An alternating current is passed to the primary of the transformer and the measurement/sensing/protection device is connected to its secondary. Nearly all window-type current transformers manufactured today are rated 600 V class. Wire wound Current Transformer can be used to measure currents in the range of 1A to 100A. Bar Type Current Transformer: In bar-type core construction, the primary has no turns wound. Outdoor wound-type current transformers (CTs) are available 5-69 kV, 60-350 kV BIL, up to 1200 A. Wound-type units are available in two distinct . 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. But in a practical current transformer, there is a phase difference between the secondary and the primary current since the primary output has also supplied the portion of the exciting current. CT may have one or more cores; typical applications of these cores are: The use of more than one core for protection is justified when the installation have two protections sets main and backup. The Specification of CT 1 . Instead, the line that runs the current flowing in the system is threaded across a hole or window in the toroidal transformer. Your email address will not be published. The current rate will determine the turn ratio, and as the primary winding commonly includes one or two turns, whereas the secondary winding can include several hundred turns, the rate between the primary and secondary can be relatively large. In the shape of a doughnut, these transformers are designed without having a primary winding as part of their construction. A window current transformer has an open area in the centre for a power line to be passed through as the primary . Figure 3. StudyElectrical.Com | Online Electrical Engineering Learning Site The construction of the current transformer can be wound or bar type. In a bar type current transformer, the core and secondary windings are the same as in a ring type transformer but the fully insulated bar conductor constituting the single turn primary winding is now an integral part of the current transformer.The insulation on the primary winding conductor may be bakelized paper tube or a resin directly moulded on the bar. The body of the CT itself provides insulation between the primary circuit and ground. The following are different types of current transformers: A current transformer having a primary winding of more than one full turn wound on core. Current-Transformer.ppt - Free download as Powerpoint Presentation (.ppt), PDF File (.pdf), Text File (.txt) or view presentation slides online. Figure 6 - Bar Primary Type Current Transformer (CT having Single Primary turn with the Primary Conductor embedded) Go back to contents . Often only 50% of the effective magnetic path is used. 50,60. It uses the actual cable or bus bar of the . So measured readings are obtained at high accuracy. 1 to former Standard 60044 to include the accuracy class X, defined on the withdrawn BS 3938:1973. Current transformers are used with ammeters, kWh meters, power factor meters and energy meters for current measurements. . Electrical Deck is a platform for learning all about electrical and electronics engineering. 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. The secondary of the transformer has many turns depending on the magnitude of the current to be stepped down. Then the primary current or load current I1 is given as. In a ring-type core structure, the secondary winding is wound on the inner core of the bakelite former. The center of the current transformer is accumulated with lamination of silicon steel. Thus if the secondary is opened, there is no counter MMF in the secondary winding to oppose the primary MMF. We develop your standard equipment, but also . Rather, a toroidal transformer has a window or hole through which the current flowing in the network passes. Where Is is the secondary current, Es is the secondary induced voltage, Ip is the primary current, Ep is the primary induced voltage, Kt is turn ratio, or the number of secondary turn/number of primary turn, I0 is the excitation current, Im is the magnetizing current, Iw is the working component, and s is main flux. Based on the function performed by the current transformer, it can be classified is follows: . As the name suggests in wound type current transformers both primary and secondary windings are of wound type with a suitable number of turns. The figure below shows thewound type transformer. The primary of a CT typically has only one turn, which is nothing but the current-carrying conductor passed through its window. Current transformers are available in various sizes and shapes and are used as an interface between high currents and measuring/sensing devices. Why are Air-Conditioners (AC) Rated in Tons, Not in kW or kVA? Table 1 Common accuracies and burden powers of CT and error limits. When applying auxiliary CTs, the user must be aware of their reflected impedance on the mainline CT. The current transformer has high electrical isolation. 1 A, 5A. The primary winding consists of very few turns and, therefore, there is no appreciable voltage drop across it. Current transformers (CT) are used in High Voltage (HV) and Medium Voltage (MV)[1] installations to give an image of electrical current to protection relays and units and metering equipment and they are designed to provide a current in its secondary proportional to the current flowing in its primary. The accuracy classed defined by IEC are 0.2 or 0,2S, 0.5 or 0.5S and those defined by IEEE are 0.15 or 0.15S, 0.3 or 0.6. Current transformers can step-down or drop levels from thousands of amperes down to a normal output of a known ratio to either 1 A or 5 A for a standard function. Required fields are marked *. Wound type current transformer. The knee point is defined as the point at which a 10% increase in voltage produces a 50% increase in magnetizing current. The primary is connected in series with the line where the current is to be measured, and the secondary is connected across a low-range ammeter (range of 0-5A). Current transformers are considered to be applied as proportional current instruments. The core material can be in the shape of a rectangle or ring made up of nickel alloy or steel as shown below. Wound-type current transformers, T-type classification, are . Common value for RF is 1.5. Basically, it is a step-down transformer (with respect to current) consisting of two windings, primary and secondary without any electrical connection between them. They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device. Wound Current Transformers: The transformers primary section is physically configured in series form with the conductor that carries the detected current flowing in the system. Exciting Current) can . The secondary part, however, may have a great number of coil rolls wound on a laminated component of low-loss magnetic material. 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 reassembly of the core halves is critical. It transforms the primary current to a proportional secondary value in terms of magnitude and phase. 2.5, 3.75. A current transformer is a device that is used for the transformation of current from a higher value to a lower value. A bar current transformer is a special type of window current transformer with a solid bar placed permanently through the window. Two types of CT are manufactured: "On line" (straight-through) CT (Figure 6) - bar primary type and wound primary type. Therefore, the secondary of the current transformer may never be open when the primary part is carrying the current. Here, we can also say that the current transformer has a primary and secondary turn ratio of 1:200. It is a very good information about CT for electrical knowledge. Special precautions must be taken when connecting CT primary (connection points are usually identified by P1 and P2), and secondary (connection points are usually identified by S1 and S2) in order to assure the correct flow of electric current and the proper functioning of the devices, what is explained in Figure 5. Difference Between Single Phase and Three Phase Transformer. The simplest form any current transformer can take is the ring type or Window type, examples of which are given in the below figure which shows three . A current transformer is similar to a electrical power transformer to some extent, but there are some difference in construction and operation principle. It is important to monitor current and voltages at the substations and generating stations which are at high values. 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. Ring type is the most usual type of core-balance current transformer (CBCT). The transformer is fundamentally a step-down transformer (with respect to current), consisting of primary and secondary windings without any electrical connection between them. When large currents pass through the primary winding, it induces small currents in the secondary winding (depending upon the turns ratio). A. true. When the current moves across the primary part, it often flows within secondary windings, and amperes turns of each section are subsequently identical and opposite. At 750 kV, cascaded current transformers are used or alternatively a coaxial shunt is used to modulate a radio frequency signal that is transmitted from the shunt placed in the high voltage line to receiving equipment on the ground, thereby overcoming the insulation problem.However. In high line voltage systems oil-immersed current transformers are used. In a split core current transformer, the core is split, each half having two finely ground or lapped gap faces.These current transformers are assembled onto the primary conductor on site for either permanent or temporary duty. The ammeters scale is adjusted according to the turns ratio. Figure 8 Schematic connection of primary CT windings with double ratio. The maximum current up to which the transformer must maintain its current ratio is 40,000 amperes. We are specialized in offering Summation Current Transformer to our clients. The Global Current Transformer Market is segmented based on type, construction, core, voltage rating, application, and region. Bar-type Current Transformer - This type of current transformer uses the actual cable or bus-bar of the main circuit as the primary winding, which is equivalent to a single turn. Types of Instrument Transformers. Indoor and Outdoor Current Transformers Overview - Specifications. What is the bar type current transformer? A current transformer (CT) is a type of transformer that is used to measure AC current. The proper combination of core type and coil arrangement can greatly reduce the total reactance, thus reducing errors. A typical connection . They are constructed to supply a current in its secondary winding related . 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. Why is a Transformer Rated in kVA, but Not in kW? The phasor diagram of the current transformer is illustrated in the figure below. The type of wire used as the main current carrying conductor in a transformer winding is either copper or aluminium. This causes excessive core losses and produces excessive heat. The secondary of a CT cannot be left open-circuited because it may cause an abnormally high voltage to appear on the secondary and will damage the core and the windings. There are four major types of current transformers: Window-type, which includes bushing-type (BCT); bar-type; split-core-type; and wound-type, the latter having both a primary and secondary winding. . The working principle of a standard current transformer is moderately different from that of an ordinary voltage transformer. The Mumetal or Permalloy is used for making cores to obtain a high degree of accuracy. . a) primary winding consists of a rod of suitable size and material. If the turns ratio between primary and secondary is 100:05; then it will step up the level of voltage in terms of 20:01 or it will step down the level of current in terms of 01:20. 3). They have very similar construction to that of normal transformer. Instrument transformers are of two types - Current Transformer (C.T.) After the secondary winding has been placed on the core, the ring type transformer is completed by exterior taping with or without first applying exterior end collars and circumferential insulating wraps. D. 5. The current transformer connected to a small line voltage system uses tape or varnish as insulation. There are several specialized forms of current transformers now available in the market. In terms of voltage, it is a step-up transformer which steps-down the level of current. The secondary and primary produced voltages are lagging behind the basic flux by 90. The primary winding and secondary winding(s) are insulated from each other and from the core(s) and are . The primary section is a simple turn winding (also introduced as a bar primary) and carries the full-load current, while the secondary part of the transformers has several turns. Good to Know: In addition to that, core losses due to high flux density would heat up the core and the windings and damage them.