Power quality is essential for smooth functioning of industrial process. As industries expand, utilities become more interconnected and usage of electrically equipment increases, power quality is jeopardized. The quality of power in the power system is severely affected by the presence of harmonics. This harmonics adversely effects the power system performance. Some of the effects are over heating of metal parts, noise in motors, low efficiency in motors etc. The effects produced by the harmonics are reduced by adopting some corrective measures.
Since last 25 years there has been an increase in the use of solid state electronic technology. This new, highly efficient, electronic technology provides product quality with increased productivity. Today, we are able to produce products at costs less than in the years passed, with the introduction of automation by using the solid state electronic technology .This new technology requires clear electric power.
The conventional speed control systems are being replaced by modern power electronic systems, bringing a verity of advantages to the users. Classic examples are DC $ AC drives, UPS, soft stators, etc. Since the thrusters converter technology is rapidly gaining in the modern industrial plants, the power supply systems are contaminated as the ideal sinusoidal current and voltage waveforms are getting distorted. This is in turn is affecting the performance of the equipment in the electrical network.
2. WHAT IS POWER QUALITY
Adequate to superior power quality is essential for the smooth functioning of critical industrial processes. As industries expand, utilities become more interconnected and usage of electronically controlled equipment increases, power quality is jeopardized. Most large industrial and commercial sites are served by overhead lines with feeders that are subject to unpredictable and sporadic events, e.g. lightning and contact with tree limbs. Most distribution circuits have resoling devices that clear temporary faults through a timed series of trip and close operations.
This minimizes the possibility of long-term outages but leads to a number of minor power disturbances. These typically occur several times a month. Many electric utilities have increased the voltage at which they distribute power. This allows a single circuit to serve more customers or deliver higher loads, and reduces energy losses in the system. But it often means the overhead distribution circuit is longer, with more exposure to disturbances. And disturbances travel farther because of lower system impedances associated with higher voltage circuits. Sophisticated new systems are providing vastly increased efficiency and control in critical processes. But with their high sensitivity even to brief variations in electric power quality, today's computer-driven devices fail when power is disturbed for even a few milliseconds.
3. HARMONICS-BASIC CONCEPTS
A pure sinusoidal voltage is conceptual quantity produced by an ideal AC generator build with finely distributed stator and field windings that operate in a uniform magnetic field. Since neither the winding distribution nor the magnetic field is uniform in a working AC machine, voltage waveform distortion is created, and the voltage time relation-ship deviates from the pure sine function. The distortion at the point of generation is very small (about 1%to 2%), but nonetheless it exists.
Because this is a deviation from a pure sine wave, the deviation is in the form of a periodic function and by definition, the voltage distortion contains harmonics. When a sinusoidal voltage is applied to a certain type of load, the current drawn by the load is proportional to the voltage and impedance and follows the envelope of the voltage wave form .These loads are referred to as linear loads (loads where the voltage and current follow one another without any distortion to their pure sine waves).examples of nonlinear loads are resistive heaters, incandescent lamps and constant speed induction and synchronous motors.
In contrast some loads cause the current to vary disproportionately with the voltage during each half cycle. These loads are classified as nonlinear loads and the current and voltage have waveforms that are non sinusoidal containing distortions where by 50 Hz waveform has numerous additional waveforms superimposed upon it creating multiple frequencies within the normal 50 Hz sine wave .The multiple frequencies are harmonics of the fundamental frequency.
Normally current distortion produce voltage distortions .However when there is a stiff sinusoidal voltage source there is a low impedance path from the power source which has sufficient capacity so that loads placed upon it will not affect the voltage one need not be concerned about current distortions producing voltage distortions Examples of non linear loads are battery chargers, electronic ballasts; variable frequency drives, and switched mode power supplies.
As nonlinear currents flows through a facility's electrical system and the distribution - transmission lines, additional voltage distortions are produced due to the impedance associated with the electrical network. Thus as electrical power is generated, distributed and utilized, voltage and current waveforms distortions are produced.
Power systems designed to function at the fundamental frequency which is 50 Hz in India are prone to unsatisfactory operation and at times failure when subjected to voltages and currents that contains substantial harmonic frequency elements. Very often the operation of electrical equipment may seem normal but under a certain combination of conditions the impact of harmonics is enhanced with damaging results.
4. THE AFFECTS
The actual problems of any building/industry will vary depending on the type and number of installed harmonics producing loads. Most electrical network can withstand nonlinear loads of up to 15% of the total electrical system capacity without concern but when the nonlinear loads exceed 15% some non expected negative consequences can be expected. .for electrical networks , they have on linear loading of more than 25% particular problems can be apparent.
The following is a short summery of most problems caused by harmonics:
Blinking of incandescent lights-transformer saturation
Capacitor failure-harmonics resonance
Circuit breaker tripping-inductive heating and over loading
Computer malfunctioning-voltage distortion
Motor failure-inductive heating
Fuses blowing for no apparent reason-inductive heating & over load
Electronic component shut down- voltage distortion
Flickering of florescent lights-transformer saturation
The heating effects of harmonic currents can cause destruction of equipment, conductors, and fires. The results can be unpredictable legal and financial ramifications apart from safety risks. Voltage distortions can lead to over heating of equipment failure, expensive down time and maintenance difficulties. Harmonic currents and voltage distortions are becoming the most severe and complex electrical challenge for the electrical industry .The problems associated with nonlinear loads were once limited to isolated devices and computer rooms, but now the problem can appear through the entire network and utility system
The point at which the harmonic limits are applied is called the point of common coupling (PCC). When the input transformer is the point of measurement then the PCC refers to this point where the facility electrical system is common to the facility of additional consumers. If there is a distortion present on the electrical power system at this point it may be experienced by the neigh boring facilities as well. So we need to avoid this situation
Users of variable frequency drives often have strict demands placed on them to mitigate harmonic distortion caused by the nonlinear loads. Many choices are available to them including line reactors, harmonic traps, 12 pulse rectifier, 18 pulse rectifiers, and low pass filters.
5.1 LINE REACTORS
The input harmonic current distortion can be reduced by simple addition of input line reactance. The inductive reactance of an input line reactor allows 50 Hz or 60 Hz currents to pass easily but presents considerably higher impedance to all other harmonic frequencies. Harmonic currents are thus attenuated by the reactance offered by the line reactor.
These reactors are also used to solve the problems in variable frequency drive installations.Eg: harmonic attenuation , drive tripping .The line reactors are always used in the line side or input of the variable frequency drives. Thus they are called the line reactors. The line reactors cannot be used at the output of the variable frequency drives
Because the reactors are over heated due to the harmonic content of the output waveform of the VFD Harmonic compensated reactors can be used on the either side of the variable frequency drives .Due to the introduction of the Harmonic compensated reactors the following problems are eliminated: motor noise, low efficiency of the motors, temperature rise in motors and variable frequency drives short circuit problem.
5.2 HARMONIC FILTERS
In some cases, reactors alone will not be capable of reducing the harmonic current distortion to the desired levels. In these cases, a more sophisticated filter will be required. The common choices include shunt connected, tuned harmonic filters (harmonic traps) and series connected low pass filters (broad band suppressors). They consist of a capacitor and an inductor which are tuned to a single harmonic frequency. Since they offer very low impedance to that frequency, the specific (tuned) harmonic current is supplied to the drive by the filter rather than from the power source. If tuned harmonic filters (traps) are selected as the mitigation technique, then multiple tuned filters are needed to meet the distortion limits which are imposed.
When employing tuned harmonic filters, we need to take special precautions to prevent interference between the filter and the power system. A harmonic trap presents a low impedance path to a specific harmonic frequency regardless of its source. The trap cannot discern harmonics from one load versus another. Therefore, the trap tries to absorb that entire harmonic which may be present from all combined sources (non-linear loads) on the system. This can lead to premature filter failure.
Since harmonic trap type filters are connected in shunt with the power system, they cause a shift in the power system natural resonant frequency. If the new frequency is near any harmonic frequencies, then it is possible to experience an adverse resonant condition which can result in amplification of harmonics and capacitor or inductor failures. Whenever using harmonic trap type filters, one must always perform a complete system analysis. You must determine the total harmonics which will be absorbed by the filter, the present power system resonant frequency, and the expected system resonant frequency after the filter (trap) is installed. Field tuning of this filter may be required if adverse conditions are experienced.
5.3 12 PULSE RECTIFIERS
12 Pulse drives are frequently specified by the engineers for heating, ventilating and air conditioning applications because their ability to reduce harmonic current distortion. In the mid 1960s when power semiconductors were only available in limited ratings, twelve-pulse drives provided a simpler and more cost effective approach to achieving higher current ratings than direct paralleling of power semiconductors.
A typical diagram of a large twelve-pulse drive appears in figure the drive's input circuit consists of two six-pulse rectifiers, displaced by 30 electrical degrees, operating in parallel. The 30-degree phase shift is obtained by using a phase shifting transformer. The circuit in figure simply uses an isolation transformer with a delta primary, a delta connected secondary, and a second wye connected secondary to obtain the necessary phase shift. Because the instantaneous outputs of each rectifier are not equal, an inter phase reactor is used to support the difference in instantaneous rectifier output voltages and permit each rectifier to operate independently. The primary current in the transformer is the sum of each six-pulse rectifier or a twelve-pulse wave form.
Theoretical input current harmonics for rectifier circuits are a function of pulse number and can be expressed as:
h = (np + 1) where n= 1, 2, 3, and p = pulse number
For a six-pulse rectifier, the input current will have harmonic components at the following multiples of the fundamental frequency.
5, 7, 11, 13, 17, 19, 23, 25, 29, 31, etc.
For the twelve-pulse system shown in figure 1, the input current will have theoretical harmonic components at the following multiples of the fundamental frequency:
11, 13, 23, 25, 35, 37, etc.
Note that the 5th and 7th harmonics are absent in the twelve-pulse system. Since the magnitude of each harmonic is proportional to the reciprocal of the harmonic number, the twelve-pulse system has a lower theoretical harmonic current distortion.
12 PULSE RECTIFIERS
Figure shows the actual measurement of input current harmonic distortion for 12 pulse rectifier supplied from a balanced 3 phase voltage source while operating at full load conditions. For test purpose transformer has delta primary and delta,wye secondary windings. To obtain the best results, the bridge rectifier is connected in series so equal dc windings. To obtain the best results, the bridge rectifier is connected in series so equal dc
The data shows when the current through both sets of the rectifiers is equal, harmonics can be as low as 10% to 12% total harmonic current distortion, at full load. Current sharing reactors will help parallel connected bridge rectifiers to share current equally. Even with balanced current harmonic current distortion can increase appreciably at light loaded conditions. Even with perfectly balanced line voltages, the resultant % total harmonic current distortion increases as the load increases. As the load reduced, that is 23% total harmonic current distortion at 20% load.
5.4 18 PULSE RECTIFIER
A typical diagram of a series connected eighteen pulse drive constructed from a standard six-pulse drive, two external rectifiers and a conventional 18 pulse isolation transformer appears in figure 1. The drive has terminals available to connect a DC link choke. These terminals are used to connect the two external rectifiers in series with the drives internal rectifier. The eighteen pulse transformer is designed to provide one third the normal input voltage to each of the three rectifiers at a 20 degree phase displacement from each other. The 20-degree phase shift is obtained by phase shifting the transformers secondary windings. The circuit in figure 1 simply uses an isolation transformer with a delta primary, and three delta connected secondary windings, one shifted + 20 degrees, one shifted -20 degrees and one in phase with the primary.
The primary current in the transformer is the sum of each six-pulse rectifier or an eighteen-pulse wave form.
Theoretical input current harmonics for rectifier circuits are a function of pulse number and can be expressed as:
h = (np Ã‚Â± 1) where n= 1, 2, 3,... and p = pulse number
For a six-pulse rectifier, the input current will have harmonic components at the following multiples of the fundamental frequency.
5, 7, 11, 13, 17, 19, 23, 25, 29, 31, 35, 37, 41, 43, 47, 49, 53, 55, etc.
For the eighteen-pulse system shown in figure 1, the input current will have theoretical harmonic components at the following multiples of the fundamental frequency:
17, 19, 35, 37, 53, 55, etc.
Note that the 5th and 7th, 11th and 13th harmonics are absent in the theoretical eighteen-pulse system. Since the magnitude of each harmonic is proportional to the reciprocal of the harmonic number, the eighteen-pulse system has a lower theoretical harmonic current distortion.
To determine how an eighteen-pulse drive system operates under unbalanced line voltage conditions, we constructed a 30 HP eighteen-pulse drive from a conventional isolation transformer and standard six-pulse drive using the series bridge connection shown in figure 1. An auto transformer could have been used in place of the isolation transformer. The auto transformer costs less and requires less mounting space, but the isolation transformer was selected because it provides better performance and is readily available as a modified standard transformer.
Care was taken in the physical construction of the transformer to balance the leakage reactance and output voltage of the three secondary windings. The system was tested with line voltage unbalance ranging from 0% to 3% and with loads ranging from 5% to 100%. The input total harmonic current distortion, THID, is shown in figure 3. THID varied from 7.4% at full load with balanced line voltages to 59% at 30% load with a 3% unbalance. The data show that the harmonic performance of eighteen-pulse drives degrades rapidly with increasing line voltage unbalance and decreasing load.
Simply focusing on harmonic performance under the best operating conditions, perfectly balanced line voltages and full load, is not a useful indicator of performance under practical operating conditions. In heating, ventilating and air conditioning applications where drives will operate for long periods of time at 30% to 60% load eighteen pulse drives to not meet expectations.
18 PULSE RECTIFIER
To determine how an eighteen-pulse drive system operates under unbalanced line voltage conditions, we constructed a 30 HP eighteen-pulse drive from a conventional isolation transformer and standard six-pulse drive using the series bridge connection shown in figure 1. An auto transformer could have been used in place of the isolation transformer. The auto transformer costs less and requires less mounting space, but the isolation transformer was selected because it provides better performance and is readily available as a modified standard transformer. Care was taken in the physical construction of the transformer to balance the leakage reactance and output voltage of the three secondary windings. The system was tested with line voltage unbalance ranging from 0% to 3% and with loads ranging from 5% to 100%. The input total harmonic current distortion, THID, is shown in figure 3. THID varied from 7.4% at full load with balanced line voltages to 59% at 30% load with a 3% unbalance. The data
show that the harmonic performance of eighteen-pulse drives degrades rapidly with increasing line voltage unbalance and decreasing load. Simply focusing on harmonic performance under the best operating conditions, perfectly balanced line voltages and full load, is not a useful indicator of performance under practical operating conditions. In heating, ventilating and air conditioning applications where drives will operate for long periods of time at 30% to 60% load eighteen pulse drives to not meet expectations.
Obviously, any unbalance in the eighteen-pulse transformer's leakage reactance and output voltage will degrade performance. Unfortunately perfect transformers can not be built. Output voltage depends on turns ratios which are limited to plus or minus one turn. As a result the output voltage of the three secondary windings cannot be perfectly balanced. Leakage reactance is a function of coil position and volume. Clever Mechanical design of the transformer windings will help to minimize the differences in leakage reactance between the three groups of secondary windings but perfect balance can not be achieved. Data for the transformer used in this test appears in Tables 1 and 2.
Winding Phase Shift
% Nominal Output Voltage
Based on Turns Ratios
0 3.67 160.00
-20 4.73 160.50
+20 5.33 160.50
Transformer Full Load Data
Degrees Secondary Phase Voltage
At Full Load Unbalance
A B C Average
0 154.3 154.4 154.1 154.26 0.10
-20 157.9 157.0 157.6 157.50 0.32
+20 156.6 155.4 156.9 156.30 0.57
Average 156.02 1.12
The addition of 5% line reactors at the input to each of the three rectifiers results in a significant improvement in the operation Drives are applied in heating, ventilating, and air conditioning applications because loads are variable and users demand energy efficiency and comfort. Varying loads result in load unbalances within building power distribution systems which add to the utility line voltage unbalance at the point of common coupling. Harmonic mitigation techniques which are not effective with line voltage unbalances of 1% to 3% at the point of utilization will not as a practical matter achieve useful results. The data in this report show that a standard six-pulse drive fed from a low pass Matrix Filter provides superior harmonic performance to an eighteen-pulse drive in applications with variable loads and line voltage unbalances ranging from 0% to 3%.
5.5 LOW PASS HARMONIC SUPPRESSORS
Low pass harmonic filters, also referred to as broad band harmonic suppressors, offer a non-invasive approach to harmonic mitigation. Rather than being tuned for a specific harmonic, they filter all harmonic frequencies, including the third harmonic. They are connected in series with the non-linear load with a large series connected impedance, therefore they donâ„¢t create system resonance problems. No field tuning is required with the low pass filter.
Due to the presence of the large series impedance, it is extremely difficult for harmonics to enter the filter / drive from the power source. Rather they are supplied to the drive via the filter capacitor. For this reason, it is very easy to predict the distortion levels which will be achieved and to guarantee the results.
A low pass (broad band) harmonic filter can easily offer guaranteed harmonic current levels, right at the drive / filter input, as low as 8% to 12% THID. (To achieve 8% maximum current distortion one can typically select the broad band harmonic suppressor based on a HP / KW rating which is 25-30% larger than the total drive load to be supplied). In most cases, this results in less than 5% THID at the facility input transformer and meets most international standards.
Fig. 6 Actual input current waveform for VFD fitted with Broad Band Harmonic Suppressor.
The low pass filter not only offers guaranteed results, it is also more economical than 12 or 18 pulse rectifier systems, active filters or in many cases even harmonic traps. They are intended for use with 6-pulse drives having a six diode input rectifier in variable torque applications. This typically means fan and pump applications. For the sake of economy, a single Broad Band Harmonic Suppressor may be used to supply several drives (VFDs). When operating at reduced load, the THID at the filter input will be even lower than the guaranteed full load values.
MOTOR TEMPERATURE REDUCTION
Motors operated on a VFD tend to run warmer than when they are operated on pure 60hz, such as in an across-the-line stator application. The reason is that the output waveform of the VFD is not pure 60hz,, but rather it contains harmonics which are currents flowing at higher frequencies. The higher frequencies cause additional watts loss and heat to be dissipated by the iron of the motor, while the higher currents cause additional watts loss and heat to be dissipated by the copper windings of the motor. Typically the larger horsepower motors (lower inductance motors) will experience the greatest heating when operated on a VFD.
Reactors installed on the output of a VFD will reduce the motor operating temperature by actually reducing the harmonic content in the output waveform. A five percent impedance, harmonic compensated reactor will typically reduce the motor temperature by 20 degrees Celsius or more. If we consider that the typical motor insulation system has a "Ten Degree C Half Life" (Continual operation at 10 degrees C above rated temperature results in one half expected motor life), then we can see that motor life in VFD applications can easily be doubled. Harmonic compensated reactors are actually designed for the harmonic currents and frequencies whereas the motor is not.
Because the carrier frequency and harmonic spectrum of many Pulse Width Modulated (PWM) drives is in the human audible range, we can actually hear the higher frequencies in motors which are being operated by these drives. A five percent impedance harmonic compensated reactor will virtually eliminate the higher order harmonics (11th & up) and will substantially reduce the lower order harmonics (5th & 7th). By reducing these harmonics, the presence of higher frequencies is diminished and thus the audible noise is reduced. Depending on motor size, load, speed, and construction the audible noise can typically be reduced from 3 - 6 dB when a five percent impedance harmonic compensated reactor is installed on the output of a PWM drive. Because we humans hear logarithmically, every 3dB cuts the noise in half to our ears. This means the motor is quieter and the remaining noise will not travel as far.
Because harmonic currents and frequencies cause additional watts loss in both the copper windings and the iron of a motor, the actual mechanical ability of the motor is reduced. These watts are expended as heat instead of as mechanical power. When a harmonic compensated reactor is added to the VFD output, harmonics are reduced, causing motor watts loss to be reduced. The motor is able to deliver more power to the load at greater efficiency. Utility tests conducted on VFD's with and without output reactors have documented efficiency increases of as much as eight percent (at 75% load) when the harmonic compensated reactors were used. Even greater efficiency improvements are realized as the load is increased.
SHORT CIRCUIT PROTECTION
When a short circuit is experienced at the motor, very often VFD transistors are damaged. Although VFD's typically have over correct protection built-in, the short circuit current can be very severe and its rise time can be so rapid that damage can occur before the drive circuitry can properly react. A harmonic compensated reactor (3% impedance is typically sufficient) will provide current limiting to safer values, and will also slow down the short circuit current rise time. The drive is allowed more time to react and to safely shut the system down. You still have to repair the motor but you save the drive transistors.
The above methods solve other problems on the load side of VFD's in specialized applications also. Some of these include: Motor protection in IGBT drive installations with long lead lengths between the drive and motor, Drive tripping when a second motor is switched onto the drive output while another motor is already running, and Drive tripping due to current surges from either a rapid increase or decrease in the load.
VFD users have many choices when it comes to harmonic filtering. Of course they may do nothing, or they may choose to employ one of the many techniques of filtering available. Each filtering technique offers specific benefits and has a different cost associated with it. Some may have the potential to interfere with the power system while others will not.
For best overall results when using reactors or harmonic filters, be sure to install them as close as possible to the non-linear loads which they are filtering. When you minimize harmonics directly at their source you will be cleaning up the internal facility mains wiring. This will also reduce the burden on upstream electrical equipment such as circuit breakers, fuses, disconnect switches, conductors and transformers. The proper application of harmonic filtering techniques can extend equipment life and will often improve equipment reliability and facility productivity.
Â¢ INDUSTRIAL REFERENCE MAY 2003 PAGE 178
1. INTRODUCTION 1
2. WHAT IS POWER QUALITY 2
3. HARMONICS BASIC CONCEPT 3
4. THE AFFECTS 5
5. SOLUTION 7
5.1. LINE REACTORS 7
5.2. HARMONIC TRAPS 8
5.3. 12 PULSE RECTIFIER 9
5.4.18 PULSE RECTIFIER 12
5.5. LOW PASS SUPPRESSORS 17
6. BENEFITS 19
7. CONCLUSION 22
8. REFERENCES 23
I express my sincere gratitude to Dr.Nambissan, Prof. & Head, Department of Electrical and Electronics Engineering, MES College of Engineering, Kuttippuram, for his cooperation and encouragement.
I would also like to thank my seminars guide Mrs. Nafeesa K. (Lecturer, Department of EEE), Asst. Prof. Gylson Thomas. (Staff in-charge, Department of EEE) for their invaluable advice and wholehearted cooperation without which this seminars would not have seen the light of day.
Gracious gratitude to all the faculty of the department of EEE & friends for their valuable advice and encouragement.