ENGINEERING ARTICLES

The main components of a wind power system are illustrated in Figure 28.1, which include a turbine rotor and blades, a yaw mechanism, a gearbox, a generator, a power electronic converter system, a transformer to connect the wind power system to a power grid, and a wind turbine generator control system. FIGURE 28.1 Main components of a wind power system. The wind turbine converts kinetic power in wind (i.e., aerodynamic power) to mechanical power by means of rotation of turbine rotor and blades. The mechanical power is transmitted from the turbine shaft directly or through a gearbox to the generator shaft, depending on the number of poles of the generator. If the generator has a low number of poles (e.g., four poles), a gearbox is commonly used to connect the low-speed turbine shaft and the high-speed generator shaft. If a generator with a high number of poles is used, the gearbox may not be necessary. The generator converts mechanical power to electrical power, which is fed into ...

In virtually every engineering application there is a need for measuring some physical quantities, such as forces, stresses, temperatures, pressures, flows, or displacements. These measurements are performed by physical devices called sensors or transducers, which are capable of converting a physical quantity to a more readily manipulated electrical quantity. Most sensors, therefore, convert the change of a physical quantity (e.g., humidity, temperature) to a corresponding (usually proportional) change in an electrical quantity (e.g., voltage or current). Often, the direct output of the sensor requires additional manipulation before the electrical output is available in a useful form. For example, the change in resistance resulting from a change in the surface stresses of a material the quantity measured by the resistance strain gauges must first be converted to a change in voltage through a suitable circuit (the Wheatstone bridge) and then amplified from the milli volt to the v...

Balancing authorities are responsible for the performance of the electric system is to ensure that at every moment of time there is sufficient generation to reliably supply the customer requirements and all associated delivery system losses. The process is complicated by the fact that the customer load changes continuously and, therefore, the generation must adjust immediately, either up or down, to accommodate the load change. Since electric power cannot be stored, the generation change must be accomplished by a physical adjustment of the equipment generating the electricity. The Balancing Authority Areas vary greatly in both geographic size and the amount of generation/ load they control. AREA CONTROL Each Balancing Authority is responsible for maintaining its own load/generation balance, including its scheduled interchange, either purchases or sales. A Balancing Authority can consist of a generator or group of generators, an individual company, or a portion of a company or a g...

The electric utility industry over time developed planning, operating, and design standards to address customer expectations of reliable service. These standards were at first local in perspective but, as interties were built and the interdependent nature of the system became apparent, many of the standards were expanded to a regional and then a national perspective. Transmission lines cannot be added helter-skelter based solely on the profits for the owner. Locations and designs for new substations selected by the distribution systems must recognize the future of the transmission system that will supply them. One cannot design a reliable low-cost automobile by having separate uncoordinated designs for the brakes, the transmission, the engine, and other essential systems. The same is true for the transmission system. It must be designed as an integrated whole. Since the effects of electrical disturbances can spread over a wide, multistate region, the need for regional coordination in ...

Reliability of a system is difficult to measure. Perhaps the best way is through evaluation of the consequences of possible consumer interruptions. Investigations have shown that the best measure of reliability is that of consumer reaction. Five conditions that have been identified impact the value an average consumer puts on an unsupplied megawatt-hour of lost energy: The activities affected by the curtailment and, therefore, the time of day and mix of customers The number of interruptions Availability of advance warning Weather conditions and, therefore, the time of year The duration of the interruption Figure 9-1 shows that this reaction increases dramatically as the frequency of outages increase, as the duration of the outage increases, and with the magnitude or extent of the outage. The following function presents a means of evaluating this reaction: R = function of {K, F, T, P, t} Where K is an empirical coefficient proportional to the consumer’s dependence on e...

The costs of electric power outages to U.S. electric customers are generally called “socioeconomic” costs. Attempts have been made to quantify these costs but the estimates vary widely. One source reports that the costs are $26 billion each year and that they have been increasing as the electric power industry is restructured. A 2001 report from the Electric Power Research Institute (EPRI) states that power outages and problems with power quality cost the U.S. economy over $119 billion per year. Costs are due to: Loss of life due to accidents (e.g., no street lights) Loss of life of the ill and elderly (death rates go up) Loss of productivity by industry Loss of sales by business Loss of wages of labor Damage to equipment in industry Fires and explosions Riots and thefts Increased insurance rates

Interruptions in the supply of electricity to customers can occur at any hour of the day or night and can last from fractions of a second to many hours or even days. Interruptions can be caused by disturbances to or malfunctions of any of the three components of the power system: generation, transmission, or distribution. They can also be caused by the unavailability of adequate resources to supply the customer load. These two attributes of reliability are characterized by NERC as security and adequacy. Data shows that over 90% of customer outages are caused by problems originating on the local distribution system. Although generation and transmission-related outages are less common than those related to the distribution system, they often have much more serious consequences because of the number of customers affected and the duration of the outage. Disturbances can be initiated by: 1) External events such as: Environmental factors, including wind, rain, lightening, i...

SMART GRID Smart Grid is one of a number of names used to describe a transmission and distribution network that uses modern sensing, computational, and communication technologies to improve the reliable and economic functioning of the bulk power system. Other names include Modern Grid, Intelli grid, and Future Grid. A subset of this notion is the development of micro grids. The Smart Grid can perhaps be described as any activity that might improve the performance and efficiency of the electric power system. Those applications being discussed seem to fit into two broad categories: 1. Those impacting the operation and control of the bulk power system 2. Those impacting the distribution system, especially activities that would involve interaction with the customer and the customer’s electric devices Besides addressing technical issues, substantive public policy issues also need attention, especially for those technologies that would alter or change the usage pattern of cust...

There are a large range of possible approaches and concepts for storing energy in electric utility systems. These are discussed in the following subsections. 1) MECHANICAL SYSTEMS HYDRO PUMPED STORAGE :  In hydro pumped storage, water is pumped from a lower to a higher elevation. The water at the higher elevation can be stored and used to generate electricity for later utility use when it flows down through a hydro turbine to drive an electric generator. Pumping and generation may also be accomplished with a reversible pump–turbine connected to a motor generator. The reservoirs needed for the pumped storage operation may be natural bodies of water, reservoirs of existing hydro plants or of water storage systems, especially constructed surface reservoirs, underground caverns, or a combination of these. Typical efficiency of this process is about 70%, with 30% used in the pumping generating cycle. The major barriers to widespread use of conventional pumped storage are siting, g...

Energy storage applications offer potential benefits to the transmission and distribution system because of the ability of modern power electronics, and some electro-chemistries, to change from full discharge to full charge, or vice versa, extremely rapidly. These characteristics enable energy storage to be considered as a means of improving transmission grid reliability or increasing effective transmission capacity. At the distribution level, energy storage can be used in substation applications to improve system power factors and economics and can also be used as a reliability enhancement tool and a way to defer capital expansion by accommodating peak load conditions. Energy storage can also be used to alleviate diurnal or other congestion patters and, in effect, store energy until the transmission system is capable of delivering the energy to the location where it is needed. Other technical applications of electric energy storage include: Grid stabilization Grid freque...

An alternate means of transmitting electricity is to use high-voltage direct current (HVDC) technology. As the name implies, HVDC uses direct current to transmit power. Direct current facilities are connected to HVAC systems by means of rectifiers, which convert alternating current to direct current, and inverters, which convert direct current to alternating current. Early applications used mercury arc valves for the rectifiers and inverters but, starting in the 1970s, thyristors became the valve type of choice. Thyristors, also called silicon-controlled rectifier (SCRs), are controllable semiconductors that conduct when their gates receive a current pulse. They can carry very high currents and can block very high voltages. They are connected is in series to form a thyristor valve, which allows electricity to flow during the positive half of the alternating current voltage cycle but not during the negative half. Since all three phases of the HVAC system are connected to the valves, ...

There are a number of designs used for substations. However, there are elements common to all: >> A BUS : is the physical structure to which all lines and transformers are connected. Buses are of two generic types: open air and enclosed. Enclosed buses are used when substations are located in buildings or outdoors where space is at a premium. They involve the use of an insulating gas such as sulfur hexafluoride (SF6) to allow reduced spacing between energized phases. Bus structures are designed to withstand the large mechanical forces that can result from fields produced by high short-circuit currents. These forces vary with the third power of the current. A bus section is the part of a bus to which a single line or transformer is connected. >> PROTECTIVE RELAYS : are devices that continuously monitor the voltages and currents associated with the line and its terminals to detect failures or malfunctions in the line or equipment. Such failures are called faults a...

Substations are locations where transmission lines, transformers, and generators are connected. They fulfill a number of functions: Allow power from different generating stations to be fed into the main transmission corridors. Provide a terminus for interconnections with other systems. Provide a location where transformers can be connected to feed power into the sub transmission or distribution systems. Allow transmission lines to be segmented to provide a degree of redundancy in the transmission paths. Provide a location where compensation devices such as shunt or series reactors or capacitors can be connected to the transmission system. Provide a location where transmission lines can be de-energized, either for maintenance or because of an electrical malfunction involving the line. Provide protection, control, and metering equipment/

The primary components of an overhead transmission line are: Conductors Ground or shield wires Insulators Support structures Land or right-of-way (R-O-W) 1) CONDUCTORS : are the wires through which the electricity passes. Transmission wires are usually of the aluminum conductor steel reinforced (ACSR) type, made of stranded aluminum woven around a core of stranded steel that provides structural strength. When there are two or more of these wires per phase, they are called bundled conductors. 2) GROUND OR SHIELD WIRES: are wires strung from the top of one transmission tower to the next, over the transmission line. Their function is to shield the transmission line from lightning strikes. 3) INSULATORS : are made of materials which do not permit the flow of electricity. They are used to attach the energized conductors to the supporting structures, which are grounded. The higher the voltage at which the line operates, the longer the insulator strings. In recent years, p...

Disturbances may also change the voltage at the generator’s terminals. In response, the generator’s automatic voltage regulating system will sense the change and adjust the generator’s field excitation, either up or down, to compensate. Transient stability or instability considers that period immediately after a disturbance, usually before the generator’s governor and other control systems have a chance to operate. In all cases, the disturbance causes the generator angles to change automatically as they adjust to find a new stable operating point with respect to one another. In an unstable case, the angular separation between one generator or group of generators and another group keeps increasing. This type of instability happens so quickly, in a few seconds, that operator corrective action is impossible. If stable conditions exist, the generator’s speed governor system, sensing the beginning of change in speed, will then react to either admit more mechanical energy into the r...

YAW CONTROL The yaw control continuously orients the rotor in the direction of the wind. It can be as simple as the tail vane, or more complex on modern towers. Theoretical considerations dictate free yaw as much as possible. However, rotating blades with large moments of inertia produce high gyroscopic torque during yaw, often resulting in loud noise. Too rapid yaw may generate noise exceeding the local ordinance limit. Hence, a controlled yaw is often required and is used. SPEED CONTROL The wind turbine technology has changed significantly in the last 25 years. Large wind turbines being installed today tend to be of variable speed design, incorporating the pitch control and the power electronics. Small machines on the other hand must have simple, low cost power and speed control. The speed control methods fall into the following categories: No speed control whatsoever. In this method, the turbine, the electrical generator, and the entire system is designed to withstand t...

Transformers are susceptible to damage by secondary short-circuit currents having magnitudes that can be many times rated load current. The damage results from the following effects: >> The  I 2 R  losses in the winding conductors are increased by the square of the current. This increases the temperature rise of the windings. >> Because protective devices limit the duration of short circuits (as opposed to overloads), the temperature rise of the winding can be calculated by dividing the total energy released by the  I 2 R  losses by the thermal capacity of the conductor. >> The short-circuit currents exclude flux in the core and increase stray flux around the core. This stray flux induces currents in metallic parts other than the winding conductors, which can be damaged thermally. >> A short circuit applied to the secondary circuit of an auto-transformer can substantially increase the voltage across the series winding ...

The design of new substations has the advantage of starting with a blank sheet of paper. The new substation will typically have many IEDs for different functions, and the majority of operational data for the SCADA system will come from these IEDs. The IEDs will be integrated with digital two-way communications. The small amount of direct input/output (hardwired) can be acquired using programmable logic controllers (PLC). Typically, there are no conventional remote terminal units (RTU) in new substations. The RTU functionality is addressed using IEDs and PLCs and an integration network using digital communications. FIGURE 7.1 SA system functional architecture diagram. In existing substations there are several alternative approaches, depending on whether the substation has a conventional RTU installed. The utility has three choices for their existing conventional substation RTUs: integrate RTU with IEDs; integrate RTU as another substation IED; and retire RTU and use IEDs an...

The following additional design trade-offs are available to the system engineer: NUMBER OF BLADES This is the first determination the design engineer must make. Wind machines have been built with the number of blades ranging from 2 to 40 or more. The high number of blades was used in old low, tip-speed ratio rotors for water pumping, and the application which needs high starting torque. The modern high, tip-speeds ratio rotors for generating electrical power have two or three blades, many of them with just two. The major factors involved in deciding the number of blades are as follows: The effect on power coefficient. The design tip-speeds ratio. The cost. The nacelle weight. The structural dynamics. The means of limiting yaw rate to reduce gyroscopic fatigue. Compared to the two-blade design, the three-blade machine has smoother power output and balanced gyroscopic force. There is no need to teeter the rotor, allowing the use of simple rigid hub. Adding the third b...

The wind power system is comprised of one or more units, operating electrically in parallel, having the following components: The tower. The wind turbine with two or three blades. The yaw mechanism such as the tail vane. The mechanical gear. The electrical generator. The speed sensors and control. The modern system often has the following additional components: The power electronics. The control electronics, usually incorporating a computer. The battery for improving the load availability in stand-alone mode. The transmission link connecting to the area grid. Because of The large moment of inertia of the rotor, the design challenges include the starting, the speed control during the power producing operation, and stopping the turbine when required. The eddy current or other type of brake is used to halt the turbine when needed for emergency or for routine maintenance. In The multiple tower wind farm, each turbine must have its own control system for operational and safet...

Electric power is one of the mainstays of our lives and the life of our nation. It differentiates advanced societies from third world nations. It touches almost every facet of our lives: our homes, our businesses, our schools, our transportation, and our leisure time. It is there when we are born, and it is there when we die. Think of the impact on our lives if we were not able to watch our favorite TV shows, use our home computers, heat and cool our homes, refrigerate our food, wash our clothes or our dishes, or read at night. Yet most people take it for granted, except during those relatively rare times when it is unavailable or when we receive our electric bills and note that the charges have suddenly and un-explainedly increased. We know we have power outlets in our homes and businesses and we may notice the distribution wires running along our streets or if we pass high-voltage transmission towers, but many of us do not know how the whole system works. Some of us are affected bec...