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 voltage transformer content dahms gmbh. To verify that the ct is working at the site we have to test the entire circuit. Or if the transformer is voltage stepdown, it is current stepup. Current transformer basics and the current transformer. A current transformer is a type of stepup transformers while voltage transformer is a type of step down transformers. Instrument transformers are used for measurement and protective application, together with equipment such as meters and relays. Higher voltage and lower current reduce the required size and cost of transmission lines and reduce transmission losses. The ratio of the primary to the secondary, the ratio of the input to the output, and the turns ratio of any given transformer will be the same as its voltage ratio.
Instrument transformers are used in ac system for measurement of electrical quantities i. The constant voltage transformer cvt has a wide application, particularly where reliability and inherent regulating ability against line voltage changes are of prime importance. The equation for current corresponding to equation 1 is 2 ip is ns np so the current is inversely proportional to the number of turns. Voltage transformer an overview sciencedirect topics. The instrument transformers like voltage transformer and current transformer are used for connecting the electrical instruments to the measuring instruments. 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. Transformers do what their name impliesthey transform voltages from one value to another the term voltage is used rather than emf, because transformers have internal resistance.
Current transformers are special transformers for the proportional transformation of high currents info direct measurable values. Ideal transformer theory equations example problems. Voltage transformers are in most situations connected between phase and ground. What is a voltage transformer and a current transformer. Current transformers perform the following functions. Current transformer steps high current down to a value applicable to a conventional ammeter. Top 15 difference between current transformer and voltage.
Voltage transformers are together with current transformers known as instrument transformers. A current transformer is a transformer designed to provide the secondary current proportional to the current flowing in its primary coil. In this case, the primary winding behaves as a coil with a core. Current and voltage transformer symbols and simplified concepts 5 i 1 i 2 v 1 v 2 symbol of a current transformer symbol of a voltage transformer conceptual picture of a current transformer. A current transformer is used to calculate current and power, to operate the protective relay and to monitor the power grid operation, etc. In voltage transformer, secondary current is directly linked with the primary current. The potential transformer is made with highquality core operating at low flux density so that the magnetising current is small. At 5% rated current through 100%, the current transformer must maintain 0. The standard that describes voltage transformers in details is iec 186. Basic function of instrument transformers is to step down the ac system voltage and current. Ct and a transformer sample for which the technical characteristics are well. Used in ac system for the measurement of current, voltage, power and energy. Note that while the pt is a stepdown device, the current transformer or ct is a stepup device with respect to voltage, which is what is needed to step down the power line current. Calculate voltage, current, andor number of turns given the other quantities.
A current transformer has a primary winding, a core and a secondary winding, although some transformers, including current transformers, use an air core. Correct operation of current current, voltage voltage, or current voltage devices usually depends on the relative instantaneous directions. Transformers transfer electrical energy between circuits completely insulated from each other and this allows using very high steppedup voltages for transmission lines, resulting in a lower steppeddown current. What will happen if secondary of a current transformer is opencircuited. The flux flows in the part of the core represented by the lefthand exciting branch of the equivalent circuit shown in figure 1. These transformers reduce the voltage and current from high value to the low value which can be measured by conventional instruments. Special transformers and applications transformers. Secondary current is dependent upon the voltage in addition to the load resistance.
Typical nominal secondary values are 5 a and 1 a for current transformers and 110 v for voltage transformers. In both examples shown, the voltage transformer and current transformers have the same ratio so that, after combining, the ratio of voltage to current stays the same, meaning that the impedance of the side port is the same as the main line. Together with instrument transformer are deliv ered. Piv it follows that if a transformer is voltage stepup, then it must also be current step down.
Mathematical models for current, voltage, and coupling capacitor voltage transformers article in ieee transactions on power delivery 151. Ceiunel current transformer rated insulation level of up to 36 kv. This is the first part of video series called as current transformer in this video i have discussed about definition of current transformer and working principle of it. Instrument transformer theory and testing steve hudson, pe. Current and voltage instrument transformers applicaton guide for current transformers, the value of the reactance x is determined in a special way so that it represents the leakage flux. The actual number of turns of wire on any winding is generally not important. Current and voltage potential instrument transformers. The transformer is an electrical device that allows to increase or decrease the voltage in an alternating current electrical circuit, maintaining the power. Current and voltage instrument transformers technical. Protection its require linearity in a wide range of voltages. Generally, the metering its require high accuracy in the range of normal oper ating voltage and current.
Routine test report together with instrument transformer are delivered. While the physical principles are the same, the details of a current transformer compared with a voltage transformer will differ owing to different requirements of the application. A secondary open current transformer operates like a loaded one with an almost infinitely high burden. A potential transformer is a conventional transformer having primary and secondary windings. All merlin gerin instrument transformers are in conformity with iec standards. Their role in electrical systems is of primary importance as they are a means of stepping down the current or voltage of a system to measurable values, such as 5a or 1a in the case of a current transformers or 110v or 100v in the case of a voltage transformer. Electrical engineering multiple choice questions and. Finds a wide application in protection circuits of power system ex. Difference between current transformer and voltage. The output of a constant voltage transformer is essentially a square wave, which is desirable for rectifier output applications while, also. Current transformers are commonly used in measuring instruments and protective relays used in the electrical power networks. Instrument transformer basic technical information and. Choosing a backup generator plus 3 legal house connection options transfer switch and more duration. Unlike a voltage transformer, the primary current of a current transformer is not dependent of the secondary load current but instead is controlled by an external load.
Our voltage and current transformers are designed with a single winding or multiple secondary windings and with the option of primary voltage or current regulation. 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. The use of current or voltage transformer for other purpose then described above is forbidden if not agreed with the producer. 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. Overall working of both the transformers is same, they both differ only in application point of view. These instruments transformers are of two types i current transformers ct and ii potential transformers pt. Which of the following does not change in a transformer. A voltage transformer is ideally a transformer under noload conditions where the load current is zero and the voltage drop is only caused by the magnetizing. Mathematical models for current, voltage, and coupling. Instrument transformers may also be used as an isolation transformer so that secondary quantities may be used in phase shifting without affecting other primary connected devices. What are the difference between power transformer and. The basics of current transformers nk technologies.
Power transformers are used to transfer large amount of power form 1 voltage level to other voltage level for obvious reasons and are used at p. Current transformers current and voltage transformers are responsible for scaling primary system signals. A transformer is an electrical apparatus designed to convert alternating current from one voltage to another. 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 that is approximately zero. Nevertheless, the primary winding is still connected to electricity and forms a closed circuit. Transformers, impedance matching and maximum power.
The meter measures only the voltage and current reaching the meter terminals. Upt primary voltage ust secondary voltage 1 fault inception 2 aperiodic damping of ust 3 periodic damping of ust time ts in ms ratio classes 3pt1 6pt1 3pt2 6pt2 3pt3 6pt3 10. The power that enters the equipment, in the case of an ideal transformer, is equal to that obtained at the output. C electrical power from one voltage to another voltage keeping the frequency same by electromagnetic induction discover the worlds research 16. The secondary current is usually rated at a standard 1 ampere or 5 amperes for larger primary current ratings. Voltage transformer which is also called as a potential transformer. Medium voltage products current and voltage instrument. Instrument transformers are also used with protective relays for protection of power system. Current transformers,voltage transformers and instrument. We will also provide different specifications on request.