Thursday, 31 January 2019

EFFECTS OF INTERHARMONICS PRESENCE

Interharmonic currents cause interharmonic distortion of the voltage depending on magnitudes of the current components and the supply system impedance at that frequency. The greater the range of the current components frequencies,the greater is the risk of the occurrence of unwanted resonant phenomena, which can increase thevolt age distortion and cause overloading or disturbances in the operation of customers' equipment and installations. Among the most common, direct, effects of interharmonics are: 

a) Thermal effects

b) Low-frequency oscillations in mechanical systems

c) Disturbances in fluorescent lamps and electronic equipment operation.  

d) Interference with control and protection signals in power supply lines. (This is now the main harmful effect of the interharmonics) 

e) Overloading passive parallel filters for high order harmonics

f) Telecommunication interference

g) Acoustic disturbance

h) Saturation of current transformers 

Sources of Interharmonics



There are two basic mechanisms for the generation of interharmonics. 

The first is the generation of components in the side-bands of the supply voltage frequency and its harmonics as a result of changes in their magnitudes and/or phase angles. These are caused by rapid changes of current in equipment and installations, which can also be a source of voltage fluctuations. Disturbances are generated by loads operating in a transient state, either continuously or temporarily, or, in many more cases, when an amplitude modulation of currents and voltages occurs. These disturbances are of largely random nature,
depending on the load changes inherent in the processes and equipment in use. 

The second mechanism is the asynchronous switching (i.e. not synchronized with the power system frequency) of semiconductor devices in static converters. Typical examples are cyclo-converters and pulse width modulation (PWM) converters. Interharmonics generated by them may be located anywhere in the spectrum with respect to the power supply voltage harmonics. 


In many kinds of equipment both mechanisms take place at the same time. 
Interharmonics may be generated at any voltage level and are transferred between levels, i.e. interharmonics generated in HV and MV systems are injected into the LV system and vice versa. Their magnitude seldom exceeds 0.5% of the voltage fundamental harmonic although higher levels can occur under resonance conditions

Basic sources of this disturbance include: 
  • arcing loads
  • variable-load electric drives
  • static converters, in particular direct and indirect frequency converters 
  • ripple controls 

Interharmonics can also be caused by oscillations occurring in the systems comprising series or parallel capacitors and transformers subject to saturation and during switching processes. 

The power system voltage contains a background Gaussian noise with a continuous spectrum.  Typical levels of this disturbance are in the range 
(IEC 61000-2-1) 


ARCING LOADS 

This group includes arc furnaces and welding machines. Arc furnaces do not normally produce significant interharmonics, except where amplification occurs due to resonance conditions. Transient operation, being a source of interharmonics, occurs most intensively during the initial phase of melting (Figure 1). 



Welding machines generate a continuous spectrum associated with a particular process. The duration of individual welding operations ranges from one to over ten seconds, depending on the type of welding machine. 

ELECTRIC MOTORS 

Induction motors can be sources of interharmonics because of the slots in the stator and rotor iron,particularly in association with saturation of the magnetic circuit (so-called „”slot harmonics“„). At the steady speed of the motor, the frequencies of the disturbing components are usually in the range of 500 Hz to
2000 Hz but, during the startup period, this range may expand significantly. Natural asymmetry of the motor (rotor misalignment, etc.) can also be a source of interharmonics – see Figure 2. 



Motors with variable-torque loading, i.e. forge drives, forging hammers, stamping machines, saws, compressors, reciprocating pumps, etc., can also be sources of subharmonics. The effect of variable load is also seen in adjustable-speed drives powered by static converters. 

In wind power plants the effect of the variation in turbine driving torque, resulting, for example, from the ”shadow effect“ of the pylon, can modulate the fundamental voltage component, thus becoming the source of undesirable, low-frequency components. 

STATIC FREQUENCY CONVERTERS -INDIRECT FREQUENCY CONVERTERS 

Indirect frequency converters contain a dc-link circuit with an input converter on the supply network side and an output converter (usually operating as an inverter) on the load side. In either current or voltage configurations the dc-link contains a filter which decouples the current or the voltage of the supply and load systems. For that reason the two fundamental (the supply and the load) frequencies are mutually decoupled. But ideal filtering does not exist, and there is always a certain degree of coupling. As a result, current components associated with the load are present in the dc-link, and components of these are present on the supply side. These components are subharmonic and interharmonic with respect to the power system frequency. 

CURRENT-SOURCE LOAD COMMUTATED INVERTERS 

Due to the semiconductor devices switching technique, these are classified as line commutated indirect frequency converters. A frequency converter (Figure 3) consists of two three-phase bridges P1 and P2 and a dc-link with reactor of inductance L . One of the bridges operates in the rectifier mode and the other in the inverter mode, although their functions could be interchangeable.



The presence of two rectifier bridges supplied from two systems of different frequencies results in the dc-link current being modulated by two frequencies–f1 & f2. Each of the converters will impose non-characteristic components on the dc link, which will appear as non-characteristic harmonics on the ac side, both in the load and in power supply system


We will See the "EFFECTS OF THE PRESENCE OF INTERHARMONICS"  in next article soon,

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Understanding INTERHARMONICS In Power System


Technically "Interharmonics"are voltages or currents with a frequency that is a non-integral multiple of the fundamental supply frequency, while each harmonic frequency is an integral multiple of the supply frequency.  


Interharmonics,always present in the power system, have recently become of more importance since the widespread use of power electronic systems results in an increase of their magnitude. Interharmonics are caused by the asynchronous switching of semiconductor devices in static converters such as cyclo converters and pulse width modulation(PWM) converters, or by rapid changes of current in loads operating in a transient state. 

Practically "Interharmonics" are explained as Electronics and communications devices in the smart grid can increase a rare, and not well-understood, distortion.

Using sophisticated power electronics and communications systems to improve power system efficiency, flexibility and reliability is increasing interharmonic distortion and putting new equipment sensitive to that distortion on the system. Understanding interharmonics is necessary to prevent them from adversely affecting system operation.

IEEE Standard 519-2014, Recommended Practice and Requirements for Harmonic Control in Electric Power Systems, defines interharmonics as any “frequency component of a periodic quantity that is not an integer multiple of the frequency at which the supply system is operating.” IEC 61000-2-1 includes a similar definition. Mathematically, with the supply (fundamental) frequency and m any positive non-integer, any signal with the frequency mf is an interharmonic of f. This is similar to the harmonic definition, nf, where f once again represents the fundamental frequency, but represents any integer greater than zero.


While interharmonic and harmonic definitions are similar, their difference that harmonics are periodic at the fundamental frequency and interharmonics are not is important. All periodic waveforms can be represented by their fundamental component and a Fourier series of harmonics with various magnitudes, frequencies and angles. Interharmonics are not periodic at the fundamental frequency, so any waveform containing interharmonics is non-periodic and any non-periodic waveform includes interharmonics. The level of interharmonic distortion can be thought of as a measure of a waveform’s non-periodicity.

We will See the sources of interhormonics in next article soon,


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Wednesday, 30 January 2019

NUISANCE TRIPPING OF CIRCUIT BREAKERS - CASE STUDY

Nuisance tripping of circuit breakers is a common problem in many commercial and industrial installations. This Application Note explains the need to use true RMS measurement instruments when troubleshooting and analyzing the performance of a power system. 

Nuisance tripping of circuit breakers is often caused by the load current being distorted by the presence of harmonic currents drawn by non-linear loads. Harmonic currents distort the current waveform and increase the load current required to deliver energy to the load. Many measurement instruments, even quite modern ones, use an averaging measurement technique that does not measure harmonic currents correctly. The readings may be as much as 40% too low, and circuit breakers and cable sizes may be underrated as a result.  

True RMS meters, which take the complete distorted waveform into account, should be used instead.  

NUISANCE TRIPPING 

Many commercial and industrial installations suffer from persistent so-called ‘nuisance tripping’ of circuit breakers. The term refers to the apparently random and inexplicable nature of these events which, although there is no apparent fault, can cause significant disruption and financial loss. Of course, there is always a reason for the nuisance tripping of a breaker and there are two common causes. The first possible cause is the inrush currents that occur when some loads, particularly personal computers and other electronic devices, are
switched on. The second possible cause is that the true RMS current flowing in the circuit has been under measured – in  other words, the true current really
is too high and the trips are valid.

INRUSH CURRENTS 

Modern electronic equipment, such as personal computers, monitors, television sets and office equipment,uses a type of power supply that converts mains electricity to the low voltage direct current without a low frequency transformer. This type of supply is known as a switched mode power supply (SMPS) and works by rectifying mains current directly and storing the direct voltage on a large capacitor which charges to the peak of the supply voltage. Conversion circuits draw current from the capacitor and generate the required low voltage, usually via a high frequency transformer to provide galvanic isolation. SMPS is very cheap, but it causes problems in installations because it produces large harmonic currents and draws very large inrush currents to initially charge the storage capacitor. 

Many PCs are never actually ‘turned off’ –in the sense of being electrically isolated from the supply – but remain powered in standby mode and can be ‘woken up’ by user input, modem activity, or by a network message. On wake up, they draw a starting current very similar to that drawn when starting from cold. 

CASE STUDY 

In a small computer room at the Reputed university circuit breakers were being tripped apparently at random but particularly during the night. Groups of 16 computers were connected to standard final circuits,each protected by a 32 A miniature circuit breaker. A preliminary investigation revealed nothing – the installation was apparently correctly installed and functioning correctly. As the problem persisted, further tests revealed that the trips occurred at the start of the nightly maintenance procedure as the computers turned back on and it was realized that the inrush current was responsible. Further inspection revealed that the MCBs were type B devices, so they were replaced with type D devices of the same rating – this will be discussed in detail later. Although this change resolved the problem, a measurement exercise was undertaken to verify the
conclusion. 

A logic controlled switch was interposed at the load side of one of the breakers together with a transient recorder to measure the applied voltage and current. The switch was capable of applying the supply voltage at a defined point on the voltage waveform. 

A number of startup cycles were conducted and the inrush currents recorded. Figure 1 shows the inrush current for a typical personal computer and (CRT) monitor set with the supply voltage applied close to the positive voltage peak. At 155 A, this was the worst case – i.e. the maximum – inrush current recorded during the testing for this configuration. The voltage waveform is shown only for clarity but it is interesting to note the resulting distortion of the supply waveform,especially on the first half cycle.The steady state current consumption was 0.75 A. The current and time resolutions of the measurements are 0.28 A and 0.8 ms respectively. 







MCB CHARACTERISTICS 

Although the case study relates to personal computers, the principle applies to most modern electronic equipment so it is prudent to design installations to survive large inrush currents. Figure 3 shows the envelope of the characteristic curves for type B, C and D MCBs. The so-called ‘inverse time’ part of the characteristic is designed to protect against over-current. It allows for substantial short-term overload without tripping, taking advantage of the inherent short time over-current tolerance of the cable. As the over-current level increases, the time to respond reduces rapidly to restrict the rise in temperature and reduce the risk of damage. The instantaneous characteristic is intended to respond very rapidly to fault current to reduce the risk of damage to load circuits.

Types B, C and D MCBs are differentiated by their instantaneous tripping current, shown as B , etc. in Figure 3.  min, B max


Taking the example of a type B MCB, Figure 3 shows that the breaker will not trip ‘instantaneously’ at any current below 3 times nominal rating, but must trip at or above 5 times nominal.In the case of a nominal 32 A device, instantaneous tripping will occur between 96 A and 160 A with a type B device, and between 320 A and 640 A for a type D device. The minimum duration required to trip is not well defined. 

From the case study results, it is clear that a type B MCB could trip due to the inrush current from one computer/monitor pair while selecting a type D device would provide a degree of protection against nuisance tripping. Changing the MCB type means that the potential fault current is increased so the loop impedance of the final circuit should be carefully checked to ensure compliance with the applicable regulations. In cases where the inrush is too high for any available breaker,problem loads should be distributed among more final circuits. 

TRUE RMS – THE ONLY TRUE MEASUREMENT 

Why do under-measurements occur frequently in modern installations even though digital test instruments are so accurate and reliable? The answer is that many instruments are not suitable for measuring distorted currents - and most currents these days are distorted. Since currents are being under-measured, the real current is much closer to the nominal breaker current than believed, leading to genuine trips that are misinterpreted as nuisance trips. 

This distortion is due to harmonic currents drawn by non-linear loads, especially electronic equipment such as personal computers, electronically ballasted fluorescent lamps and variable speed drives. Figure 6 shows the typical current waveform drawn by a personal computer. Obviously, this is not a sine wave and all the usual sine wave measurement tools and calculation techniques no longer work. This means that, when troubleshooting or analyzing the performance of a power system, it is essential to use the correct tools for the job – tools that can deal with non-sinusoidal currents and voltages.

Figure 4 shows two clamp-meters on the same branch circuit. Both the instruments are functioning correctly and both are calibrated to the manufacturer’s specification. The key difference is in the way the instruments measure. 

Figure 4

Figure 4 - One current, two readings. Which do you trust? The circuit feeds a non-linear load with distorted current. The True RMS clamp (left) reads correctly but the average responding clamp (right) reads low by 32%.The left-hand meter is a true RMS instrument and the right-hand one is an averaging reading RMS calibrated instrument. Appreciating the difference requires an understanding of what RMS really means.

WHAT IS RMS? 

The ‘Root Mean Square’ magnitude of an alternating current is the value of equivalent dir ect current that would produce the same amount of heat in a fixed resistive load. The amount of heat produced in a resistor by an alternating current is proportional to the square of the current averaged over a full cycle of the waveform.In other words, the heat produced is proportional to the mean of the square of the current, so the equivalent current value is proportional to the root of the mean of the square or RMS. The polarity is irrelevant since the
square is always positive.

For a perfect sine wave, such as that seen in Figure 5, the RMS value is 0.707 times the peak value. Or the peak value is 2 (= 1.414) times the RMS value. In other words, the peak value of 1 A RMS pure sine wave current will be 1.414 A. If the magnitude of the waveform is simply averaged (inverting the negative half cycle), the mean value is 0.636 times the peak, or 0.9 times the RMS value. There are two important ratios shown in Figure 5: 



When measuring a pure sine wave – but only for a pure sine wave – it is quite correct to make a simple measurement of the mean value (0.636 x peak) and multiply the result by the form factor 1.111 (making 0.707 times peak) and call it the RMS value. This is the approach taken in all analogue meters (where the averaging is performed by the inertia and damping of the coil movement) and in all older and many currently available digital multi-meters. This technique is described as ‘mean reading, RMS calibrated’ measurement.The problem is that the technique only works for pure sine waves and pure sine waves do not exist in the real world of an electrical installation. The waveform in Figure 6 is typical of the current waveform drawn by a personal computer. The true RMS value is still 1 A, but the peak value is much higher, at 2.6 A, and the average
value is much lower, at 0.55 A. 


If this waveform is measured with a mean reading,RMS calibrated meter it would read 0.61 A, rather than the true value of 1 A, nearly 40% too low.  Figure 7 below gives some examples of the way the two different types of meters respond to different wave shapes. 



A true RMS meter works by taking the square of the instantaneous value of the input current, averaging over time and then displaying the square root of this average. Perfectly implemented, this is absolutely accurate whatever the waveform. Implementation is, of course, never perfect and there are two limiting factors to be taken into account: frequency response and crest factor. 

For power systems work it is usually sufficient to measure up to the 50th  harmonic, i.e. up to a frequency of about 2500 Hz. The crest factor, the ratio between the peak value and the RMS value, is important; a higher crest factor requires a meter with a greater dynamic range and therefore higher precision in the conversion circuitry. A crest factor capability of at least three is required for accurate measurement in power installations. It is worth noting that, despite giving different readings when used to measure distorted wave forms, meters of both types would agree if used to measure a perfect sine wave. This is the condition under which they are calibrated, so each meter could be certified as calibrated – but only for use on sine waves. 

True RMS meters have been available for at least the past 30 years, but they used to be specialized and expensive instruments. Advances in electronics have now resulted in true RMS measurement capability being built into many handheld multi-meters. Unfortunately, this feature is generally found only towards the top end of most manufacturers ranges, but they are still cheap enough to buy as ordinary instruments for everyone and every day. 

THE CONSEQUENCES OF UNDER MEASUREMENT 

The limiting rating for most electrical circuit elements is determined by the amount of heat that can be dissipated so that the element or component does not overheat. 

Cable ratings, for example, are given for particular installation conditions (which determine how fast heat can escape) and a maximum working temperature. Since harmonic polluted currents have a higher RMS value than
that measured by an averaging meter, cables may have been under-rated and will run hotter than expected;the result is degradation of the insulation, premature failure and the risk of fire.

Busbars are sized by calculating the rate of heat loss from the bars by convection and radiation and the rate of heat gain due to resistive losses. The temperature at which these rates are equal is the working temperature of the busbar, and it is designed so that the working temperature is low enough so that premature ageing of insulation and support materials does not result. As with cables, errors measuring the true RMS value will lead to higher running temperatures. Since busbars are usually physically large, skin effect is more apparent than for smaller conductors, leading to a further increase in temperature. 

Other electrical power system components such as fuses and the thermal elements of circuit breakers are rated in RMS current because their characteristics are related to heat dissipation. This is the root cause of
nuisance tripping – the current is higher than expected so the circuit breaker is operating in an area where prolonged use will lead to tripping. The response of a breaker in this region is temperature sensitive and may appear to be unpredictable. As with any supply interruption, the cost of failure due to nuisance tripping can be high, causing loss of data in computer systems, disruption of process control systems, etc.

CONCLUSION 

This paper has described two common causes of nuisance tripping. In each case there are some simple preventive steps. Avoiding tripping due to inrush currents simply requires selection of the correct type of breaker and sensible distribution of loads among circuits. Under-measurement is easily avoided by ensuring that true RMS meters are used routinely. Knowledge of the real current in a circuit allows corrective action to be taken. For example, redistributing loads across final circuits.  


Case Study Courtesy - ECI online Article 

Tuesday, 29 January 2019

Why to Compensate Reactive Power ? & Control & Regulations of Reactive Power

There are many simultaneously active loads in a conventional electricity network. Many are resistive, some have a capacitive component, i.e. the current curve hurries a bit ahead of the voltage curve (leading), and others have an inductive component, i.e. the current lags behind the applied voltage (for a more detailed explanation, see Annex on inductances, capacitances and reactive power).Resistive-inductive loads prevail in most networks, resulting in a resistive-inductive overall current. This incessant, undesired, oscillation of energy means an additional flow of current in cables and transformers. It causes additional resistive-losses and uses a potentially large part of their capacity.
Therefore the basic reasons for compensating are to avoid: 
  • The undesired demand on transmission capacity (additional current)
  • The energy losses caused by such
  • The additional voltage drops caused by such 

These extra voltage drops in the system are significant; a reactive current flowing in a resistance causes a real power loss. Even where the impedance is largely reactive, rapid changes in the reactive current  may cause flicker. A good example of this is a construction crane connected to a relatively small distribution transformer when a new home is erected in a residential area. The cranes are usually driven by relay-controlled three-phase induction motors which are quite frequently switched from stop to start, from slow to fast, and from downwards to upwards. 

The start-up currents of these motors are very high, several times the rated current, and have a very high inductive component, the power factor being around cos ϕ ≈ 0.3 (or even smaller with bigger machines). The voltage drop in the transformer is also largely inductive, so it has more or less the same phase angle as the start-up current of the motor. It will  add much more to the flicker than the same current drawn by a resistive load (Figure 1). Fortunately, this also means that this flicker can easily be mitigated by adding a capacitor to compensate the inductive component of the motor’s start-up current.  
CONTROL AND REGULATION OF REACTIVE POWER 
It is normally desirable to compensate reactive power. This is quite easy to achieve by adding an appropriate capacitive load parallel with the resistive-inductive loads so that the inductive component is offset. While the capacitive element is feeding its stored energy back into the mains,the inductive component is drawing it, and vice versa, because the leading and the lagging currents flow in opposite directions at any point in time. In this way, the overall current is reduced by adding  a load. This is called parallel compensation. 

To do this properly requires knowledge of how much inductive load there is in the installation, otherwise over-compensation may occur. In that case, the installation would become a resistive capacitive load which in extreme cases could be worse than having no compensation at all. If the load –more precisely its inductive component – varies, then a variable compensator is required. Normally this is achieved by grouping the capacitors and switching them on and off group-wise via relays. 

This of course causes current peaks with the consequent wear of the contacts, risk of contact welding and induced voltages in paralleled data lines. Care must be taken in timing the switch on. When voltage is applied to a fully discharged capacitor at the instance of line voltage peak, the inrush current peak is equal to that of a short circuit. Even worse, switching on a short time after switching off, the capacitor may be nearly fully charged with the inverse polarity, causing an inrush current peak nearly twice as high as the plant’s short circuit current peak! If there are many switch-mode power supply loads (SMPS) being operated on the same system, then a charged compensation capacitor, reconnected to the supply, may feed directly into a large number of discharged smoothing capacitors, more or less directly from capacitance to capacitance with hardly any impedance in between. The resulting current peak is extremely short but extremely high, much higher than in a short circuit! 

There are frequent reports about the failure of devices, especially the contacts of the relays controlling the capacitor groups, due to short interruptions in the grid which are carried out automatically, e.g. by auto-reclosers, to extinguish a light arc on a high or medium voltage overhead line. It is often suggested that this doubling of peak value cannot occur with capacitors that are equipped with discharge resistors in accordance with IEC 831. However, the standard requires that the voltage decays to less than 75 V after 3 minutes, so they have little effect during an interruption as short as a few tens of milliseconds up to a few seconds. If, at the instant of re-connecting the capacitor to the line voltage, the residual capacitor voltage happens to equal the supply voltage, no current peak occurs. At least this is true if the compensator is viewed as a pure capacitance and the incoming voltage as an ideal voltage source, i.e with zero source impedance. But if the self-inductance of the system is taken into account, certain resonances between that and the capacitance may occur. 

Assume the following case: the residual voltage of the  capacitor is half the peak value and equal to the instantaneous line voltage, which is at 45° after the last zero voltage crossing, i.e. 




At this point in time the current in the capacitor is expected to be: 





However, this is not the case, because the capacitor has been disconnected from the supply up to this point in time. At the instant of connection, neglecting the system’s inductance, the current would rise up to this value immediately, and nothing would happen that would not have happened anyway in the steady state. But a real system is not free of inductance, so the current will assume
this value only hesitantly at first, then speed up and – again due to the inductance, its ‘inertia’ – shoot beyond the target way up to nearly double the expected value. Then it will come down again and so on, and thus perform a short period of oscillation that may be attenuated down to zero well within the first mains cycle after connection. The frequency of such oscillation may be rather high, since the mains inductance is low, and may cause interference to equipment in the installation. Only if the instantaneous line and residual capacitor voltages are both at their positive or negative peaks at the instant of re-connecting the capacitor, at which point in time the instantaneous current
would be zero anyway, will the resistive-inductive current start without oscillation. 

More precisely, there are two conditions to be fulfilled. Firstly, the sum of voltages across the capacitance and its serial reactance (be it parasitic or intentional detuning) must be equal to the line voltage. Secondly, the supposed instantaneous current, assuming connection had already taken place long before, has to equal the actual current, which of course is zero until the instant of switching. This second condition is fulfilled only at line voltage peak, which therefore has to equal the capacitor voltage. To achieve this, the capacitor is pre-charged from a supplementary power source. This practice has a  secondary minor advantage in that it makes sure that there is always the maximum possible amount of energy stored in the capacitor while not in use, so that at the instant of turn-on it may help to mitigate some fast voltage dip and prevent the subsequent flicker. 

Relays, however, are too slow and do not operate precisely enough for targeted switching at a certain point of the wave. When relays are used, measures have to be taken to attenuate the inrush current peak, such as inrush limiting resistors or detuning reactors. The latter are frequently used anyway for other reasons, and are sometimes required by utilities. Although this series reactor
replaces the inrush current peak at switch-on with a voltage peak (surge) at switch-off, it is still the lesser evil, since the reactive power rating of the reactor is just a fraction of the capacitor rating and so the energy available is less. 

Electronic switches, such as thyristors, can be easily controlled to achieve accurate point-on-wave switching. It is also possible to control switching so as to mitigate a fast flicker caused by a large unstable inductive load, such as the crane motor mentioned previously, an arc furnace or a spot welder. 

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Monday, 28 January 2019

What Is Power Quality? Whose Responsibility is it?

Power Quality
Electric power quality, or simply power quality, involves voltage, frequency, and waveform. Good power quality can be defined as a steady supply voltage that stays within the prescribed range, steady a.c. frequency close to the rated value, and smooth voltage curve waveform (resembles a sine wave). In general, it is useful to consider power quality as the compatibility between what comes out of an electric outlet and the load that is plugged into it.
India and most developing countries continue to struggle for 24×7 power supply, good Power Quality (PQ)environment, and Energy Efficient (EE) economy.
WHO IS RESPONSIBLE FOR MAINTAINING GOOD POWER QUALITY?
Answer to this is usually divided and depends upon whom we pose this question. Network Operator will blame his end-customers, Device Manufacturer will blame both the Network Operator and the end-customer, and end-customer usually has not much awareness, and believe that its supply problem from Network Operator.
In addition to above 3 key stakeholders, there are others like – Designers, Commissioning Engineers and Maintenance Engineers that also plays role in sustaining good PQ environment.Commissioning Engineer play the role in ensuring the quality of installation based on certain design standards. Good installation is one of the necessities to maintain power quality during operations and to minimize voltage drop, sparking, overheating, etc. Maintenance engineers are subsequently responsible for preventing any glitches in operations which can result in power quality issue. For ex. loose connections lead to sparking;poor quality of wires results in voltage drops at customers premise, etc.
Further on, we shall focus upon 3 key stakeholders – Customers, Network Operators and Equipment Manufacturers.

INTER-CONTENTEDNESS OF OUR GRID AND POWER QUALITY 

Power Quality is a measure of quality of power supply on the grid. A PQ disturbance occurs in case of any deviation of voltage and current wave forms from the ideal. Voltage disturbances commonly originate in the network and affect the customers. On the other hand, current disturbances originate at a customers installation and affect the network components and other installations. Therefore, VOLTAGE QUALITY is considered to be primary responsibility of the network operator, while CURRENT QUALITY is primary responsibility of the end-customers.
Because of interconnected grid, PQ disturbances are caused both by upstream and downstream elements. Across various PQ disturbances

CUSTOMERS AND PQ INTERCONNECTION

Today’s customers are highly dependent on digital technology and power electronic devices, with increasing use of various types of electronic appliances, ballasts, variable speed drives, etc. These devices when used produce current distortions in the network due to their non-linear operating characteristics. These disturbances then travel upstream because of insufficient isolation of each customers from the grid. This increase current in turn causes additional energy losses in the system and also pose increased demand of apparent power to individual customer and also entire network faces the risk of premature aging and failure.
Some of commonly reported PQ complaints from end-customers:

Equipments affected by poor PQExternal Manifestation of poor PQElectrical Manifestation of poor PQ
IT equipmentsComputer lock-ups and data lossPresence of earth leakage current causing small voltage drops in earth conductors
Variable speed drives, telecom equipments, arc furnace, welding equipment, relays, static converters, security and access control systems, etc.Motors and drives malfunctioning, computer screen freeze, loss of dataCapacitor bank failure, shocks due to neutral voltage, Flickering of lights, noise in telecom lines
Motors and process devicesMalfunctioning of motors and process devices. Extra heating, decreased operational efficiency and premature aging of the equipment.Presence of voltage and current harmonics in the power supply
Relays, circuit breakers and contactorsNuisance tripping of protective devicesDistorted voltage waveform because of voltage dip
Sensitive measurements of process control equipmentLoss of synchronization in processing equipmentSevere harmonic distortion creating additional zero-crossings within a cycle of the sine wave
Table 1. Customers reported problem due to poor PQ Environment (Source: 2. Sharmistha Bhattacharyya and Sjef Cobben, Technical University of Eindhoven)
With increased awareness, Customers can take below precautions to support building healthy PQ environment:
  • Maintain power factor within prescribed limits to reduce reactive power demand, which in turn will balance the voltage in their premise and also overall network
  • Reduce harmonic currents while using more energy efficient equipments at their premise
  • Keep a log of faced power disturbances at premises, which may come handy in finding effective solutions

‘NETWORK OPERATORS’ AND PQ INTERCONNECTION

The Network Operators design and maintain key network characteristics like feeder length, number and sizing of Distribution Transformers (DT), DT load balancing etc. which in turn determine grid impedance and that influence the PQ level in the network. With high impedance in the network, PQ issues (mainly flicker and harmonics) become more prominent. Further, DT winding configurations and earthing problems also add to the harmonic behavior and voltage dips in the network. 
Thus, technical Loss reduction and fixing PQ environment are strongly interrelated, and could be addressed though same investments. The main network components which get affected in terms of faster wear and tear through PQ disturbances are:

  • Transformers
  • Cables
  • Power-factor correction (PFC) Capacitors
  • Protective Devices, Digital Relays
  • Revenue Meters
The Network Operator can streamline its loss reduction initiatives with improving PQ environment in following ways:
  • Controlling voltage level at customers point of connection by reactive power management and take appropriate steps at the broader network level.
  • Maintaining load balance at Feeder and DT level, and reduce current losses. This will ensure increased power availability.
  • Doing regular PQ Measurements with advancement in technologies like SCADA, Smart Metering, etc. and designing relevant dashboards to facilitate timely actions
  • Isolating customer loads and their variations from main grid through use of Capacitor banking. Different variants like automatic power factor correcting devices, switched capacitors, Statics VAR compensators, dynamic voltage regulators etc. are available.

EQUIPMENT MANUFACTURERS’ AND PQ INTERCONNECTION

Organized and branded Equipment Manufacturers usually specify PQ immunity (EMI/EMC) of their equipment in terms of harmonic current emission and other parameters, as applicable under some Standards. However, in a real life situation, the network voltage is already distorted (and is non-sinusoidal) because of harmonic current emissions from other loads and customers in the network. This can result into distortions from their devices exceeding the ‘compatibility level’ of the system.
The optimum performance of Equipment Manufacturers’ device is not guaranteed when the supply voltage is distorted. Experiments show that devices produce higher harmonic currents when the supply voltage is distorted. Below table compares total harmonic current distortions (THD) of some households’ devices under sinusoidal and distorted supply voltage condition.

Device
THD with respect to the total RMS current drawn by the device
Under clean voltage condition
Under distorted voltage condition (THD = 6%)
TV
48%
55%
Personal Computer (PC)
87%
89%
Refrigerator
10%
18%
CFL
72%
79%
Table2. THD of devices under clean and distorted network voltage conditions (Source: 2. Sharmistha Bhattacharyya and Sjef Cobben, Technical University of Eindhoven)
The device manufacturer, however, cannot be blamed directly for such a situation as he is not responsible for his devices’ operations under a distorted supply voltage condition. But at the same time the manufacturers also need to ensure the immunity of equipments they manufacture against Electro Magnetic Interference (EMI) and specify the tolerance limits for same.
Question is should Manufacturer built in this isolation into its equipments, with resulting price hike, or should customer take collective facility isolation from main grid interference? While answer to this will be driven by market and regulations, still Manufacturers together with their installation teams could start giving weight to PQ issues during their installations and checking some basics like – capacitor bank, cables with larger neutral conductors, adjusting under voltage relays, etc.

CO-CREATING AN ECOSYSTEM FOR BETTER PQ MANAGEMENT

As we saw, poor PQ environment is caused by Customers, Network Operators and also Equipment Manufacturers. At the same time, poor PQ impacts all of them negatively – Network Operator has to face high losses, customers has to face increased break-down of equipments and higher bills, and Equipment Manufacturers has to face increased warranty costs. Therefore, to implement PQ mitigation, a systematic approach needs to be followed starting with to identify the responsibilities of each stakeholder in the network. 
With growing complexities both on the on end-customer side (via increasing usage of electronic appliances) and Network Operator side (via higher adoption of smart grids, smart meters etc), it is important that each stakeholders understand their contribution and impact from PQ, and take appropriate measures. End-Customers have to become more aware and demanding in their procurements of both power and also equipments, as at the end, it is them who pays out for these in-efficiencies. In US and Europe, there are clear SLAs for voltage supply and harmonics emission at the point of supply between the Network Operator and end-customers. Improved Regulation, policies, standards and end-customer awareness and reinforcement will play key role in guiding market for optimum equilibrium for good PQ environment.







Source: A web Article from APQI