Power quality describes the electrical grid's ability to supply a clean and stable power supply, and the ability of your electrical equipment to consume the energy being supplied. Good power quality is characterized by a steady power supply that has a smooth sinusoidal waveform and is consistently within voltage and frequency tolerances. Poor power quality, on the other hand, may negatively affect your business and cause:
Malfunction or breakdown of machines
Electronic equipment overheating
High maintenance costs
So how can you tell if you have poor power quality?
The Verdigris Analytics Dashboard measures and tracks power factor, voltage, and total harmonic distortion (THD), all of which are primary indicators of power quality. Read on for an explanation of these metrics and how our analytics can alert you when problems arise.
A Motor Control Center (MCC) monitored by Verdigris EV1 at 8KHz sampling rate.
In this article, we use power quality screenshots from a commercial office building (350,000+ sq. ft.) located in the Pacific Northwest. We've selected panel MCC1 for illustration purposes, which is a 277V, 3-phase, 4-wire, Motor Control Center, as seen in Figure 1 above. All dashboards screenshots conform to the same time period (month of October) and data interval (1 minutely).
Power Factor (PF) is defined as the ratio of real power flowing to the load and the apparent power in the circuit, as shown in the power triangle diagram in Figure 2. Only real (or "true") power is used to do work; reactive power is not used to do work. In other words, excessive reactive power charges useless power to the system and results in low PF.
Power triangle diagram describes how apparent power = real power + reactive power.
Low PF means lower operating efficiency, which requires increased equipment capacity and results in more electrical losses. Low PF means higher reactive power; generally higher reactive power means higher voltage. Performance diminishes and expenses increase. Thus a higher PF is generally desirable, with the ideal PF of 1.00 (or 100%) in most cases, where apparent power and real power are equal. With that said, some devices such as motors, transformers, and lighting ballasts naturally cause reactive power, in which case a lower PF may be expected.
While utilities typically bill customers only for real power, many utilities may add a surcharge to your invoice (often called an "adjustment") if there’s excessive reactive power in order to cover the associated expenses, or they may add a credit to your account for a high PF as an incentive.
Utilities use a number of different methods to calculate your power factor adjustment but it's usually some combination of the adjustment charge or credit rate, total kWh above or below the threshold, and the calculated % Power Factor ($$ x kWh x PF). For example, one California utility requires a minimum power factor of 85% or 0.85, and a surcharge is added for each kWh used below this level. With Verdigris, you can identify when your total PF becomes too low and save money by isolating the exact location of the problem. You might even be able to convert a monthly expense into savings!
Verdigris "Classic" Dashboard shows building-level power factor along with a sorted list of top 5 significant contributors.
Our example building is potentially in trouble with their utility and should investigate the top contributors to the poor power factor.
Pro tip: For accurate comparison, make sure to select the same time interval in the Verdigris Dashboard as your utility data collection interval (1m, 15m, 1h, 1d)
Fortunately, there are a number of ways you can improve your power factor, and many utilities offer rebates to help cover the costs!
Resize electric motors
Add variable frequency drives
Install power factor correction capacitors
Resize step-up, step-down transformers
Replace older magnetic ballasts in lighting
Power quality is heavily dependent on the quality of the voltage of the electricity supply. Ideally, you're looking for a steady supply voltage that stays within a prescribed range. Higher than nominal voltage can be damaging to equipment performance and longevity, while lower than nominal voltage may cause brownouts and a reduction in productivity. The Max Voltage in your quick glance dashboard should always be within ±5% of the nominal voltage (120V or 277V in North America).
The Voltage Dashboard is shown below displays voltage by the hour of the day. Unsteady supply may indicate a problem. Voltage “sag,” or a drop below nominal voltage can indicate a heavy load or an undersized transformer. Ensure that your transformer has an output rating that matches or exceeds the amount of power required on that circuit. Voltage “swells” or “spikes,” which are sudden and abrupt increases in voltage can indicate large inductive loads being shut off. A daily, recurring and predictable sag could indicate a problem at the grid or utility level.
Verdigris Analytics Dashboard displays voltage (rms) trends of the selected panel.
Because AC voltage is a sine wave around 0, attempts to find the average value would yield 0. Hence, the root mean squared (RMS) voltage is used instead, providing an effective value for quantifying the voltage per phase.
Verdigris Analytics Dashboard displays phase unbalance % of the selected panel.
The three phases of the RMS Voltage, represented by each line in Figure 4, should be well balanced. Each phase should follow a similar pattern, and the difference between the highest and lowest voltage should never be more than 4% of the lowest phase. In Figure 5, we can see the average phase unbalance for ~0.5% without spikes on a minutely interval, this building's phases seem well balanced over the past week.
Total Harmonic Distortion
Total harmonic distortion (THD) is how much an electrical wave deviates from its fundamental sinusoidal form. Non-linear loads such as electrical motors, transistors, and variable frequency drives can draw current that is not perfectly sinusoidal, creating disturbances and distorting the voltage waveforms. Unwanted distortion can cause excessive heating and core loss in motors and increased power usage. Distortions in the power supply are especially damaging for manufacturing or research facilities where calibration is critical to the equipment’s performance accuracy. It is paramount for these facilities to receive clean power for equipment efficiency.
How a waveform becomes harmonically distorted by the harmonic disturbance of a non-linear load.
The less distortion the better. If the THD is greater than 0.03, there may be an issue in at least one of several levels. If you find that your entire building is affected, this could indicate bad power quality from your provider. If found at the panel level, this can indicate a load imbalance, a bad transformer, or even faulty equipment. Sharp changes in demand or supply can cause distortion in harmonics, and can often be traced to electrical equipment such as variable frequency drives or other motor speed controllers.
Verdigris Analytics Dashboard displays trended THD of the selected panel.
With the Verdigris System, you can gain immediate insights to the health of your building’s power quality and take action to prevent equipment damage and save money.