ENGINEERING ARTICLES

The main purpose of preparing a specification for an item of equipment is to ensure that the purchaser, who may also become the owner, obtains the equipment required, rather than what the supplier or manufacturer thinks the purchaser should have. In many situations the difference in perception of the requirements may be small and insignificant. However, for complicated equipment such as high-voltage switchgear and generation systems the differences may be very significant. In order to satisfy both the requirements of the owner and the available options from the supplier, it is necessary to describe the requirements in various degrees of detail. The degree of detail will be a function of the type of equipment. Complex equipment such as large motors, generators, high-voltage switchgear and variable speed drive systems will need a more detailed description than the more standardized equipment such as power cables, low-voltage motors and, to some extent, low-voltage motor control centers. ...

The term ACE (area control error) is used to describe the instantaneous difference between a balancing authority’s net actual interchange flow and the scheduled interchange flow, taking into account the effects of frequency and metering error. The term flat tie line control is used when only tie line flows are closely monitored in consideration of the actual interchange flow. The term flat frequency control is used when only frequency is carefully controlled. When both tie line flow and frequency are carefully controlled by AGC (automatic generation control), the term is called tie line bias. Tie line bias allows the balancing authority to maintain its interchange schedule and respond to interconnection frequency error. The AGC system is part of the energy management system (EMS). (Note: the computer program tools used by system operators that make up the EMS, System Control Centers and Telecommunications). Tie line bias is carefully monitored and reported for all tie lines. Bias is th...

The most frequently used fuels for large-scale power generation are oil, natural gas, and coal. Figure3 illustrates the principal elements of a fossil fuel power plant. Fuel handling includes transport by rail, on ships, or through pipelines. A power plant usually maintains several days of fuel reserve at any one time. Oil and gas are stored in large metal tanks, and coal is kept in open yards. The temperature of the coal layer must be monitored carefully to avoid self-ignition. Oil is pumped and gas is fed to the burners of the boiler. Coal is pulverized in large mills, and the powder is mixed with air and transported by air pressure, through pipes, to the burners. The coal transport from the yard to the mills requires automated transporter belts, hoppers, and sometimes manually operated bulldozers. Two types of boilers are used in modern power plants: the sub-critical water-tube drum-type and the super-critical once-through type. The former operates around 2500 psi, which is belo...

Electrical power can be produced in many ways, including chemical reactions, heat, light, or mechanical energy. Most electrical power produced today is through hydroelectric plants and nuclear energy, and by burning coal, oil, or natural gas. Fossil fuel and nuclear-fission plants use steam turbines to deliver the mechanical energy required to rotate large three-phase generators, which produce massive quantities of electrical power. Generators used in such facilities usually are classified as high-speed units, operating at 3600 rpm to produce a 60 Hz output frequency. Hydroelectric systems use hydraulic turbines, mounted vertically to intercept the flow of water to produce electrical energy. Most hydroelectric facilities use low speed generators, operating at from 120 to 900 rpm to produce 60 Hz. It follows that a larger number of poles are required for a low-speed generator. Fossil fuels, used as a source of heat, are burned to produce steam in a boiler system. The steam then drives o...

In addition to the basic components of a synchronous generator (the rotor, stator, and their windings), auxiliary devices are used to help maintain the machine’s operation within acceptable limits. These devices include the following: GOVERNOR The function of the governor is to control the mechanical power input to the generator. The control is via a feedback loop where the speed of the rotor is constantly monitored. For instance, if this speed falls behind the synchronous speed, the input is insufficient and has to be increased. This is accomplished by opening a valve to increase the amount of steam for turbo-generators or the flow of water through the penstock for hydro-generators. Governors are mechanical systems and, therefore, usually have some significant time lags (many seconds) compared to other electromagnetic parameters associated with the machine. DAMPER WINDINGS (armortisseur windings). These windings are special conducting bars buried in notches on the rotor surface (...

The process of interrupting the current in an ac system is aided by the fact that ac current goes through zero every half-cycle, or approximately every 8 ms in a 60 Hz system. The absence of a natural current zero in dc makes it difficult to develop a dc circuit breaker. There are three principal problems that must be addressed: • Forcing current zero in the interrupting element • Controlling the over voltages caused by large changes in current as a function of time (di/dt) in a highly inductive circuit • Dissipating large amounts of energy (tens of mega joules is not uncommon) The second and third problems are solved by the application of zinc oxide varistors connected line to ground and across the breaking element. The first is the major problem, and several different solutions have been adopted by different manufacturers. Basically, current zero is achieved by inserting a counter voltage into the circuit. In the circuit shown in Figure, opening CB (an air-blast circuit breaker) caus...

In cases where HVDC is selected on technical considerations, it may be the only practical option, as in the case of an asynchronous interconnection. However, for long-distance power transmission, where both ac and HVDC are practical, the final decision is dependent on the total costs of each alternative. The total cost of a transmission system includes the line costs (conductors, insulators, and towers) plus the right-of-way (R-O-W) costs. A dc line with two conductors can carry almost the same amount of power as a three-phase ac line with the same size of line conductors. However, dc towers with only two conductors are simpler and cheaper than three-phase ac towers. Hence, the per-mile costs of line and R-o-W will be lower for a dc line. Power losses in the dc line are also lower than for ac for the same power transmitted. However, the HVDC system requires converters at each end of the line; hence, the terminal costs for dc are higher than for ac. The variation of total costs for ac ...