If for some reasons, the ampere meter is disconnected from the secondary windings, it is necessary to short circuit the secondary winding. A current transformer (CT) is a type of transformer that is used to reduce or multiply an alternating current (AC). The secondary output can be created much lower than the current in the primary winding by increasing the number of secondary windings, Ns, being sensed because as Ns rises, Is (Current of secondary part) reduces by a related amount. The proper combination of core type and coil arrangement can greatly reduce the total reactance, thus reducing errors. In practice, they are intended to be used over insulated conductors when the conductor voltage exceeds 600 V. It is common practice to utilize a 600 V class window type in conjunction with air space between the window and the conductor on higher-voltage systems. Typically, the single straight line considers as primary winding. Accuracy class for relaying CTs: Relaying CTs requires lower accuracy. The accuracy classed defined by IEC are 5P, 10P, PR, PX or TP and those defined by IEEE are C 100-800. Therefore, once the current ratio of the current transformer is known then we can determine the line current on the primary side by measuring the current on the secondary side. Why is a Transformer Rated in kVA, but Not in kW? It is also being introduced as the series transformer. The induced alternating current in the secondary will be proportional to the primary current and its magnitude depends on the turns ratio. Either way, the primary conductor is a single turn through the window, fully insulated from the secondary winding. Window-type current transformers generally have a round window opening but are also available with rectangular and oval openings. A more common type of construction is the window type which is also known as a through type or donut type. Therefore, if we know the current ratio and the rating of AC ampere meter, we may determine the line current. Current transformer is used to step down the current of power system to a lower level to make it feasible to be measured by small rating Ammeter (i.e. 1 A, 5A. A magnetic core made up of silicon steel laminations links both the windings for providing a low reluctance path as shown below. The core, if of a nickel-iron alloy or an oriented electrical steel is almost certainly of the continuou, At very high voltages, the insulation of the current carrying conductor from the measuring circuit becomes an expensive problem. The phasor diagram of the current transformer is illustrated in the figure below. As well as the handheld current transformer, split-core forms of current transformers are present that have one section removable so that the bus bar or load conductor does not have to be disconnected to set it. 5. This bar assembly can be permanently attached or held in place with brackets. Home > Transformer > What is the Current Transformer? If you are facing with any issues or want to request any article please feel free to contact us, and also check out our privacy policy. Current transformers are used extensively in metering and circuit protection. The bar must be sized to handle the continuous current to be passed through it, and it must be mechanically secured to handle high-level short-circuit currents without incurring damage. Burden in VA. 1. Toroidal Current Transformers, Bar type Current Transformers, and . [1] Being Un the rated voltage of the network: HV Un 60 kV; MV 1 kV < Un 49.5 kV. The windings must be designed to withstand, without damage, the large short circuit forces that are caused when a short circuit takes place on the system in which the current transformer is connected. 2. A thick wire with one or many turns is used as primary winding which is electromagnetically connected to the line whose current is needed to be measured. So, for instance, a current transformer with a ratio of, say, 300/5 can be modified to another of 150/5 or even 100/5 by passing the basic primary conductor within its interior window two or three times. The current transformer can be used in commercial-grade applications, such as control panels and panelboards, and for most industrial metering and relay applications in switchboards, switchgear and motor control centres. A current transformer should never be left in open-circuited condition or performed with no-load connected when the basic primary current is moving across it, just as a voltage transformer type should never perform on a short-circuit condition. A current transformers primary winding is often configured in series form with the main conductor. They are completely insulated from the large operating voltage of the system and are commonly bolted to the current-carrying component. The windings should be close together to reduce the secondary winding leakage reactance as the leakage reactance increases the ratio error. The current transformer (CT) is a type of instrument transformer designed to measure high-value current in the power system. The distribution of the cut(s) and gap(s) helps control the magnetizing losses. placed on panel boards can be used (Fig. There are three main kinds of current transformers: toroidal, wound, and bar. Usually HV CT use oil or gas (SF6) as insulation medium and MV CT use synthetic resins. Save my name, email, and website in this browser for the next time I comment. CT secondary circuit must be grounded, and grounded at one point only. 2.7.4 Current transformer construction. Itproduces an alternating current (AC) in its secondary which is proportional to the AC currentin its primary. These small currents in the secondary winding are then measured by the low-range ammeter connected across it. The primary of a CT typically has only one turn, which is nothing but the current-carrying conductor passed through its window. Also, consider that the number of rolls and the current in the secondary and primary windings are related by an inverse ratio. In the case of a large number of secondary winding turns, requiring more than one winding layer, the frequently adopted technique is to sectionalize the secondary winding so as to considerably reduce the peak voltage between layers. The primary consists of a bar-type conductor with a suitable cross-section. The center of the current transformer is accumulated with lamination of silicon steel. In both types, the core and windings are enclosed in a porcelain structure. . These contain a copper or aluminium busbar surrounded by the secondary winding wound over the ferromagnetic core. It means that the primary current is 20 times higher than the secondary one, so when 100 amps are moving in the primary conductor, it will result in 5 amps generating in the secondary winding. Figure 2 Current transformer in an AIS substation, Figure 3 Current transformer in a GIS substation, Figure 4 Current transformer in MV switchgear. Good to Know: It is a very good information about CT for electrical knowledge. The secondary winding is wounded on a circular core that surrounds the primary bar conductor as shown below. It transforms the primary current to a proportional secondary value in terms of magnitude and phase. They are directly connected to the current-carrying conductor. In some, the bar carrying high current may act as a primary. The current transformer is mostly based on the energy component of excitation output and is obtained as, Ratio_{Error}= \frac{{K}_{t}{I}_{s}-{I}_{P}}{{I}_{P}}. The primary current and secondary output of the current transformers are related to each other. Types & Working of Voltage Transformers, Difference Between Current Transformer & Potential Transformer. The bar may be of rectangular or circular cross-section. The secondary winding is fully insulated from the primary that is at CT rated voltage. They have separate primary and secondary winding. It transforms the large-voltage current into the low-voltage current, due to which the heavy current moving across the transmission lines are safely controlled by the ammeter. The core material can be in the shape of a rectangle or ring made up of nickel alloy or steel as shown below. Bar type current transformers consist of a single bar as a primary winding. All Rights Reserved. Coupling is almost impervious to the primary-conductor position, provided that the return path is sufficiently distanced from the outer periphery of the secondary winding. The magnitude of the primary and secondary voltages depends on the number of rolls on the windings. The instruments (ammeter etc.) This is sometimes provided to fit a specific bus arrangement found in the rear of switchgear panels or on draw out-type circuit breakers. A bar-type current transformer which has 1 turn on its primary and 160 turns on its secondary is to be used with a standard range of ammeters that have an internal resistance of 0.2. In a low voltage wound type current transformer the secondary winding is wound on a bakelite former or bobbin and the heavy primary conductor is either wound directly on top of secondary winding, suitable insulation being first applied over the secondary winding or the primary is wound entirely separately, taped with suitable insulating material and then assembled with the secondary winding on the core. Then the primary current or load current I1 is given as. The turns ratio of a current transformer is made such that the full load current in its primary give a secondary current of either 5A or 1A. 2.5.3 Core-Balance Current Transformers. if(typeof ez_ad_units!='undefined'){ez_ad_units.push([[300,250],'electricalclassroom_com-box-3','ezslot_2',190,'0','0'])};__ez_fad_position('div-gpt-ad-electricalclassroom_com-box-3-0');A current transformer is a type of instrument, used to measure or sense large alternating currents by scaling it to a smaller, safer and measurable value. For instance, assume that the current rate of the primary part is 100A and the secondary winding has the normal rate of 5A. The primary has few numbers of the turn of heavy wire, and the secondary of the current transformer has many turns of fine wire. In other words, the secondary of the current transformer should be never left open under any circumstances. Also, the voltage in the secondary winding will be 200 times the primary voltage. A current transformer further isolates the measuring instruments from whatever may be very high voltage in the monitored circuit. Such use may be seen in isolated-phase bus compartments. Also important to consider in a CT is the magnetizing curve that is similar to the one shown in Figure 9. 3: Bar Type CT. Precautions in the use of C.T. At very high voltages, the insulation of the current carrying conductor from the measuring circuit becomes an expensive problem. 5A ammeter). It is difficult to make measurement devices such as an ammeter or a wattmeter(kWh) meter and protection relays that can carry hundreds or thousands of amperes. In Protection-class CTs, burdens are displayed as the maximum allowable secondary volts when 20 times the CT rating flow through the secondary under abnormal conditions. The most common type is that wound on a toroidal core. The split half consists of 2 finely grounded or lapped gap faces. 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