Fonte (Source): Consulting – Specifying Engineer
Por (By): Debra Vieira, PE, LEED AP, ATD, CH2M
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Many industrial, commercial, and service businesses are sensitive to power quality problems because they affect a company’s ability to compete in a global economy.
- Explain the definition of, and need for, high-quality power.
- Identify the costs related to a power quality disturbances.
- Recognize the value of gathering and interpreting data from power monitoring systems.
The generally accepted definition of clean power is “current and voltage waveforms that are purely sinusoidal.” However, this clean, or high-quality power does not have to be absolutely sinusoidal. So, what is the definition of high-quality power? Does the mere presence of harmonics on a power system indicate poor power quality? What about intermittent transients? Is that poor power quality?
Technically, there is no single accepted definition of “quality power.” Standards exist that help define criteria that can be measured, such as voltage. However, the real measure of power quality is determined by the performance and productivity of end-user equipment. If the equipment is not performing correctly, verification of proper mechanical and electrical installation and maintenance is necessary. A faulty piece of equipment, bad bearings, or poor internal connections can affect performance. If this doesn’t resolve the problem then power quality is most likely inadequate.
When we discuss power quality, what we really mean is voltage quality—because it’s the quality of the voltage that we can address. Power is the rate of delivery of energy and is proportional to the product of the voltage and current. The equipment itself—not the distribution system—defines how much power is drawn from the system. The power system defines the quality of the voltage delivered. Because there is a close relationship between voltage and current, we must address the current to understand many of the power problems that exist. For example:
- A short circuit can cause a voltage sag—or cause voltage to even disappear completely—due to extremely high current passing through the system impedance.
- Lightning generates high impulse voltages that can travel on the power distribution system.
- Distorted currents from harmonic loads also cause the voltage to distort as the current passes through the system impedance.
Sensitive electronic devices
We are heavy users of electronic devices, which are inherently sensitive to power quality. This sensitive equipment accentuates the limitations of the power system, limitations that have always existed but were rarely observed with less sophisticated equipment. Power quality issues are not easily identified. Determining how much of equipment malfunction and downtime is due to poor power quality is difficult to analyze. A power monitoring system that provides wave capture with a time stamp of the power anomaly, along with equipment status, can help facility operators correlate the cause and effect between power events and equipment malfunctions.
Since the advent of electricity, reliable, high-quality power has been desirable. In the late 1980s, computers became commonplace in our offices and homes. In the 1990s, we were able to network this equipment together to increase equipment performance.
Today, we face new problems, such as faster processing speeds, increased computer chip densities, and equipment that is more sensitive to the quality of power it receives. Factories, offices, hotels, shopping centers, hospitals, and homes depend heavily on microprocessor-based loads, such as lighting controls, computers, copiers, appliances, scanners, control systems, monitoring devices, etc. It’s difficult to find equipment that lacks a microprocessor. While this electronic equipment is relatively small in size and power consumption, it is large in quantity and is in close proximity to one another.
We are interested in power quality because of its economic impact. An increasing majority of industrial, commercial, and service businesses are sensitive to power quality problems because they affect a company’s ability to compete in a global economy. Businesses that depend on high quality power, such as information technology or the continuous process industry with its programmable logic controllers, distributed control systems, industrial computers, human-machine interfaces, variable frequency drives (VFDs), motion controllers, and sensors, can suffer huge financial losses along with loss of productivity and competitiveness when power disturbances occur.
The costs related to a power quality disturbance can be categorized as direct costs, indirect costs, and inconveniences.
Direct costs: include reduced equipment efficiency, loss of raw material and production, equipment/product damage, corrupt data communications/storage, and nonproductive employee wages.
Indirect costs: more difficult to quantify and may include missed delivery deadlines, which may cause future orders to be lost.
Inconvenience: Items in this category are not expressed in lost revenue dollars but rather in how much someone is willing to pay to avoid having to deal with the inconvenience.
Ultimately, the end user is responsible for preparing appropriate performance criteria for the equipment as well as for the proper installation and correction of inadequacies in the power and grounding system. Unfortunately, many end users are unaware of the installation pitfalls and need assistance.
Many times, the local utility company can provide guidance on how to properly install sensitive electronic equipment as well as modifications to the power and grounding system. The utility is motivated to provide customer service in regard to power quality to help build and maintain confidence in its distribution system. Utility engineers can provide troubleshooting analysis of harmonic issues because of the connection requirements stipulated in IEEE 519-2014: Recommended Practice and Requirements for Harmonic Control in Electric Power Systems, which defines the maximum allowed reflective harmonics allowed from a customer. However, some specialized equipment may be beyond the abilities of the utility and may require the assistance of an engineer who is knowledgeable in power quality issues.
Having quality power is not accidental; it is well planned and carefully installed. The basics of power quality start by creating a solid foundation of grounding, bonding, and wiring; then layer on surge protection and power conditioning; and lastly, monitor power quality trends to provide a baseline for preventive maintenance activities to correct internal power quality problems and to define external power quality problems (see Figure 1).
Grounding, bonding, and wiring
Around 80% of all power quality problems are related to grounding, bonding, and wiring problems within a facility. Is this percentage exaggerated? Possibly, but many power problems are resolved simply by fixing a few grounding connections or replacing a couple of grounding cables.
Grounding and bonding are not the same. However, they are closely related. Why do we ground and bond electrical systems? The answer can be found in NFPA 70-2014: National Electrical Code (NEC), Article 250.4(A)(1) Electrical System Grounding, which states “Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.” Furthermore, grounding stabilizes the voltage to earth during normal operations for grounded systems. Electrical equipment is grounded in accordance with Article 250.4(A)(2) Grounding of Electrical Equipment, which requires that “Normally noncurrent-carrying conductive material enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials.”
Noncurrent-carrying conductive materials that may become energized are bonded together to establish a low-impedance ground-fault current path in accordance with NEC Article 250.4(A)(3) Bonding of Electrical Equipment, which indicates that “Normally noncurrent-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that established an effective ground-fault current path.” Bonding of equipment facilitates the operation of overcurrent devices, such as fuses, circuit breakers, or relays, during fault conditions as required by NEC Article 250.4(A)(5). Multiple parallel paths result in low impedance to facilitate overcurrent device operation.
Electrical systems do not need to be grounded to function. In fact, not all electrical systems are grounded. But when discussing electrical systems, usually the voltages are with respect to ground. The term “ground” represents a reference point—or a zero potential point—to which all other voltages refer. A zero reference voltage is critical for proper operation of electronic equipment. Unfortunately, the least amount of attention is focused on grounding, bonding, and wiring. Typical wiring and grounding problems include:
- Faulty, loose, or resistive connections that result in heating, and potential arcing and burning of the insulation
- Missing equipment grounds that create situations where there is no effective return path for ground-fault current
- Power supply conduits insulated from computer cabinets with PVC conduit fittings
- Conduit runs that end before reaching the equipment with a conductor running exposed the remaining distance
- Green, insulated grounding conductors that are run separately from the phase conductors.
Another common grounding problem is using an isolated ground with the idea of obtaining a clean ground. Isolated grounds are typically misunderstood and misapplied because they are not actually isolated, but rather insulated, thereby eliminating parallel return paths. Installation of an isolated ground conductor is governed by NEC Article 250.146(D) Isolated Ground Receptacles and is used to reduce electrical noise on the grounding circuit by purposely insulating the receptacle from the mounting means. The receptacle grounding terminal is instead connected to an insulated equipment grounding conductor run with the circuit conductors, and is permitted to pass through one or more panelboards without connection to the panelboard grounding terminal bar. This isolated ground conductor terminates at the source of the separately derived system or service. The conductor is not isolated from the electrical grounding system.
Sometimes isolated or dedicated grounds are recommended by equipment manufacturers. These recommendations can compromise the safety and performance of the equipment, are dangerous, violate the NEC, and are unlikely to solve power quality problems.
Safety and equipment performance depend on the proper selection and installation of the power and electronic equipment grounding and bonding system. In all circumstances, the equipment and grounding system must comply with the NEC and local installation codes. A solidly grounded ac system with insulated equipment grounding conductors should be used to feed electronic loads. All metal parts of equipment enclosures, raceways, and grounding conductors are to be effectively and permanently bonded to each other and to the power system grounding electrode system at the service entrance and at each separately derived system.