ENGINEERING ARTICLES

Because of the sensitive nature of the vital and essential consumers with regard to personnel safety and production continuity, it is established practice to supply their associated switchboards with dual, or occasionally triple, feeders. For non-essential switchboards it may be practical to use only one feeder. For switchboards other than those for the generator or intake feeders it is established practice to add some margin in power capacity of their feeders so that some future growth can be accommodated. The margin is often chosen to be 25% above the TPPL. If the feeders are plain cables or overhead lines then it is a simple matter to choose their cross-sectional areas to match the current at the 125% duty. For transformer feeders there are two choices that are normally available. Most power transformers can be fitted with external cooling fans provided the attachments for these fans are included in the original purchase order. It is common practice to order transformers initially ...

Since the 1980s the power MOSFET has superseded the BJT in inverters for drives. Like the BJT, the MOSFET is a three-terminal device and is available in two versions, the n-channel and the P-channel. The N-channel is the most widely used, and is shown in Figure 2.18. The main (load) current flows into the drain (D) and out of the source (S). (Confusingly, the load current in this case flows in the opposite direction to the arrow on the symbol.) Unlike the BJT, which is controlled by the base current, the MOSFET is controlled by the gate source voltage. To turn the device on, the gate-source voltage must be comfortably above a threshold of a few volts. When the voltage is first applied to the gate, currents Flow in the parasitic gate-source and gate-drain capacitances, but once these capacitances have been charged the input current to the gate is negligible, so the steady-state gate drive power is minimal. To turn the device 0v, the parasitic capacitances must be discharged and the ga...

In order to reduce the mechanical (due to electro-dynamic forces) and thermal stresses on the object to be protected, the current must be interrupted right during the initiation of the short-circuit, before the full prospective value can be attained (as for example to avoid the welding of the contactor contacts). This is achieved by: • Quick opening of the main contacts. • Rapid build-up of a high arc-voltage (move the arc quickly away from the contact tips and guide it to the arc chamber). The effects of the reduced let-through values are: • Reduction of the electro-dynamic forces on the bus-bars (as for example increased spacing between supports). • Reduction of thermal stresses. The welding of the contacts of contactors can be prevented. Over-dimensioning of the contactors can be avoided or at least kept within reasons. The result is reflected in the short-circuit co-ordination tables - compact starter combinations with components selected mostly on the basis of their rated current...

A metal oxide varistor ( MOV ) is built from zinc oxide disks in series and parallel arrangement to achieve the required protective level and energy requirement. One to four columns of zinc oxide disks are installed in each sealed porcelain container, similar to a high-voltage surge arrester. A typical MOV protection system contains several porcelain containers, all connected in parallel. The number of parallel zinc oxide disk columns required depends on the amount of energy to be discharged through the MOV during the worst-case design scenario. Typical MOV protection system specifications are as follows. The MOV protection system for the series capacitor bank is usually rated to withstand energy discharged for all faults in the system external to the line section in which the series capacitor bank is located. Faults include single-phase, phase-to-phase, and three-phase faults. The user should also specify the fault duration. Most of the faults in EHV systems will be cleared by the pr...

Since most loads are inductive and consume lagging reactive power, the compensation required is usually supplied by leading reactive power. Shunt compensation of reactive power can be employed either at load level, substation level, or at transmission level. It can be capacitive (leading) or inductive (lagging) reactive power, although in most cases as explained before, compensation is capacitive. The most common form of leading reactive power compensation is by connecting shunt capacitors to the line. SHUNT CAPACITORS: Shunt capacitors are employed at substation level for the following reasons: 1. VOLTAGE REGULATION: The main reason that shunt capacitors are installed at substations is to control the voltage within required levels. Load varies over the day, with very low load from midnight to early morning and peak values occurring in the evening between 4 and 7 pm. Shape of the load curve also varies from weekday to weekend, with weekend load typically low. As the load varies, vol...

Except in a very few special situations, electrical energy is generated, transmitted, distributed, and utilized as alternating current (AC). However, AC has several distinct disadvantages. One of these is the necessity of reactive power that needs to be supplied along with active power. Reactive power can be leading or lagging. While it is the active power that contributes to the energy consumed, or transmitted, reactive power does not contribute to the energy. Reactive power is an inherent part of the “total power.” Reactive power is either generated or consumed in almost every component of the system, generation, transmission, and distribution and eventually by the loads. The impedance of a branch of a circuit in an AC system consists of two components, resistance and reactance. Reactance can be either inductive or capacitive, which contributes to reactive power in the circuit. Most of the loads are inductive, and must be supplied with lagging reactive power. It is economical to supp...

The fact that interconnections between power systems are increasingly common does not imply that they are as simple as connecting a few wires. Interconnections obviously entail the expense of constructing and operating transmission lines and substations, or in the case of HVDC, converter stations. Interconnections also entail other costs, technical complexities, and risks. For AC interconnections especially, a power system interconnection is a kind of marriage, because two systems become one in an important way when they operate in synchronism. To do this requires a high degree of technical compatibility and operational coordination, which grows in cost and complexity with the scale and inherent differences of the systems involved. To give just one example, when systems are interconnected, even if they are otherwise fully compatible, fault currents (the current that flows during a short circuit) generally increase, requiring the installation of higher capacity circuit breakers to maint...

There are number of technical rationales for grid interconnections, many of which have economic components as well. Technical rationales for grid interconnection include: • Improving reliability and pooling reserves: The amount of reserve capacity that must be built by individual networks to ensure reliable operation when supplies are short can be reduced by sharing reserves within an interconnected network. • Reduced investment in generating capacity: Individual systems can reduce their generating capacity requirement, or postpone the need to add new capacity, if they are able to share the generating resources of an interconnected system. • Improving load factor and increasing load diversity: Systems operate most economically when the level of power demand is steady over time, as opposed to having high peaks. Poor load factors (the ratio of average to peak power demand) mean that utilities must construct generation capacity to meet peak requirements, but that this capacity sits idle m...

REACTIVE POWER: Reactive power is a concept used by engineers to describe the background energy movement in an Alternating Current (AC) system arising from the production of electric and magnetic fields. These fields store energy which changes through each AC cycle. Devices which store energy by virtue of a magnetic field produced by a flow of current are said to absorb reactive power; those which store energy by virtue of electric fields are said to generate reactive power. Power flows, both actual and potential, must be carefully controlled for a power system to operate within acceptable voltage limits. Reactive power flows can give rise to substantial voltage changes across the system, which means that it is necessary to maintain reactive power balances between sources of generation and points of demand on a 'zonal basis'. Unlike system frequency, which is consistent throughout an interconnected system, voltages experienced at points across the system form a "voltage ...

Shunt reactors are connected in parallel with the rest of the power network. Shunt reactor can be treated as a device with the fixed impedance value. Therefore the individual phase current is directly proportional to the applied phase voltage (i.e. I=U/Z). Thus during external fault condition, when the faulty phase voltage is lower than the rated voltage , the current in the faulty phase will actually reduce its value from the rated value. Depending on the point on the voltage wave when external fault happens the reduce current might have superimposed dc component. Such behavior is verified by an ATP simulation and it is shown in Figure 17. Figure 17: External Phase A to Ground Fault, Reactor Phase Currents As a result, shunt reactor unbalance current will appear in the neutral point as shown in Figure 18. However, this neutral point current will typically be less than 1 pu irrespective of the location and fault resistance of the external fault. Figure 18: External Pha...

The shunt reactors are generally designed for natural cooling with the radiators mounted directly on the tank. However sometimes it is required to have some control action in the cooling circuit depending on the status of the shunt reactor circuit breaker. The control action can be initiated by the circuit breaker auxiliary contact or by operation of an overcurrent relay set to 50% of the reactor rated current. By using overcurrent relay secure control action is obtained when reactor is energized regardless the circuit breaker auxiliary contact status. In order to improve power system performance, lately it is often required by the electrical utilities to perform automatic shunt reactor in and out switching, by monitoring the busbar voltage level. This functionality is quite easy to integrate into multifunctional, numerical relay. However user must carefully check relay performance regarding the following points: • over/under voltage relay with reset ratio or 1% or better is required f...

Similarly to the power transformers, HV oil immersed shunt reactors typically have build-in mechanical devices for internal fault or abnormal operating condition detection. Typically the following built-in mechanical fault detection devices can be encountered within shunt reactor: • gas detection relay (i.e. Buchholz relay) with alarm and trip stage • sudden pressure relay • winding temperature contact thermometer with alarm and trip stage • oil temperature contact thermometer with alarm and trip stage • low oil level relay These mechanical relays are excellent compliment to the electrical measuring relays previously explained. Typically it is recommended to arrange that these mechanical relays trip reactor circuit breaker independently from electrical relays. However signals from mechanical devices shall be connected to binary inputs of numerical relays in order to get time tagging information, disturbance recording and event reporting in case of their operation.

Turn-to-turn faults in shunt reactor present a formidable challenge to the protection engineer. The current and the voltage changes encountered during such fault are very small and therefore sensitive and reliable protection against turn-to-turn faults is difficult to achieve. At the same time the longitudinal differential protection offers no protection at all for such faults. Hence special protection schemes shall be employed. One such scheme, often used in certain countries, utilizes a fact that the HV shunt reactor winding is often made of two half-windings connected in parallel (i.e. the HV lead is brought out at the mid point of the winding, and the two neutral leads at the bottom and the top of the winding). This gives the opportunity to install two CTs in the winding star point (i.e. one in each winding part). Then so-called split phase differential protection can be utilized to detect turn-to-turn faults. However this protection scheme have the following drawbacks: • this ...

1) LINEARITY: For normal operating voltages there is a linear relationship between applied voltage and reactor current (i.e. a small increase in voltage will result in a proportional increase in current). Magnetic fluxes and flux densities are also proportional to the time integral of the applied voltage. With a voltage of sinusoidal shape the fluxes and flux densities are also proportional to the voltage. The deviation from a true sinusoidal shape in line voltage is in general negligible for normal operating voltages. As the magnetic flux to a great extent has its path in magnetic core steel the core steel will get saturated for flux densities above a certain level, the saturation point. Below and up to the saturation point only a small current is needed to magnetize the core steel and the extra current needed to reach a marginal increase flux density is small. Once above the saturation point the extra current needed to further increase the flux density will be large. 2) HARMONIC C...

Steam turbine power plants can use coal, oil, natural gas, or just about any combustible material as the fuel resource. However, each fuel type requires a unique set of accessory equipment to inject fuel into the boiler, control the burning process, vent and exhaust gases, capture unwanted byproducts, and so on. Some fossil fuel power plants can switch fuels. For example, it is common for an oil plant to convert to natural gas when gas is less expensive than oil. Most of the time, it is not practical to convert a coal burning power plant to oil or gas unless it has been designed for conversion. The processes are usually different enough so that switching will not be cost effective. Coal is burned in two different ways in coal fired plants. First, in traditional coal fired plants, the coal is placed on metal conveyor belts inside the boiler chamber. The coal is burned while on the belt as the belt slowly traverses the bottom of the boiler. Ash falls through the chain conveyor belt a...

High pressure and high temperature steam is created in a boiler, furnace, or heat exchanger and moved through a steam turbine generator (STG) that converts the steam’s energy into rotational energy that turns the generator shaft. The steam turbine’s rotating shaft is directly coupled to the generator rotor. The STG shaft speed is tightly controlled for it is directly related to the frequency of the electrical power being produced. High temperature, high pressure steam is used to turn steam turbines that ultimately turn the generator rotors. Temperatures on the order of 1,000°F and pressures on the order of 2,000 pounds per square inch (psi) are commonly used in large steam power plants. Steam at this pressure and temperature is called super heated steam, sometimes referred to as dry steam. The steam’s pressure and temperature drop significantly after it is applied across the first stage turbine blades. Turbine blades make up the fan shaped rotor to which steam is directed, thus...

Power plants produce electrical energy on a real-time basis. Electric power systems do not store energy such as most gas or water systems do. For example, when a toaster is switched on and drawing electrical energy from the system, the associated generating plants immediately see this as new load and slightly slow down. As more and more load (i.e., toasters, lights, motors, etc.) are switched on, generation output and prime mover rotational shaft energy must be increased to balance the load demand on the system. Unlike water utility systems that store water in tanks located up high on hills or tall structures to serve real-time demand, electric power systems must control generation to balance load on demand. Water is pumped into the tank when the water level in the tank is low, allowing the pumps to turn off during low and high demand periods. Electrical generation always produces electricity on an “as needed” basis. Note: some generation units can be taken off-line during light load c...

Devices that are connected to the power system are referred to as electrical loads. Toasters, refrigerators, bug zappers, and so on are considered electrical loads. There are three types of electrical loads. They vary according to their leading or lagging time relationship between voltage and current. The three load types are resistive, inductive, and capacitive. Each type has specific characteristics that make them unique. Understanding the differences between these load types will help explain how power systems can operate efficiently. Power system engineers, system operators, maintenance personnel, and others try to maximize system efficiency on a continuous basis by having a good understanding of the three types of loads. They understand how having them work together can minimize system losses, provide additional equipment capacity, and maximize system reliability. The three different types of load are summarized below. 1) RESISTIVE LOAD: The resistance in a wire (i.e., cond...

Power system stability is a single problem, however, it is impractical to deal with it as such. Instability of the power system can take different forms and is influenced by a wide range of factors. Analysis of stability problems, including identifying essential factors that contribute to instability and devising methods of improving stable operation is greatly facilitated by classification of stability into appropriate categories. These are based on the following considerations: Ø The physical nature of the resulting instability related to the main system parameter in which instability can be observed. Ø The size of the disturbance considered indicates the most appropriate method of calculation and prediction of stability. Ø The devices, processes, and the time span that must be taken into consideration in order to determine stability. Figure 7.1 Possible classification of power system stability into various categories and subcategories. 1) ROTOR ANGLE STABILITY:  Ro...

While it is true that power systems are designed to be transiently stable, and many of the methods described above may be used to achieve this goal, in actual practice, systems may be prone to being unstable. This is largely due to uncertainties related to assumptions made during the design process. These uncertainties result from a number of sources including: LOAD AND GENERATION FORECAST: The design process must use forecast information about the amount, distribution, and characteristics of the connected loads as well as the location and amount of connected generation. These all have a great deal of uncertainty. If the actual system load is higher than planned, the generation output will be higher, the system will be more stressed, and the transient stability limit may be significantly lower. SYSTEM TOPOLOGY: Design studies generally assume all elements in service, or perhaps up to two elements out-of-service. In actual systems, there are usually many elements out-of-servic...

Transient stability is an important consideration that must be dealt with during the design of power systems. In the design process, time-domain simulations are conducted to assess the stability of the system under various conditions and when subjected to various disturbances. Since it is not practical to design a system to be stable under all possible disturbances, design criteria specify the disturbances for which the system must be designed to be stable. The criteria disturbances generally consist of the more statistically probable events, which could cause the loss of any system element and typically include three phase faults cleared in normal time and line-to-ground faults with delayed clearing due to breaker failure. In most cases, stability is assessed for the loss of one element (such as a transformer or transmission circuit) with possibly one element out-of-service in the pre-disturbance system. In system design, therefore, a wide number of disturbances are assessed and if th...

Many factors affect the transient stability of a generator in a practical power system. From the small system analyzed above, the following factors can be identified: Ø The post-disturbance system reactance as seen from the generator. The weaker the post-disturbance system, the lower the Pmax will be. Ø The duration of the fault clearing time. The longer the fault is applied, the longer the rotor will be accelerated and the more kinetic energy will be gained. The more energy that is gained during acceleration, the more difficult it is to dissipate it during deceleration. Ø The inertia of the generator. The higher the inertia, the slower the rate of change of angle and the lesser the kinetic energy gained during the fault. Ø The generator internal voltage (determined by excitation system) and infinite bus voltage (system voltage). The lower these voltages, the lower the Pmax will be. Ø The generator loading before the disturbance. The higher the loading, the closer the unit will be...

1) MODELING: The basic concepts of transient stability presented above are based on highly simplified models. Practical power systems consist of large numbers of generators, transmission circuits, and loads. For stability assessment, the power system is normally represented using a positive sequence model. The network is represented by a traditional positive sequence power flow model, which defines the transmission topology, line reactances, connected loads and generation, and pre disturbance voltage profile. Generators can be represented with various levels of detail, selected based on such factors as length of simulation, severity of disturbance, and accuracy required. The most basic model for synchronous generators consists of a constant internal voltage behind a constant transient reactance, and the rotating inertia constant (H). This is the so-called classical representation that neglects a number of characteristics: the action of voltage regulators, variation of field flu...

Denotes the ability of an electric power system, for a given initial operating condition, to regain a state of operating equilibrium after being subjected to a physical disturbance, with most system variables bounded so that system integrity is preserved. Integrity of the system is preserved when practically the entire power system remains intact with no tripping of generators or loads, except for those disconnected by isolation of the faulted elements or intentionally tripped to preserve the continuity of operation of the rest of the system. Stability is a condition of equilibrium between opposing forces; instability results when a disturbance leads to a sustained imbalance between the opposing forces. The power system is a highly nonlinear system that operates in a constantly changing environment; loads, generator outputs, topology, and key operating parameters change continually. When subjected to a transient disturbance, the stability of the system depends on the nature of the dist...

OPERATION OF FUEL CELL POWER PLANTS: Telecommunications installations with backup fuel cell power often incorporate fuel cells and batteries. As the system voltage changes, rectifiers or controllers switch between the primary power source and the backup power sources. In the absence of grid power or another primary alternating current (AC) power source, the fuel cells, or a combination of fuel cells and batteries, provide direct current (DC) power to run the equipment. The fuel cells have internal batteries that provide temporary “bridge” power until the fuel cell reaches peak power production and takes over the load. When the primary power source is restored, the fuel cells shut down, and the load is returned to the primary source. When the hydrogen fuel supply in a fuel cell is low, a self-checking alarm remotely alerts the operator to replenish the storage containers. The operator can resupply the fuel cell via “hot swapping” or “bumping.” In a “hot-swap” resupply, operators deliv...

Telecommunications providers rely on backup power to maintain a constant power supply, to prevent power outages, and to ensure the operability of cell towers, equipment, and networks. The backup power supply that best meets these objectives is fuel cell technology. The telecommunications industry relies on an elaborate network of cell phone towers and field facilities to transmit phone calls and provide services. To operate effectively, each of these towers and field facilities requires a constant and highly reliable electrical power supply. The industry transmits voice and electronic data through wired and wireless networks. To provide these services, facilities require substantial electrical power, which usually comes from the electrical grid but may also be converted to direct current (DC) power at -48 volts for wired networks and +24 volts for wireless networks. Adequate, effective backup power is essential because the electrical grid is subject to disruption by natural and man...

Fuel injection is a system for mixing fuel with air in an internal combustion engine. A fuel injection system is designed and calibrated specifically for the type of fuel it will handle. Most fuel injection systems are for diesel applications. With the advent of electronic fuel injection (EFI), the diesel gasoline hardware has become similar. EFI’s programmable firmware has permitted common hardware to be used with different fuels. Carburetors were the predominant method used to meter fuel before the widespread use of fuel injection. A variety of injection systems have existed since the earliest usage of the internal combustion engine. The primary difference between carburetors and fuel injection is that fuel injection atomizes the fuel by forcibly pumping it through a small nozzle under high pressure, while a carburetor relies on low pressure created by intake air rushing through it to add the fuel to the air stream. The fuel injector is only a nozzle and a valve: the power to inj...