Energy Meter With Harmonics Energy Measurement

Abstract

Prepared by Shan ?.B.S, AEE,KSEB

Due to steep rise in the use of non linear equipments by the domestic and industrial consumers, the magnitude of harmonics in the power system has increased to such alarming levels that it has become increasingly difficult to maintain the quality of the power supply. As harmonics injection to the supply system is from the consumer side, consumers need to be encouraged to use more distortion free equipments and for that harmonics energy consumption shall be included in the energy billing. But this is not so simple. Due diligence shall be applied to segregate the harmonics power components consumed and injected into the system. Only those consumers who are injecting harmonics power to the system are causing system pollution while those connections which are absorbing harmonics power are actually using linear equipments and are the victims of this power system pollution. This paper attempts to put forward an algorithm for the above problem of separating and measuring the harmonics energy absorbed and injected into the power system using a static energy meter.

Introduction

AC Electrical power system is intended to generate, transmit, distribute and utilize with voltages and currents having sinusoidal waveform. But due to the wide spread use of nonlinear equipments, distortions due occur in the sinusoidal waveform of voltages and currents. Magnetic circuits are inherently nonlinear in nature due to the nonlinear hysteresis behavior. These distorting components are generally called harmonics. All Electrical systems are designed for a particular frequency. This frequency is 50 HZ for many countries in Asia and Europe, while in USA, Canada etc this frequency is 60Hz. Even though the harmonics frequencies can occur in any frequency ranges, in this paper only integral multiples of fundamental frequency alone is considered.

Nowadays harmonics generated by electronics equipments, which are nonlinear in nature, by the domestic and industrial consumers consumer equipments are creating havoc in the electric supply system. Distortion in the load currents has rose to such high magnitudes that it is capable of causing distortion in the supply voltage itself, deviating the voltage waveform from pure sinusoidal to distorted sinusoidal. Even linear loads, when supplied with a distorted voltage draws distorted currents. When distorted voltages is supplied to consumers who are using linear/non harmonic equipments, that equipments tends to draw 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, these generated harmonics currents causes excessive heating in the lines, transformers and capacitors leading to higher distribution loss.

The resonance conditions may also occur because of the presents of higher harmonics which leads to drawing of excessive RMS current than the actual power rating of the equipments. Occurrences of resonance with capacitors, used for power factor improvement, voltage regulations etc, leads to the heavy current flow through it, creating a highly dangerous situation, which frequently results in the blasting of capacitors. Such is the situation that the conventional practice of recommending the use of capacitors for power factor control had became unwise to follow. In addition, the magnitude of the true RMS current may become much higher than the current magnitude shown in ammeters as these meters are inherently can measure only fundamental current magnitudes. As power losses depends on the magnitude of the true RMS current values and not limited to the fundamental component alone, total transmission losses in the system may be much higher than the anticipated values. Transmission operating engineers shall not receive any advance alerts to take swift preventive measures. For example with high harmonics, a power transformer winding temperature may be very high even though the transformer ammeter may be showing currents lower than rated value. Temperatures at feeder joints may rise steeply due to harmonics and may burn down at these points.

The only solution to discourage the use of harmonics generating equipments, is to incorporate penalizing components in the billing system so that the consumers shall find it necessary to use only non-distorting equipments. The ‘Total Harmonic Distortion’(THD) value, which is generally measured is 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 (+/-) may be helpful for analyzing purpose but it cannot be used for billing purposes. Inorder to integrate this penalizing component in the billing system, the quantity taken for such purpose should be simple to understand, easy to measure and shall be aggregating in nature. For this purpose the energy meter should be capable of measuring harmonics energy injected into the system.

Sources of Power Harmonics

Power systems are designed to operate at a particular frequency of 50/60 hz. However, certain types of loads produce currents and voltages with frequencies that are multiples of the fundamental frequency. These other frequency components are a form of electrical pollution known as power system harmonics.

Today, the most common sources of harmonics are power electronic loads such as adjustable speed drives (ASDs) and switching power supplies in addition to laptops and computers. 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.

Harmonics Generating Consumer Equipments

?????? Equipments using inverter technologies like inverter Fridges ,inverter A/C etc

?????? Static power converters using thyristor or SCRs to control the drives.

?????? Arc furnaces, Arc welding sets and ovens.

?????? Ballast in high power discharge, mercury vapor lamps, high-pressure sodium vapor lamps and metal Halide lighting etc.

?????? Switching or phase controlled AC to DC power supplies, battery chargers and UPS for computers and computer-controlled machines.

?????? Transformers operating near saturation

?????? Solid state frequency converters for induction heating and cyclo converters

?????? Induction furnaces and electrolysis plants.

Harmonic current, voltage and power

The very principle equations of the current, voltage and power contains portions for fundamental and harmonic components. 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

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.

Energy meter calculations

From the above discussion, it is clear that, if it was possible to find the product of instantaneous voltages and currents and summing over a time, this exact same method can be used for finding the energy consumption of different harmonics components. But this is not realizable. Present day static meters actually doesn't do any actual measurements but take samples of only voltages and current waveforms and rest of the values are obtained using various approximated formulas to do all the calculations using firmware programs. But the computation capacities of the microcontrollers available are not enough to do large number of calculations for the entire samples taken. So for the measurement of voltage and currents samples of only very few waveforms are taken and in most firmwares the highest magnitude of these samples of voltage and current are taken as the Vmax and Imax. Vrms and Irms are found

simply by dividing the max values with? (1.141). These methods shall invariably fail if harmonic components are present in the voltages and currents.

ENERGY METER MEASUREMENTS WITH DISTORTION- SUGGESTIONS

Harmonic distortion is a steady state phenomena. So the present sampling rates used in static energy meters can be followed for harmonics energy measurements also. For energy meter measurements, it is sufficient to consider the harmonic range from 2nd to 25th though most standards recommend upto 50th.

In a balanced three phase loads, harmonics in any one phase is indicative of the harmonic content in the other phases. But this is an ideal situation and typically consumer loads are unbalanced, so each phase must be measured. For identifying the direction of the harmonics energy flow, phase angles of each harmonic are required. The direction of the active energy flow is very important as it is to be used for identifying the real pollution causing loads.

Sign convention used in this paper

A linear load supplied with distorted voltage draws load current which shall be distortive in nature. In this case, the load is not causing distortion but as the supply voltage contains harmonics, this linear load is forced to draw current in proportion to the supply voltage waveform. So active power/energy calculated is positive. These types of linear loads absorbs harmonics and that consumers shall not be penalized.

If non linear loads exists at the consumer side, the load current drawn by that load contains harmonics even if the supply voltage is purely sinusoidal in nature. In such situations these loads are the only source of harmonics and contribute to the power system pollution. Such load inject/supply harmonics energy into the system and in such cases active power/energy calculated is negative.

While for a distorted supply voltage a non linear load, draws active power for some harmonics and for the rest of the harmonics it supplies active power to the network. In such cases each harmonics energy/ power components has to be found and depending on the phase angle, each harmonics energy component may be absorbed by the load that harmonics energy component shall be positive and for those active power components injected back into the power system, its sign shall be negative.

So when energy meter measurements is taken for a period may contain, both the positive and negative harmonic active energy components. The negative active energy indicates that the load is harmonic source of that much energy. Or in other words the negative active energy denotes the energy component that load contributed to the power system pollution.

The linear loads which are not sources of harmonics will draw current proportional to the voltage. Thus it will draw some nonlinear current whens input voltage is nonlinear. So for such loads the harmonics energy measured shall be positive.

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.

Harmonics power injecting and consuming consumers

When the magnitude of the distorted load current flowing through the transformer increases, the output voltage waveform of the transformer get distorted. Consequently the supply voltage, which effectively is the secondary voltage of the distribution transformer becomes distorted. This distorted voltage when supplied to the consumer, increases the harmonics contents of the load current, which again causes the increase of distorted load current in the transformer, further distorting the transformer output voltage. This can be understood easily when a distorted voltage is applied to a resistance load. In this case the current drawn by the resistance load shall be proportional to the applied voltage and if the applied voltage contains harmonics the resistance load also draws same harmonics load currents, adding to the harmonics in the power system.

Energy Meter Requirement

Energy recording of energy meters due to harmonics is a long debated controversial subject due to the above mentioned reasons. The solution is to separate the harmonics power consumption of injecting/ polluting connections and that connections which are non polluting/linear in nature. This problem was difficult to solve due to the lesser computational capabilities of microcontrollers used in energy meters. Higher capacity microcontrollers were there but using such types increases the cost of energy meter many fold. Nowadays cost of such microcontrollers has reduced to accessible limits and so a new algorithm is proposing for the development of energy meters with harmonics energy consumption.

Standards Considered

IEEE Std 1459 TM -2010 (Revision of IEEE Std 1459-2000)

IEEE Standard Definitions for the Measurement of Electric Power Quantities Under Sinusoidal, Non sinusoidal, Balanced, or Unbalanced Conditions are mentioned in IEEE Std 1459. This standard suggests a number of equations for metering purpose and from amoung these the following equations were selected based on the computational adaptability for the metering micro-controllers capacity.

The basic equations for power mentioned in the above standard are

P = P 1 + P H-------------------- (1)

where???????????? P is the total power,

P1 is the fundamental power P H is the harmonics power

Equation for harmonic active power suggested in the above standard is

PH = V0I0 + Svh Ih cos qh =? P- P 1--------------------- (2)

Eventhough the above equations hold good for the calculations for metering purposes, there are some practical difficulties which can be solved using the proposed equations mentioned in the same IEEE document above. The limitations are given below.

1.????? V0I0 component of the equation represents the DC component. As generally in power distribution, DC component is generally very small and so can be neglected.

2.????? The second part of the eq (2) ie the harmonics component can be used only if the voltage and current harmonics components contains an appreciable magnitude of both voltage and current. This means that this part can be used only when the supply voltage contains harmonics, which may not be actually the situation. But this can be used in cases where the supply voltage is distorted.

Algorithm Proposed

1.????? V0I0 component of the equation can be neglected.

2.????? Case 1: when the supply voltage is distorted

In this case, Svh Ih cos qh part of the equation can be used, but for separating the injecting power component and the absorbing power component the qh has to computed such that for the values of 90>qh > 270 , the harmonics power is absorbed by the load and is not adding to the harmonic pollution/distortion. This harmonics power consumption occurs only because of the voltage distortion. So only for values of 90<qh < 270 the equation Svh Ih cos qh has to be added up. Or in other words harmonics power component summed for values of 90>qh > 270 shall be taken as negative and for values of 90<qh < 270 the harmonics power summation shall be taken as positive. Here the positive part adds to the harmonics distortion or pollution of the power system and may be used for penalizing for discouraging the use of distorting equipments. This situation can be understood from the power flow diagram shown below.

3.????? Case 2: when the supply voltage is sinusoidal but load current is distorted

In this case, Svh Ih cos qh part of the equation cannot be used because here the vh part becomes zero as the supply voltage doesnot contain harmonics. Here the load is solely responsible for the current harmonics and so in this situation entire harmonics adds to the system distortion. So the here the equation to be used is (1).

PH = P- P 1

?

Power Flow Diagram

Conclusion

By using higher end micro controllers with programs using the above proposed algorithm, the harmonics injecting/polluting energy component can be found using energy meters commonly used in consumer premises.

References

1.????? IEEE Transactions on Power Delivery, Vol. 11, No. 1, January 1996 Practical Definitions for Powers in Systems with Non sinusoidal Wave forms and Unbalanced Loads :A Discussion.

2.????? IEEE Transaction on Power Delivery. v 10 n 3 Jull995, p 1693-1698, Alex E. Emanuel, "On the Assessment of Harmonic Pollution".

3.????? Power Direction Method Cannot Be Used for Harmonic Source Detection WiIsun Xu, Senior Member University of Alberta.

4.????? IEEE Std 280 TM -1985 (Withdrawn), IEEE Standard Letter Symbols for Quantities Used in Electrical

5.????? IEEE Working Group on Non-Sinusoidal Situations, “Practical definitions for powers in systems with non-sinusoidal waveforms and unbalanced loads,” IEEE Transactions on Power Delivery, vol. 11, no. 1, pp. 79–101, Jan. 1996.

6.????? IEEE Std 1459-2010 IEEE Standard Definitions for the Measurement of Electric Power Quantities Under Sinusoidal, Nonsinusoidal, Balanced,or Unbalanced Conditions

7.????? IEEE Guide 519-1992, IEEE Recommended Practice for Harmonic Control in Electric Power Systems

8.????? International Conference on Advanced Electronic Science and Technology (AEST 2016) Study on electrical energy meter for energy measuring under harmonics condition.

9.????? IEEE Transactions on Power Delivery, Vol. 11, No. 1, January 1996 Practical Definitions for Powers in Systems with Non sinusoidal Wave forms and Unbalanced Loads :A Discussion.

10.? IEEE Transactions on Power Delivery, Vol. 11, No. 1, January 1996 Practical Definitions for Powers in Systems with Non sinusoidal Wave forms and Unbalanced Loads :A Discussion. %You need to replace "rsc" on this line with the name of your .

11.? IEEE Transaction on Power Delivery. v 10 n 3 Jull995, p 1693-1698, Alex E. Emanuel, "On the Assessment of Harmonic Pollution".

12.? Power Direction Method Cannot Be Used for Harmonic Source Detection WiIsun Xu, Senior Member University of Alberta.

13.? IEEE Std 280 TM -1985 (Withdrawn), IEEE Standard Letter Symbols for Quantities Used in Electrical

14.? IEEE Working Group on Non-Sinusoidal Situations, “Practical definitions for powers in systems with non-sinusoidal waveforms and unbalanced loads,” IEEE Transactions on Power Delivery, vol. 11, no. 1, pp. 79–101, Jan. 1996.

15.? IEEE Std 1459-2010 IEEE Standard Definitions for the Measurement of Electric Power Quantities Under Sinusoidal, Nonsinusoidal, Balanced,or Unbalanced Conditions

16.? IEEE Guide 519-1992, IEEE Recommended Practice for Harmonic Control in Electric Power Systems

17.? International Conference on Advanced Electronic Science and Technology (AEST 2016) Study on electrical energy meter for energy measuring under harmonics conditionEnergy Meter With Harmonics Energy Measurement

Prepared by Shan Balakrishnan Saraswathi for publishing

in IEEE

Abstract

Due to steep rise in the use of non linear equipments by the domestic and industrial

consumers, the magnitude of harmonics in the power system has increased to such alarming

levels that it has become increasingly difficult to maintain the quality of the power supply. As

harmonics injection to the supply system is from the consumer side, consumers need to be

encouraged to use more distortion free equipments and for that harmonics energy

consumption shall be included in the energy billing. But this is not so simple. Due diligence

shall be applied to segregate the harmonics power components consumed and injected into

the system. Only those consumers who are injecting harmonics power to the system are

causing system pollution while those connections which are absorbing harmonics power are

actually using linear equipments and are the victims of this power system pollution. This

paper attempts to put forward an algorithm for the above problem of separating and

measuring the harmonics energy absorbed and injected into the power system using a static

energy meter.

Introduction

AC Electrical power system is intended to generate, transmit, distribute and utilize with

voltages and currents having sinusoidal waveform. But due to the wide spread use of

nonlinear equipments, distortions due occur in the sinusoidal waveform of voltages and

currents. Magnetic circuits are inherently nonlinear in nature due to the nonlinear hysteresis

behavior. These distorting components are generally called harmonics. All Electrical systems

are designed for a particular frequency. This frequency is 50 HZ for many countries in Asia and

Europe, while in USA, Canada etc this frequency is 60Hz. Even though the harmonics

frequencies can occur in any frequency ranges, in this paper only integral multiples of

fundamental frequency alone is considered.

Nowadays harmonics generated by electronics equipments, which are nonlinear in

nature, by the domestic and industrial consumers consumer equipments are creating havoc in

the electric supply system. Distortion in the load currents has rose to such high magnitudes

that it is capable of causing distortion in the supply voltage itself, deviating the voltage

waveform from pure sinusoidal to distorted sinusoidal. Even linear loads, when supplied with

a distorted voltage draws distorted currents. When distorted voltages is supplied to

consumers who are using linear/non harmonic equipments, that equipments tends to draw

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,

these generated harmonics currents causes excessive heating in the lines, transformers and

capacitors leading to higher distribution loss.