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WORLDTEK

QUALITY POWER SOLUTIONS

PRODUCT RANGE: CAPACITORS DETUNED REACTORS THYRISTOR SWITCH MODULES

AUTOMATIC POWER FACTOR CONTROLLERS

PRESENTATION ON HARMONICS

HARMONICS GENERATION IN LT AC NETWORKS – AND SOLUTION

A pure sinusoidal waveform is the most important theoretical assumption on which circuit analysis is carried out in conventional electrical engineering. Due to various reasons, such as increasing usage of non-linear loads, rapidly time-varying loads etc., this waveform has now become more or less extinct and in its place distorted waveforms are commonly found in most electrical networks. The distorted waveforms can be decomposed into various components using the Fourier Series. These individual components are referred to as harmonics. The order of harmonic “n” is defined as the ratio of the frequency of a harmonic to the fundamental network frequency or rated frequency. The type of loads used in the particular network influence the nature of harmonic distortion. In the modern context these loads could be static converters, variable speed drives, UPS systems, computer systems, new generation fluorescent lighting systems etc.

Each of these loads have their own characteristics in terms of the type and magnitude of harmonic distortion that results from their use. Hence the nature and magnitude of harmonic distortion in low voltage networks has become quite varied and complex. 1) LOW VOLTAE SYSTEMS Industrial and commercial power systems which operate at low voltage levels i.e.415 Volts AC/230 Volts AC etc., have some unique differences in comparison to distribution systems. They can briefly be stated as follows. 1.1 The percentages of non-linear loads i.e. harmonic producing loads are generally higher. 1.2 The variety of non-linear loads are quite high due to the use of diverse power electronic equipment within the installation. 1.3 The content of resistive type load is generally small. As a result the ability of the load to provide damping close to resonant frequencies is limited Harmonic distortion can be very high if resonance is close to a generated harmonic within the installation.

1.4 The frequency response of the network is greatly influenced by the presence of significant quantity of capacitor banks, which are used for power factor

• improvement. As a result the probability of resonance occurring near lower order

harmonics is quite high. 2) TYPES OF HARMONICS The most common sources of harmonics in low voltage net works are

• AC/DC Converters (Rectifiers)
• AC/DC (Frequency) Converters
• Uninterrupted Power Supplies (UPS)
• New generation fluorescent lighting systems.
• Welding machines
• Saturated transformers

The AC/DC converters or rectifiers are mainly used for feeding DC motors, battery chargers, electrolysis plants etc and are normally 3 phase, 6 or 12 pulse type. The characteristic harmonic current generated by rectifiers can be calculated based on the Fourier theorem using the formula as follows.

n= ((p) x (k) +/- 1 where p = number of the pulse and k= 1,2,3,4,5 …… which gives n= 5,7,11,13,17,19,23,25 for 6 pulse rectifier. Similarly, the 12 pulse rectifier generates n=11,13,23,25……. Where ”n” is the

• rder
• f

the harmonics.

The AC/AC converters are mainly used for feeding high frequency induction furnaces, frequency modulated variable speed drives etc., the converter normally consists of 6 or 12 pulse AC/DC converter on the AC network side and DC/AC inverter on the load side. The order and magnitude of the characteristic harmonics can be calculated in a similar way as for AC/DC converters. It is however, quite common that in this application “Inter-harmonics” are generated due to the modulation of the fundamental current by the drive frequency.

Inter-harmonics are those harmonic frequencies which are non- integral multiples of the fundamental frequency, for e.g. n=5.5 i.e. 5.5 x 50 Hz=275 Hz. The power supplies for IT (Information Technology) and other sensitive equipment are usually single phase connected between phase and neutral. Such power suppliers are normally SMPS (Switch Mode Power Supply) type and typically give feed back of significant quantity of “Triplen Harmonics.” Triplen harmonics are defined as odd harmonics that are multiples of 3. e.g. 3,9,15,21 …. It is now self evident that when several different types of loads are used in a given installation, the resultant harmonic spectrum will consist

• f

a wide range

• f

harmonic and Inter-Harmonic

• frequencies. Some typical harmonic spectra measured in low

voltage installations are shown in Fig. 1 & 2.

3) ILL-EFFECTS OF HARMONIC DISTORTION

3.1 Effects on rotating machines Motors and generators can be adversely affected by presence of harmonic voltages and currents due to increased heating caused by an increase in iron and copper losses at the harmonic frequencies. In addition to this harmonic currents can increase audible noise emission, reduce machine efficiency and torque developed. These effects combined together can increase energy consumption and reduce machine life considerably. 3.2 Effects on Transformers Harmonic currents cause increased copper losses and stray flux losses, whereas voltage harmonics cause an increase in iron losses. In addition the audible noise level shall increase. These effects generally result in high temperature rise within the transformer, thereby causing premature aging of the insulation and reduction in transformer life.

3.3 Effects on Switchgear and Power Cables Increased heating due to skin effect caused by the presence of harmonic currents is a common occurrence. Reduction in switchgear life and increase in cable faults are therefore quite common. 3.4 effects on Protective Relays High harmonic distortion levels are likely to cause mis-operation of relays due to change in the operating characteristics of the relay itself. Relays have a tendency to operate slower and / or with higher pick up values in such conditions. As a result, protection of the electrical installation can be compromised. 3.5 Effects on Capacitors Since capacitive reactance is inversely proportional to the applied frequency, harmonic currents are attracted into capacitor banks. This results in abnormal overload conditions on the capacitors thereby causing premature failure.

3.6 Effects on Power electronic Equipment Malfunction of power electronic equipment can occur due to conditions caused by increased Harmonic distortion. This is often caused due to shifting of voltage zero crossing, or a point at which one phase to phase voltage becomes greater than another phase to phase voltage. This leads to misfiring of thyristors followed by failure of expensive semiconductor devices used in power electronic equipment. 3.7 Effects on Control and Instrumentation Electronics Sensitive equipment such as computers, programmable controllers, electronic instruments etc., can operate erratically when subjected to higher level of harmonic content in the power supply. This erratic performance can result in malfunctions with serious consequences for process control and productivity etc. The combination of these effects results in

• Frequent equipment failure
• Increased down time of plant
• Higher energy productivity
• Increased operation cost.

4) CONVENTIONAL SOLUTION FOR REDUCING HARMONIC DISTORTION

The solution, which is in use, is called a harmonic filter. A harmonic filter generally means equipment comprising of reactors, capacitors, and resistors if required. The filter is designed so as to give defined impedance over a specified frequency range. There are broadly three types of filters in use. Tuned filters : A tuned filter works on the principle of providing least impedance path for one or two harmonic frequencies and has a tuning frequency which is within +/- 10/% of the harmonic frequency to be filtered. Detuned Filters : A detuned filter works on the principle of avoiding resonance by achieving an inductive impedance at the relevant harmonic

• frequencies. The tuning frequency is generally lower than 90% of the

lowest harmonic frequency whose amplitude is significant.

Damped filters : A damped filter is one, which offers low and predominantly resistive impedance over a wide band of harmonic frequencies. Since these filters are impedance based, they are referred to as PASSIVE FILTERS. The design of such filters is mainly dependant on the impedance characteristics of the network in which they are to be installed. The analytical modeling of harmonic source can be determined from the single line diagram by creating a NORTON equivalent as seen by the harmonic source. Fig. 3.

I gn GENERATED n-th HARMONIC CURRENT I fn n-th HARMONIC CURRENT ABSORBED BY THE FILTER I nn n-th HARMONIC CURRENT INJECTED TO THE NETWORK Z fn FILTER IMPEDANCE AT n-th HARMONIC Z nn NETWORK IMPEDANCE AT n-th HARMONIC

RECTIFIER FILTER NORTON EQUIVALENT CKT.

I gn I fn z fn z I nn

A single line diagram showing the typical connection scheme of a passive filter is shown in fig. 6. Where more than one dominant harmonic frequency is present, a separate filter for each frequency is generally used. Filtering of Harmonics using Passive Filters

Z HARMONIC GENERATING LOAD OTHER LOAD FILTER

Passive filters are in use for several years now. However, due to the increasing complexity of harmonic distortion in terms of variety and magnitude of different harmonics generated, the efficiency and reliability of this technology now has several limitations. 5) LIMITATIONS OF PASSIVE FILTERS

• NOT REGULATABLE : The filtering efficiency is a function of filter

impedance in relation to all other impedances in the network and a predetermined level of harmonic presence. Hence, the filtering efficiency will drop if the magnitude of harmonic presence and / or network impedance change with respect to the initial design assumptions. Filtering of harmonic currents, therefore, can not be regulated on a real time basis. DEDICATED TO ONE OR TWO HARMONIC FREQUENCIES : A passive filter may be used to filter only one or two harmonic components. A separate filter is generally required for each additional harmonic component to be filtered. This is a limitation in the context of multiple harmonic frequencies and inter-harmonics generated by modern power electronic loads.

• SENSITIVE TO FREQUENCY VARIATIONS : Supply frequency variation causes

changes in filter and network impedance. This can result in overloading and premature failure of passive filters.

• SUSCEPTIBILITY TO NETWORK CHANGES : Later network changes, for

example reconfiguration of the supply network the addition of a new large converter/

• ther loads within the existing installation etc., will alter the impedance configuration

and could result in severe overloading of the passive filter.

• SHIFT IN SYSTEM NATURAL RESONANT FREQUENCY : The natural resonance

frequency is a function of the “L” and “C” components of the filter and the network impedance configuration. Changes mentioned above can alter the resonant frequency and thereby cause resonant/near resonant conditions, resulting in current amplification, which will prove extremely harmful to the installation.

• LOCATIONAL LIMITATION:

Passive filters cannot be generally located directly across harmonic generating loads of the power electronic type. This is due to the risk of malfunction in the semiconductor circuitry of such loads arising out of interaction with the “L-C” component of the filter. Consequently, passive filters are generally located at the supply bus. Thereby, the benefit of harmonic filtering are realized only upstream of the filter location.

• REACTIVE POWER COMPENSATION : Since passive filters are a

combination of reactors and capacitors , they provide reactive power compensation with respect to the fundamental. Consequently, this can become a limitation in sizing the filter for an installation where the need for reducing harmonic distortion is more critical than providing power factor improvement. In certain cases this can prove a critical constraint for reducing harmonic distortion. 6) THE NEW SOLUTION – ACTIVE FILTERS The aforesaid limitations of the passive filters are effectively overcome by the advent of “ACTIVE FILTER’ technology. This is achieved primarily by the following.

• Incorporation of high end insulated gate Bipolar Transits or (IGBT)

switching technology to generate required frequency spectrum.

• Use of advanced digital signal processors (DSPs)to enable real time

control.

• Elimination of “L-C Impedance” in the principle of filtering.
• Immune to network frequency changes / variation.
• Can be installed at any location.
• Easy capacity enhancement due to modular design.
• Optional control of reactive power compensation.

The active filter technology thus represents a breakthrough in the mitigation of harmonic problems by effectively eliminating the critical limitations of the conventional passive filters. The wide variety of features of this technology, enable the user to easily achieve a reliable solution for reduction/ elimination of harmonic presence combined with optional variable reactive power compensation.

7) PRINCIPLE OF THE ACTIVE FILTER The active filter is based on the principle of measuring the harmonic currents and using his measurement on a real time basis to generate a harmonic current spectrum in phase opposition to the measured

• spectrum. This has the effect of canceling the original harmonic
• currents. Figure 7 illustrates the principle of the active filter.

The active filter uses a CPU (Central Processing Unit) for detecting the

• rder and magnitude of the harmonics present in the load and injects a

compensating current on a real time basis. The CPU can also determine the extent of reactive power factor correction. The control system is such that it is necessary to choose only the current rating of the filter out of several standard ratings available. Extensive knowledge of the network is generally not required as in the case of passive filters.

8) COMPONENTS OF THE ACTIVE FILTER

The active filter monitors the line current in real time and converts the data to digital signals in a Central Processing Unit (CPU). The current generator and the control system are the key elements of the active filter. The compensating current is generated by an IGBT bridge that can generate any required waveform using Pulse Width Modulation (PWM)

• technology. The source for the IGBT Bridge is a DC link capacitor which is charged

simultaneously with the generation of compensating current to the network. The generated

• utput is injected into the network via a reactor/filter circuit. The principle of connection is

shown as under.

Supply system Harmonic Generating load DC link Capacitor Coupling Reactor IGBT Bridge

9) ACTIVE FILTER OPERATION The waveforms recorded in respect of a lift drive during acceleration mode are shown below fig. 9 represents the waveform as measured without the active waveform as measured without the active filter whereas fig. 10 represents the same waveform as measured after installing the active filter. It can be observed that the active filter is able to remove the distortion and achieve a sinusoidal wave shape as seen from the supply side. Active filters can be easily connected across individual loads as shown in Fig. 11 or alternatively connected on the supply bus as shown in fig. 12

Since active filters are modular by construction the ability to reduce distortion can easily be enhanced by adding more filters in parallel.

M

M Active Filter

50Hz 440V

Active Filter

50Hz 440V 3 3

PRECAUTION OF MEASURE 1)The best option to limit the harmonics content to an acceptable level by the selection of a suitable rectifier type or circuit. 2)To add line reactors at drive inputs. SAFE ZONE If the sum of the non-linear load is up to the 20% of the total load, then Yes there must be some harmonics but it is not going to affect much at least on your p.f. HOW TO APPROACH 1)To carry out the harmonics analysis of the system. 2)To provide suitable filers either passive or active according to the requirement.

CONCLUSION

The harmonic spectra resulting in low voltage installations have become more complex due to wide variety of power electronic loads. The amplitude of various harmonics present has also become a dynamic variable due to increasing time varying characteristics of modern power electronic loads. While PASSIVE FILTERS have been used effectively in the past, and will continue to be used in very specific applications, their limitations in the context of low voltage applications have now become an important issue. ACTIVE FILTER have therefore emerged as the new generation solution for mitigating harmonic problems in low voltage networks. Given the modular design, elimination of network analysis, ease of adapting to changing network conditions, and coupled with the fact that they are environmentally friendly precuts. ACTIVE FILTERS represent a reliable, user-friendly “Plug and Play” solution for solving harmonic problems.

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