What is Power Quality?
Power Quality has various definitions. For instance, a technical definition, based on the voltage/amperage/distortion of the sinusoidal waveforms in an alternating current system. As well as, a supplier management definition, which usually applied to a utility supplier by a customer.
While utilities don’t usually talk to their customers about providing a quality service, there are specific government requirements for delivering electricity to customers. Additionally, these indicators (SAIDI, SAIFI, and CAIDI) are defined in utility terms, not customer friendly terms:
Three of the Utility Reliability indices defined:
- SAIFI (System Average Interruption Frequency Index): Sum of outage Customers Interrupted/Total number of customers served.
- SAIDI (System Average Interruption Duration Index): Sum of outage Customer Minutes of interruption/Total number of customers served.
- CAIDI (Customer Average Interruption Duration Index): Sum of outage Customer Minutes of interruption /Sum of outage Customers Interrupted.
SAIFI measures how often a customer can expect to experience an outage. SAIDI measures average outage duration per customer. CAIDI measures average outage duration if an outage is experienced or average restoration time. These definitions are critical for customers to understand what to expect from their utility supplier.
(Look for a future APT blog post on these utility indices and how to work with your utility supplier to understand what they mean to you as a consumer and customer of your local electric utility.)
A more technical approach is to define Electric Power Quality as the degree to which the voltage, frequency, and waveform of a power supply system conform to established specifications, see Wikipedia (https://en.wikipedia.org/wiki/Electric_power_quality).
Furthermore, good power quality may be defined as a steady supply voltage that stays within the prescribed range, a steady Alternating Current frequency close to the rated value, and smooth voltage and current waveforms (comparable to a perfect sine wave).
Below find APT’s helpful definitions of ten (10) commonly used terms in Power Quality
10 Power Quality Terms
1. Voltage Sag
A voltage sag refers to the decrease in voltage for a short duration(usually less than two seconds). Several factors commonly cause voltage sags, including short circuits (faults) on the electric power system, large load additions by customer or utility, and motor starting.
Sags can cause some sensitive equipment and computers to shut down or malfunction. Undervoltage refers to sags that last longer than two minutes, which can then damage electrical equipment.
2. Voltage Swell
A voltage swell refers to an increase of more than 10% of the standard voltage levels (typically for short durations).
Voltage swells that last longer than 2 minutes are known as overvoltage. If it is too high can damage electrical equipment.
The typical cause of a swell is due to large load changes and power utility line switching. Although designed to limit voltage swells, utility voltage regulating equipment may not react fast enough to avoid damage.
Transients refer to a sudden increase in the value of voltages and currents that typically last less than ½ second.
The cause of transients can be from load switching, faulty wiring, lightning, or electrostatic discharge. All of these can damage electronic and electrical equipment. Additionally, transients can cause problems with computer data.
In the United States, the normal frequency of utility systems is 60 Hertz or cycles per second. For instance, frequency, displayed in sinewave graphs, can be viewed below.
Frequency is usually a stable factor in electricity production and transmission. It may cause a problem with smaller production generators. Subsequently, if this goes unchecked may lead to problems with electronic equipment and speed drives of motors.
5. Distortion (Harmonics)
Distortion, caused by harmonic frequencies added to the 60 Hertz voltage or waveform, may make the waveforms appear less rough or jagged.
The harmonics are caused by non-linear loads, typically found in computers, lighting, adjustable speed drives, and rectifiers. Consequently, distortion can cause problems with motors, computers, transformers, and conductors. Problems arise by components heating up and ultimately causing malfunctions to occur.
Interruptions may be momentary or sustained. They typically occur when voltage levels drop to zero.
Momentary interruptions are short duration (from 30 cycles to 3 seconds) on one or more phase conductors, which usually restore automatically. Subsequently, if momentary interruptions go unnoticed, they may cause computers to shut down and impact sensitive electronic equipment.
On the other hand, sustained interruptions are medium duration, defined as voltage loss (for 3 seconds to 2 minutes), which typically restored automatically. Futhermore, longer duration interruptions (of more than 2 minutes) result in fieldwork to restore the power.
7. CEBEMA & ITIC Curve
The CEBEMA curve, developed by the Computer Business Equipment Manufacturers Association, represents the withstand tolerance capabilities of computers in terms of magnitude and duration of voltage variations on a power system.
The curve intends to identify ways to provide system reliability for electronic equipment. Further, it is one of the standards used for measuring performance for all types of electronic equipment and power systems.
A newer curve called the ITIC ( Information Technology Industry Council) has been developed and offers additional information. For instance, an ITI curve looks at sags, swells, transients, interruptions, and steady voltage, designed to look for ways to improve electronic equipment reliability.
8. Nominal Voltage
Nominal voltage refers to a value assigned to a circuit or system to designate its voltage class (120,240,277,480 etc.). The actual operational voltage circuit that operates within can vary from the nominal voltage and still have satisfactory performance.
9. Linear Load
A linear load refers to an electrical load where the relationship between current and voltage waveforms are sinusoidal and the current at any time is proportional to the voltage (as when in a steady-state of operation). Examples of a linear load include motors, capacitors, and transformers.
10. Non-Linear Load
When a load becomes considered non-linear, its impedance varies with the applied voltage throughout the cycle waveform. Moreover, the changing impedance means that the non-linear load current draw will not be sinusoidal. These non-sinusoidal currents contain harmonics that interact with and distort the voltage of power distribution system equipment and loads connected to them.
On the other hand, typical non-linear loads are single-phase circuits connected to electronic lighting ballasts and power conversion technologies. For example, Switch Mode Power Supply (SMPD), found in most power electronic devices (computers, printers, monitors, electric vehicle chargers, telecom systems, etc.).
How APT Can Help With Power Quality
APT’s business focuses on helping customers work with their utility suppliers to understand the expected “quality of service” the electric utility will provide. This defines how to size generators, transfer switches, and Uninterruptible Power Supplies (UPS) to ride through interruptions and other deviations in service quality.
Furthermore, APT specializes in helping customers reduce costs and, in many cases, prevent impact to critical loads. Our twenty-five-year history as a service provider allows us to provide a unique and valuable service for customers concerned about their electric power quality of service and its impact on their operations.
Let APT help you and your operation speak the same language as your utility supplier and prevent poor power quality from affecting your critical loads.
Nick Skarvelis Senior Electrician APT