TRANSMISSION LINES DESIGN and ELECTRICAL ENGINEERING HUB

The Electric Field, also called the electrostatic fi eld, the electric field is not as commonly known per se as the magnetic fi eld. In the same way that current is connected to the magnetic field, voltage is connected to the electric field. That leads to a good rule of thumb to remember:  Current is magnetic and voltage is electric . The electric fi eld comes from electric charges, both positive and negative. In a way that is analogous to the way like poles on magnets repel and opposite poles attract, like charges repel and opposite charges attract. Any molecule or atom can be neutral (no net charge), positively charged, or negatively charged. The accumulation of these charges is what is known as voltage. One way to think of it is that the charges are the voltage making the electric field, and movement of those charges is called current and creates the magnetic field. Similarly to the way an inductor is a way of concentrating a magneti...

Electronic filters have many applications in the telecommunications and data communications industry. One such application, which involves a multiple channel communications system employing a technique known as time-division multiplexing (TDM).  In this system several channels are transmitted through a medium such as an optical fi ber, as shown here, or through a coaxial cable or waveguide. Multiplexing means combining several signals into one, and this is accomplished in TDM by allocating time slots for each channel so that each channel is transmitted at a particular time.  If the signals are synchronized correctly there will be no interference between them. At the transmitter end a multiplexer is used to combine the signals, while at the receiver end a demultiplexer is used to separate the original channels. However, when the channel signals arrive at the receivers they have deteriorated in shape and amplitude. In order to clean them up they are reconstructe...

Standing waves appear when a length of line is excited at a frequency for which the electrical line length is a significant part of an electrical wavelength. They result from the constructive and destructive interference of forward and refl ected waves on the line. Traveling Waves The behavior of the line can be determined by solving the applicable differential equations relating the line parameters to the exciting frequency. The solution of the equations for a line with losses is rather complex and adds little to the practical considerations, so the lossless line will be analyzed instead. In the lossless line, L is the series inductance per unit length, and C is the shunt capacitance. If a differential length, dx , is considered, the inductance for that length is L dx , and the voltage in that length is e _ – L dx ( di/dt ). Since e = ( de/dx ) dx , the equation can be written as dx ( de/dx ) =  – L dx ( di/dt ). Similarly, dx ( di/dx ) = – C dx ( de/dt ). Dividing out t...

William Gilbert William Gilbert is today's EE Hero. Our Electrical Engineering Hero for this entry is considered as the first electrical engineer. William Gilbert is famous for his works on electricity and magnetism. In fact, Gilbert's book De magnete (in English called On the Magnet, Magnetick Bodies Also, and on the Great Magnet the Earth), published in 1600, is considered as a cornerstone works in this branch of engineering. William Gilbert, or Gylberde,was born on the 24th of May 1544 at Colchester, where his father, Hierome Gilbert, became recorder. Educated at Colchester school, he entered St. John's College, Cambridge in 1558, and after taking the degrees of BA and MA, in due course, graduated MD in 1569, in which year he was elected a senior fellow of his college. He is also considered as the most distinguished man of science in England during the reign of Queen Elizabeth I. One of the most notable fact related to our ee hero is that the Gilbert (G) CGS un...

The pulling and braking systems should operate smoothly and should not cause any sudden jerking or bouncing of the conductor. Each system should be readily controllable and capable of maintaining a constant tension. Pullers and tensioners may be mounted separately or in groups for bundled conductor installation. The controls should allow the independent adjustment of tension in each conductor. It is recommended that the tensioner have an independently operated set of bullwheels for each subconductor when stringing bundled conductor, particularly when more than two subconductors per phase are being installed. Pullers should be equipped with load-indicating and load-limiting devices. The load-limiting device should automatically stop the puller from acting further if a preset maximum load has been exceeded. Tensioners should be equipped with tension indicating devices. Capacity selection of the puller and tensioner is dependent upon conductor weight, the length to be strung,...

It is important, from several standpoints, to establish the existing pattern of electrical usage and to identify those areas where energy consumption could be reduced. A month-by-month record of electricity usage is available from electric bills, and this usage should be carefully recorded in a format (possibly graphic) that will facilitate future reference, evaluation, and analysis. The following list of items (where appropriate) should be recorded in the electric usage history: 1) Billing month 2) Reading data 3) Days in billing cycle 4) Kilowatthours (or kilovoltamperehours, if billed on this basis) 5) Billing kW demand (or kVA demand, if billed on this basis) 6) Actual kW demand (or kVA demand, if billed on this basis) 7) Kilovats (actual and billed) 8) Kilovar hours (actual and billed) 9) Power factor (average or peak, as billed) 10) Load factor (average use compared to peak use) 11) Power bill (broken down into the above categories along with fuel cost)...

Hazardous energy control is not optional these days. It is required by law for all employees who work on de-energized equipment where there is potential for injury if the equipment is unexpectedly re-energized. This is an extremely important part of the overall electrical safety program, not only because it is the law, but also because it is a key effective method toward ensuring that employees have the electrically safe work condition. It is often called a lockout/tagout program. OSHA regulation 29 CFR 1910.333(b)(2) states: “While any employee is exposed to contact with parts of fixed electrical equipment or circuits which have been deenergized, the circuits energizing the parts shall be locked out or tagged out or both in accordance with the requirements of this paragraph.” That paragraph covers the following subjects: a) Establishment and maintenance of written procedures for lockout/tagout; b) Establishment of safe procedures for de-energizing equipment; c) Requirem...

The following tasks are some examples of possible exposure to energized conductors: a) Measuring, testing, and probing electrical system components; b) Working near battery banks; c) Opening electrical equipment enclosure doors or removing covers; d) Inserting or pulling fuses; e) Drilling, or otherwise penetrating, earth, walls, or ßoors; f) Pulling conductors in raceways, cable trays, or enclosures; g) Lifting leads or applying jumpers in control circuits; h) Installing or removing temporary grounds; i) Operating switches or circuit breakers; j) Working inside electronic and communications equipment enclosures. Evaluating the degree of hazard Each of the tasks mentioned above should be evaluated for the degree of electrical hazard involved. For example, opening an enclosure door of a 120 V control panel containing many relays and terminals does expose a person to an electrical hazard. But the probability of serious injury may be small due to the lower...

An insulation fault, irrespective of its cause, presents hazards for: human life; preservation of property; availability of electrical power; the above all depending on dependability. Electric Shock of persons A person (or animal) subjected to an electrical voltage is electrified. According to the gravity of the Electric Shock, this person may experience: discomfort; a muscular contraction; a burn; c cardiac arrest (this is Electrocution) Since protection of persons against the dangerous effects of electric current takes priority, Electric Shock is thus the first hazard to be considered. The current strength I -in value and time-, passing through the human body (in particular the heart) is the dangerous aspect. In LV, the impedance value of the body (an important aspect of which is skin resistance) virtually changes only according to environment (dry and wet premises and damp premises). In each case, a safety voltage (maximum acceptable contact voltage for at least 5 s...

Recognising that the experience gained on HVDC transmission systems could be of value throughout the industry, CIGRÉ Study Committee 14 established Working Group 04, Performance of DC Schemes, with terms of referencewhich included an obligation to collect information on all systems in commercial service. It was considered that such information could be useful in the planning, design, construction and operation of new projects. It was also envisaged that the sharing of operational performance data could be of benefit to those concerned with the operation of existing HVDC links or those planning new HVDC links. It was clear that such reports were best prepared in accordance with a standardized procedure so that, with time, the accumulated data from several systems would establish a basis against which performance could be judged. General information collected includes a system description, main circuit data and a simplified one-line diagram for each scheme. This descriptiv...

Back-to-back HVDC converter stations are integrated within ac transmission links and play a role similar to that of transmission tie stations (or substations) on an ac transmission system. Often, the back-to-back HVDC converters are permitting power interchange between two weak and/or unsynchronized ac systems. In some applications, the back-to-back tie is used for only a few hours per day, and power may flow either way. In these cases, the “availability” is a logical RAM parameter to specify as one of the design goals, and the specification should treat the entire back-to-back tie as a single system. Other RAM specification terms should be similar to the terms used in IEEE Std 859-1987. In other applications, it might be desirable to transmit the maximum amount of energy possible at all times and in only one direction. In such cases, one may treat the HVDC converter station as a generator, and “energy availability” may be a better parameter to specify than “availability.”...

The metal-oxide (MO) arrester offers an alternative solution to limit TOVs and can act alone or in conjunction with convertor control and the switching of shunt capacitors and ac filters. The basic idea behind the use of MO arresters to limit TOVs at the convertor station is to exploit the high energy-absorption capability offered by the MO equipment. Various solutions are possible. Two basic approaches adopted in recent dc projects are discussed below: a) MO arresters with an extremely low protective level are used to limit the TOV to values of typically 1.4 pu and permanently connected to the ac busbar. To achieve this low protective level and because of the given MO material characteristic, signiÞcant currents flow through the arrester at normal operating voltages. The arrester, therefore, needs special cooling to avoid overheating during continuous operation. The use of such permanently connected MO arresters is recommended where the initial two or three peaks of t...

GENERATOR PROTECTION As with electrical motor protection, generator protection schemes have some similarities and overlap. This is advantageous, since not all generators have all of the protection schemes listed in this section. In fact, there are many protection schemes available; only the more common ones are discussed here. Generator Over-Current Over-currents in the windings due to over-loads or faults will cause extensive damage. The generator must be separated from the electrical system and field excitation removed as quickly as possible to reduce this damage to a minimum. During run-up and shutdown, the field may accidentally be applied while the frequency is below 60Hz. Under these conditions normal protections may not work or may not be sensitive enough. A sensitive over-current protection called supplementary start over-current is usually provided when the frequency is less than about 56Hz. Generator Differential Protection Differential protection can be us...

The most frequent failure in electrical equipment is the degradation and breakdown (flashover) of the insulation. Hence, it is necessary to mention in this module n basic electrical theory the effect of operating environment on electrical machine insulation. Electrical insulation can be liquid or solid, organic or inorganic. Organic insulation material consists of enamels, varnishes, resins, or polymers that are applied to the steel surface to provide high inter-laminar (between windings) resistance as found on most air-cooled machinery and some oil-immersed transformers. Larger transformers are oil-filled with pure mineral oil to provide higher insulation capability and more effective heat dissipation when equipped with external radiators, fans and pumps. Physical insulation inside these transformers is often in the form of oil-impregnated paper wrapped around the conductors. Inorganic insulation material can include a combination of magnesium oxide, silicates, phosphates...

Busway Construction Originally a busway consisted of bare copper conductors supported on inorganic insulators, such as porcelain, mounted within a nonventilated steel housing. This type of construction was adequate for the current ratings of 225Ð600 A then used. As the use of busways expanded and increased loads demanded higher current ratings, the housing was ventilated to provide better cooling at higher capacities. The bus bars were covered with insulation for safety and to permit closer spacing of bars of opposite polarity in order to achieve lower reactance and voltage drop. Feeder Busway Feeder busway is used to transmit large blocks of power. It has a very low and balanced circuit reactance to minimize voltage drop and sustain voltage at the utilization equipment Feeder busway is frequently used between the source of power, such as a distribution transformer or service drop, and the service entrance equipment. Industrial plants use feeder busway from the service...

The fundamental performance or reliability criterion is the acceptable failure rate. This criterion is based on the consequence of failure and on the expected life of the equipment. Therefore, the failure rate of transmission lines and substation equipment may be different. Transmission Lines The performance/reliability criterion for lightning is normally specified as the number of flashovers per 100 km-years. For switching surges, the flashover rate is normally specified in terms of flashovers per number of switching operations. However, the highest magnitude switching surges normally occur when reclosing, which is normally caused by a fault associated with lightning. Thus, the two separate criteria may not be appropriate in specifying the line reliability. Another criterion, denoted as the storm outage rate, is the number of unsuccessful reclosures per year and is obtained by multiplying the lightning flashover rate for the line in units of flashovers per year by the sw...

Anything we learn about the behavior of a circuit from the connections among its elements can be understood in terms of two constraints known as Kirchhoff’s laws (after the 19th-century German physicist Gustav Robert Kirchhoff). Specifically, they are Kirchhoff’s voltage law and Kirchhoff’s current law. Their application in circuit analysis is ubiquitous, sometimes so obvious as to be done unconsciously, and sometimes surprisingly powerful. While Kirchhoff’s laws are ultimately just concise statements about the basic physical properties of electricity, when applied to intricate circuits with many connections, they turn into sets of equations that organize our knowledge about the circuit in an extremely elegant and convenient fashion. Kirchhoff’s Voltage Law Kirchhoff’s voltage law (often abbreviated KVL) states that the sum of voltages around any closed loop in a circuit must be zero. In essence, this law expresses the basic properties that are inherent in the definition of ...

To better understand how harmonic currents affect transformers one must first understand the basic construction. For power transformers up to about 50 MVA, the typical construction is core form.  The low-voltage winding is generally placed next to the core leg, with the high-voltage winding wound concentrically over the low-voltage winding. For some high-current transformers, these windings may be reversed, with the low-voltage winding wound on the outside over the high-voltage coil. The core and coils are held together with core clamps, and the core and coil is generally enclosed by a tank or enclosure. Losses in the transformer can be broken down into core loss, no-load loss, and load loss. Load losses can be further broken down into I^2R loss and stray loss. Stray loss can be further broken down into eddy current losses and other stray losses. Electromagnetic fields from the ac currents produce voltages across conductors, causing eddy currents to flow in them. This ...

Temperature Limits According to ANSI standards, modern distribution transformers are to operate at a maximum 65˚C average winding rise over a 30˚C ambient air temperature at rated kVA. One exception to this is submersible or vault-type distribution transformers, where a 55˚C rise over a 40˚C ambient is specified. The bulk oil temperature near the top of the tank is called the “top oil temperature,” which cannot be more than 65˚C over ambient and will typically be about 55˚C over ambient, 10˚C less than the average winding rise. Hottest-Spot Rise The location in the transformer windings that has the highest temperature is called the “hottest spot.” Standards require that the hottest-spot temperature not exceed 80˚C rise over a 30˚C ambient, or 110˚C. These are steady-state temperatures at rated kVA. The hottest spot is of great interest because, presumably, this is where the greatest thermal degradation of the transformer’s insulation system will take place. For calculat...

Mineral Oil Mineral oil surrounding a transformer core-coil assembly enhances the dielectric strength of the winding and prevents oxidation of the core. Dielectric improvement occurs because oil has a greater electrical withstand than air and because the dielectric constant of oil (2.2) is closer to that of the insulation. As a result, the stress on the insulation is lessened when oil replaces air in a dielectric system. Oil also picks up heat while it is in contact with the conductors and carries the heat out to the tank surface by selfconvection. Thus a transformer immersed in oil can have smaller electrical clearances and smaller conductors for the same voltage and kVA ratings. Askarels Beginning about 1932, a class of liquids called askarels or polychlorinated biphenyls (PCB) was used as a substitute for mineral oil where flammability was a major concern. Askarel-filled transformers could be placed inside or next to a building where only dry types were used previously....

There are many different accessories used to monitor and protect power transformers, some of which are considered standard features, and others of which are used based on miscellaneous requirements. A few of the basic accessories are briefly discussed here. Liquid-Level Indicator A liquid-level indicator is a standard feature on liquid-filled transformer tanks, since the liquid medium is critical for cooling and insulation. This indicator is typically a round-faced gauge on the side of the tank, with a float and float arm that moves a dial pointer as the liquid level changes. Pressure-Relief Devices Pressure-relief devices are mounted on transformer tanks to relieve excess internal pressures that might build up during operating conditions. These devices are intended to avoid damage to the tank. On larger transformers, several pressure-relief devices may be required due to the large quantities of oil. Liquid-Temperature Indicator Liquid-temperature indicators measure ...