Paper on recent trends in power electronics

POWER ELECTRONICS             
GENESIS, TRENDS AND IMPROVEMENT.


ABSTRACT
The latest buzzword, in energy conscious industries, is the improvement of power quality.  Good quality of electric power can not only result in handsome savings in power bills but also helps in avoiding unwarranted breakdowns.  This paper aims at analysing the reasons for poor quality of power and solutions for improving it. The change in magnitude and power factor can be corrected without much of difficulty with only problem being that of maintenance of shape, which is distorted by harmonics. We look at the factors causing in the production of harmonics, what impact these harmonics have on other equipments in the load serenity. We look at the types of harmonics, their effects and problems caused by them. Active filters are the ultimate solution for reduction of harmonics which use the IGBT technology in additions to the pulse width modulation technology (PWM) which combines with control techniques to emerge successfully with improved power quality by fighting with the problems of harmonics to an extent that the THD (total harmonic distortion) is within acceptable limits i.e. <5%, these filters are the ultimate solution and far win over their counter parts i.e. passive filters. 

INTRODUCTION:

Electric power quality, in general, means how pure is the quality of power being used, without any abnormalities, which cause disturbances in it.

The voltage, which is the primary driving force behind all electrical equipment in the system, is an alternating sinusoidal and is characterized by its magnitude, frequency and wave shape.  Sags, Surges, spikes and power failures all these disturbances are created due to modification of amplitude and can be measured and removed by using voltage stabilizing, conditioning and UPS systems.  To ensure good power quality apart from the requirement of magnitude and frequency to be within acceptable limits, it is important that the wave shape is maintained as a ‘sine wave’ throughout the electrical network.

HOW POWER QUALITY DETERIORATES:
Although power is generated in the pure form, electrical consumers or loads distort it progressively due to the manner in which it is consumed.  This problem of power pollution was not heard of in the past but is taking a serious form due to extensive use of energy efficient power electronic (non-linear) loads. 
In the past major loading of supply system was made of large slowly changing electrical loads like motor, transformer, which were linear in nature i.e. the current consumed was directly proportional to the supply voltage.  The current waveforms being sinusoidal with only power quality problem being that of power factor. 
In the last 10 to 15 years, the gradual and never ending prolification of computers, micro-processor system and power electronics in all kinds of industrial, commercial and domestic facilities have completely changed the nature and profile of supply loading.  These nonlinear loads consume non-sinusoidal current when sinusoidal voltage is applied to them by conducting only during a fraction of the fundamental period. The current drawn is characterized by sharp rise and fall during only a fraction of the fundamental period .The current waveform contains a considerable amount of odd harmonics, the magnitude of which may be higher than the fundamental current component. In the past all these non-linear loads were confined to industries and hence the power quality by and large was satisfactory. 
The number of such polluting electrical gadgets is increasing at an alarming rate thereby polluting the power quality in modern electrical networks.  Problems in computers due to poor power quality are
·       Damage to computer chips, causes computer downtime, computer errors etc.,
·       UPS system do not address transient problems effects
·       Poor power quality can cut the lifetime of motor to half.
The distorted current is a result of several sine wave currents at multiples of fundamental frequency.  These high frequency components are called as ‘Harmonics’.  When the harmonic currents flow through the power distribution system, harmonic voltages are developed. 
These harmonic voltages are superimposed over the incoming pure supply voltage causing distortion in the voltage.  Since voltage is common to all loads (linear or non-linear), harmonic distortion spreads in the electrical system causing varying degree of damage depending upon the susceptibility and sensitivity of the electrical equipment. 

SOURCES OF HARMONICS

·    Switched mode power supplies (SMPS)

·       Electronic fluorescent lighting ballets
·       Variable speed drive
·       Uninterrupted power supply (UPS)
·       Magnetic cored devices.
·       Power converters and rectifiers
·       Electric arc furnaces and welders
*    Induction furnace
*   Transformer magnetization non-linearity
An industrial unit generating harmonics becomes a source of pollution not only for itself but also other power consumers within its vicinity.

HARMONICS

    1) Types:
a)   Less then fundamental (sub harmonics)
b)   Greater than fundamental
a)   Multiple of fundamental harmonics i.e. odd & even
b)   Non-multiple of fundamental (Inter harmonics)
             [Fundamental harmonics
              50 Hz + 1 Hz (normally)
              50 Hz + 3 Hz (exited)]
2) Nature of Occurrence
              a) Steady
              b) Intermittent
3) Effects
A)   NUISANCE
i)                  Communication noise.
ii)                Light flickers
iii)              T.V. interference
iv)              Computer mal-operation
v)                Transformer noise
B)   ECONOMICS
i)                  Meter error
ii)                Reduction of equipment life
iii)              Increased cost
C)   DAMAGE
i)                  Overheating
ii)                Over voltage
iii)              Capacitor failure
iv)              Motor vibration
EFFECT OF HARMONICS ON CAPACITORS:
Electric Utilities i.e. Electricity boards in India are offering attractive incentives for power factor (PF) improvement to industrial consumers installing power capacitors in their electrical network to achieve power factor to unity [1.0] Power factor is displacement of current waveform in time with reference to voltage wave forms. Capacitor banks are even present in many industrial plants for reactive power compensation purposes.
The power factor correction i.e. improvement in power factor is basically done by capacitors.  These power factor correction capacitors react adversely in the presence of harmonic distortion in the system.  Although capacitors, being linear loads, do not generate harmonics, they tend to amplify existing harmonics. 
The amplification of harmonics by capacitors is caused by the electrical phenomenon of ‘Resonance’. Which is similar to symphonic vibration in a stringed musical instrument.  These ingredients cause electrical resonance viz., capacitors, inductors, non-linear loads.  Elimination of any one of these avoids occurrence of resonance.  As majority of modern electrical loads are both inductive and non-linear, their elimination is not possible.  Elimination of capacitance is impossible, as it would reduce PF below acceptable limits.

REASONS FOR LIMITING HARMONICS

Harmonic pollution causes a number of problems. A first effect is the increase of RMS value and the peak value of distorted waveform. This is illustrated in the figure below (next page), which shows the increase of these values as more harmonic components are added to an initially undistorted waveform. The RMS-value and peak value of undistorted waveform are defined as 100%. For example let us consider fundemental, 3rd, 5th harmonics with the help of the fig 1. in the next page. The increase of RMS-value increases heating of the electrical equipment. Furthermore, circuit breakers may trip due to higher thermal or instantaneous levels. All fuses may blow and capacitors may get damaged. KWH meters may get fault readings. The winding and iron losses of motors increase and they experience perturbing torques on the shaft. Sensitive electronic equipment may be damaged. Equipment, which uses supply voltage, as reference may not be able to synchronize properly, and either apply wrong firing pulses to switching elements or switch off.  Interference with electronic communication equipment may occur. Excessive amount of harmonics leads to premature ageing of the equipment.
FIGURE 1
Some more effects of harmonics are
              * Reduced efficiency
              * Insulation damage
              * Malfunctioning of equipment
              * Increased copper losses etc.
These are the motivations to take action against harmonics. Harmonic pollution may not only effect equipment of polluting plant but may also effect equipment in other plants. In order to limit this disturbance, maximum allowable distortion limits have been defined in standards and recommendations. Also the, International Electromechanical Commission (IEC) has issued technical reports which outline assessment procedures to determine whether distorting loads may be connected to the supply system. In many Cases, the regulations impose a limit for the total harmonic distortion (THD) of the voltage or current
Present at the point of common coupling (PCC). The PCC is the location at which the plant is connected to the public power system (generally at the primary of the main transformer(s)). The THD expresses the relative importance of the harmonics with respect to the fundamental Component. It is expressed as a ratio of total harmonic voltage to the fundamental voltage and is called total harmonic distortion (THD).  Generally 5% THD is considered an allowable limit for an electrically clean supply.             

ADVENT OF HARMONIC FILTERS
In order to improve power factor without causing harmonic amplification, harmonic filters are to be used.  Harmonic filters are suitably modified capacitors used together with series reactors, which are specially designed to improve power factor and suppress harmonic amplification.  These types of harmonic filters are the passive type and extensively used in place of conventional power factor correcting in harmonic rich environments. A passive filter basically consists of a series circuit of reactors and capacitors
The figure below illustrates schematically the description of passive filtering with a harmonic generator; impedance representing all other loads a filter and a network.
e.g. Blocking Reactor: Blocking Reactor sometimes called as re-tuned filter or low pass filter. Harmonic currents generated by, for example, a frequency converter are shunted by this circuit designed to have low impedance at a given frequency compared to the rest of the network. The main function of this is to protect the capacitor unit firm being exploited by the excessive level of power harmonics in electric network. Although passive filters are cost effective there are some disadvantages like they provide certain amount of reactive power which is not desirable when the loads to be compensated are ac drives which already have a good power factor they are susceptible to overload, sensitive to frequency changes and network changes and each passive filter can be used to filter only one harmonic component.  These limitations can be overcome by the use of active filters.

ACTIVE FILTERS – THE ULTIMATE SOLUTION


Active filters are a relatively new technology and overcome the disadvantages of passive filters. They make use of IGBT semiconductors with various control loops to increase power factor and reduce harmonics, thereby reducing amperage, KVA, circulating currents, harmonic resonance, line voltage imbalance and closed delta-winding losses. The active filter measures the harmonic currents and generates actively a harmonic current spectrum in opposite phase to the original distorting current that was measured. The original harmonics are thereby cancelled. THE PRINCIPLE OF ACTIVE HARMONIC FILTERING IS AS SHOWN BELOW

It allows for a concept that may not be overloaded. The compensating currents created by a 3 phase Insulated Gate Bipolar Transistor (IGBT) inverter bridge that is able to generate any given voltage waveform with PWM (pulse width modulation) technology. The IGBT Bridge uses a DC voltage source realized in the form of DC capacitor. The generated voltage is coupled to the network via reactors and small filter circuit. We use IGBT because they offer high switching frequencies that allow the generation of high frequency harmonic currents, and relatively low on state losses when compared to MOSFET’s. The PWM reactors transform voltage source inverter into current source. Finally the output filter absorbs the high frequency components introduced by the PWM switching action. With the help of Pulse width modulators (PWM) it is possible to control the dc voltage, but in case of active filters , an AC voltage needs to be created and controlled from a DC source. This can be achieved with a time varying duty cycle. Any duty cycle may be used providing that its frequency is strictly inferior to half that of carrier frequency. The system is called as pulse width modulation since, the ac output voltage , which has the same shape as duty cycle, is generated by controlling the width of the voltage pulses. Now a days closed loop control techniques are being implemented in which the filter directly its effect on the filtration by comparing the resultant waveform with the pure one which is given as a reference, with the help of feedback, furthermore the closed loop control system ensures that measurement errors do not result in a higher distortion apart from improved efficiency and accuracy. Open and closed loop current control as performed by active filters is shown in the figure below.
 
An open loop control system needs higher-class current sensors thereby increasing the risk of inaccuracy and reduced efficiency. Active filters reduce the energy cost by minimizing harmonics (THD<5%), increase power factor up to 100%, correct voltage imbalances and increase system reliability due to improvement in power quality. Active filters generate counter harmonic currents in opposite direction so as to totally cancel the harmonic currents generated by the non-linear loads.  Active filters are the most efficient type harmonic filtering solution and are being used in critical areas where harmonic control is crucial to the reliability of the installation.

BENEFITS OF ACTIVE FILTERS
·       Reduced energy costs
·       Elimination of higher and lower order harmonics
·       Improve load power factor
·       Voltage improvement and stability
·       Surge and transient protection
·       Reduce voltage unbalance
·       Reduce power line losses.
CONCLUSION
Considering all the above facts, it may not be surprising if electrical utilities across the country start penalizing industrial consumers for high harmonic levels like they do for poor power factor.  This is because high harmonics are similar to poor power factor in terms of reducing the efficiency of power distribution system.  These days when industrial efficiency standards are being continuously improved, the concept of power quality management is going to play a very significant role in the near future.  Thankfully there are qualified consulting engineering firms who should be able to help decide the right solution for a particular situation.
REFERENCES:
[1] IEEE Standard 519-1992, "IEEE Recommended Practices and Requirements for Harmonic Control in
Electric Power Systems", IEEE, New York, NY, USA, 1993.
[2] Recommendation G5/3, "Limits for Harmonics in the UK Electricity Supply System", The Electricity
Council Chief Engineers Conference, United Kingdom.
[3] Technical Report IEC 1000-3-6, "ELECTROMAGNETIC COMPATIBILITY (EMC) - Part 3: Limits - Section 6:Assessment of emission limits for distorting loads in MV and HV power systems", International
[4] E..W.Kimbark, High Voltage Direct Current Transmission –John Wiley & Sons, 4th edition.
[5] Jacob Millman & Taub, Pulse Digital And Switching Waveforms – Tata Mc-Graw Hill Publications.

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1 comments:

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