??????????? The ‘Total Harmonic Distortion’(THD) value, which is generally measured are not helpful in many cases, as no mitigation or precautions can be taken based on the THD values. A spectrum of power harmonics from fundamental to about 50th harmonics with direction (+/-) will be helpful both for analysis and penal calculations. As harmonics distortion are steady state in nature the? measurements? taken at an interval of 15 minutes is adequately enough for the smart grid project of the utility.
??????????? Generally the effect of harmonics is studied in voltage or current. The presents of? harmonics? in the system results into power harmonics. Power harmonics are then measured by energy meters. Though current harmonics are high, power harmonics may be small and contribution to energy is insignificant. However, power harmonic losses may be a substantial component of total losses in the system. The power harmonics direction is opposite to fundamental power flow at the source of harmonics and so total power measured is less than fundamental power. The linear loads consumes harmonics power from the system.
HISTORY
??????????? Our power systems are designed to operate at frequency of 50 hz. However, certain types of loads produce currents and voltages with frequencies that are integer multiples of the fundamental frequency. These higher frequencies are a form of electrical pollution known as power system harmonics.
??????????? Power system harmonics are not a new phenomenon. During 1910s the main concern was third harmonic currents caused by saturated iron in transformers and machines. A text published by Steinmetz in 1916 devotes considerable attention to the study of harmonics in three-phase power systems.? He was the first to propose delta connections for blocking third harmonic currents. After Steinmetz’s important discovery, and as improvements were made in transformer and machine design, the harmonics problem was largely solved until the 1930s and 40s. By then, with the advent of rural electrification and telephones, power and telephone circuits were placed on common rights-of-way. Transformers and rectifiers in power systems produced harmonic currents that inductively coupled into adjacent open-wire telephone circuits and produced audible telephone interference. These problems were gradually alleviated by filtering and by minimizing transformer core magnetizing currents. Isolated telephone interference problems still occur, but these propower measureblems are infrequent because open-wire telephone circuits have been replaced with twisted pair, buried cables, and fiber optics.
??????????? Today, the most common sources of harmonics are power electronic loads such as adjustable speed drives (ASDs) and switching power supplies. Electronic loads use diodes, silicon controlled rectifiers (SCRs), power transistors, and other electronic switches to either chop waveforms to control power, or to convert 50/60Hz AC to DC. In the case of ASDs, DC is then converted to variable-frequency AC to control motor speed.
Introduction
??????????? A well designed power plant generates sinusoidal alternating current or electrical power, thus a utility delivers sinusoidal power or fundamental power to consumers. The loads at consumer end are nonlinear in nature and cause distortion in the wave shape and generate other components at frequencies that are integral multiples of the fundamental frequency. The combined wave shape of all the frequencies is not a sinusoidal wave shape and make an irregular shape but of repetitive nature. The repetitive deviation of either the voltage or current waveform from a pure sinusoid is usually referred as harmonic distortion. The combined wave shape is a complex wave shape and can be decomposed by Fourier transform method.
??????????? The distortion components can be integral multiple of the fundamental frequencies in a power system and cause distortion in the sinusoidal wave shape and are called harmonic distortion but generally the integer multiples are referred as harmonic components and are harmonic distortion. The harmonic components may be different in current and voltage waveforms.
?Harmonics? Generation
??????????? Harmonics are generated by any load, which draws current not proportional to the voltage applied. Nowadays most of the loads are somewhat nonlinear in nature, but among them some generates more and higher level harmonics than others. These include the following types of loads
Harmonic current, voltage and power
The sinusoidal voltages or fundamental value of voltage is defined by the equation
' E=Em Sin wt
' I=Im Sin wt
where Em and Im are the peak values.
The ratio of RMS values to peak values called form factor has a value of? 0.707 for pure sinusoidal waveform or in this case for each harmonic component.
Let the complex voltage waveform be represented by the equation
'E = E1m Sin wt+E2m Sin 2wt + E3m Sin 3wt + E4m Sin 4wt +...+ Enm Sin nwt
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is applied to a circuit.
Let the equation of the resultant current wave be
'I= I1m Sin (wt+1) +I2m Sin (2wt+2) + I3m Sin (3wt+3) + I4m Sin (4wt+4)+...+ Inm Sin(nwt+n)
The instantaneous value of resultant power in the circuit is
p=ei watt
For obtaining the values of power, we have to multiply every term of the voltage wave, in turn, by every corresponding integral term in the current wave. The average power supplied during a cycle would be equal to the sum of the average power values over one cycle of each individual product term. Hence total power supplied by a complex wave is the sum of the average power supplied by each harmonic component acting independently.
Total power is
P= E1I1cos1 + E2I2 Cos 2 + E3I3 Cos 3 + ......EnIn Cosn
The first component is the fundamental power and other components are harmonic components. Hence the total power is a sum of fundamental power and harmonic power. The direction of flow of individual component should be considered appropriately.
??????????? The direction of the? individual component powers are important as negative components reduces the total power shown in ordinary meters leading to revenue loss. Moreover as negative power components are generated by the sources of harmonics and by knowing these components adequate?? steps can be taken to mitigate these problems. These negative power components affects the voltage waveform which leads to degrading/pollution of supply voltage fed by the utility.
UTILITY ENERGY MEASUREMENTS
??????????? Harmonic distortion is? steady state in characteristics of voltage and current. Therefore, generally when harmonics or power quality study is conducted, characterizing harmonic distortion levels is accomplished with profiles of harmonic distortion taken with harmonics analyzers, over a time of 24hrs. Harmonic current and voltages may be measured for every 10minutes or 15 minutes span for 24hrs. For most applications, it is sufficient to consider the harmonic range from 2nd to 25th but most standards specify upto 50th.
??????????? For the utility measurements, the PCC can be the supply/ metering point. The amount of current /voltage distortion at the PCC depends upon several facts such as voltage level, type of non linear loads and its percentage composition in the total load. In most cases, measuring harmonics in any one phase is indicative of the harmonic content in the other phases if it is three phase balanced industrial load. However, typically consumer loads are unbalanced, so individual phases must be measured.
??????????? Harmonic measurements should be made under a variety of load conditions. Harmonics may not be produced continuously, but the addition of high power equipment may be the occasion for drastic increase in harmonics .A reduction in plant loads may increase harmonics. Peak load periods may be a time of lower harmonics, because the high current leads may shunt off some harmonic currents. So specific harmonics may be capable of producing drastic effects. So inorder to prevent accidents due these harmonics, real time indicators should be implemented. For this adequate measuring instruments should be employed.
??????????? While measurement of harmonic voltages and currents, it is important to note phase angles because harmonics with similar phase angles will superimpose, raising the harmonic level, while those with opposite phase angles will compensate each other, thereby lowering the harmonic level. Also, to evaluate harmonic flows throughout the system, to identify harmonic source and sinks, phase angles of each harmonic are required.
??????????? The direction of the active power flow is also important as it can be used to find the location of the harmonic sources. A linear load supplied with distorted voltage draws active power for each harmonic. In this case, active power calculated is positive.(Active power of nth harmonic,P(n)>=0).If non linear loads exists at the customer side, the active power for some harmonics can be supplied to the network. Active power calculated is negative. (P (n) < 0).
Accordingly, voltage distortion source can be located at-
i. upstream of PCC
ii. downstream of PCC
iii. Both sides of PCC
??????????? Some of the harmonic active powers may be positive and some others may be negative. The negative active powers indicate that the load is harmonic source of that particular order.? So the direction of each active power harmonic has to be indicated.
??????????? The linear loads which are not sources of harmonics will draw current proportional to the voltage. Thus it will draw some nonlinear current as its input voltage is also nonlinear.? This complex current drawn by linear loads will be in phase with the voltage applied and thus the total power measured by the meter will be the fundamental power consumed by the load plus harmonic power drawn due to the supply voltage . Thus the harmonic energy is flowing from the utility to the consumer and the power metered may be more than the power actually consumed in the load.
??????????? So inorder to find the real harmonics producing consumers , power spectrum for different harmonics has to be obtained. Smart meters capable of recording? power /energy spectrum for different harmonics can be a solution for this purpose.
??????????? Energy meters uses internal CTs? which act as low pass filters as it offers high impedance to high frequencies. Its energy measured may be erroneous when the supply voltage is distorted as there is no such filtering applied to voltage. So Energy meters using special low pass filters which will filter the harmonics components and shows only the fundamental energy consumed which will be lesser than the true power consumed by the harmonic load.
Meter Readings Required from smart meter
??????????? For effective monitoring in smart grid and for suitable warning indications at the right time in the present world of high harmonic electrical system, the following quantities shall be displayed and transmitted at suitable periods as decided for the smart metering project.