AMAZON multi-meters discounts AMAZON oscilloscope discounts
Thomas Edison and his associates were incorrect in thinking they could build a large scale electrical distribution system that would work on direct current. The only way their dc system could function would be if it consisted of a huge number of small neighborhood-based power plants with large diameter copper conductors providing power to the individual buildings, not to mention streetlights and other loads. Moreover, there could be no such thing as three-phase power.
Electrical power is the product of amperes and volts. This means that for a given resistance in a length of power line and the load, if the applied voltage is increased, the amperage will decrease. Since the ampacity of wire is measured by the amount of current it can carry without overheating, it is clear that electrical power can be transmitted more efficiently at higher voltage levels. Therefore, the strategy will be to use higher voltages for long transmission lines, dropping the voltage to a more useable level at the destination.
The only ways to change dc voltage levels are electronically and by means of motor generator sets, neither of which is feasible for numerous high-power applications. AC voltage, on the other hand, can be changed efficiently and relatively inexpensively by means of transformers. The moving magnetic flux that is necessary for inductive coupling is created not mechanically but because of the changing voltage polarity (and consequently current direction) in the primary winding of a transformer. Large amounts of power can be stepped up or stepped down merely by varying the ratio of the number of primary to secondary winding turns.
Consequently, Tesla and Westinghouse prevailed, and today enormous ac electrical grids power the world. With a rated capacity of 830,000 megawatts, the North American Grid is the largest machine ever built. It provides electrical power for most residences and businesses in the United States, Canada, and parts of Mexico. A major design goal is reliability, and this is achieved by pooling the power from numerous generating plants. The electricity combines to make a single output for each of the three separate sectors of the North American Grid, in which the ac is synchronized and phase locked at 60 Hz. The three sectors-the Eastern, the Western, and the Electric Reliability Council of Texas-are linked by huge dc interconnects, so that when a single power source goes down, others make up the deficit. Considering the size and complexity of the Grid, widespread outages are few and, for the most, part brief.
The first step in servicing many types of nonfunctioning tools and appliances can be to fasten an ohmmeter to the plug. Stick the probes through the holes in the prongs and hold them in place with a wrap of electrical tape. Put the ohmmeter in honk mode and work the power switch or wiggle the cord. For a motorized tool such as a drill, you need to have continuity through the cord, switch, and associated wiring, and through the brushes all the way to the armature.
To understand how these connections work, we must look at the way in which two or more ac generators can be synchronized. To begin, ac power sources must be the same voltage and frequency. Furthermore, the phase sequences and angles must be identical.
Then, with the two sources electrically isolated (breakers in the OFF position) the speed and hence frequency of one of the sources is adjusted until the waveforms of the two coincide, at which precise moment they are placed together online so that they are connected in parallel.
Once they are so joined, the smaller power source tends to remain in synch with the larger one, and useable power is available from the combination.
Problem
Solving In this way, all ac power sources within each of the three sectors of the North American grid are phase locked, although the three sectors are not synchronized with each other because they are connected only by dc lines.
Some power sources, such as utility-scale photovoltaic arrays, produce dc output, which has to be changed by means of a synchronous inverter to ac of the proper voltage and frequency so that it can be synched into the grid.
Each subcircuit within the grid is composed of three parts: generation, transmission, and distribution. Providing the links between each of the stages and between elements within them are the ubiquitous substations that we see as we drive along roads anywhere in the world (see Fig. 5-1).
In addition, many substations are invisible to us because they are within buildings or underground. When substations are utility-owned and operated, they are not under National Electrical Code (NEC) jurisdiction; however, if they are privately owned within a large industrial complex that includes power transmission and distribution, then they are subject to NEC mandates. In either case, the design goals are essentially the same: transformation, short-circuit and overcurrent protection, switching, monitoring frequency and other electrical parameters, metering, lightning protection, capacitors for power factor correction, lighting, alarm, communication, etc.
Transformation is usually of three-phase power, often accomplished by an array of three transformers (see Fig. 5-2), or in the case of an open Delta, two transformers (see Fig. 5-3). I like to stand outside a substation enclosure and see if I can visually trace the wiring. This may seem like a strange affinity, but perhaps no more so than playing bingo or attending a rock concert.
Figure 5-2 Three-phase transformer array.
Figure 5-1 Substation with grounded fence.
When putting in a new service, put a very small dab of corrosion inhibitor on each terminal of the meter socket. This makes it easier to insert and remove the meter.
Short-circuit and overcurrent protection are overriding concerns. The principle is the same as in a small-scale service, but the breakers are very expensive, costing thousands of dollars. A difficult design objective is extinguishing the arc. If voltage and current are high, as the contacts open electrical energy will continue to flow between them because an ionized path through the air is established, becoming hotter and more intense even as the gap increases. This is essentially what happens in a lightning event, and various strategies are employed to extinguish the arc before it burns up the equipment. Some possibilities are compressed air pressure, oil or sand immersion, magnetic deflection, and mechanical intervention.
Concerning switching, it must be said that this capability is essential within a substation. It is necessary to isolate and de-energize discrete portions of the load so that repairs or new construction can be done without causing a widespread outage.
Metering is also essential within any substation. It is an important troubleshooting tool in attempting to pin down load irregularities. As customer loads become increasingly nonlinear, harmonics problems tend to surface, causing unwanted heating of the neutral.
Effective metering can help resolve such issues.
Lightning protection has progressed rapidly in recent years, and it has grown in importance as power lines become longer, with more customer outlets. There is a lot we still do not know about lightning, including the exact mechanism for the formation of charge clusters within clouds, and what triggers the release of such awesome quantities of electrical energy. Nevertheless, researchers have come to understand that damage to electrical equipment can be prevented by minimizing bends in transmission lines and, above all, creating low-impedance grounding at key locations, both utility- and customer-owned.
Capacitors may be placed online as needed for power factor correction. They function to compensate for large inductive loads that cause current to become out of phase with voltage.
Figure 5-3 Single phase transformer.
All of the substation properties, plus internal wiring, alarm, and communication must be documented and mapped out to facilitate new construction and upgrades.
SCADA and Remote Monitoring
Because substations are usually not maintained by fulltime on-site workers, there has to be a mechanism for remote monitoring. This is accomplished by Supervisory Control and Data Acquisition (SCADA). It is used in many applications including gas and oil refineries and pipelines, water treatment plants, wind farms, airports, and so on.
The substation can transfer data to the monitoring facility when solicited or on its own initiative, so this protocol is well suited for sensing as well as control. Numerous SCADA software programs are offered, and they can be simply installed in computers at the monitoring facility. These programs include provisions for PLCs, human interface capability with user-friendly graphics, and other bells and whistles. Often it is possible, when trouble arises, to take remedial action electronically without dispatching workers to the site.
Alternatively, if it is necessary to send a crew to the substation, they will know what tools and materials to take with them.
For substations as elsewhere in electrical systems, grounding serves more than one purpose. Principal benefits of good low-impedance grounding are as follows:
• It mitigates the harmful effects of lightning, which besides damaging substation equipment can create a voltage surge that will travel throughout the distribution network to impact customers' electrical equipment and threaten buildings and the people within them with the twin demons of electrical fire and shock.
• It makes for a reliable return path to the electrical supply to facilitate the operation of overcurrent devices.
• It stabilizes voltages so they will not float over or under the design standard, either of which will damage motors as well as sensitive electronic equipment.
Where there is an electrical distribution system of any significant size, there will be three-phase power. For many non-electricians, the idea is foreign and incomprehensible, mainly because it is not part of the ordinary residential setup. Moreover, most backyard mechanics, by and large a knowledgeable bunch, are surprised to learn that the familiar automotive alternator is in truth a three-phase generator, with an internal diode network that outputs a dc voltage appropriate for charging the car battery as well as powering the onboard computer and other dc loads.
Telephone servicing is much easier with three simple tools: a test set, tone generator, and wand. Some technicians use an ohmmeter instead, but the foregoing reveal sound quality issues as well.
For electricians and utility workers, the good news is that three-phase connections are simple to understand and, on a comparative kilowatt basis, actually easier to install because the wire sizes are smaller than for single phase. Troubleshooting and repair are straightforward-just look for the dropped phase.
Wye and Delta
Three-phase power begins at the generator, and its unique characteristics are due to the rotary nature of that power source. The three generator windings are spaced equally around the rotating armature or field, creating three outputs of the same frequency and voltage.
There are six wires emanating from the three coils, but within the generator housing or immediately outside, they are connected to make a three-wire or four-wire output. They are connected neither in series nor in parallel, but in either of two unique configurations known as Wye or Delta. Each of these has particular advantages for the supplier and end user.
In the Wye configuration, one end of each winding is connected to a common terminal, which is usually grounded and becomes the neutral. The other end of each winding becomes one of the three hot legs. Four wires make up this power supply, and single-phase power (using one of the legs and the neutral) and three-phase power are available in the same entrance panel.
The other somewhat less common but nevertheless very useful three-phase configuration is the Delta, so called because its schematic representation resembles the Greek letter Delta, an equilateral triangle with its apex at the top.
The center-grounded Delta has one winding center-tapped and grounded. The corner opposite the center-grounded winding is called the high leg because it has a higher voltage to ground than the other two legs. The high leg is always color-coded orange.
In another version of the Delta configuration-the open Delta-one winding is omitted, but there are still three hot legs. Because of this "virtual winding," the three voltages are present but the overall capacity is reduced.
If three single-phase transformers are used in a Delta bank to provide stepped-down voltage for customer use, and if one of the transformers fails, the configuration becomes an open Delta and continues to function with no outage on a temporary emergency basis, although at reduced capacity of 57.7 percent of full power until a replacement can be installed.
Three-phase power is 150 percent more efficient than single-phase power. Conductors are 75 percent the size of single-phase conductors. Motors, raceways, and other components are also smaller, so there are savings in material and labor. A problem that is encountered especially in rural areas, however, is that three-phase power is not always available from the utility, and costs for a line extension may be prohibitive for all but the largest facilities.
Eventually, there may be a move back to dc power, and Edison will have been vindicated, although in a way he never conceived. This is because of the enormous potential for photovoltaic power, produced locally, that could revolutionize the way electricity is brought to the home and business. At present, there are three models, with subdivisions and variations.
• Stand-alone systems are primarily seen in remote, off-grid locations. They are used for rural residences, where it is not feasible to bring in a utility line, and for isolated research facilities and data collection posts with environmental instrumentation and communication capability. Stand-alone PV systems are ideally suited for spacecraft where solar radiation is intense and continuous and the ambient temperature is low.
• Cogeneration systems are used where there is the potential for utility connection. The cost of a synchronous inverter is generally less than the cost of a battery bank.
• Utility-scale solar PV generation is seeing widespread use and as the price of solar cells continues to drop, it is realistic to expect that these installations will proliferate.
Solar technology falls into two main categories-crystalline silicon and thin film. With advances in manufacturing methods, the price per installed kilowatt is falling rapidly, making both of them increasingly attractive.
If sunlight strikes a material that is neither completely reflective nor completely transparent, there will be increased particle activity within it, in proportion to the amount of energy absorbed, that is, not reflected back toward the sun and not transmitted. This will be seen as a temperature rise in the material, but there will be no useable electrical output unless the material is a semiconducting photosensitive diode with leads attached.
As we have seen, a diode consists of lightly doped silicon that has a p-n junction. If leads are connected to both sides of the cell, there will be a measurable voltage and if a load is connected, there will be current flow as long as photons strike the surface of the cell and are absorbed.
Electrodes with leads must be attached to extract electricity, and this is not a problem on the bottom surface, but at the top where the solar radiation has to enter the cell, a simple metal plate is not possible because it would block the incoming light. A metallic grid must be constructed, its parts of sufficient mass so that the current will not be limited too much, but also not shading out an excessive amount of the cell's area. A strategy that has been successful is to make the conductor with a rectangular cross-section and place it so that the greater dimension is perpendicular to the surface of the cell so that sufficient light can get through and yet the conductor will not be overly resistive due to a small size.
The cells are wired in series-parallel configurations, creating panels and finally a complete array with the desired voltage and current capability.
Most electrochemical activity becomes more robust at higher temperatures, but PV generation is the opposite. It is stronger by a considerable factor when the ambient temperature is lower. For this reason, solar technology is suitable for cold northern areas where there may be less sunlight but colder daytime temperatures. Many people assume that intense direct sunlight is necessary for PV activity, but in fact, there is considerable available energy on a cold, cloudy day.
Code Navigation
For most of us, it is not possible to memorize the provisions of the NEC. Instead, learn to access answers to questions on an open-book basis. It is suggested that you spend a lot of time reading and understanding the table of contents and the index so that you know which of these to consult in various cases, and what the keywords may be. You should also know the contents of particularly important articles such as 240, 250, and 430.
The other main category of PV generation-thin-film technology-operates on the same fundamental principles, but the details differ. The silicon is laid on a metal, plastic, or glass substrate by the action of vapor deposition, followed by placement of a transparent conductive oxide coating that constitutes the topside electrode. The thin-film cell has good characteristics for the collection of light and export of electrons.
Thin-film technology, also known as amorphous silicon, is somewhat less efficient than crystalline silicon, but it is cheaper to manufacture. Kilowatt for dollar, it may be the better deal, depending on the vendor.
At present, thin-film solar PV comprises only about 20 percent of the market, but in a highly volatile environment, it could surge ahead. What is interesting, from the point of view of the builder, is that thin-film technology is well suited for building integrated photovoltaics (BIPV). This is because, as the name suggests, thin-film solar PV products, in some versions, are flexible and may be attached to either curved or flat surfaces that form the facades of contemporary commercial buildings. In BIPV, the thin-film array is not mounted on the roof with standoff hardware, but it actually becomes the roof, siding, or windows.
In the not distant future, we may see as a norm use of BIPV to supply on-site dc power.
It is perfectly feasible to manufacture appliances and lighting that operate on dc, so the entire concept of a power distribution network could come to be eclipsed by autonomous stand-alone power generation.
For troubleshooting and repair of any solar PV system, thin film or crystalline, special considerations and techniques come into play, particularly on a commercial scale where there are higher voltage levels and greater amounts of available fault current.
Solar Difficulties
Of course, an individual solar cell is a sealed unit, wherein the conversion of radiant energy to electricity takes place on a submicroscopic level, so internal repair is not possible.
However, in many situations, one or more cells could become open or shorted, making the array in varying degrees dysfunctional. Other failure modes could involve storage batteries, inverter, utility connection, system wiring, or problems in downstream premises wiring or loads. Troubleshooting techniques-user interview, analysis of symptoms, use of documentation and technical assistance, and half-splitting circuit sleuthing-all play a role in locating and fixing the problem.
However, where solar PV systems differ is that the system must be considered always live on both sides of the disconnecting means, even if it is locked out. This is so even while the array is covered by an opaque material (a cumbersome method at best) because of the possibilities of unexpected voltage due to system capacitance and backfeed from battery storage or utility power. Beware also of the backup generator that, sensing an outage, could come online unexpectedly. On a large commercial site, such a generator could be too distant to hear when it roars to life.
To begin, check input and output voltages at the inverter. If the system appears dead only at the downstream end, it is likely an inverter malfunction, and if measurements indicate this is so, you will need to look inside the enclosure. Using manufacturer schematics and documentation, trace the power flow, looking for any damage that is apparent visually, such as blown fuses or burnt or loose wires. Do a thorough cleaning and check for any airflow blockage that would inhibit cooling.
If there is no dc supply to the inverter, the next place to look is on the roof or other array location. Check wire nut or crimp connections within junction boxes, as they are often the problem due to moisture infiltration. If a single part of the array is found to be defective, look for blown fuses or circuit breakers that may need to be reset. A roof-mounted array is a natural target for lightning damage, and this is not always visually apparent.
Continue to work upward into the array and it should be possible to determine the cells or connections that are faulty. A frequent cause of voltage loss within modules is dust or pollen buildup on the upper surface. Also, be aware that over the years trees will grow, causing unexpected shading. This will first become apparent in spring or early summer when the trees are leafing out or there is new growth. New construction can also cause increased shading.
In the course of upgrading a service or reworking existing wiring, it is often found that the panel directory is in poor shape due to inaccuracies, bad calligraphy, or tattered condition. You can print a new one on your computer using heavy paper cut to fit. Use the Excel program to make rows and columns. If you do not use Excel, you can use Microsoft Word, but it is more difficult to line up the spaces for double-pole breakers.
After doing one of these jobs, save the directory in your computer. Then, the next time you can just enter new data. Be sure to include your logo with phone number.
An entirely different problem in a PV system is with the load. Breakers, fuses, or switches could be at fault. If an overcurrent device cuts out, remember there may be an underlying cause. Motors often have internal fusing, even imbedded in a winding, or an open coil or wiring fault could be the culprit. An easy test is to replace the motor with another load and note the result.
Poor grounding and faulty neutrals can create difficulties that are sometimes difficult to locate. Keep taking voltage and current measurements at various accessible points and soon you will prevail.
As part of the customer interview, determine if additional loading has been brought online, as this may cause poor system performance and may require an inverter, battery bank, wiring and overcurrent protection, or solar array upgrade.
Many of the above troubleshooting guidelines are equally applicable to wind power and fuel cell systems.