Category: Education
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25.7 Introduction to protective relaying
Circuit breakers used in residential, commercial, and light industrial service are self-tripping devices: they internally sense the amount of electric current going through them, automatically opening when that current exceeds a pre-determined level. Circuit breakers used in medium-voltage (2.4 kV to 35 kV) and higher-voltage applications, however, must be triggered to trip by external devices. This remote-control…
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25.6 Electrical Sensors: Potential Transformers (PTs) and Current Transformers (CTs)
The two “process variables” we rely on most heavily in the field of electrical measurement and control are voltage and current. From these primary variables we may determine impedance, reactance, resistance, as well as the reciprocals of those quantities (admittance, susceptance, and conductance). Other sensors more common to general process measurement such as temperature, pressure, level, and flow…
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25.4 Single-line electrical diagrams
Electrical power grids primarily consist of three-phase AC circuits. This means most power lines (transmission and distribution) have at least three conductors, and power transformers are either three-phase units or banks of single-phase transformers connected in Delta and/or Wye primary and secondary winding configurations. Of course, diagrams must be drawn to document how all these conductors and…
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25.3 Interconnected generators
Any power grid large enough to meet the demand of an entire nation must have multiple generators drawing from multiple energy sources supplying that power. Connecting these generators together so as to equitably share loads in the grid is no trivial task. The basic concepts involved with interconnecting generators, however, are independent of the distance…
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25.2 Understanding Electrical Power Grids
The term “grid” refers to the conductors and equipment interconnecting power sources to power loads in a wide-spread electrical system. Generating stations (i.e. “power plants”) convert various forms of energy such as fossil fuel, solar, wind, elevated water, and nuclear into electrical power; which is then sent through step-up transformers to raise the voltage and…
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Chapter 25 Electric Power Measurement and Control Systems
Electrical power is a commodity in the modern world, bought and sold on the open market like any other. Thus, it is important to be able to measure and control electricity, not only for reasons of efficiency but also for sale, taxation, safety, equipment protection, and reliability of service. As with any other quantity we…
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24.4 Mechanical vibration switches
A much simpler alternative to continuous vibration sensors (displacement or acceleration) and monitoring equipment suitable for less critical applications is a simple mechanical switch actuated by a machine’s vibration. These switches cannot, of course, quantitatively analyze machine vibrations, but they do serve as qualitative indicators of gross vibration. The following photograph shows a Robertshaw “Vibraswitch”…
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24.3 Vibration Monitoring hardware
The following photograph shows a large air blower in a wastewater treatment facility equipped with a Bently-Nevada model 3300 vibration monitoring rack (located left-center on the foreground panel): Five vibration measurement and display cards are installed in this rack, each card capable of processing up to two displacement sensor signals. A six-channel temperature monitor card…
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24.2 Vibration sensors
Sensors used to measure vibration come in three basic types: displacement, velocity, and acceleration. Displacement sensors measure changes in distance between a machine’s rotating element and its stationary housing (frame). Displacement sensors come in the form of a probe that threads into a hole drilled and tapped in the machine’s frame, just above the surface of a rotating…
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Chapter 24 Machine vibration measurement
Unlike most process measurements, the measurement of a rotating machine’s vibration is primarily for the benefit of the process equipment rather than the process itself. Vibration monitoring on an ammonia vapor compressor, for instance, may very well be useful in extending the operating life of the compressor, but it offers little benefit to the control of the…
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23.12 Safety gas analyzers
Process analyzers measure the concentration of specific substances for the purpose of measuring and/or controlling those concentrations in a process stream. Safety analyzers detect the presence of dangerous concentrations of specific substances to warn personnel of threats to life or health. While there is virtually no end to the different types of process analyzers in…
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23.11 Analyzer sample systems
Some analyzers measure the composition of a process stream by directly immersing the sensing element in that stream. This is called in situ measurement, which is a Latin phrase meaning “in the place.” A pH probe inserted into a process pipe, an oxygen probe inserted into the stack of a combustion furnace, and a GFC analyzer measuring…
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23.10 Fluorescence Measurement
Recall that an exothermic chemical reaction is one that releases a net sum of energy, as opposed to an endothermic reaction which requires a greater input of energy than it releases. Combustion is a common class of exothermic reactions, with the released energy being very obviously in the forms of heat and light, with heat being the predominant form.…
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23.9 Fluorescence Measurement
When a high-energy photon strikes an atom, it may eject one of the lower-level electrons from its shell, leaving a vacancy to be filled by one of the electrons already residing in a shell higher than the vacancy but lower than that of the ejected electron. When that medium-level electron falls down to fill the…
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23.8 Laser spectroscopy
A laser50 is a light source emitting waves of light that not only share the exact same frequency (color), but are also in-phase with each other. A beam of light consisting of just one frequency (color) is called monochromatic. A beam of light consisting of waves in perfect synchronization with each other is called coherent. Laser light is both…