PQ FAQs

From the power user perspective, power quality may be defined as any electrical parameter or connection that affects the operation of the equipment. This includes all electrical parameters, connections and grounds, and can be influenced by utility power, local electronic equipment and other electricity users.

From the power quality market or industry perspective, power quality is defined as any product or service that is supplied to users or utilities to measure, treat, remedy, educate engineers or prevent power quality issues, problems and related items.

Power quality is measured by the consistency of the voltage supplied at your meter. The amount of voltage available may fluctuate or change due to controllable or uncontrollable circumstances. Measuring, recording and diagnosing power quality problems requires specialized equipment, as power quality awareness is not typically a consideration in designing standard local electric infrastructure.

1.3) Why is power quality such a problem today? Top
Today's electrical power is much more reliable than it was ten years ago. However, today's high-tech equipment and control units are much more sensitive to electrical disturbances and are more important to the critical functions of many businesses. Therefore, power quality problems today are more frequent and more costly than ever before, despite the overall improvements in electricity reliability. A simple example is digital vs. analog clocks - a momentary power "blink" will disrupt a digital clock, but not an analog model.

Any type of electrical equipment can be affected by power quality problems. Standard building systems such as lighting, HVAC, and communications equipment are frequently affected. However, the most costly power quality problems often involve more sensitive high-tech equipment, such as computer and data systems, which can be affected by their direct power supply or through interactions with other electrical equipment.

Are you experiencing premature failures of electronic equipment or motors and drives?
Do you have adjustable speed drives that frequently trip off-line?
Do your computers lock up inexplicably?
Do your computer monitor screens jitter?
Do you have equipment problems during or shortly after lightning storms?
Are uninterruptible power supplies frequently going to battery?
Are circuit breakers tripping without being overloaded?
Are transformer cases extremely hot to the touch?
Does equipment frequently malfunctions at the same time of day?
Do automated systems fail for no apparent reason?
Do certain electronic systems work in one location but not in another location?
Do lights dim or flicker?

2.3) Are power quality problems always noticeable? Top
No - in many cases, disturbances can cause imperceptible damage to circuits and other components, a phenomenon known as "electronic rust." This is a major cause of premature equipment failure and problems like computer lockups. Unfortunately, many power quality problems go unresolved until a significant monetary loss is actualized, and even then masked problems like failed electrical/electronic equipment hide the underlying power quality problems that are the root cause of some of these failures.

Internal factors: Approximately 80 percent of electrical disturbances originate within a building. Potential causes may include large equipment start-up or shut-down, improper wiring and grounding flaws, overloaded circuits or harmonics.

External factors: About 20 percent of power quality problem originate within the utility transmission and distribution system. Although utility companies generate a pure, steady electric power, many things can happen as it travels to your facility. The most common causes of external power quality problems include lightning strikes, equipment failures, vehicle accidents, weather conditions, neighboring businesses, and abnormal operation of utility equipment. External causes are not necessarily from electrical lines - any wire entering a building can cause problems, including communication and data lines, telephone and coaxial lines, and satellite connections.

3.2) What are the types of electronic irregularities that cause power quality problems? Top
Voltage fluctuations (Sags and Swells): Changes or swings in the steady-state voltage above or below the designated input range for a piece of equipment. Fluctuations include both sags and swells. They can be caused by large equipment start-up or shut-down, sudden change in load, improper wiring and grounding, utility protection devices. Fluctuations typically range from 3 to 30 cycles and can be single or three phase.

Transients: also called surges, glitches, spikes and impulses, are sub-cycle disturbances of very short duration that vary greatly in magnitude. When transients happen, thousands of volts can be generated into the electrical system, causing problems for equipment down the line. Causes include lightning, normal operation of utility protection devices, equipment start-up and shut-down, welding equipment. While lightning generates the most impressive transients, switching large motors and transformers or capacitors creates transients sufficient to destroy today's delicate microprocessor based equipment. Note that common industry practice is to refer to transients as "surges," even though the word "spike" is more descriptive. The term "surges" refers technically to longer duration over-voltages.

Noise: high-frequency interference caused by a number of factors, including arc welding or the operation of some electric motors. Causes include operation of electronic equipment, welding equipment, radio and television broadcasts, radar transmitters, loose electrical connections, thermostats, heaters, variable frequency drives.

Harmonics: the periodic steady-state distortions of the standard sine wave due to equipment generating a frequency other than the standard 60 cycles per second. Causes include electronic ballasts, non-linear loads like switch-mode power supplies used in computers, variable frequency drives. In general harmonics cause magnetic portions of the electrical system to overheat. Such as transformers, line reactors, magnetic relays and power factor capacitors.

Power outages: total interruptions of electrical supply. Ice storms, lightning or wind are some of the more typical causes of power failures. Utilities have installed p2rotection equipment that briefly interrupts power to allow time for a disturbance to dissipate. For example, if lightning strikes a power line, a large voltage is instantly induced into the lines. The protection equipment momentarily interrupts power, allowing time for the surge to dissipate.

4.1) How can power quality problems impact a business' financial performance? Top
Power quality is an increasingly important issue for all businesses. Problems with powering and grounding can cause data and processing errors that affect production and service quality.

Maintenance: Reacting to a voltage disruption can include everything involving restoring production, diagnosing and correcting the problem, clean-up and repair, disposing of damaged product and, in some cases, environmental costs.

Hidden costs: If the impact of a voltage sag is a control error, a product defect may be discovered after customer delivery. The costs of losing repeat sales, product recalls and public relations can be significant and hard to quantify.

4.2) How much does it cost to fix power quality problems? Top
It's difficult to estimate an average cost, because of the wide variety of causes and solutions. Instead, it's often more useful to think of power quality services in terms of an investment - in almost every case, the amount invested in improving power quality is paid back in improved performance, production and even energy consumption. Many businesses see a significant payback in just a few months.

5.3) How are power quality problems fixed? Top
The solution to a power quality issue depend on the nature of the problem, but may include the installation of power-conditioning devices such as uninterruptible power supplies, transient voltage surge suppressors or isolation transformers. Other potential solutions include facility rewiring or replacement of damaged or degraded equipment.

Critical consumer loads are usually protected with a back-up source of power, commonly referred to as Un-interruptible Power Supply (UPS) systems. There are three UPS system types: static, interactive standby, and rotary. Static UPS systems are composed of a rectifier, batteries and an inverter. Interactive standby UPS systems differ from static UPS design in that a ferroresonant transformer is added to the inverter output. The ferroresonant transformer acts as a "magnetic flywheel" to store energy for up to four milliseconds A rotary UPS system is similar to the interactive UPS system except that the ferroresonant transformer is replaced with an AC motor and an AC generator.

5.6) What is the purpose of emergency systems and how do they differ from standby systems? Top
Emergency systems are intended to supply electric power for essential life safety requirements. This includes some levels of lighting, fire detection, elevators, fire pumps, and communication. Standby systems are defined as power supply systems that help fire fighters or other building hazard prevention that prevents financial loss in the event of electric service interruption. Power is generated using engine-generator sets or storage batteries and AC inverters. Engine generator sets are commonly required in hospital settings and batteries with AC inverters (UPS systems) are commonly used in large computer installations.

5.7) What is the difference between a TVSS, a UPS and a SPS? Top
Transient suppression is a common technique for reducing or mitigating transient voltages. A Transient Voltage Surge Suppressor (TVSS) is like a very fast switch. It is the most common type of power conditioning equipment.

TVSSs have been around in their current form for many years. The basic concept of a TVSS is to either limit the peak transient voltage through absorption, or divert the energy to another path. TVSSs are available in many different sizes and are typically one of three different technologies. These are gas discharge tubes, silicon avalanche diodes, and metal oxide varistors.

For protection inside facilities, the most common technology is metal oxide varistors, or MOV. These can be purchased in sizes ranging from stand alone products for receptacles to large systems for panel or service entrance applications. For lightning protection use equipment that conforms to ANSI and UL guidelines only. The IEEE Emerald book provides additional information regarding the use and installation of lightning surge arrestors.

An uninterruptible power supply, or UPS, is a device that provides continuous, acceptable power to its loads regardless of the input power supplied. UPS systems come in many types and sizes.

A UPS is used to provide clean, conditioned continuous power to critical electronic equipment. It can protect the electronic equipment from most conditions experienced on the power system. The UPS will provide power to the load even in the event of a total outage. The UPS supplied power will last only as long as the systems battery bank will allow. These systems are most commonly used to protect computer systems from momentary outages such as those experienced during stormy conditions.

The way a UPS works depends upon what type of unit it is. There are four basic types of UPSs on the market. They are: ferroresonant, line interactive, double conversion, and rotary units. While each type has unique features, each will contain the following:

Rectifier or Charging Unit - takes the utility AC power and converts it to DC. Also charges the batteries.
Inverter - takes the DC from the rectifier or batteries and converts it to AC for use in the computer system.

The rotary UPS system uses a motor generator (MG) set in conjunction with a static ups. The MG is used to provide total isolation from the external supply source. Under normal conditions the MG would supply 100% of the power for the load. In the event of an outage the MG would continue to spin for a very short period due to inertia. This would allow enough time for the inverter to come on line and supply power to the motor generator. The rotary UPS offers the highest level of protection, but is the most costly unit as well.

A ferroresonant UPS utilizes a ferroresonant transformer to condition and regulate the incoming power. When there is an outage, the unit will utilize the stored energy in the ferroresonant transformer to supply power until the inverter is brought on line by the battery bank. In this unit the inverter only comes on in the event of an outage. Ferroresonant units are used almost exclusively on single phase applications.

The line interactive UPS uses a bi-directional inverter, battery bank, power conditioner and an onboard computer. The bi-directional inverter runs continuously. The inverter supplies AC power to the power conditioner, and charges the batteries at the same time. The amount of load the inverter handles is controlled by the on board computer. In the event of an outage the inverter is supplied by the battery bank and handles all of the load.

Double conversion UPS systems use a rectifier charger, battery bank, and inverter. In this unit the inverter is continuously operating and supplies 100% of the load. If utility power is available the inverter power is supplied by the rectifier charger. In the event of an outage the inverter is supplied from the battery bank, with no interruption to the load.

A stand-by power supply, or SPS, uses utility power to supply electricity to computers under normal conditions. When utility power is unavailable due to a momentary interruption, the SPS continues to supply power through a battery system for a limited time. These units are available for small computer systems such as personal computers or work stations.

The main reason to purchase an SPS is to protect against the momentary outages or "blinks" that often accompany storms and windy conditions. The SPS will provide you with enough time to save data and shutdown your PC in an orderly fashion.

The electricity you receive from the utility is alternating current or AC. Batteries such as the one in your car, provide direct current or DC. The SPS takes the AC power from the power company and supplies it directly to your computer as long as it is available. When the AC gets too low or disappears, the SPS automatically switches over to an onboard battery. The SPS converts the battery DC to AC for use in your computer. The length of time that Q power is available from the battery will depend on the size of the battery and the load of your computer. It is important to remember that the SPS does not condition the power in any way during normal utility operation.

When considering an SPS there are three questions that must be answered. First, how long does the battery need to keep the system running, and how long does it take to store up sufficient energy? Let's say there's an interruption once a day, and a backup time of at least 5 minutes is needed. If the SPS provides the 5 minutes, but takes 3 days to recharge, then it will not meet the stated needs.

Second, what is the transfer time to go from source to battery? Depending on this time, the SPS may or may not keep the system running during an interruption. If the electronic load can only ride-through an interruption of 8 milliseconds, and the SPS requires 10 milliseconds to transfer, then this SPS does not help.

Third, what determines when the transfer is made from source to backup, and from backup to source? It is possible that the sensing circuitry may respond to sags which the load could easily ride-through. By responding, though, it uses some of its energy. After some amount of time, the next sag or real interruption may find the SPS unable to provide any energy, and the load crashes.

The objective of a power quality audit is to find a solution to a problem. This is accomplished by checking wiring and grounding, which causes a significant amount most commercial and industrial problems, and checking the quality of both the internal and external voltage. This data is then used to determine an appropriate solution.

Does the facility's wiring and grounding meet code and, more importantly, is it adequate to meet your power needs from a power quality standpoint?
What is the quality of the ac voltage supplying the equipment?
What is the impact of the electric utility's power system?
What solutions are indicated by the audit data?

The vast majority of disturbances that affect the flow of data within and between electronic devices come from inside a facility. In fact, the cause of most data flow problems can be tracked down to wiring and grounding.

Anytime sensitive loads are used, great care should be taken to ensure the wiring and grounding are appropriate. Every aspect of the power system should meet, or exceed, the National Electrical Code requirements. It is important to realize that the Code is not concerned with power quality, rather its concern is with safety. If the Code says rigid metal conduit systems make an acceptable ground, it's OK to still run a separate ground conductor.

In the case of local area networks (LANs), grounding is even more important. In computers, the internal digital reference is usually tied directly to the input power ground. Therefore, the digital reference of PC #1 on the LAN is the outlet it is plugged in to. The same applies to PC #2. If a voltage difference exists between these two ground points, then current will flow through the data cable connecting the two PCs. This can disrupt data flow, cause data errors, and even damage the I/O ports of the PCs.

If wiring is found that violates the Code, it should be fixed immediately. Oftentimes, bringing the electrical system up to Code resolves many power quality problems. ConEdison Solutions PQRx team can audit your site to find any violations to electrical system code or violations that could impede on your facility getting the proper risk mitigation from power quality incidents.

5.10) Why do my lights go off, come back on a few seconds later and then go off again? Top
The utility power distribution system is not unlike the wiring in your home. There are large lines to carry large quantities of power and smaller lines for smaller quantities. These lines are protected by fuses and circuit breakers, just like the lines in your home.

If something comes in contact with the power line, such as a tree limb or small furry mammal, the power line reacts to this "short circuit" by increasing the current flow to and through the line.
The increased current flow is sensed by a circuit breaker. The breaker interrupts the power to the line for a very short time - usually less than a second - to allow the short circuit to go away. This protects the line from serious damage and prevents a longer term outage. After the short interruption the breaker resets itself and restores power to the line.

If the circuit is okay again the power stays on. If the short circuit is still present, the process is repeated to give the line one more chance to clear. If the line doesn't clear then the breaker trips and must be manually reset after the line has been checked and or repaired. This is why you will often see the lights blink twice before they go out for longer periods.

Occasionally, utilities must change the way power feeds into the lines serving your home. This may be due to a temporary overload, damaged equipment or maintenance work being done on the lines. When utilities make the switch you may experience a short power outage - again usually less than a second. This outage is more than enough to cause your computer, VCR or clock to lose data or at least the information stored in its Random Access Memory.

Proper use of surge protectors, or transient voltage surge suppression (TVSS) equipment will help protect equipment from all transients except a direct lighting strike. It is a good idea to install TVSS devices to protect all your sensitive electronic equipment. Here are a few symptoms you can look for to determine if you have had a transient condition:

TVSS equipment acts as a voltage sensitive switch. It constantly monitors incoming voltage. Under normal conditions nothing happens, but when the voltage difference between the power source and the protected line gets high enough, the switch closes and diverts the transient away from your equipment. This switching voltage is often called the "clamping voltage" of the TVSS.

There are two basic varieties, plug-in and hard-wired. Plug-in TVSS units are plugged into wall outlets to protect specific appliances or electronic devices. Hard-wired TVSS units are used as a primary defense for the entire building or to protect sensitive circuits and are installed by an electrician at your service entrance or in sub-panels within your building.

6.1) Are there national or international standards of Power Quality? Is Power Quality regulated by government standards? Top
There are many standards that relate to the parameters of power quality. Several of the more important standards and references include:
Ansi c84.1-1995 - Electrical Power Systems and Equipment Voltage Ratings (60 Hz)
Ansi c65.1 - Electric Power Systems and Equipment Voltage Ratings (60 Hertz)
IEEE Standard 100-1996: IEEE Standard Dictionary of Electrical and Electronics Terms

Power Quality – Frequently Asked Questions