A current transformer ( CT ) is a type of transformer used to measure alternating current (AC). This produces a secondary current which is proportional to the current in its primer.
The current transformer, together with a voltage or potential transformer, is an instrument transformer. Instrument transformers scale the values ​​of large voltages or small currents, easily handled standard values ​​for instruments and protective relay. Instrument transformers isolate measurements or protection circuits from high-voltage primary systems. A current transformer provides a secondary current that is accurately proportional to the current flowing in its primer. The current transformer presents a meaningless load to the primary circuit.
The current transformer is the current sensor unit of the electrical power system and is used in power stations, electrical substations, and in the distribution of industrial and commercial electricity.
Video Current transformer
Function
Like transformers, current transformers have primary windings, nuclei and secondary windings, although some transformers, including current transformers, use an air core. In principle, the only difference between a current transformer and a voltage transformer (normal type) is that the first is fed with a constant current while the latter is fed with a 'constant' voltage, where 'constant' has a tight circuit. the meaning of theory.
Alternating current in the primer produces an alternating magnetic field at the core, which then induces alternating current in the secondary. The main circuit is largely unaffected by CT insertion. Accurate current transformers require close coupling between the primary and the secondary to ensure that the secondary current is proportional to the main current over a wide range of currents. The current in the secondary is the current in the primary (assuming one main rotation) divided by the number of secondary rounds. In the illustration to the right, 'I' is the current in the primer, 'B' is the magnetic field, 'N' is the number of turns on the secondary, and 'A' is the AC ammeter.
Current transformers usually consist of silicon steel ring core wounds with many copper wire windings as shown in the picture on the right. The conductor carrying the main stream is passed through the ring. The primary CT, therefore, consists of one 'turn'. The 'main winding' may be a permanent part of the current transformer, ie a heavy copper rod to carry current through the core. The current type of window transformer is also common, which can have a circuit cable running through the middle of the opening in the core to provide a single-turn primary winding. To aid accuracy, the main conductor should be centered in the aperture.
CT is determined by the ratio of their currents from primary to secondary. The secondary stream of identifiers is usually standardized at 1 or 5 amperes. For example, a secondary winding CT 4000: 5 will supply an output current of 5 amperes when the main current of the winding is 4000 amperes. This ratio can also be used to find the impedance or voltage on one side of the transformer, given the corresponding value on the other side. For CT 4000: 5, secondary impedance can be found as Z S = NZ P = 800Z P , and the voltage secondary can be found as V S = NV P = 800V P . In some cases, secondary impedance is referenced to the primary side, and found as Z S ? = N 2 Z P . Referring impedance is done simply by multiplying the value of the initial secondary impedance with the current ratio. Secondary CT scrolls can have taps to provide various ratios, five common taps.
The shape and size of the current transformer varies depending on the end user or switching gear manufacturer. The current low voltage current ratio of a single current ratio is a type of ring or plastic mold.
Separate core-level transformers have two-piece or core cores with removable parts. This allows the transformer to be placed around the conductor without having to remove it first. Split-core current transformers are commonly used in low current measuring instruments, often portable, battery operated, and hand-held (see bottom right illustration).
Maps Current transformer
Use
Current transformers are widely used to measure current and monitor the operation of power grids. Along with lead voltages, class-income CTs drive watt-hour electric utility meters on much larger commercial and industrial supplies.
High voltage current transformers are installed on porcelain or polymer insulators to isolate them from the ground. Some CT configurations slip around a high voltage transformer bus or circuit breaker, which automatically centralizes the conductor inside the CT window.
The current transformer can be mounted at low voltage or high voltage from the power transformer. Sometimes parts of the bus bar can be removed to replace the current transformer.
Often, some CTs are installed as "stacks" for various uses. For example, protection devices and revenue measurements may use separate CTs to provide isolation between meter and protection circuits and enable current transformers with different characteristics (accuracy, excessive performance) to be used for the device.
The load (load) of the impedance shall not exceed the specified maximum value to avoid the secondary voltage exceeding the limit for the current transformer. The main current rating of the current transformer should not be exceeded or its core can enter its non-linear region and eventually saturate. This will occur near the end of the first half of each half (positive and negative) of the AC sine wave in the primer and will compromise accuracy.
Security
Current transformers are often used to monitor high currents or currents at high voltages. Technical standards and design practices are used to ensure the safety of the installation using a current transformer.
The secondary of the current transformer should not be disconnected from the load while the current is in the primary, since the secondary will seek to continue to drive the current into an infinite impedance effective to an isolator break and thus compromise operator safety. For a particular current transformer, this voltage can reach several kilovolts and may cause a curve. Exceeding the secondary voltage can also decrease the transformer's accuracy or destroy it. Giving current transformer energy with a secondary open circuit is equivalent to energizing a voltage transformer (normal type) with a secondary short circuit. In the first case the secondary tries to generate infinite voltage and in the second case the secondary tries to produce an infinite current. Both scenarios can be dangerous and damage the transformer.
Accuracy
CT accuracy is influenced by a number of factors including:
- Loads
- Grade load/saturation class
- Ranking factors
- Load
- External electromagnetic fields
- Temperature
- Physical configuration
- Selected tap, for multi-ratio CT
- Phase change
- Capacitive coupling between primary and secondary
- Primary and secondary endurance
- Core magnetizing current
Class accuracy for various types of measurements and at standard loads in secondary circuits (loads) is defined in IEC 61869-1 as classes of 0.1, 0.2, 0.2, 0.5, 0.5, 1 and 3. The class designation is an approximate measure of CT accuracy. The ratio (primary to secondary current) error of Class 1 CT is 1% in the rated current; the error ratio of Class 0,5 CT is 0.5% or less. Error in phase is also important, especially in circuit power meter. Each class has the maximum allowable phase error for the specified load impedance.
Current transformers used for protective relays also have accuracy requirements in overcurrent currents exceeding normal ratings to ensure accurate relay performance during system errors. A CT rated of 2.5L400 is determined by the output of the secondary winding twenty times its rated secondary current (usually 5Ã, A ÃÆ'â € "20 = 100 AA ) and 400Ã,V (IZ drop) the output accuracy will be within 2.5 percent.
Load
The secondary load of the current transformer is called "load" to distinguish it from the primary load.
The load in the CT metering circuit is mostly resistive impedance presented to its secondary winding. The typical load ratings for IEC CT are 1.5 VA, 3 VA, 5 VA, 10 VA, 15 VA, 20 VA, 30 VA, 45 VA and 60 VA. The ANSI/IEEE load rating is B-0.1, B-0.2, B-0.5, B-1.0, B-2.0 and B-4.0. This means a CT with load rating of B-0.2 will maintain an expressed accuracy of up to 0.2? on the secondary circuit. This specification diagram shows the accuracy of the parallelogram on the grid that combines the magnitude and magnitude of the angular scale at the load of the CT value. The items that contribute to the current measurement circuit load are switch-blocks, meters and intermediate conductors. The most common cause of overload impedance is the conductor between meter and CT. When the meter substation is far from the meter cupboard, the excessive cable length creates great resistance. This problem can be reduced by using thicker cables and CT with lower secondary currents (1 A A), both of which will result in fewer voltage drops between the CT and the metering device.
The knee-core saturation stress points
The knee-voltage of the current transformer is the magnitude of the secondary voltage above which the output current stops linearly following the input current in the stated accuracy. In the test, if the voltage is applied across the secondary terminals, the magnetization current will increase in proportion to the applied voltage, until the point of the knee is reached. The knee point is defined as the voltage at which a 10% increase in applied voltage increases the magnetic current by 50%. For a voltage greater than the knee point, the magnetization current increases significantly even for a small increase of voltage at the secondary terminals. The knee point voltage is less applicable for measuring the current transformer because its accuracy is generally much higher but is limited in a very small range of current transformer values, typically 1.2 to 1.5 times the rated current. However, the concept of knee-point voltage is closely related to the protection of current transformers, since they are always exposed to fault current of 20 to 30 times the measured current.
Phase shift
Ideally, the primary and secondary currents of the current transformer must be in phase. In practice, this is not possible, but, at normal power frequencies, phase displacements of a few tenths of a degree can be achieved, while simple CT may have a phase shift of up to six degrees. For current measurements, the phase shift is immaterial because ammeters only display the magnitude of the current. However, in wattmeters, energy meters, and power factor meters, phase shifts generate errors. For power and energy measurement, errors are considered negligible on the power factor of unity but become more significant when the power factor approaches zero. At the zero power factor, each power is shown completely due to the phase error of the current transformer. The introduction of electronic strength and energy meters has enabled the current phase error to be calibrated.
Construction
The bar-type current transformer has a terminal for source and load connections from the main circuit, and the current transformer body provides insulation between the primary and ground circuits. By using oil insulation and porcelain bushings, such transformers can be applied to the highest transmission voltages.
A ring-type transformer transformer is mounted on top of a bus bar or insulated cable and has only a low insulation level on the secondary coil. To obtain a non-standard ratio or for any other special purpose, more than one round of primary cables can be passed through the ring. Where a metal shield is present in the cable jacket, it must be stopped so that no net current flows through the ring, to ensure its accuracy. Current transformers used to detect soil disturbance currents (zero order), as in a three-phase installation, may have three main conductors passed through the ring. Only an unbalanced net current produces a secondary current - it can be used to detect faults from the energy conductor to the ground. Ring type transformers typically use a dry insulation system, with rubber or hard plastic boxes on top of secondary windings.
For temporary connections, a split-ring current transformer can be routed through a cable without disconnecting it. This type has a laminated iron core, with a hinged part that allows it to be mounted on top of the cable; nucleus connects the magnetic flux generated by a single primary turn curve to the secondary wound with multiple turns. Because gaps in hinged segments introduce inaccuracies, they are not normally used for earning measurement.
Current transformers, especially those intended for high voltage substation service, may have some taps on the secondary winding, providing multiple ratios in the same device. This can be done to allow the reduction of inventory of the reserve unit, or to allow the growth of the load in the installation. The high-voltage current transformer may have multiple secondary windings with the same primer, to allow separate gauges and protection circuits, or for connection to various types of protective devices. For example, a secondary can be used for overcurrent protection over branches, while a second winding can be used in the bus differential protection scheme, and the third winding used for power and current measurements.
Special types
Special built wideband current transformers are also used (usually with an oscilloscope) to measure the waveform of high frequency or pulsed current in a pulsed power system. Unlike the CT used for power circuits, wideband CTs are rated in the output voltage per primary current ampere.
If the load resistance is much smaller than the inductive impedance of the secondary winding at the measurement frequency, the current on the mainline secondary track and the transformer provides an output current proportional to the measured current. On the other hand, if the condition is not correct, then the transformer is inductive and gives differential output. The Rogowski coil uses this effect and requires an external integrator to provide an output voltage that is proportional to the current being measured.
Standard
Ultimately, depending on the needs of the client, there are two main standards designed by current transformers. IECÃ, 61869-1 (in the past IECÃ, 60044-1) & amp; IEEE C57.13 (ANSI), although Canadian and Australian standards are also recognized.
High voltage type
Current transformers are used for protection, measurement and control in high voltage electrical substations and power grids. Current transformers can be mounted inside a switchgear or in an apparatus busing, but often free outdoor current transformers are used. At the switchyard, the live tank transformer has a substantial portion of their enclosure which is energized at line voltage and must be mounted on the insulator. Dead tank The current transformer isolates the measured circuit from the enclosure. CT live tanks are useful because of short main conductors, which provide better stability and higher short-circuit ratings. The main of the entanglement can be distributed evenly around the magnetic core, which provides better performance for overload and transients. Because the main isolation of the tank life current transformer is not exposed to heat of the main conductor, life insulation and thermal stability are improved.
High voltage current transformers may contain multiple cores, each with secondary windings, for different purposes (such as meter circuitry, control, or protection). A neutral current transformer is used as a protector of Earth's disturbance to measure the current of noise that flows through the neutral line from the neutral point of the transformer.
See also
- Instrumentation
- Transformer type
- Current sensing technique
References
-
Guile, A.; Paterson, W. (1977). Power System, Volume One . Pergamon. p.Ã, 331. ISBNÃ, 0-08-021729-X.
External links
- Intro to Transformer Now
- Testing Current Transformer
Source of the article : Wikipedia
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