Instrument transformers are high accuracy class electrical devices used to isolate or transform voltage or current levels. Potential transformer voltage transformer is used for protection and measurement. Whenever an alternating current flows through the primary winding, alternating magnetic flux is produced, which then induces alternating current in the. Explain and interpret information found on transformer name plates. Allentown, pa abstract more and more substations are retrofitted with numerical relays, meters and monitoring devices. This is a common method used in portable current measuring instruments but permanent installations use more economical types of current transformer. Cts current transformers pts potential transformers. Some other differences between the current and the potential transformer are explained below in the the comparison chart. Indentify the schematic symbols of potential and current transformer. Secondary current is dependent upon the voltage in addition to the load resistance. A secondary current rating of 2a and 1a may also be used in some cases if the number of secondary turns is low and the ratio cannot be adjusted within the required limits by the addition or removal of one turn, if the length of secondary connecting lead is such that the burden due to them at higher secondary current would be excessive. The basic principle of the current transformer is the same as that of the power transformer. Mersen pt fuses are currentlimiting fuses with high interrupting rating, used for the primary winding protection of potential transformer. Potential transformer three phase potential transformer.
Like the power transformer, the current transformer also contains a primary and a secondary winding. The polarizing current may be obtained in a number of different ways, among which are. Voltage transformer is necessary for directional, voltage, distance protection. An optical voltage transformer exploits the faraday effect, rotating polarized light, in optical materials. The guiding system, the new way to create your electrical installations a comprehensive offer of products with consistent design. The terminal of the transformer should be designed so that the variation of the voltage ratio with load is minimum and the phase shift between the input and output voltage is also minimum. Automatic current and potential transformer testing instrument type wdst00 current transformer test part nominal currents of the ct to be tested x 5, 2, and 1 a. Such specially constructed transformers with accurate turns ratio are called as instrument transformers. Feb 12, 2017 in this video you will learn about what is current transformer, important concepts of current transformer, how current transformer work, use of current transformer. Current transformers are represented by the following symbol. Current transformers in the tertiary of the power transformer 3.
The capacitor voltage transformer cvt uses a capacitance potential divider and is used at higher voltages due to a lower cost than an electromagnetic pt. Routine test report together with instrument transformer are delivered. Automatic current and potential transformer testing instrument type wdst00 current transformer test part nominal currents of the ct to be tested x 5, 2. A current clamp uses a current transformer with a split core that can be easily wrapped around a conductor in a circuit. Jun, 2019 secondary current is dependent upon the voltage in addition to the load resistance. A typical connection diagram of a current transformer is shown in figure below. This section is complemented with photographs and describes the specific classes and uses for. Equivalent current transformer circuit x 1 primary leakage reactance r 1 primary winding resistance x 2 secondary leakage reactance z 0 magnetizing impedance r 2 secondary winding resistance z b secondary load note. The current transformer in addition to the potential transformer is referred as instrument transformer. May 22, 2017 such specially constructed transformer with accurate turns ratio are called as instrument transformer. They are small dimension ferrule type fuses and mount in standard clips. Commercially available relays and meters used for protection and metering, are designed for low voltage. Automatic current and potential transformer test set. Most often, the older relay gets replaced with a newer relay with the rest of the installation such as current transformer ct and lead.
Then to summarise this transformer basics tutorial. 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. If a ct test switch is used, the switch must have a makebeforebreak contact pattern to. The potential transformer is made with highquality core operating at low flux density so that the magnetising current is small. As discussed we step down the value of primary voltage by decreasing the turns in secondary winding. This ground loop typically creates a potential across the operating coil of the differential relay causing the relay to pick up as though a fault. This means that when 480 volts is applied to the primary winding, 120 volts will be induced on the secondary. A ct has large number of turns on its secondary winding, but very few turns.
Potential transformer testing ptvt valence electrical. Potential transformer or voltage transformer gets used in electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings meters and relays. Our clients can avail from us high quality potential transformer. One of the major difference between them is that the current transformer convert the high value of current into low value whereas the potential or voltage transformer convert the high value of voltages into low voltage.
In this article we are going to discuss in detail about the potential transformers. Actually they are protective type voltage transformer. The open secondary may lead to failure of the transformer. Ensure the ct is connected to system properly in all respect primary and secondary. Current transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the actual electrical current flowing in an ac. Dec 30, 2018 potential transformer or voltage transformer gets used in electrical power system for stepping down the system voltage to a safe value which can be fed to low ratings meters and relays. You can transform the voltage or current down with a transformer whose turns ratio is accurately known, then measuring the stepped down magnitude with a normal range instrument. Current transformer basics and the current transformer. The current transformers reduce the level of current to the instrument or relay operating range, whereas potential transformers transforms the high voltage to a circuit operating low voltage. Current transformers are constructed in various ways. A general overview of abbs current and voltage transformers. Jun 01, 2015 the current transformers reduce the level of current to the instrument or relay operating range, whereas potential transformers transforms the high voltage to a circuit operating low voltage. Compare and contrast the performance of three phase transformer connections.
This is a simplest form of potential transformer definition. The electromagnetic potential transformer is a wirewound transformer. Such specially constructed transformer with accurate turns ratio are called as instrument transformer. The use of current or voltage transformer for other purpose then described above is forbidden if not agreed with the producer. Working of potential transformer the electrical portal. This transformer step down the voltage to a safe limit value which can be easily measured by the ordinary low voltage instrument like a voltmeter, wattmeter and watthour meters, etc. The original magnitude can be determined by just multiplying the result with the transformation ratio. The result is a significant current flow through the ct circuit that is not representative of the primary current. We have broken them up into several different groupings. Current transformer, potential transformer working instrument. Instrument transformers convert signal levels from dangerous high voltage or inconvenient high current, or current at high voltage to levels appropriate for metering.
Current transformer, potential transformer working. Current transformers ct and potential transformers pt are used to measure the current and voltage in a circuit of the order of hundreds of amperes and volts respectively. Thepoten tialcircuitofthewattmetermaynowbeusedasavoltmeterto. Due to the covid19 outbreak, people in ohio have been ordered to shelter in place until april 6, 2020.
Supreme accuracy current transformer super accuracy for 0. Brooks testingpotentialtransformers 421 needstobemeasuredwithonlymoderateaccuracy. A transformer is a device which consists of two windings called primary and secondary. List characteristics on these devices and explain how. Potential transformer testing ptvt i cowrote an article for the neta world magazine in 20042005 that covers several different manual techniques for potential transformer testing that explains several lowtech solutions for potential transformer testing. To confirm the physical condition and electrical characteristics of current transformer installed in the installation.
Transformer types instrument transformers instrument transformers comprise a large category of current and potential transformers for various voltage, frequency and physical size ranges. In the latter case, the device is known as a coupling capacitor voltage transformer ccvt, and when the transformer primary winding is directly connected to the. In this video you will learn about what is current transformer, important concepts of current transformer, how current transformer work, use of current transformer. A potential transformer is a conventional transformer having primary and secondary windings. It is important to note that one of the main function of instrument transformers is t. Nov 15, 2019 the value of the rated secondary current is 5a. Difference between current transformer and potential. Current transformer in the power transformer neutral 2. Checked against enormous quality parameters, this potential transformer is highly demanded among our clients. Identify transformer polarity using dot and conventional labeling. Instrument transformer basic technical information and. 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.
Current polarization may be used at those points in the system where power transformers having suitably grounded neutrals are located. A transformer consists of two electrically isolated coils and operates on faradays principal of mutual induction, in which an emf is induced. Current transformer circuits for power electronics. Transformers used for voltage potential measurement steps down the voltage to the level of voltmeter. 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. Instrument transformers comprise a large category of current and potential transformers for various voltage, frequency and physical size ranges. Voltage transformer or potential transformer theory. Industry standards have established 120 volts as the secondary rating of potential transformers having primary ratings up to 24,000 volts and 115 volts as the secondary rating of pts having ratings above 24,000 volts. Current and voltage potential instrument transformers. A transformer changes the voltage level or current level on its input winding to another value on its output winding using a magnetic field. Basics, maintenance, and diagnostics vii contents continued page 4. A combination of potential transformer and voltmeter helps us to find out the higher values of potential across the line. Current transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the actual electrical current flowing in. The basics of current transformers nk technologies.
Ferrule mounting for standard clips and interchangeability with other brands. One method is quite similar to that of the potential transformer in that there are two separate windings on a. The instrument transformers like voltage transformer and current transformer are used for connecting the electrical instruments to the measuring instruments. It transfers energy from one side to another with suitable change in the level of current or voltage. The most common usage of instrument transformers is to operate instruments or metering from high voltage or high current circuits, safely isolating secondary control circuity from the high voltages or currents. Generally, the secondary side current rating is 1 ampere or 5 amperes. Potential transformer pt definition the potential transformer may be defined as an instrument transformer used for the transformation of voltage from a higher value to the lower value. Finds a wide application in protection circuits of power system ex. As their names suggest, these are used to transform current and and voltage to the value which then can be measured andor observed by the connected instruments. This potential transformer is manufactured using premium quality material and advanced machines under the supervision of our ingenious professionals. Current transformer circuits for power electronics applications. Used in ac system for the measurement of current, voltage, power and energy.
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