CTs selection for protection plans. endstream endobj 151 0 obj <> endobj 209 0 obj <> endobj 162 0 obj <>/Font<>/ProcSet[/PDF/Text]/ExtGState<>>>/Type/Page>> endobj 152 0 obj <> endobj 208 0 obj <>stream Find the secondary voltage ES on the vertical scale of the excitation curve and read over to the 200 line and down to the horizontal scalefor the secondary exciting current IE.

Disturbances and monitoring of the electrical values of a network. First, the calculation of the actual burden of the CT, including connection wires and protection relay impedance, is presented. This protects connected metering devices from overloading in the presence of fault level current flows. Current transformers use the term Accuracy to describe what would generally be considered Regulation with a power transformer. Current transformers (CT's) provide a simple, inexpensive and yet accurate means of sensing current flow in power conductors. Transformer feeder protections. Calculation of the Current Transformer 1MRS 755481 Accuracy Limit Factor 1. Current Transformer White Paper The Basics of Current Transformers Ratio The CT ratio is the ratio of primary current input to secondary current output at full load. They are available in 3 basic configurations: 1. application/pdf

The primary winding is usually a very low impedance and therefore treated as a "brute force" constant current source.

The transformer must have an accurately-defined current ratio to allow for interchangeability with other transformers of the same rating. When current flow exceeds that rating, the metering core will become saturated, thereby limiting the amount of current level within the device.

current range designated. The current transformers and accessories covered by this specification shall comply with the requirement of the latest edition of the following standards unless otherwise stated in this specification. • … Differential protections. Mostly, the primary of a CT is a straight- through bar, meaning the number of turns on the primary is just 1.

A current transformer operates with a set primary current and will try to output a constant current to the load, independent of the load. 3-2 The current in the secondary coil always changes by the inverse of the ratio by which the voltage changes. for measuring, metering and/or protective purposes. If the voltage is raised to 10 times its original value by the transformer, the current in the secondary coil will be reduced to one-tenth the value of the current in the primary coil. •60/80/100//112 MVA for 55/65oC temperature rise units; 12% increase in power rating for 65oC rise from 55oC rise, •24/12/12 MVA for three-circuit units (e.g. Ring Core CT's are available for measuring currents from 50 to 5000 amps, with windows (power conductor opening size) from 1" to 8" diameter. Burden defines the connection made to the secondary winding to differentiate it from the primary connection that is generally described as the Load.

The voltage developed across the instrumentation resistor will be the secondary current times the instrumentation resistor. IP = NS / NP x (IE + IS). An example would be an ampere meter that uses an external current transformer.

Figure 2. A current transformer follows all the standard physical laws for electrical transformers. To protect the windings of a transformer against overcurrent, use the percentages listed in Table 450.3(B) and its applicable notes. For example, a CT with a ratio of 300:5 is rated for 300 primary amps at full load and will produce 5 amps of secondary current when 300 amps flow through the primary. IS:2705 (Part-I-IV) : Specification for current Transformers. The primary current will be the turns ratio times the sum of the exciting current and the secondary current. 2008-12-18T09:30:26+01:00 An example would be a printed circuit-board-mounted current transformer that inputs into an operational amplifier circuit. The primary winding is usually a very low impedance and therefore treated as a "brute force" constant current source.

It is important to remember that Burden and Accuracy are interdependent; generally the lower the Burden resistance, the better the Accuracy. H}C3����T=툕��v��X��Ȭ�$j�ꆮ�������9V.-e��I���AL"r�����_M(p$ Ƹ!�[�����_x{�k�=k���, Current transformers: how to specify them. Summation Auxiliary Current Transformer 2.2 Specifying Protection Class Current Transformers In order to specify or check the ratings of a current transformer, a thorough understanding of the application is required. The IR losses act as a resistance RS in series with the secondary winding. The Ip/5 A ratio current transformer delivers at the secondary a current (Is) of 0 to 5 A that is proportional to the current measured at the primary (Ip). Data sent by current transformers are processed by protection, control and monitoring units that send signals to operate switchgear and/or information to a supervisory unit or to a central control room. CT specification examples. Motor feeder protections. Take China as an example. As Figure 1 illustrates, the eddy current and hysteresis losses act to shunt current across the transformer secondary and are defined as excitation losses IE. The number of 500 kV substations in the North China power grid is almost 2 times higher than in the past decade. IS:4201 : Application guide for current transformers. 2. Transformer feeder protections. %PDF-1.6 %����

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