Dealing with line
|Figure 1 - A three-phase motor is a linear load and, as a result, does not cause significant waveform distortion.||Figure 2 - A pulse-width modulated (PWM) drive draws a pulse of current whenever the input voltage exceeds the DC bus voltage by 0.7 V (the voltage drop across a diode).|
In contrast, VFDS, as well as electronic ballasts, uninterruptible power supplies (UPSs), and other nonlinear loads, have dissimilar, nonlinear voltage and current waveforms. They draw their current from only the peaks of the ac line, flattening the top of the voltage waveform. Fig. 2 shows how a voltage waveform, measured at the drive input terminals, will be affected by the current drawn by a PWM VFD.
This is explained by examining the schematic of the power circuit of a typical PWM VFD (Fig. 3). The three-phase input feeds a six-diode rectifier bridge that charges the dc filter capacitor. That capacitor, in turn, is discharged by current flowing to power the motor.
|Figure 3 - In a typical PWM VFD, the three-phase input feeds a six- diode rectifier bridge that charges the DC filter capacitor. Since six pulses of current low during each cycle of the three-phase AC sine wave, this is commonly called a "six-pulse" rectifier.|
Since six pulses of current flow during each cycle of the three-phase ac sine wave, this is commonly called a six pulse rectifier. As shown in Fig. 2, a pulse of current flows into the dc filter capacitor whenever the voltage sine wave exceeds the dc bus voltage by 0.7 V (the voltage drop across a diode). Reactive impedance (XL) "in front" of the diodes helps to slow the rate of rise of current when the diode snaps ON. This helps explain the important benefits Of XL in harmonics reduction. But more about that later.
The term line harmonics refers to sinusoidal frequency components contained on the line voltage waveform. The components are multiples of the line input frequency (usually 60 Hz in the U.S.). A pure sine wave carries no harmonics, but harmonics are present in the distorted sine wave. The harmonics of primary concern from a PWM VFD are the 5th, 7th, 11th, and 13th. The frequencies of these harmonics would be 300 Hz, 420 Hz, 660 Hz, and 780 Hz.
Devices connected to an input ac line with these harmonics will act as if they are connected to a 60 Hz line with the actual line voltage, plus the harmonic frequencies with voltages associated with the magnitudes of the harmonics. The actual waveform will appear on an oscilloscope as a single, distorted wave. Fig. 4 shows a badly distorted (18% THD) voltage waveform with the following harmonic content: 5th harmonic (15%); 7th harmonic (6%); 11th harmonic (6%); 13th Harmonic (4%).
|Figure 4. - This very badly distorted (18% THD) voltage waveform has 15% 5th harmonic, 6% 7th harmonic, 6% 11th harmonic, and 4% 13th harmonic.|
Even harmonics (i.e., the 2nd, 4th, etc.) are not usually a problem because the waveform is symmetrical about the X-axis. Likewise, harmonics divisible by three (triplen harmonics) are not generally significant on a balanced three-phase system. High-magnitude even or triplen harmonics are symptoms of a line imbalance that should be found and corrected.
In most systems, higher-order harmonics, such as the 17th, 19th, 23rd, and so on, will be of lower magnitude than those mentioned above. The table on p 73 shows some typical values of the current harmonics as a percentage of the fundamental current for a six-pulse PWM VFD. Higher-order harmonics are further diminished by the distributed capacitance of the input wiring.
It is essential to distinguish current harmonics from voltage harmonics. While current harmonics are actually generated by the pulsing current flow mentioned previously, the flow of the current through the source impedance of the distribution system causes voltage distortion.
The percentage of current harmonics in a system is dependent mainly upon the following factors for a PWM VFD:
Six-pulse Rectifier Current Harmonics
|Figure 5 - Increase reactive impedance "in front" of a PWM VFD reduces harmonic currents. The 5th harmonic is reduced from 68% to 25% of DC current by increasing XL from 0.256% to 5.61%.|
|Figure 6. Where many VFDs are installed, connecting half the drives to a wye-connected transformer and the other half to a delta-connected transformer produces a 30 degree phase difference between input feeders to the two. The resulting twelve-pulse system helps to reduce 5th and 7th harmonic levels.|
An increase in reactive impedance in front of a PWM VFD helps to reduce the harmonic currents. As shown in Fig. 5, the 5th harmonic current is reduced from 68% to 25% of the fundamental current by increasing normalized XL from 0.256% to 5.61%. Reactive impedance can be added in the following ways:
There is little added benefit to increasing the %XL beyond about 6%. This means, for example, that line reactors will not provide much additional reduction in current harmonics if the cable from the source to the drive is long and already provides the required reactance. In fact, excessive reactance can cause more voltage drop. This can increase the current required to maintain the same load horsepower, which will actually result in an increase in harmonic levels!
Likewise, line reactors and isolation transformers used together will not usually result in a harmonic reduction benefit over just one or the other.
Phase-shifting of the VFD current pulses to build a twelve-pulse system can reduce the harmonics generated by PWM drives. Virtually all PWM VFDs in the 1 hp to 150 hp range use the six-pulse diode arrangement shown in Fig. 3. However, where many VFDs are installed, it may be preferable to connect the drives to two distribution transformers to reduce the 5th and 7th harmonics in the system (Fig. 6).
Note that both transformers have delta primary connections. One transformer has a wye secondary connection and the other one has a delta secondary. The output current of a wye-secondary transformer is 30 degrees out of phase with that of a delta-secondary connected to the same source. The pulses drawn by the VFDs connected to these out-of-phase secondaries will appear to be spaced every 30 degrees rather than every 60 degrees at the source. If the current drawn from the two transformers is equal, the 5th and 7th harmonics are reduced by 90% from a six-pulse configuration. However, if the loads are not equal, the benefit is reduced. (This drawback is overcome by some larger drives that can be built specifically for twelve-pulse power feeds.)
Once current harmonics are minimized by an optimum value for X, the percentage of current harmonics for the system will depend on the current capacity of the source transformer. A larger transformer will have more current capacity and the percentage of current harmonics will be less for the system.
Remember, however, that the source transformer is a factor in the %XL of the system. A larger transformer may make it necessary to have additional reactance between the source transformer and the drive.
Also, larger transformers supply greater fault current. Make certain that your system complies with applicable codes and the drive manufacturer's limits for maximum fault current capacity. Failure to observe these codes and limits can result in injury to personnel.
Recall that it is the flow of harmonic currents through a source impedance that causes voltage distortion. It should be clear that a reduction in source impedance to the point of common coupling (PCC) will result in a reduction in voltage harmonics. This may be done in the following ways:
While harmonics above the 13th have lower amplitudes than the lower-order harmonics, VFD distortion can produce harmonic voltages and currents that extend above 100 kHz, and some electronic systems are sensitive to even small amplitudes of these frequencies. Fortunately, the higher harmonics are often easy to mitigate. Low-pass filters, available from many drive manufacturers and third party sources, can be connected to the input line close to the VFD. These pass lower frequencies while shunting higher frequency components to ground. The cost and size of such filters is often quite reasonable. T-type tuned filters are available where harmonics of a known, high frequency are responsible for a problem.
The Institute of Electrical and Electronics Engineers (IEEE) has helped users of nonlinear loads define acceptable limits of current harmonic distortion and voltage harmonic distortion for various types of systems.
IEEE 519-1991 Table 10.1 lists low voltage system classification and distortion limits. Voltage distortion in general systems should be limited to 5% total harmonic distortion (THD), according to the standard. Special, critical applications should be limited to 3% THD.
IEEE 519-1991 defines current distortion limits for various ratios of source short circuit current (ISC) to load current (IL)- (The short circuit current for a transformer may be obtained by dividing the full load amps [FLA] by the output impedance [%Z].) The standard further limits total current harmonic distortion to 5% of the full load current of a transformer, unless the transformer is specifically designed for operation under loads with higher harmonic content.
As a practical matter, many users specify IEEE voltage and current harmonic distortion limits of 5% for their entire power system. (Or, for large systems, a complete branch served by a substation transformer.) They provide details of the proposed installation including the following:
A simple one-line diagram of the VFD system can provide this information for analysis (Fig. 7). Most VFD manufacturers have computer programs that can use this data to determine whether reactors, isolation transformers, or phase shifting transformers are required for calculated compliance with IEEE 519-1991.
Fig. 8 and 9 show the relationships among VFD horsepower, source capacity, total voltage and current harmonic distortion, and optimized XL values. These figures will help you estimate the voltage and current harmonic distortion in your installation.
The graphs show an estimate of the effect of increased line reactance and increased transformer size (for a given VFD load). For example, a 50 hp drive would meet both of the 5% limits under the following conditions:
|Figure 7. A simple one-line diagram of the VFD system can aid analysis. Most VFD manufacturers have computer programs that can use this data to determine whether reactors, isolation transformers, or phase-shifting transformers are required for calculated compliance with IEEE 519-1991.|
|Figure 8 - Adding line reactance and increasing transformer size (for a given variable frequency drive load) helps reduce current harmonic distortion.||Figure 9 - As in the case above, adding line reactance and increasing transformer size (for a given VFD load) helps reduce voltage harmonic distortion. In these two cases input reactors have been added to provide 5.6% XL between the PCC (point of common connection) and the input to the VFDs, and the source transformer has a full-load kVA capacity of at least eight times the horsepower rating of the connected VFD load.|
What follows are some practices, based upon the principles in this article, that have yielded successful installations for many users of variable frequency drives:
Document number D-5062