There are many products that address power quality problems. Some devices, such as the surge arrestor, are good at providing protection against specific problems, while other devices, such as power conditioners, can provide protection from a wider variety of problems. The following provides a brief overview of the most common products used for power quality protection.
Autotransformers, unlike an isolation transformer, have the primary and secondary coils built onto a common core and offer virtually nothing in terms of power conditioning unless they are integrated into a voltage regulator or power conditioner. They are an inexpensive way to change one voltage into another (e.g. to convert 208V to 120V).
A battery is an energy-storage device. The heart of the UPS is its battery. Batteries are typically dry-type (sealed) lead acid batteries and wet-type (open) lead acid or nickel-cadmium batteries. Batteries in power conditioning devices provide electrical backup in the event of an interruption, but they are relatively expensive, and have limited correction time capacity and a short lifespan (discharge/recharge cycles).
A buck-boost transformer is a small, dry-type transformer with a dual-voltage primary and a dual-voltage secondary. Output voltage can be bucked (decreased) or boosted (increased) by 5% to 20% by de-energizing the system and manually changing the internal wiring to the unit. These units are good for adjusting output voltage levels when input voltage remains fairly constant. Buck-boost transformers cannot automatically adjust voltage, and they are not capable of correcting power quality problems other than seasonal voltage fluctuations.
Constant Voltage Transformer (CVT) – See Ferroresonant Transformer
Electronic Tap Changer
Also known as electronic tap switchers, electronic tap changers connect all taps of an isolation or autotransformer to the output through SCRs (Silicon Controlled Rectifier) that act as switches to ensure that only one tap is connected to the output at any given time. Incoming voltage sensors feed information to a microprocessor that controls the SCRs and output voltage.
These units offer fast voltage correction because they have no moving parts and can non-sequentially jump from one tap to another. Some versions may have overload-capacity and power-factor limitations.
The “ferro” or CVT works by having a portion of its core in a saturated condition. Transformers create a magnetic field, or flux, that is a function of the applied voltage. To a large extent, the magnetic flux is proportional to the voltage except at high or low voltage extremes. At voltage extremes, a relatively large change in voltage will result in little change in the magnetic flux, with the high voltage extreme described as the “saturation” region. To better understand how this works, picture a large tank of water with a small outlet pipe at the bottom. As water flow into the tank changes, the flow of water out of the small pipe will remain fairly constant due to the large volume of water in the tank.
The majority of ferros are used in applications below 1.5 kVA, although some models accommodate up to 25 kVA.
Ferros can provide good noise attenuation and are reliable. On the downside, ferros can add significant harmonic distortion, falter at times of high inrush current (>170%), and be inefficient at less than full load. They also tend to be hot and noisy during operation.
Harmonic filtering means removal of harmonics from incoming power. Passive filters are designed to remove harmonics of specific frequency and amplitude, but provide little protection when the harmonic’s characteristics change. Active filters (compensators) analyze incoming power for deviations from the desired waveform. When a harmonic is detected, the unit superimposes an opposite signal upon the waveform, canceling the harmonic. Passive filters are relatively inexpensive, but they have limited use. Active filters are expensive and are used in special circumstances where harmonics are a problem. In most cases, harmonics are reduced to tolerable levels through isolation transformers or other power conditioning devices.
An inverter is a device that converts DC power to AC power – a basic component of the UPS and other power conditioners. On its own, an inverter has no power conditioning capability.
An isolation transformer electrically isolates each winding so that current will not pass from one winding to another via conduction. This isolation prevents minor input transients and noise from being transmitted to the output. Isolation transformers are often fitted with shielding between the primary and secondary to further suppress noise and transients. These units are primarily used to step up or step down voltage. They offer some protection from noise, but little else in the way of power conditioning.
The term “line conditioner” is most commonly used to refer to a voltage regulator or power conditioner used in audio, video, graphics and computer applications. Although a line conditioner typically serves the same purpose as a power conditioner, it tends to be used at smaller KVA ratings.
Line isolation means isolation between the primary and secondary sides of an isolation transformer.
Mechanical Tap Changer
Mechanical tap changers use a motor to drive mechanical components to change taps or alter the turns ratio. Generally, the motor moves a set of brushes or contactors from one position to another to physically engage each tap.
Mechanical tap changers can achieve very precise output voltage regulation (<1%) and are relatively inexpensive. On the other hand, these units are slow in making large voltage corrections due to mechanical limitations and the necessity of sequential tap changing. Modern electronic devices tend to require correction times that are much shorter than those provided by mechanical tap changers.
The term “power conditioner” refers to a voltage regulator that is capable of more than simple voltage regulation. In addition, power conditioners often provide varying degrees of line isolation, noise attenuation, surge suppression, harmonic filtering, and phase balancing.
Primary refers to transformer input. Secondary refers to transformer output.
A rectifier is a device that converts AC power to DC power – a basic component of the UPS and other power conditioners. On its own, a rectifier has no power conditioning capability.
Sag ride-through products are designed to counteract (ride-through) electrical sags. These products can be classified as energy storage devices or non-energy storage devices.
Energy storage devices store energy electrically or mechanically and discharge this energy to boost or augment voltage when a sag occurs. These devices store energy in capacitors, batteries or rotating masses. They are expensive and have limitations on how long or frequently they can correct sags.
Non-energy storage devices draw extra current from the system, even at extremely low voltages, to synthesize the voltage necessary to counteract a sag. These devices are substantially less expensive than energy storage devices, and some provide sag correction without limitations on time or frequency of occurrence.
Surge Arrestor (Suppressor)
Surge arrestors are metal oxide varistors (MOV) that clip (truncate) voltages above a predetermined threshold. Essentially a non-linear resistor, the surge arrestor is placed between a phase and ground. At normal voltage levels, the resistance of the surge arrestor is very high so that negligible current flows to ground. At voltage levels above the threshold, the resistance of the surge arrestor is low to create a short circuit and divert the surge to the ground. Surge arrestors provide valuable protection against dangerous voltage levels.
Tap changing refers to the process of changing taps on a transformer to adjust output voltage.
To change the taps in a typical transformer, the unit must be de-energized and isolated, the unit opened, and the wiring or cable physically moved from one set of taps to another. These units are known as off-load tap changers.
Units that can change taps while maintaining power to a load are known as on-load or on-line tap changers. These units are typically mechanical or electronic.
The normal function of a transformer is to change an input voltage level to a different output voltage level, e.g., 480v to 208v. The voltage transformation ratio is determined by the numbers of “turns” on the primary and secondary sides of the transformer (turns ratio). Because the exact voltages in any given application can vary, transformers are frequently provided with “taps” that permit the turns ratio to be adjusted. A tap is simply an electrical connection to a transformer coil. By having taps above and below the nominal number of turns for a given coil, the turns ratio can be altered and the output voltage changed. With two 2.5% taps above and two below the nominal turns ratio, the output voltage can be adjusted by as much as ±5%.
TVSS or Transient Voltage Surge Suppressor – See Surge Arrestor
Uninterruptible Power Supply – UPS
For small single-phase applications, the uninterruptible power supply (UPS) is often the power conditioner of choice. The UPS can provide power conditioning as well as battery backup in the event of a power interruption. This permits time for ride-through of the interruption or for safe backup or shutdown of systems. The UPS must be sized for the peak current required by downstream devices and again for the length of time that full load current is required for system operation or shutdown. The physical size and cost of the UPS grow quickly as the size of the load and “battery time” increase. For large three-phase applications, UPSs can be room-size units requiring sophisticated maintenance and monitoring systems to prolong battery life. UPS battery replacement can be required as often as every two years, cost as much as 65% of the initial cost of the unit, and, depending upon the battery type, may require special disposal of the old batteries.
UPSs come in four basic versions: the standby (or offline), the line interactive, the ferroresonant and the double conversion.
The standby UPS only offers protection in the event of a long sag or an interruption. In normal mode, the standby battery and inverter are off, and the downstream devices operate off of unconditioned line power. When a sag or interruption occurs, a mechanical switch turns on the UPS to provide power to downstream devices. Once the event is cleared, the UPS goes back to standby charge mode. Many standby UPSs also incorporate surge protection. These types of units are frequently used to protect individual devices, such as workstations.
The line interactive works much the same way as the standby UPS does, but it is capable of providing some degree of voltage regulation (buck-boost) of line power for downstream devices.
The ferroresonant UPS replaces the buck-boost portion of the line interactive UPS with a ferroresonant transformer that provides a higher degree of line power conditioning for downstream devices.
The double-conversion UPS provides the highest degree of power conditioning of any UPS. Although the previous three types of UPSs utilize batteries and an inverter in the event of sags and interruptions, the double-conversion UPS uses two inverters. This UPS takes incoming AC power, converts it to DC, and then converts it back to AC power. In the event of a sag or interruption, the output inverter uses DC battery power to supply the load. The double conversion, with backup provided by batteries, eliminates many power quality problems, although it comes at a high initial cost, operating costs, and maintenance costs.
One of the primary functions of voltage regulators and power conditioners is maintaining the voltage level for downstream devices within a certain set of limits. For example, one unit might have an input range of +10%/-25% and a regulation to ±5%. This means that if incoming voltage is within the nominal voltage plus 10% and nominal voltage minus 25%, then the unit will regulate voltage output to within nominal voltage plus or minus 5%. If the nominal voltage is 480 volts, as long as input voltage is no higher than 528 volts and no lower than 360 volts, the output voltage will be between 504 and 456 volts. Input range and regulation vary by manufacturer, technology and price range. Large input ranges and small regulation ranges tend to have higher price tags. Most applications can be satisfied regulation of ±3 to 5%. Mechanical tap changing voltage regulators frequently offer output regulation in the range of ±1/2 to 1%, although these types of units have a significantly slower response time than other technologies.
A voltage regulator adjusts incoming voltage to ensure that output voltage levels fall within a required range.
The voltage stabilizer is the British equivalent of the voltage regulator.