Power Quality at the service panel

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Understanding that these symptoms tell us something is wrong with the power system is helpful. But where, oh where, do you begin the search to isolate the exact cause of these power quality problems?

Just like cars have a single connection point to monitor vital functions, the electrical system has a similar connection point: the electrical service panel. As a common point for branch circuit distribution, the service panel is also a convenient place to take the pulse of your electrical system.

Several problems can be located right in the service panel itself. For issues elsewhere in the system, measurements at the service panel can tell you where to look next. A quick visual inspection can catch some problems, while others require that measurements be taken.

Finding the Cause of the Problem

This article outlines a step-by-step process for locating and fixing potential trouble spots.

1. Voltage level (steady state) and voltage stability (sags)
2. Current balance and loading
3. Harmonics
4. Grounding Hot spots: loose connections/terminals
5. Bad or marginal branch circuit breakers

Depending on the voltage and measurement requirements, you can use various tools for power quality troubleshooting, from digital multimeters to handheld single and three-phase power quality analysers that perform many calculations automatically.

Voltage Level and Stability

The first step in checking to see if voltage levels and stability are the culprits is to measure voltage levels of the branch circuits, phase-to-neutral, at the load side of the branch circuit breakers.
Note: For safety's sake, when making voltage measurements, always keep a circuit breaker between you and the fault current ampacity of the feeders.

If voltage levels are low at the breaker, they'll be even lower at the receptacle. This could be caused by low tap settings at the transformer. Other likely culprits include loose connections, long feeder runs, and overloaded transformers, which create excessively high source impedance (impedance from the load to the source). Source impedance and voltage drop are two sides of the same coin.

If intermittent voltage sags are suspected, start at the panel to isolate the cause of sags: Are the sags the result of loads on the same branch circuit, or are they caused by loads elsewhere in the distribution system (including utility-generated sags)? We can start isolating the sag’s source with a multi-channel recording instrument, such as a Fluke power quality analyser, that can trend voltage and current simultaneously.

Upstream, Downstream

What information are you looking for from the trendplots?

• If a voltage sag occurs simultaneously with a current surge, the sag is caused by a load on the branch circuit (Figure 2). In other words, the cause of the sag was downstream of the measurement point and, therefore, can be thought of as a load-related disturbance.
• If, on the other hand, the voltage sag coincides with a very small change in current, the sag was likely caused by something upstream of the measurement point and can be thought of as a source-related disturbance. Typical source-related disturbances are heavily loaded three-phase motors started across the line or sags originating on the utility feed. If the sag is deep and approaches an outage, the cause is more likely to be the utility. The event probably reflects a fault and breaker trip followed by automatic breaker reclosure.

Current Balance and Loading

To check the current balance and loading, measure each feeder phase and current on each branch circuit. When making these measurements, it is critically important to use a true-rms clamp or true-rms digital multimeter (DMM) with a clamp-on accessory. An average responding clamp-on meter will not provide an accurate measurement as the combination of fundamental and harmonic current makes this a distorted waveform. A lower-cost average-sensing meter will tend to read low, leading you to assume that the circuits are more lightly loaded than they are.

Harmonics

To check for the presence and level of harmonics, measure the current on the feeder neutral. This will typically be in the 80 to 130 percent range of the feeder current because the third harmonic will add up in the neutral. Figure 5a shows some readings made in an office at a lightly loaded panel. These waveforms were captured with a single-phase power quality analyser. Note that the neutral current (Figure 5b) exceeds what would be expected from unbalanced currents alone.

Although most of us are increasingly aware that third harmonic currents (also called triple or zero sequences) generated by non-linear, single-phase loads add up in the neutral, we often wonder why.

Grounding

Neutral ground bonds in subpanels are a violation of the NEC as well as of power quality performance wiring, but they are also quite common. Neutral ground bonds should be made at the transformer (although the NEC permits it to be made at the main panel). It should never be made downstream of the main panel, whether at a subpanel or a receptacle. When a neutral ground bond is made at a subpanel or receptacle, the ground path becomes a parallel return path for normal load current, resulting in measurable current on the ground.

Hot Spots

Poor connections and the resulting heat losses are the single most significant source of system inefficiency (according to a 1995 study by the then Washington State Energy Office). From the power quality point of view, loose terminations significantly contribute to excessive source impedance. Fortunately, they are easy to locate with a simple infrared thermometer.

Infrared (IR) measurements with tools like the Fluke 60 Series are a safe and effective technique for the non-contact detection of panel hot spots. However, some key concepts are crucial to understand if we make these measurements.

Circuit Breakers

A lot of people don't think of breakers as having finite lifetimes. In reality, contacts and springs wear out. Measurements of circuit breaker voltage drop can help us determine the condition of the breaker. Measure across the line-to-load side of the branch breaker. If the voltage drop exceeds 100 mV, the breaker should be replaced. Readings should be documented and trended in the 35 to 100 mV range.

In summary, the service panel is the crossroads of the building's electrical system, where an experienced electrical troubleshooter can start down the right path to locate and fix any problems.