DC voltage stabilizers
A voltage regulator is designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.
Electronic voltage regulators are found in devices such as computer power supplies where they stabilize the DC voltages used by the processor and other elements. In automobile alternators and central power station generator plants, voltage regulators control the output of the plant. In an electric power distribution system, voltage regulators may be installed at a substation or along distribution lines so that all customers receive steady voltage independent of how much power is drawn from the line.
Many simple DC power supplies regulate the voltage using either series or shunt regulators, but most apply a voltage reference using a shunt regulator such as a Zener diode,avalanche breakdown diode, or voltage regulator tube. Each of these devices begins conducting at a specified voltage and will conduct as much current as required to hold its terminal voltage to that specified voltage by diverting excess current from a non-ideal power source to ground, often through a relatively low-value resistor to dissipate the excess energy. The power supply is designed to only supply a maximum amount of current that is within the safe operating capability of the shunt regulating device.
If the stabilizer must provide more power, the shunt regulator output is only used to provide the standard voltage reference for the electronic device, known as the voltage stabilizer. The voltage stabilizer is the electronic device, able to deliver much larger currents on demand.
Measuring instruments, such as ammeters, voltmeters, kilowatt-hour meters, etc , whether electromechanical or electronic, meet insuperable design problems if faced with the high voltages or high currents commonly used in power systems.
An instrument transformer is an electrical device intended to supply measuring instruments such as meters, relays and other similar apparatus. There are two types of instrument transformer: Current Transformer ( CT ) and voltage (or potential) transformers (VT or PT).
- Current Transformer ( CT ), in which the secondary current is under normal working conditions, practically proportional to the primary current and phase shifted from it by an angle close to zero in the appropriate direction for connections. Current transformers are therefore used with the measuring instruments to: (a) Isolate the instruments from the power circuits. (b) Standardise the instruments, usually at 5 amps or 1 amp. The scale of the instrument (according to the C T ratio), then becomes the only non-standard feature of the instrument. A current transformer (CT) is used for measurement of alternating electric currents. When current in a circuit is too high to apply directly to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry. For the IEC standard, accuracy classes for various types of measurement are set out in IEC 61869-1, Classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1 and The class designation is an approximate measure of the CT's accuracy. The ratio (primary to secondary current) error of a Class 1 CT is 1% at rated current; the ratio error of a Class 0.5 CT is 0.5% or less. Errors in phase are also important especially in power measuring circuits, and each class has an allowable maximum phase error for a specified load impedance.
- Voltage Current transformers used for protective relaying also have accuracy requirements at overload currents in excess of the normal rating to ensure accurate performance of relays during system faults. A CT with a rating of 2.5L400 specifies with an output from its secondary winding of 20 times its rated secondary current (usually 5 A × 20 = 100 A) and 400 V (IZ drop) its output accuracy will be within 2.5 percent. Transformer ( VT ) also known as a Potential Transformer ( PT ). In which the secondary voltage is under normal operating conditions, practically proportional to the primary voltage and phase shifted from it by an angle close to zero in the appropriate direction for connections. The primary of a voltage transformer is connected to the terminals between which the voltage is to be measured, and the secondary is connected to the voltage circuits of one or more measureing devices connected in parallel. Voltage transformers are more like power transformers than current transformers are. For reasons of construiction and insulation, VTs are normally made with a rectangular core and the secondaries (if more than one) are wound on the same core. Unlike CTs, they are therefore not independent, and the load of one secondary influenced the accuracy of the other. VTs may be used to measure the volrages between phases or between a phase and earth. In this case one end of its primary winding will be directly earthed, inside or outside the transformer.
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