Ct is a very dependable and accurate device for the most part except when in goes in to saturation. Impact of transient saturation of current transformer during cyclic. Supreme accuracy current transformer super accuracy for 0. This protection element is commonly used on generator stepup transformers. Polarity refers to the instantaneous direction of the primary current with respect to the secondary current and is determined by the way the transformer leads are brought out of the case. The minimum primary inductance required l p is a function of how accurate and linear the volts per ampere versus current transfer function of the transformer needs to be. Current transformer saturation leads to protection devices misoperation as the measured.
These standards represent the state of the art of classical instrument transformers used in the different electrical. Unfortunately, standard tests usually do not drive transformer cores into deep saturation and may lead to large errors in the estimation of the air core inductance affecting signi. When saturation occurs, the secondary current will not be linearly proportional to the primary current, which may lead to maloperation of protection devices. Polarity all current transformers are subtractive polarity. What happens in current transformer core during ct saturation. Current transformer saturation electrical engineering.
A current transformer can be modeled as a constant current source where ratio current is injected into a magnetizing impedance in parallel with the burdens shown in figure 1. Sorting through this vast array of information to piece together a complete understanding of. It produces a current in its secondary which is proportional to the current in its primary. The capacitive voltage transformers were standardized through the iec 6004451 and 600446 was the additional transient accuracy specification for current transformers.
Transient behaviour of conventional current transformers used as. Pdf saturation of current transformers and its impact on digital. Current transformer ct saturation is not a new topic, and. This thesis demonstrates a method for transformer core diagnostics. Estimation of transformer saturation characteristics from inrush current waveforms article pdf available in ieee transactions on power delivery 211. The component of ct saturation caused by high remanent flux or a preexisting. The transformer model used for this experiment is the equivalent circuit from the transformer model experiment, except we. The bottom line is that the current waveform on the secondary of a saturated current transformer may be diverse depending on what components are present on the primary current and also on the burden wired to the secondary circuit of the current transformer. Modeling current transformer ct saturation for detailed. Pdf modelling of current transformer saturation for power. The current transformer function is different than that of a voltage transformer.
The basics of current transformers nk technologies. This data should not be used for simulation purposes carelessly, as noload test results usually provide voltages and currents in rms values. The fundamentals of current sense transformer design patrick a. The impact of high fault current and ct rating limits on. The purpose of a current transformer ct is to produce an exact replica of the primary current waveform with its magnitude reduced in proportion to the turns ratio. Current transformer errors and transformer inrush as. Power transmission and distribution author page 244 200708. Ct saturation is a term used to describe the state wherein a ct is no longer able to reproduce an output current in propotional to its primary current or as per its ratio. Standardization in the field of current and voltage. Pdf improper selections of current transformers will result in cts saturation during fault conditions which can affect protection relays. A routine series of diagnostics and testing for your current transformer should include ct saturation testing to identify the knee point against ieee or iec standards, ct ratio tests to identify that the voltage ratio is within expected parameters and current transformer resistance testing for both the transformer windings and ct insulation.
Thompson, and brad heilman, schweitzer engineering laboratories, inc. A practical guide to ct saturation ariana hargrave, michael j. Mathematical model for current transformer based on jilesatherton theory and saturation detection method current transformer saturation will cause the secondary current distortion. Finally, we explain how ct saturation can affect relay operation. Calculation of the current transformer accuracy limit factor. Current transformer saturation voltage disturbance. Seen in some magnetic materials, saturation is the state reached when an increase in applied external magnetic field h cannot increase the magnetization of the material further, so the total magnetic flux density b more or less levels off. Prior to joining schweitzer engineering laboratories, inc. Figure 5 shows the result of the curve fitting for core 1, considering the y axis to be the current and x axis to be the voltage, the following expression give the standard. When secondary current vector lags the primary current vector. Current transformer ct saturation leads to inaccurate current measurement and, therefore, may cause malfunction of protective relays and control devices that use currents as input signals.
The lower current ratio required on the high side of the transformer, together with longer leads, conspires to cause saturation during offset, while the well rated low side cts have a higher ratio and remain linear. Pdf saturation of current transformers and its impact on. Thus i s would be a scaled down version by a factor of n. More specifically, if 120 volts were applied to the secondary of a 6005 current transformer 120. A current transformer ct is used to measure current in a shunt inductor connected on a 120 kv network. Saturation is a characteristic of ferromagnetic and ferrimagnetic materials, such as. The subsequent subsections discuss problems encountered by different techniques, a new current transformer saturation detection algorithm, testing of the proposed ct saturation detection algorithm using field data 5. Mohanan achary transformers and electricals kerala ltd. A current transformer operates with a set primary current and will try to output a constant current to the load, independent of the load. As vs increases because of the current or the burden impedance increasing, the excitation current increases and the magnetic flux in the core also increases. Correlation between saturation of high current power. Secondly, simulated waveforms of ct saturation were injected into a transformer.
Using a reactance to represent the magnetizing leg of a ct, as shown in figure 1, is a useful visual concept. 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. While some players feel core saturation affects the sustain and distortion an amp produces, it can have an adverse affect on tone, reducing bass and dulling highs. The current source represents the magnetic core saturation, as shown by a block diagram format in figure3. Simple approach integration of ideal ct secondary voltage the analysis of ct flux levels under the presence of a mix of symmetrical ac and other nonsymmetrical current wave forms, especially if any modeling of ct saturation is to be included, is a rather involved process. Protection element vhz ansi 24 can be used to protect the transformer against over excitation as this can cause damage due to overheating excessive core loss. Basic principles of control of transformer core magnetic. Pdf this work investigates the effect of current transformer ct saturation on protection schemes. Current transformer saturation, ratio and polarity test set present on the primary winding would be 1 volt. Current transformers, along with voltage or potential transformers, are instrument transformers. This leads to increased harmonic distortion, increased reactive power. If you were really applying 230v there would be a fire you are more likely to be applying a current through the ct that is sourced from an ac voltage and that voltage happens to be 230v. Using the noload current for the diagnostic gives high sensitivity. Without a magnetic field, the magnetic domains in the core material are pointing in random directions.
In the linear region of operation, magnetizing current i m1 is very small and hence i p i m is approximately equal to i p. Current transformer saturation ct saturation can be explained using the simplified equivalent circuit shown in figure 3. At this level if we increase input voltage then more current produced in core that will burn your transformer. Sizing current transformers for line protection applications. The method uses the noload current of the transformer as an indicator, and gives different characteristic signatures for different types of faults or defects.
Magnetization and primary injection test of current transformer for protective relaying jacob tsado 8 current transformers. Calculation of cts if the current transformer has primary to secondary current ratio of 100. A condition where a transformers core is fully magnetized and is producing maximum magnetic flux. There are a host of adverse effects that can accompany the presence of a direct current in a transformers windings.
Stand alone merging unit, ct saturation with extremely low burden. Example the distance between the cts and the protection relay is 15 metres, 4 mm 2 cu conductors in 4wire connection are used. It continues to increase very slowly due to the vacuum permeability. The transformer core can saturate due to, high voltage, low frequency or a combination of both. Pdf estimation of transformer saturation characteristics. Modeling the transformer saturation in pscad emtdc. Instrument transformers scale the large values of voltage or current to small. Correlation between saturation of high current power transformer core and load. So the real issue is how much current is the primary being fed. The primary winding which consists of a single turn passing through the ct toroidal core is connected in series with the shunt inductor rated 69.
This typically happens when the transformer is not large enough for the application. Current transformer saturation, ratio and polarity test. If a ct test switch is used, the switch must have a makebeforebreak contact pattern to. Calculation of the current transformer 1mrs 755481 accuracy limit factor 3. Current transformer saturation davidbeach electrical 21 sep 06 16. You can see the primary current occurring at the same time as the secondary current, but then the primary current has a substantial peak of about 1.
Current transformer ct saturation is not a new topic, and there have been many papers, books, application guides, and tutorials written on the subject. In figure4, the voltage across the first winding is represented by e1 and the voltage across the second winding is represented by e2. Power transmission and distribution author page 250 200708. Abstract current transformer ct saturation, while a fairly common occurrence in protection systems, is not often clearly. Analogue current input elements, protection ieds, conventional cts, transient response. Magnetizing currents in power transformers measurements. Telk, angmally introduction current transformers cts are used to transform large primary currents to a small secondary current suitable for instrumentation and protective relay systems. And the saturation characteristic in transformer block should be filled by flux vs current. A current transformer ct is a type of transformer that is used to reduce or multiply an alternating current ac. The current transformer is designed to connect in series with the line to transform the line current to the standard 5 amperes suitable for the. The transformer core has been driven well into saturation. Current transformer, simplified equivalent circuit. However, the relay detects the second harmonic present in the false differential current and blocks tripping for external faults. B2 current and voltage transformers schneider electric.
Current transformer saturation steadystate saturation with ac current transient saturation with offset current. This test is performed automatically during a ct saturation test or manually with. The predominant effect that is witnessed is half cycle saturation. The current transformer will operate into either a short circuit or a resistive load until the voltage induced is enough to. Magnetization and primary injection test of current. And the effect of load current and load power factor on the transformer currents.
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