The TV announcer for an MLB game attributed a team’s losing streak to a failure in implementing the fundamentals. The pitchers weren’t getting the ball in the strike zone, the infielders weren’t getting in front of the ground balls, the outfielders weren’t watching the ball off the bat to start running in the right direction and the batters weren’t picking up the spin on the ball and following it to the bat. These are all basics taught to Little League players, but someone needed to remind the professionals if they were going to change Ls into Ws. 

The same is true for trying to solve power quality problems. Fortunately, the fundamentals always apply, no matter how simple or complex the cause and effect of the PQ problem is. In addition, it doesn’t matter if you are new to the field or have been doing troubleshooting and benchmark surveys for decades. In that vein, here are a few fundamentals to help win the battle with PQ phenomena.

Know the laws

While I have only dedicated 5% of my articles over the past years to Ohm’s and Kirchhoff’s laws, they are indispensable and should be well understood. They apply to every situation, whether it involves voltage variations, harmonics, transients, noise or unbalance. While most people just think of Ohm’s Law in terms of resistance and the relationship between voltage and current, it also works with real-world situations of impedance. Rarely are circuits and loads purely resistive—the capacitive and inductive characteristics should also be accounted for in the analysis and formulation of solutions. 

Kirchhoff’s Laws cover voltage and current. Voltage sags result when the current causes a voltage drop in all of the impedances between the source and load, and Kirchhoff’s voltage law can determine what the impact will be on the loads. If the current measurements of all the conductors into a node don’t sum out to zero as per Kirchhoff’s current law, look for a leakage path somewhere.

Following the NEC doesn’t mean good PQ 

The purpose of the National Electrical Code is clearly spelled out in the beginning, where Article 90 states it is for “the practical safeguarding of persons and property from hazards arising from the use of electricity, … contains provisions that are considered necessary for safety … but not necessarily efficient, convenient, or adequate for good service, (and) is not intended as a design specification or an instruction manual for untrained persons.”

Some aspects of the Code do promote good PQ practices, such as bonding the neutral (grounded conductor) and ground (grounding conductor) at only one point at the service entrance, except where there are separately derived sources. This helps prevent ground loops and the potential to raise the voltage on the equipment grounding conductor by having current that normally flows in the neutral to flow through the grounding conductor. As above, current through impedance generates voltage. 

However, the requirements are based on safety, not PQ. For example, supporting of cables is based on distance and current within the cables that can generate heat and degrade the insulation. Stapling or bundling multiple cables together results in coupling of the voltage and current fields. Electrical voltage fields decrease as the square of the distance, and current-causing magnetic fields decrease as the cubed value of the distance. Devices such as cable clip stackers can help provide that distance to reduce coupling effects from noise, transients and harmonics, but aren’t required by the Code. Isolating “offending” cables in properly grounded conduit can further reduce the interference.

Being recorded doesn’t make it real

In the past four decades, power quality instruments have improved greatly through innovation by design engineers and in conjunction with PQ standards. The latter provides a means for verifying that what is measured is done accurately. Not all instruments in use fully comply with the standards. A simple example is when measuring current harmonics. 

Two things are needed. The current probe or transducer (CT) must be able to pass the signals over the full measurement bandwidth required and the measuring circuitry must limit its measurements to that bandwidth. Until recently, that bandwidth was often 2 kHz. If higher harmonics are present and not filtered, they can “fold back” and show up in the harmonic spectrum being measured. Likewise, the newer standards go to 9 kHz. If the sampling rate of the analog to digital conversion isn’t at least twice that rate for each channel, the signals wouldn’t be accurately measured. When measuring on higher voltage circuits, the primary current transformers being measured on their secondary output often were not designed with harmonics in mind.

The instrument isn’t the only contributor to nonreality data. Improperly matching CTs with the proper voltage and direction of current flow produces bad data. Not connecting the voltage probes to an electrically clean and secure measuring point can produce negative, probe-created voltage transients, not electrical system issues.

“Fundamental” can be defined as “serving as, or being an essential part of, a foundation or basis.” That goes for PQ too. Forgetting the fundamentals can lead to prolonged investigations or worse if there is a significant financial impact from implementing the wrong solution or more lost productivity.

JOHN / STOCK.ADOBE.COM