Instrument transformers comprise a large category of current and potential transformers for various voltage, frequency and physical size ranges. Such specially constructed transformers with accurate turns ratio are called as instrument transformers. A potential transformer is a conventional transformer having primary and secondary windings. Voltage transformer or potential transformer theory. Current transformers reduce high voltage currents to a much lower value and provide a convenient way of safely monitoring the. A current transformer ct is used to obtain reduced current signals for purpose of measurement, control and protection. Knee point voltage of current transformer electrical.
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. List characteristics on these devices and explain how. Instrument transformer basic technical information and. Supreme accuracy current transformer super accuracy for 0. If a ct test switch is used, the switch must have a makebeforebreak contact pattern to. Current transformer basics and the current transformer. A current transformer operates with a set primary current and will try to output a constant current to the load, independent of the load. The key operation of the current transformer is absolutely no different from those of a regular transformer. To check pricing and availability for a specific potential transformer model, give us a call at 614 8896152 or use our quote request form. 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. 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. For the most current product performance data visit ep. The basic principle of the current transformer is the same as that of the power transformer. 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.
May 22, 2017 such specially constructed transformer with accurate turns ratio are called as instrument transformer. This means that when 480 volts is applied to the primary winding, 120 volts will be induced on the secondary. A current transformer ct is necessary if you need to measure the current, energy, frequency, and other parameters of the power system and when one or. Transformers used for voltage potential measurement steps down the voltage to the level of voltmeter. Since the invention of th e first constantpotential transformer in 1885, transformers have become essential for the transmission, distribution, and utilization of alternating current electrical. The use of current or voltage transformer for other purpose then described above is forbidden if not agreed with the producer.
Below is a list of all potential transformers available through flexcore, however, not all models are instock. Compare and contrast the performance of three phase transformer connections. Special transformers and applications transformers. What are a potential transformer and a current transformer. Current transformer ct and potential transformer pt are instrument transformers basically used for measurement and protection purpose in power systems.
Indentify the schematic symbols of potential and current transformer. 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. Nov 15, 2019 a current transformer is an instrument transformer, used along with measuring or protective devices, in which the secondary current is proportional to the primary current under normal conditions of operation and differs from it by an angle which is approximately zero. Difference between current transformer ct and potential. In this video you will learn about what is current transformer, important concepts of current transformer, how current transformer work, use of current transformer.
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. Automatic current and potential transformer test set. What are the uses of current and potential transformer. Used in ac system for the measurement of current, voltage, power and energy. Varnish impregnated for added protection and strength application the auxiliary current and voltage transformers change the. 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. Basics, maintenance, and diagnostics vii contents continued page 4. 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 transformers are represented by the following symbol. Instrument transformers potential transformer, current transformers for high voltage and current measurement. Instrument transformers scale the large values of voltage or current to small.
Auxiliary current or voltage transformer indoor, 2500 volts test voltage instrument transformers product features. Current transformer, potential transformer working instrument transformers. Current transformer vs voltage transformer potential transformer a transformer is a device that transfers electrical energy from one circuit to another using electromagnetic induction through inductively coupled conductors, also called the transformers coils. Current transformers are constructed in various ways. Instrument transformers convert signal levels from dangerous high voltage or inconvenient high current, or current at high voltage to levels appropriate for metering. The current transformer function is different than that of a voltage transformer. 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.
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 figure below shows the connections for testing a potential transformer using a resistance potential divider. One method is quite similar to that of the potential transformer in that there are two separate windings on a. Ensure the ct is connected to system properly in all respect primary and secondary. Cts are instrument transformers used for converting primary current into a reduced secondary current for use with meters, relays, control equipment and other instruments the importance of instrument transformer tests is often underestimated. In the current transformer, the primary winding is connected in series of the load whereas, in the potential transformer, the primary winding is. Potential transformer testing ptvt valence electrical. A current transformer ct is a type of transformer that is used to reduce or multiply an alternating current ac. An optical voltage transformer exploits the faraday effect, rotating polarized light, in optical materials.
The instrument transformers like voltage transformer and current transformer are used for connecting the electrical instruments to the measuring instruments. To confirm the physical condition and electrical characteristics of current transformer installed in the installation. Current transformers, along with voltage or potential transformers, are instrument transformers. Current transformers reduce high voltage currents to a much lower value and. The current transformer is used to measure the current so it is called a current transformer whereas the potential transformer is used to measure voltage so it is called a potential transformer. 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 potential transformer is made with highquality core operating at low flux density so that the magnetising current is small. Ct and a transformer sample for which the technical characteristics are well. The current transformer will operate into either a short circuit or a resistive load until the voltage induced is enough to. Such specially constructed transformer with accurate turns ratio are called as instrument transformer. This section is complemented with photographs and describes the specific classes and uses for. It produces a current in its secondary which is proportional to the current in its primary. In current transformer current and density vary over a wide range but in potential or voltage transformer it varies over a small range.
The original magnitude can be determined by just multiplying the result with the transformation ratio. The burden with which the transformer is to be tested is connected across the secondary winding, and the normal primary voltage, at the. Current transformer, potential transformer working instrument. Whereas in the past, due to the high burden of electromechanical relays, it was the rated power of the current transformers ct and potential transformers pt. Routine test report together with instrument transformer are delivered. Amran instrument transformers is a leading manufacturer of low voltage and medium voltage instrument transformers current and voltage transformers with multiple manufacturing plants in usa and india.
To start with, both are instrument transformers basically used for measurement and protection purpose in power systems. A current transformer ct is another special instrument transformer designed to step down the current through a power line to a safe level for an ammeter to measure. Cts current transformers pts potential transformers. Commercially available relays and meters used for protection and metering, are designed for low voltage. A potential transformer pt is a special instrument transformer designed to provide a precise voltage stepdown ratio for voltmeters measuring high power system voltages. Current and voltage transformers are responsible for scaling primary system signals. Like the power transformer, the current transformer also contains a primary and a secondary winding. Some other differences between the current and the potential transformer are explained below in the the comparison chart. 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.
A potential transformer or voltage transformer is a special kind of transformer used to measure voltages higher than you would want to measure directly. 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. 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. This is a simplest form of potential transformer definition. Figure 1 is a stepdown transformer and figure 2 is a stepup transformer. Most often, the older relay gets replaced with a newer relay with the rest of the installation such as current transformer ct and lead. Stepup and stepdown transformer problems and solutions. Lets say, for instance, that you have a 100kv grid and want to know how much powerenergy you. Current transformers play an important role in the monitoring and protection of electrical power systems.
Identify transformer polarity using dot and conventional labeling. The result is a significant current flow through the ct circuit that is not representative of the primary current. Difference between current transformer and potential. Amrans product portfolio includes standard products as well as numerous custom products specifically designed and developed for oem specific. All the readings are noted in table and a curve is drawn between applied voltage v and excitation. Calculation of cts if the current transformer has primary to secondary current ratio of 100. Types of instrument transformer instrument transformer has two type 1 current transformer 2potential transformer 5 6. 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. Whenever an alternating current flows through the primary winding, alternating magnetic flux is produced, which then induces alternating current in the. Current transformer circuits for power electronics applications. S tepup transformer serves to raise the electrical voltage while the stepdown transformer to reduce electrical voltage. 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. 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.
For drawing ct saturation curve, we apply voltage in the ct secondary keeping primary open in step of 10% of rated voltage till 120% and read the secondary current using clamp meter. The electromagnetic potential transformer is a wirewound transformer. As their names suggest, these are used to transform current and and voltage to the value which then can be measured and or observed by the connected instruments. The basics of current transformers nk technologies. In this article we are going to discuss in detail about the potential transformers. This is step up transformer because when we step up, the voltage increases and current decrease. A general overview of abbs current and voltage transformers.
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