Analyzing and quantifying the effects of electric power quality
With the increased use of electronic switching devices in modern manufacturing facilities, there is a need to introduce new industrial standards for electrical system power quality. These new standards should establish requirements to exceed the “IEEE recommended practices for harmonic control in electrical power systems” (IEEE Std 519-1992). The quality of electrical power directly impacts the performance of our rotating equipment in terms of efficiency, vibration, sound power levels, and expected life. In the presence of deregulation, power quality can also affect the price we pay for electrical energy. This article discusses the reasons why these standards should be developed and how they can be achieved.
Since the 1980s the industrial and commercial manufacturing sectors have greatly increased their use of electronics and electronic switching devices. Examples of these devices are switch mode power supplies, ac motor inverter drives, computers, high frequency welders, and uninterruptible power supplies (UPSs).
These types of electrical loads are nonlinear. Nonlinear loads cause distortion of the voltage and current waveforms of ac power, resulting in a flow of harmonic currents and voltage distortion in an ac power system. These power quality problems can cause interference with communication circuits; overheating, vibration, and I losses in ac motors; and overheating and I losses in transformers, switchgear, and power cables.
Electronic equipment is susceptible to poor performance and premature failure caused by harmonic distortion. High levels of harmonics result in erratic and sometimes subtle malfunctions of the equipment, having serious consequences in some cases (Ref. 1, sec. 6, art. 6.6).
Harmonics generated internally, which flow back to the utility power grid, can have an adverse effect on other customers. In states where deregulation legislation is being considered or soon to be passed, the quality of power that is received will be a cost issue, as will the quality of power returned to the utility grid. Recommendations for power quality outlined in the IEEE 519-1992 may soon become regulations for industrial customers to follow.
The energy required to produce electrical power is greatly influenced by power quality. Electricity flows into and out of a facility. The power company consumes other forms of energy to maintain this flow. Industrial customers that have a poor internal power factor force the utility to use more energy to maintain electrical flow. Currently, for example, the electric utility in the Chicago area expects its industrial customers to maintain an 85% power factor.
Penalties for poor power factor may be an increasing cost. A poor power factor also subtracts from the electrical system capacity and allows higher motor temperatures and additional I losses in conductors due to higher current. Poor power factor can contribute to increased electrical demand charges with higher motor inrush currents.
In any ac motor, many driving forces, both mechanical and electromagnetic, exist simultaneously and produce vibration. These vibration forces can cooperate to weaken a motor and shorten its useful life. In many cases, the root causes of these vibration forces are unintentionally built in at the manufacturing source, and/or can be the result of poor power quality.
High levels of inductive imbalance in a newly manufactured motor can be attributed to poor quality control in the manufacturing of a motor. Motors with high levels of inductive imbalance, or motors run with poor power quality, will not operate smoothly. They will produce many mechanical vibration forces, low frequency pulsations, two times the slip frequency, two times the line frequency, and rotor/stator harmonics. These are due to imperfections in the rotor, stator, bearing center position, and other electrical system anomalies.
Slip frequency vibration
Assume a motor is perfectly symmetrical mechanically. The radial magnetic force between the rotor and stator core at each of the magnetic poles tends to distort the core periodically as the magnetic field rotates. Since a force of attraction exists regardless of the polarity of the pole, a vibration force of twice the line frequency always results. This second harmonic driving force is independent of the number of poles, but is more prominent in two-pole motors because the distance between poles is relatively greater.
Consider a two-pole motor in which the center of rotation of the rotor coincides with the center of the stator, but in which the rotor is not round. This means that there is a point of minimum air gap that travels at rotor speed.
In a 60-Hz, two-pole motor, the revolving field rotates at 60 revolutions per second (rps), or 3600 rpm. If it is assumed that the rotor speed is 59 rps and the point of minimum air gap starts at time zero in the center of the north pole, then, when the north pole has made one revolution, the point on the rotor will have made a 59/60 revolution (Fig. 1.). The rotor will keep falling behind, and when the field has made 30 revolutions, the point on the rotor will have made 29.5 revolutions, therefore lining up with the south pole.
When the field has made 60 revolutions, the rotor will have made 59, and the point on the rotor will coincide with the north pole again. In one revolution of slip, the point of minimum air gap has come under the influence of maximum flux density twice, and therefore the frequency of the unbalanced magnetic pull between the rotor and stator is twice the slip frequency. This vibration frequency is superimposed on the normal twice the line frequency driving force and modulates it at twice the slip frequency.
A revolving point of minimum air gap can be caused by other anomalies such as bent shaft or dynamic unbalance combined with a weak shaft. With dynamic unbalance, the geometrical center of the rotor does not coincide with the axis of rotation but lies on a circle about this axis. The diameter of this circle depends upon the amount of unbalance and the stiffness of the shaft.
Electrical anomalies in the rotor, such as a defective rotor bar, also cause modulation. Normally the ampere-turns of the stator and rotor tend to balance one another except for the excitation ampere-turns. However, if it is assumed that a rotor bar is open circuited, then the current flowing in it and the ampere-turns produced will be greatly reduced.
The net unbalanced stator ampere-turns will produce an air gap flux density greater than normal at the point of the defective rotor bar. This increased flux density causes a higher induced voltage and a higher current in the bars adjacent to the defective bar. The net effect is that the stator and rotor ampere-turns still balance one another, but the flux distribution in the air gap is altered. This change in flux distribution is most pronounced when the defective bar is under a magnetic pole and least effective when it is between poles.
Since the defective bar is passed by a magnetic pole twice for each revolution of slip, the inherent twice line frequency is modulated at two times the slip frequency. The motor torque also pulsates at this same frequency. Pulsation caused by modulation is summarized in Fig. 2.
Next, consider the vibration forces that contribute to the phenomenon of beats. In addition to the inherent twice line frequency vibrations, certain mechanical asymmetries would also cause twice the line frequency driving forces. Assume that the center of the rotor rotation coincides with the center of the stator and that the rotor is round, but the stator is slightly elliptical. This can be the result of a slight manufacturing flaw or a soft motor foot. As the field rotates, it passes a point of minimum air gap twice each revolution and produces a vibrational force of twice line the frequency. If the rotor and stator are round but the rotor is not perfectly centered in the stator, the same frequency of vibration results.
Several mechanical driving forces exist which can cause vibrations of two times the running frequency. For example, a shaft that has a noncircular cross section caused by a keyway, loose bearing-to-housing fits, coupling misalignment, and elliptical bearings can cause these vibrations.
This second harmonic driving force is not particularly important except in two-pole motors in which the harmonic differs from twice line frequency by two times the slip frequency. These two driving forces alternately reinforce and interfere with each other and produce a beat. The conditions producing this beat are summarized in Fig. 3.
Another factor that must be included when considering the causes of the beat phenomenon is the space harmonics in the main flux wave. These harmonics produce torque and radial forces, which tend to cause noise and vibration. For example, consider the stator fifth harmonic that travels around the stator in the direction opposite that of the fundamental. The fifth harmonic has five times as many poles as the fundamental, or ten poles.
The time variation of all the harmonics in the air-gap flux is the same as for the fundamental since the same current produces them all. Therefore, with sinusoidal-impressed voltages, the revolving harmonic component fields in the air-gap caused by the windings revolve more slowly than the fundamental field.
The fifth harmonic revolves 1/5 as fast as the fundamental, or 12 rps for a 60-Hz, two-pole motor. With ten poles revolving at 12 rps, a vibrational driving force of 120 Hz is produced. In the same manner, all of the stator harmonics will produce radial forces of twice the line frequency — one reason why they make the pulsation worse.
The frequency of the voltage induced in the rotor by a space harmonic is equal to the line frequency times the slip of the rotor with respect to the harmonic.
For the fifth harmonic, which rotates in the opposite direction of the fundamental, consider the following equations:
f 2 = f 1 (6-5 S 1)
Where: f 1 = frequency of the fundamental, S 1 = slip of the rotor with respect to the fundamental, and P 1 = number of poles of the fundamental.
If a slip of 5%, or 3 rps, for a 60-Hz, 2-pole motor is assumed, the frequency induced by the fifth harmonic in the rotor is:
f 2 = (60)(6-5×0.05) = 345 Hz
For the seventh harmonic, which travels in the same direction as the fundamental, consider the following equation:
f 2 = f 1 (-6+7 S 1 )
The frequency induced by the seventh harmonic is:
f 2 = (60)(-6+7×0.05) = -339 Hz
Numerically, these two frequencies differ by 6 Hz, or two times the slip frequency. The negative sign for the seventh harmonic merely means that its motion relative to the rotor is opposite that of the fifth harmonic. Thus, a point on the rotor is influenced by two revolving fields, which are aiding one another when they are in phase and bucking one another when they are out of phase. The net result is that a torque pulsation occurs, which varies at two times the slip frequency.
Other harmonics, such as the 11th and 13th, 17th and 19th, 23rd and 25th, etc., also beat with one another at the same time (Ref. 3).
The proper winding pitch can suppress the magnitude of space harmonics. The most desirable pitch is 83%, which reduces the fifth and seventh harmonics to approximately 26% of that with full pitch. Since small two-pole motors are very difficult to wind with this long of a throw, the best compromise is 55% to 57%, which reduces the seventh harmonic to a small value. If the winding is such that prominent space harmonics are present, the pulsation can occur even if the rotor, stator, and air gap are perfect (Ref. 3).
In order to produce more electromechanical force (emf), some motor windings are full pitch. Using a full pitch allows the maximum amount of emf. However, the use of full pitch windings will not suppress harmonics. And the presence of harmonics in a motor will produce more vibration and heat, which shortens a motor’s useful life.
Many motors are built using a lapped winding arrangement. This type of winding improves the waveform, saves copper in the coil ends, and reduces the inductance of the windings. These are results from the lesser mutual inductance between those conductors, which lie in slots also containing conductors of either of the two other phases.
In Fig. 4(a), E 1 is the emf induced in the conductors comprising one side of a coil, and E 2 is the emf induced in the conductors comprising the other side of the coil. E 1 is equal to E 2 numerically, as the same number of conductors cutting the same flux at the same speed induces each. Figure 4(a) gives the relationship of the induced emfs E 1 and E 2 in the two coil sides when a full pitch coil is used. When one side of the coil is under a north pole, the other side is in a corresponding position under a south pole. The induced emfs differ by 180 deg in phase, but the coil connection is such that these emfs add, with their sums being E as shown.
When a 5/6 pitch is used, the coil spread is equal to 5/6 of 180 deg, or 150 electrical space deg. The emfs E 1 and E 2 will differ in phase by 30 electrical time deg, as shown by angle B in Figure 4(b). The total emf E , which is their vector sum, is slightly less than when a full pitch coil is used.
The pitch factors for the harmonics are considerably less than that for the fundamental so that the harmonics are reduced much more proportionately than the fundamental. For example, a 2/3 pitch will eliminate the third harmonic, a 4/5 pitch the fifth, etc. Therefore, with a fractional-pitch winding, the waveform is improved and less vibration is produced (Ref. 4).
Harmonics waste energy
Harmonics produced by electronic devices are present in most modern manufacturing facilities. When combined with all other harmonic producing factors previously explained, the harmonic effect can be amplified. The torque pulsation and vibration present in the motor can also occur at the precise time as a workload pulse or natural vibration frequency of a component in the driven machine. When these forces combine, a resonant vibration condition can occur for these other machine components. Increased vibration, heat, and energy losses are the most relevant effects. But reduction in equipment performance, reliability, and expected life are the most costly.
Exceed the 519 standard
The IEEE Standard 519-1992 is the recommended practice and requirements for harmonic control in electrical power systems. The specification defines voltage distortion limits at the point of common couple for 69 kV and below to be 3% for individual voltage distortion, and 5% for total voltage distortion. These limits should be clearly understood as design values for the “worst case” as stated (Ref. 1).
Operating a facility with voltage distortion this high will cause significant energy losses and shortened equipment life. This author recommends that under normal conditions, total voltage distortion should not exceed 1.5%.
Edited by Jack Smith, Senior Editor, 630-320-7147, firstname.lastname@example.org
Electrical data acquisition
At the M&M Mars plant in Burr Ridge, IL, electrical data acquisition is accomplished using a permanently installed electrical power monitoring system consisting of 70 circuit monitors and system software residing on a Windows NT server and four clients. With this system, power monitoring and logging is accomplished for all plant feeders and mains. For locations where the electrical power monitoring system is not installed, a portable circuit monitor with its associated software is used for electrical data acquisition. The portable circuit monitor also provides an excellent means of extracting individual motor circuit data for post processing.
In 1994, through the Cooperative Research and Development Agreement (CRADA) between M&M/Mars and the Oak Ridge National Laboratory (ORNL), it was discovered that some high efficiency motors, specifically cost-optimized designs, have unique susceptibilities. When cost-optimized high efficiency motors are used with less than high-quality power, poor motor performance and a shorter motor life can be expected. These results were written and presented by the ORNL at the 1995 national meeting of the Institute for Ammonia Refrigeration Engineers. ORNL wrote a second paper based on laboratory testing results titled “A comparison of two energy efficient motors.” This was a comparison of a premium efficiency motor to a regular (cost-optimized) energy efficiency motor. The results indicated that the premium efficiency motor performed better than a standard energy efficient motor, but it could also be susceptible to power quality problems (Ref. 2).
Any ac motor operating in a poor power quality environment is susceptible. Increased heat, vibration, and efficiency losses are the results. The long-term effects are a higher operating cost and shorter motor life.
Power analyzer software
On July 6, 1998, M&M/Mars Burr Ridge contracted the ORNL to develop advanced software algorithms that would calculate energy losses due to electrical system anomalies. These algorithms take into account measurable quantities such as harmonics, voltage unbalance, and total harmonic distortion (THD). The algorithms calculate the savings in energy and improvement in equipment life that can be achieved if the power anomalies are removed. The resulting Power Analyzer Software (PAS) and intellectual property are the sole property of M&M/Mars.
The hypothesis surrounding this technology was based on the findings completed in 1995. The goal is to improve the performance and extend the expected life of all electrically driven machines in the facility. A machine is expected to operate more efficiently and consume less electrical energy.
PAS accomplishes this analysis through five steps:
The three phases of raw voltage and current waveforms are captured simultaneously on a motor circuit, motor control center (MCC), distribution feeder, or main.
The waveform data is exported in a comma separated variable (CSV) format to a spreadsheet template. The template is used to organize the data for import into PAS.
The algorithms in PAS process this data to display the quantity of electrical losses in terms of total unbalanced power distortion (TUPD) and kW.
PAS provides a process for building a motor model based on motor nameplate information.
The processed data can then be plugged into the motor model as captured, or can be precleaned by removing some or all of the power anomalies. This pre-cleaning process can be performed as individual custom reductions for each harmonic quantity and power anomaly or as a total cleaning and power-balancing process.
The uncleaned and cleaned power data can be processed individually into the motor model. PAS will deliver a prognosis of expected motor life and energy savings based on the quality of the power fed into the motor model. The prognosis delivered from PAS can also help identify mechanical problems in the motor or driven machine.
PAS provides the ability to identify cost saving opportunities. These tools provide the specific knowledge for application of corrective equipment to reduce energy losses and improve machine reliability. After processing the electrical data for the Burr Ridge site, PAS revealed an energy savings potential of at least 5%.
Real time reactive compensation (RTRC)
After determining the energy savings and reliability improvement potential of using clean electrical power, the goal was then to determine which type of power conditioning equipment was needed.
The Burr Ridge facility acquired a real-time reactive compensation system, which provides instantaneous power factor correction, load stabilization, and harmonic filtering. The controller has a data acquisition and response time of one cycle (16.7 msec), which is fast enough to compensate for even high-speed loads. The result is prevention of voltage sag and flicker, as well as increased system capacity and energy savings.
The system includes custom designed iron core reactors in series with each three-phase capacitor module. The series capacitor/reactor combination tunes the network below the first dominant harmonic (usually the fifth, or 300 Hz). The system can absorb as much as 50% of the fifth harmonic. This reduces harmonic distortion and balances voltage, thus improving overall network conditions.
Energy savings and benefits
Energy savings are experienced through the reduction of high inrush currents from highly cyclical loads, harmonic reduction in ac motors, improved power factor, improved voltage balance, and lower I
The improvement in power quality has reduced heat, vibration, and noise in our ac motors, resulting in improved machine performance, reliability, and longer life expectancy.