Harmonics are distortions of voltage and current that occur when a rectifier is used to convert AC to DC as occurs with a VFD. IEE-519-2014 sets the standard for harmonic distortion limits, but each utility company will set specific limits for their customers.
Harmonics explained in the simplest terms are multiples of the utility frequency.
In North America, the utility delivers power at 60Hz. Harmonics are simply electrical currents that occur at multiples of 60Hz. They are described as the numerical multiple, such as the 3rd (180Hz), the 5th (300HZ). All else being equal, the higher the harmonic, the more damage and disruption it's presence can cause. IEEE519-2014 recognizes this and lower amounts of higher harmonics are permitted in the standard.
Harmonic distortion, when allowed to propagate across the grid, can lead to unreliable grid infrastructure, equipment damage, costly fees, or even service termination by the utility company. Additionally, harmonic currents increase energy costs and unwanted heat which reduces the life of power distribution equipment.
Reactors resist rapid changes in current which stabilizes the current, and significantly reduces harmonic currents. They are most effective at reducing higher-order harmonics above the 7th order.
Because reactors also introduce an impedance, there is a slight voltage drop across them. This voltage drop and the diminishing returns that come with higher reactance, limit the amount of reactance we can use. A three percent reactor is considered the standard. In practice, some companies request 5% reactor, but the improvement in harmonic mitigation is negligible unless there is a certain problem with higher-order harmonics that is being solved.
A 3% choke on a 100 HP drive, provides 43% reduction in THD, and a 21% reduction in total current.
Reactors can be implemented on the line side (AC Reactor) or the DC Bus of the drive (DC Reactor). From a harmonics mitigation standpoint, there is no practical difference between the two. However, the AC or Line Reactor, as it is often referred to, has the advantage of providing a level of protection for the rectifier from line-side disturbances. The DC reactor offers no such protection.
While inductive reactors are generally not a complete solution to harmonic issues, the low cost, simplicity, increased efficiency, and added protection of AC line reactors make them the recommended first step for all drives of every size.
The passive filter works by providing a low-impedance path-to-ground for the particular harmonic frequencies it is is tuned for. It consists of a primary inductor with relatively high impedance as well as a shunt-tuned reactor which is in series with capacitors.
The tuned reactor is designed to allow higher frequencies to pass through it and into the capacitors which capture the harmonic currents keeping them off the grid, while the fundamental current continues unabated.
Standard VFDs use a 6 pulse full-wave rectifier to supply the DC bus. The rectifier and the intermittent current draw that it employs per phase is the primary source of harmonics. A 12-pulse drive draw current through a special phase-shifting transformer to power a second 6 diode rectifier.
The current supplied to the second rectifier is phase-shifted 30 degrees, which causes harmonic currents to cancel each other out. It is especially effective with those of the 5th and 7th orders.
The 18-pulse drive uses an even more expensive phase-shifting transformer to power 3 separate 6-pulse rectifiers that are 20° apart.
This cancels the 5th through the 13th harmonics and is able in theory to eliminate the harmonics for the 5th through 13th and reduce the THD to around 5%.
Under ideal conditions, the 18-pulse drive can deliver results that meet IEEE-519 standards. However, like the 12-pulse, the harmonic performance is tied to the phase to phase voltage balance and the loading of the drive. If either of those conditions is less than ideal, the harmonic mitigation performance of the drive will deteriorate significantly. Additionally, the special transformer that powers the drive requires forced air cooling which necessitates regular maintenance to remove dirt build-up around the transformer. The expensive transformer and the forced-air cooling it requires, make the 18-pulse drive a poor fit for dirty environments such as the oil field.
The AFE instead uses an inverter bridge made up of insulated gate bipolar transistors (IGBTs.) IGBTS are high-speed electronic switches. These same switches are the same type the drive uses to create the AC current that regulates the speed of the motor, so the AFE essentially consists of two drives. The AFE controls the input IGBTs so that a clean sinewave is drawn from and output back to the mains, and yields a THD as slow as 2-3% for harmonics in the 3rd to 50th range, which is well below IEEE-519 standards.
Unlike the multi-pulse drives, AFEs are immune to voltage imbalances and can achieve excellent results at any load level.
Currently, AFEs are the only technology that can provide a voltage boost for the DC bus when needed. Other benefits include being able to achieve unity power factor and handle regenerative power.
Harmonics are a serious problem for utilities supplying the oilfield. But they are a problem with a solution. Let's talk about the solution that's right for you.
