ELECTRIC POWER CONSULTANTS, LLC - 1821 Curry Road ! ... The maximum amount of a harmonic current from a three-phase drive is 1/(harmonic order).
Harmonic Misconceptions Conrad St. Pierre - Electric Power Consultants, LLC There have been many harmonic articles in trade magazines and the problems that can occur because of them. Due to limited space and the technical level of the readers, many simplifying generalizations are made. These generalizations become the rule, because they are repeated articles after an article, without explanation or technical detail. These rules can be misapplied and point to harmonics as the culprit when they are not. The following are samples of these rules, which are true for some conditions but not true for all conditions. The reason for some of these rules will be explained. The maximum amount of a harmonic current from a three-phase drive is 1/(harmonic order). That is, the maximum 5th harmonic is 1/5 or 20% of the fundamental. This is a theoretical rule that applies for the following particular set of conditions. 1.
The drive is a properly operating 6-pulse full-wave bridge drive operating at full-load with a step wave output. The 12-pulse rectifier bridge drive will have greatly reduced harmonics.
2.
The DC bus voltage or current is constant. - meaning it has sufficient inductance or capacitance.
3.
The source impedance is low.
A good exception to this rule is a PWM (pulse width modulated) drive witha constant magnitude varying pulse width output. These drives can have the 5th harmonic current up to 80% of the fundamental current with a low series or internal reactance. The 5th harmonic current decreases to approximately 22% with a higher series or internal impedance. The percent harmonic current also dependents upon the impedance being on the input ac or dc portion of the converter. Figure 1 shows the change in harmonics on a PWM drive when external impedance is added.
Percent Fundamental Current
100.0
80.0
5th 60.0
7th 11th
40.0
13th 17th
20.0
19th 0.0
0.1
1
10
Percent Impedance (Source plus Reactor or Transformer)
Fig. 1 - PWM Drive Harmonic Content
ELECTRIC POWER CONSULTANTS , LLC - 1821 Curry Road ! Schenectady, NY
12306 ! Ph 518-356-9665
Triplex harmonics do not flow through a wye-delta transformers. The fact is that triplex harmonic currents can flow through and do flow through wye-delta transformers. The amount that flows depends on the phase angle and magnitude of the currents in each of the three phases. For example, 100% of the third harmonic current would pass through the transformer if they were equal in magnitude and displaced 120o from each other. This flow of current would be just like a balanced three-phase 60 Hertz current as shown in Fig 2. On the other extreme, if the third harmonic current produced is in phase with the third harmonic currents in the other two phases, the third harmonic current will add in the neutral of a wye connected transformer. Most of third harmonic currents in a 60-Hz system are the latter. The third harmonic current in the wye legs of a transformer will circulate in the delta winding to balance the amp-turns. Figure 3 shows this condition. This misconception is caused by the generalization that all third harmonics behave similarly to fundamental frequency zero-sequence currents. A 60-Hertz three-phase system can have positive, negative and zero sequence currents when the phases are not balanced. Likewise, the third harmonic currents can have positive, negative and zero sequence currents. When the third harmonic current in each phase is balanced and displaced by 120o, the current is positive sequence. When the third harmonic currents are balanced and inphase with each other, the current is zero sequence. In most three phase systems the third harmonic current produced are closer to being in-phase, but with a slight unbalanced in magnitude and phase angle. Therefore, most of the third harmonic current will flow in the transformer neutral and the remainder through the transformer.
1.0
1.0
0.0
0.0
-1.0
-1.0
TIME
1.0
1.0
0.0
0.0
-1.0
TIME
-1.0
TIME
TIME
1.0
1.0
0.0
0.0 1.0
-1.0
-1.0
TIME
TIME
0.0
-1.0
TIME
Fig. 2 - Out-of-Phase Third Harmonic Current Flow (Displaced by 120 degrees)
ELECTRIC POWER CONSULTANTS , LLC - 1821 Curry Road ! Schenectady, NY
12306 ! Ph 518-356-9665
1.0
1.0
0.0
0.0
-1.0
-1.0
TIME
TIME
1.0
1.0
0.0
0.0
-1.0
-1.0
TIME
TIME
1.0
1.0
0.0
-1.0
TIME
1.0
0.0
0.0
-1.0
TIME
-1.0
TIME
Fig. 3 - In-of-Phase Third Harmonic Current Flow (Displaced by 0 degrees)
Harmonic currents can cause an induction motor to run backwards. Besides running backwards, it is commonly said that motors will overheat due to the additional horsepower required to overcome the harmonic "torque fight.” - a term used to describe the backward torque on the motor. This misconception comes from the fact that the 5th harmonic currents have negative sequence characteristics. By using the simple equivalent circuit for an induction motor shown in Figure 4, the torque due to a 5th and 7th harmonic voltage can easily be quantified. S
XS
R
R S = 0.003 PU X S = 0.08 PU
V
RM
XM
R
R
SLIP
R R = 0.01 PU X R = 0.08 PU R M = 20 XM = 5 SLIP = 0.01 PU
Fig. 4 - Induction Motor Equivalent Circuit
Figure 5 shows the per unit torque for an induction motor subject to 10%, 5th and 8.5%, 7th harmonic voltages caused by adjacent SCR controlled drives. The harmonic voltage will result in 12% 5th and 6% 7th harmonic current in the motor based on rated current or the total rms current being 101% of rated. The ELECTRIC POWER CONSULTANTS , LLC - 1821 Curry Road ! Schenectady, NY
12306 ! Ph 518-356-9665
negative torque of the 5th harmonic at approximately rated speed is approximately 0.003% of the fundamental torque, (Trq . I2*RR/s = 0.122*0.01/6 = 0.00003). Also, the 0.00001% forward torque produced by the 7th harmonic will cancel a potion of the backward torque produced by the 5th harmonic voltage. Clearly, the motor with this very low negative torque will not run backwards. The additional fundamental current required to supply the "torque fight" phenomenon is insignificant, but the increase in eddy currents and hysteresis losses within the motor due to harmonics are not. Many IEEE papers have quantitatively addressed this temperature rise and they appear to indicate a greater possibility of motor overheating due to unbalanced voltage rather than from harmonics currents.
Per Unit Torque
4.0
-6
-5
-4
-3
-2
-1
Fundamental Torque
3.0 2.0
7th Harmonic Torque 1.0 0.0
0
1
2
3
4
5
6
7
8
Per Unit Speed (Base 60 Hz) -1.0 5th Harmonic Torque -2.0 -3.0 -4.0
Fig. 5 - Induction Motor Torque
Harmonic filters are always required when VSD (Variable Speed Drives) are used. Several factors should be considered before a harmonic filter is installed. If the amount of drives is small (
