Central Maine Power is proud of its reputation for providing safe, reliable, electric power. However, severe storms, lightning, high winds, power equipment failures, cars hitting utility poles - even small animals climbing on utility wires - can cause power line disturbances.
Electrical load changes within your facility can also affect the quality of power. There are a number of things you can do to protect your sensitive electronic equipment from power disturbances. This information has been prepared to help you understand the causes and results of these disturbances and how to protect against them.
Equipment in your home or office that can be affected includes computers, digital clocks, answering machines, VCR’s, electronic cash registers and security systems. Other equipment impacted by power quality problems include energy management systems, variable speed drives and phone systems.
Most electrical devices can tolerate short-term power disturbances without any noticeable effects. However, more serious power disturbances can cause data loss, memory loss, altered data, product loss, and other functional errors-as well as equipment damage. These problems often cause expensive downtime, inefficiency, lost orders, scheduling problems and accounting problems. It is often necessary to troubleshoot to determine the cause of these problems. Having the right kind of power protection for your electronic systems becomes more important every day. It is difficult to predict when a minor power-related problem might become a major problem for your home or business.
Since power disturbances are almost always intermittent problems, they can be difficult to identity.
Once a problem has been isolated as a power problem, it is important to identify the type of power disturbance so that the cause can be found and a solution can be implemented. Sometimes identifying the cause of a power disturbance can point to a low or no-cost solution.
The kind of protection you need depends on the type of equipment you have and how you operate. Faults, which occur on the utility system or within your facility’s distribution network, may be caused by squirrels, birds, auto accidents, tree branches, overloads or other occurrences that trip a protective device. When faults occur, one, two, or all three phases of your electrical supply may be affected. However, you may not notice when only one phase is interrupted, a condition known as “single phasing.”
During single phasing, motors and other equipment may continue to run, but excessive current will flow in the two energized phases causing motors to overheat and possibly burn out. The only way single phasing can be identified quickly, before equipment is damaged, is to install a monitoring system to alert you. If you wait until you notice that your equipment is overheating, most likely the damage has already been done.
The National Electrical Code requires that three-phase motors and equipment have adequate electrical protection against power disturbances or interruptions. CMP’s customers are responsible for installing single phasing and ground fault protection for three-phase equipment to prevent damage in the event of a disturbance or interruption. Without proper protection, disturbances or interruptions, known as faults, may damage three-phase motors and equipment.
CMP strongly recommends that you review your motor protection with an electrician, the equipment manufacturer, or a consulting engineer to ensure that your equipment has the proper protection. The protective equipment should be sensitive enough to detect abnormal voltage conditions on any or all phases.
Types of Disturbances
There are three types of irregularities, which could affect your power supply:
1. Voltage fluctuations
2. Switching transients
3. Power outages
|| Voltage Fluctuations
In many states, including Maine, Public Utility Commissions have established allowable voltage tolerances for electric service. In Maine, these tolerances require that the voltage provided to residential and commercial customer’s facilities not exceed plus or minus 5% of the “expected voltage” for periods longer than one minute. For example, a residential customer’s expected voltage is 120. To stay within the requirement, CMP must maintain its voltage requirement of between 114 and 126 volts. For industrial customers, the normal voltage variation is plus or minus 10% of the standard voltage for any period longer than a minute.
Some electrical equipment has been developed with very sensitive power quality requirements. This equipment may be so sensitive that even fluctuations within the tolerance limits can cause problems.
Sometimes, power problems cause the voltage to fluctuate outside the allowable tolerance. These voltage fluctuations can usually be detected by a visible flickering of lights. Overvoltage or undervoltage conditions can result in damage to equipment, loss of data, and erroneous readings in monitoring systems.
Undervoltage, also known as voltage sag or “brownout,” can result from overloaded power circuits. Intermittent low voltage can be caused by starting large motors or by momentary tree contacts on power lines. CMP requires that any customer-owned motor not cause the primary voltage to decrease by more than 3 percent upon starting.
If the sag lasts for more than a few cycles, motor contactors, fluorescent lighting or programmable logic controllers may fail to operate properly.
Overvoltage conditions, also known as voltage swells, are less common but may be more damaging to equipment. These swells occur more frequently in facilities which rapidly switch large electrical loads off and on.
|| Switching Transients
Transients are by far the most common sort of power disturbance. Electrical transients are short duration disturbances typically caused by switching electrical equipment on and off. You may notice a similar situation if you are in the shower and someone turns on another faucet in your home. The water pressure in your shower is decreased momentarily while the system accommodates the increased use. This is similar to the disturbance on your electrical system. Transients are sometimes difficult to detect since they last such a short time.
|| Voltage Spikes
Short duration impulses, in excess of the normal or expected voltage are called spikes. Although the duration is very brief, a spike may significantly exceed the normal voltage. Spikes can wipe out data stored in memory, produce output errors, or cause extensive equipment damage. In addition to the noticeable immediate damage, spikes may also cause harder to detect problems such as reducing the service life of your equipment. Subsequent random failures can be particularly annoying and expensive.
One major day-to-day cause of energy spikes is the switching on and off of electrical motors. This is called inductive load switching. Air conditioners, electric power tools, furnaces, electrostatic copy machines, arc welders and elevators are particularly guilty of creating voltage spikes. The problems created by inductive load switching are very common in industrial plants.
Larger spikes are typically caused by lightning. Though the chances of a lightning bolt hitting your facility or machinery directly are small, a lightning strike several miles away may be transmitted through power lines and show up as a voltage spike in your facility. Grounding your equipment appropriately is your best defense against voltage surges.
|| Electrical Noise
Electrical noise is any unwanted signal traveling on electrical wires. Electrical noise can be caused by radio transmitters, fluorescent lights, computers, business machines, motors, dc drives, arc welders, heavy equipment, load switching, and loose electrical connections. Although some electronic equipment does have internal noise filtering capability, equipment located in very noisy environments may still encounter interference, such as glitches or malfunctions with computer, electronic, and communications equipment.
Electrical noise generators cause harmonic currents. Harmonic currents are currents that alternate faster than (on a frequency greater than 60 Hz) the standard current most equipment is designed for. These faster alternating currents appear as electrical noise to other devices on the same circuit. The only way to get rid of electrical noise is through the use of “dedicated circuits” and/or appropriate filtering equipment discussed later. IEEE Standard 519-1992 provides allowable harmonic limits for both customers and the utility.
Power Outages, often referred to as Power Interruptions, can best be defined as a complete loss of voltage for a few seconds or longer. Sensitive electronic equipment generally does not respond well to any type of power interruption.
Momentary (short duration) Outages generally range from less than one cycle to a few seconds. If the momentary interruption is caused by an event outside of ones home or business, the interruption is likely caused by a device known as a recloser.
A recloser turns off the power in response to a short circuit or an electrical fault on the utility system, commonly referred to as the power line. These faults are often caused by trees or animals coming into contact with the utility system. Though momentary outages are considered a nuisance, reclosers provide many benefits. In addition to protecting the public and utility personnel, reclosers are designed to reenergize the utility system if the fault was able to clear. This feature reduces the frequency of extended power outages.
Extended power outages are caused by larger scale problems, such as trees coming down on the utility system during a storm or vehicular accidents. The duration of the outage will vary based on the extent of the damages.
Power Protection: What you can do
Dedicated Circuit-Investigate Installing a Dedicated Circuit: computer equipment should be served from dedicated circuits that serve only computers. The dedicated circuit, with an insulated ground, should originate from the main switch. The dedicated circuit will reduce voltage fluctuations caused by other equipment.
When a variety of equipment shares a circuit, voltage fluctuations may be caused by the equipment switching on and off. By putting more sensitive equipment on its own circuit, you reduce the wear and tear and help to ensure the quality of power to that equipment.
There are two main categories of power protection equipment: power enhancement and power synthesis. Power enhancement equipment modifies and improves the incoming power while power synthesis equipment utilizes the incoming power as an energy source to create “new” power.
Power enhancers refine incoming power through a filtering process and work to overcome transient power problems. In many cases, different types of power enhancement devices are used in conjunction with one another to be sure that electrical problems are reduced or eliminated. Examples of power enhancers include:
These units protect against transients by reducing high-voltage impulses to a level that is safe for your equipment. Surge suppressors can cope effectively with virtually any transients, although actual capability depends on the quality of the unit. Because surge suppressors are relatively inexpensive, it makes sense to install them as insurance against potential equipment damage, particularly from lightning strikes.
CMP recommends installing a quality surge suppressor that has a UL listing, and a UL Standard 1449 voltage “clamping” label. The best units can “clamp” a voltage transient to under 300 volts. The surge suppressor must clamp transients between line to neutral and neutral to ground. This level of protection is adequate for most applications.
Power line filters are the most widely used method for electrical noise reduction. A filter would be designed for a specific electrical noise problem. The design of the filter may require technical assistance from the equipment manufacturer or a power quality consultant.
Quality voltage regulators maintain voltage output within narrow limits in spite of wide input fluctuations.
These units are specifically designed to prevent electrical noise from being passed through to the protected equipment.
Hybrid power conditioners combine two or more functions in one device. A power conditioner will combine the advantages of both voltage regulators and isolation transformers. Some may also add surge protection. The term “power conditioner” is applied rather loosely, so study the manufacturer’s technical literature before you buy to be sure the product is capable of performing properly for your situation.
Power synthesis takes utility power and changes or synthesizes it into “new,” undistorted power. Since this is a much more complex process, these devices are more expensive than power enhancers. Example of power synthesizers include:
Magnetic Power Synthesizer
This is a magnetic ferroresonant system made up of transformers, inductors and capacitors that reconstruct the desired AC output. This new output is free from all power line disturbances. These units do not store power, so they cannot protect sensitive equipment from outages.
This system buffers power to sensitive equipment by virtually regenerating it. It uses utility power to drive a motor, which, in turn, drives a generator, which provides electricity to the electronic equipment. Because motor-generators isolate the incoming power source from the eventual recipient of the power, they deal effectively with noise and transients on the incoming power supply. Motor-generators provide “ride-through” of momentary power interruptions up to one-half second, with actual capability varying with the design of the unit.
Uninterruptible Power Source (UPS)
Standby Power Source (SPS)
Although both an uninterruptible power source (UPS) and a standby power source (SPS) protect against blackouts, the cost and the degree of protection they provide differ considerably. A true UPS is always on-line, providing continuous, regulated and noise-free power under all AC-line conditions. An SPS, on the other hand, is always off-line and switches on-line only when AC power is lost. In fact, an SPS is sometimes called an off-line UPS.
An alternate power source, like a UPS, contains a battery and an inverter to convert DC to 120 volts AC. But that’s where the similarities end. An SPS also has a transfer switch. Normally, the inverter is at rest and the primary AC power passes through the switch to the equipment. However, when the AC voltage drops below a pre-set transfer point, the transfer switch transfers the load to the output of the inverter, which then supplies 120 VAC power to the equipment.
Several factors must be considered before selecting a UPS or SPS. If a power outage is the only problem to be rectified, an SPS provides the most economical solution. As a minimum, the SPS should meet the IEEE 587 standard.
However, if voltage fluctuations occur frequently (causing havoc with a computer being used in a more critical application) a UPS is the best choice.