Have you ever paid lots of money for a product that failed to perform? Perhaps you bought a carpet cleaner that left stains? Or a sleeping pill that kept you awake? How about a weight-loss miracle that actually added pounds?
Failed solutions are everywhere, and they’re no fun.
The same thing happens on a larger scale when companies invest millions in a technology that turns out to be obsolete. Their intentions are good and they invest in good faith, but the results are sadly disappointing.
Take for example, the problem of electrical harmonic distortion—that “noise” that happens when non-linear power loads draw current with a waveform that doesn’t match the supply voltage waveform. It’s an increasingly common problem, and most oil and gas companies are searching for smart solutions.
Some configurations attempt to mitigate harmonic distortion with a twelve-pulse rectifier operation. In this configuration, harmonics are controlled when voltage is filtered through a 12-pulse transformer. Three-phase power passes through a phase-shifting rectifier to a six-pulse converter bridge on each of the two outputs. When the current passes through the transformer there is a phase shift of 30 electrical degrees.
A 12-pulse circuit produces very little harmonic current below the 11th harmonic, and so the effect is passive cancellation of the fifth and seventh harmonics in the core of the transformer.
On the surface it may appear that this circuit is a cost-effective solution, but when we consider the pricing and installation costs for each of the necessary components, the value is diminished. And don’t forget the cost to maintain the safety of the system; to guard against mineral oil leaks or spills.
But even beyond the cost of purchasing, mounting and maintaining the cumbersome equipment, the bottom line is that it’s not a solution at all.
Here’s why: Voltage balance is very critical with the multi-pulse solutions; if the phase-to-phase voltage is not perfectly synched, the solution is compromised. Whenever one phase emits more or less voltage than others, the current varies and harmonics are not effectively cancelled.
With this method, the best result is10-12 percent current distortion, but 8 percent current distortion is generally what IEEE 519 standards will accept under practical operating conditions.
The sad truth is, some companies spend a lot of money for such an ineffective solution.
We have seen customers who have purchased incomplete “packages” that lack the most essential component: the transformer. As remarkable as this sounds, it happens all too often. Some customers are uninformed as to what it takes to make systems work, and unscrupulous salespeople take advantage of their lack of knowledge. That’s how they’re able to sell 12-pulse drives without a phase-shifting transformer. In effect they’re selling six-pulse drive systems with a fancy name -- and lots of harmonic distortion.
To mitigate negative harmonic effects, you may have investigated multiple solutions, such as active front-end variable frequency drives, passive harmonic filters and 18- or 24-pulse drives. Out in the field, you may have even seen older technologies such as those ineffective 12-pulse drives.
Confusing? SPOC Automation is ready to solve your problems and support you with expert assistance. We specialize in manufacturing leading-edge VFDs and in providing technology solutions to harmonic distortion.
Ready for some real solutions? Give us a call and let’s start a conversation today.