|
Short Circuit Ratio (synchronous Generator)
In a synchronous generator, the short circuit ratio is the ratio of field current required to produce rated armature voltage at the open circuit to the field current required to produce the rated armature current at short circuit A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit .... This ratio can also be expressed as an inverse of the saturated direct-axis synchronous reactance (in p.u.): : SCR = \frac 1 Effects of SCR values Higher SCR requires lower reactance X_S that in practice means a larger air gap. Both high and low levels of SCR have their benefits: * low SCR: ** in case of a short circuit, the current is proportional to SCR, therefore generators with low SCR require less protection and thus are cheaper; ** low SCR allows shorter air gap and lower excitation field, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Synchronous Generator
An alternator (or synchronous generator) is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field with a stationary armature.Gordon R. Selmon, ''Magnetoelectric Devices'', John Wiley and Sons, 1966 no ISBN pp. 391-393 Occasionally, a linear alternator or a rotating armature with a stationary magnetic field is used. In principle, any AC electrical generator can be called an alternator, but usually, the term refers to small rotating machines driven by automotive and other internal combustion engines. An alternator that uses a permanent magnet for its magnetic field is called a magneto. Alternators in power stations driven by steam turbines are called turbo-alternators. Large 50 or 60 Hz three-phase alternators in power plants generate most of the world's electric power, which is distributed by electric power grids. History Altern ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Electric Current
An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge carriers, which may be one of several types of particles, depending on the Electrical conductor, conductor. In electric circuits the charge carriers are often electrons moving through a wire. In semiconductors they can be electrons or Electron hole, holes. In an Electrolyte#Electrochemistry, electrolyte the charge carriers are ions, while in Plasma (physics), plasma, an Ionization, ionized gas, they are ions and electrons. In the International System of Units (SI), electric current is expressed in Unit of measurement, units of ampere (sometimes called an "amp", symbol A), which is equivalent to one coulomb per second. The ampere is an SI base unit and electric current is a ISQ base quantity, base quantity in the International System of Qua ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Voltage
Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), work needed per unit of Electric charge, charge to move a positive Test particle#Electrostatics, test charge from the first point to the second point. In the SI unit, International System of Units (SI), the SI derived unit, derived unit for voltage is the ''volt'' (''V''). The voltage between points can be caused by the build-up of electric charge (e.g., a capacitor), and from an electromotive force (e.g., electromagnetic induction in a Electric generator, generator). On a macroscopic scale, a potential difference can be caused by electrochemical processes (e.g., cells and batteries), the pressure-induced piezoelectric effect, and the thermoelectric effect. Since it is the difference in electric potential, it is a physical Scalar (physics ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Field Current
A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a electric motor, motor or electrical generator, generator. It consists of a coil of wire through which the field current flows. In a rotating machine, the field coils are wound on an iron magnetic core which guides the magnetic field lines. The magnetic core is in two parts; a stator which is stationary, and a rotor (electric), rotor, which rotates within it. The magnetic field lines pass in a continuous loop or magnetic circuit from the stator through the rotor and back through the stator again. The field coils may be on the stator or on the rotor. The magnetic path is characterized by ''poles'', locations at equal angles around the rotor at which the magnetic field lines pass from stator to rotor or vice versa. The stator (and rotor) are classified by the number of poles they have. Most arrangements use one field coil per po ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Short Circuit
A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit. The opposite of a short circuit is an open circuit, which is an infinite resistance (or very high impedance) between two nodes. Definition A short circuit is an abnormal connection between two nodes of an electric circuit intended to be at different voltages. This results in an electric current limited only by the Thévenin equivalent resistance of the rest of the network which can cause circuit damage, overheating, fire or explosion. Although usually the result of a fault, there are cases where short circuits are caused intentionally, for example, for the purpose of voltage-sensing crowbar circuit protectors. In circuit analysis, a ''short circuit'' is defined as a connection between two nodes that forces them to be at the sam ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Reactances Of Synchronous Machines
The reactances of synchronous machines comprise a set of characteristic constants used in the theory of synchronous machines. Technically, these constants are specified in units of the electrical reactance (ohms), although they are typically expressed in the per-unit system and thus dimensionless. Since for practically all (except for the tiniest) machines the resistance of the coils is negligibly small in comparison to the reactance, the latter can be used instead of (Complex number, complex) electrical impedance, simplifying the calculations. Two reactions theory The air gap (electrical machine), air gap of the machines with a salient pole rotor is quite different along the pole axis (so called ''direct axis'') and in the orthogonal direction (so called ''quadrature axis''). Andre Blondel in 1899 proposed in his paper "Empirical Theory of Synchronous Generators" the two reactions theory that divided the Armature (electrical), armature magnetomotive force (MMF) into two component ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Per-unit System
In the power systems analysis field of electrical engineering, a per-unit system is the expression of system quantities as fractions of a defined base unit quantity. Calculations are simplified because quantities expressed as per-unit do not change when they are referred from one side of a transformer to the other. This can be a pronounced advantage in power system analysis where large numbers of transformers may be encountered. Moreover, similar types of apparatus will have the impedances lying within a narrow numerical range when expressed as a per-unit fraction of the equipment rating, even if the unit size varies widely. Conversion of per-unit quantities to volts, ohms, or amperes requires a knowledge of the base that the per-unit quantities were referenced to. The per-unit system is used in power flow, short circuit evaluation, motor starting studies etc. The main idea of a per unit system is to absorb large differences in absolute values into base relationships. Thus, rep ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Air Gap (electric Machine)
Air gap in magnetic circuits is a term used to define an intentional gap left in the magnetic material. In stationary devices, like inductors and transformers, the air gap is used for a few purposes: * to minimize the magnetic saturation of their cores due to the direct current (DC) that might be flowing through the coils. Without saturation the inductance (and thus the blocking capability) of a choke stays constant regardless of the DC current flowing; * counter-intuitively, if a DC magnetization is present in an inductor, an increased (up to some limit) air gap actually incrementally increases the effective inductance; * in a shunt reactor an air gap is used for two reasons: ** with an ungapped core the reluctance is small, so very little reactive power is obtained with the disproportionate effect of the iron loss; ** an increase of the gap reduces the ratio of the total loss to the reactive power, with the limiting factor being the increased heating due to the copper los ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Contingency (electrical Grid)
In an electrical grid, contingency is an unexpected failure of a single principal component (e.g., an electrical generator or a power transmission line) that causes the change of the system state large enough to endanger the grid security. Some protective relays are set up in a way that multiple individual components are disconnected due to a single fault, in this case, taking out all the units in a group counts as a single contingency. A scheduled outage (like maintenance) is not a contingency. The choice of term emphasizes the fact that a single fault can cause severe damage to the system so quickly that the operator will not have time to intervene, and therefore a reaction to every single fault has to be defensively pre-built into the system configuration. Some sources use the term interchangeably with "disturbance" and "fault". Contingency analysis The contingency analysis application periodically runs on the computers at the operations centers providing suggestions to the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Power Angle
A power system consists of a number of synchronous machines operating synchronously under all operating conditions. Under normal operating conditions, the relative position of the rotor axis and the resultant magnetic field axis is fixed. The angle between the two is known as the power angle, torque angle, or rotor angle. During any disturbance, the rotor decelerates or accelerates with respect to the synchronously rotating air gap magnetomotive force, creating relative motion. The equation describing the relative motion is known as the swing equation, which is a non-linear second order differential equation that describes the swing of the rotor of synchronous machine. The power exchange between the mechanical rotor and the electrical grid due to the rotor swing (acceleration and deceleration) is called Inertial response. Derivation A synchronous generator is driven by a prime mover. The equation governing the rotor motion is given by: J\frac = T_a = T_\text - T_\text, where: ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
