Construction can be one single primary turn as in Toroidal, Doughnut, or Bar types, or a few wound primary turns, usually for low current ratios. The current transformer basically consists of an iron core upon which primary and secondary windings are wound. Normally the secondary ratings are of the order 5 A, 1 A, 0. The wound transformer is mainly used for measuring the current from 1amps to 100 amps. Like the power transformer, the current transformer also contains a primary and a secondary winding. A neutral current transformer is used as earth fault protection to measure any fault current flowing through the neutral line from the wye neutral point of a transformer. Clamp meters open and close around a current carrying conductor and measure its current by determining the magnetic field around it, providing a quick measurement reading usually on a digital display without disconnecting or opening the circuit.
The secondary winding of the transformers has a large number of turns. Window-type current transformers are also common, which can have circuit cables run through the middle of an opening in the core to provide a single-turn primary winding. In the United States, engineers typically use a 5-amp output. Hence current transformers are used to supply those devices with currents of magnitude proportional to those of power. The principal function of a current transformer is to produce a manageable level of voltage and current, proportional to the current flowing through its primary winding, for the operation of measuring or protective devices.
For example, assume that the current rating of the primary winding is 100A. Current transformers used for protective relaying also have accuracy requirements at overload currents in excess of the normal rating to ensure accurate performance of relays during system faults. For 600 amp applications, use 800 amp current transformer mounting base. The ammeter is required to give a full scale deflection when the primary current is 800 Amps. The value of rated secondary current shall be 5 A.
Its secondary winding then provides a much reduced current which can be used for detecting overcurrent, undercurrent, peak current, or average current conditions. Ring-type current transformers are installed over a bus bar or an insulated cable and have only a low level of insulation on the secondary coil. This means that the primary current is 20 times greater than the secondary current so when 100 amps is flowing in the primary conductor it will result in 5 amps flowing in the secondary winding. Instead, the line that carries the current flowing in the network is threaded through a window or hole in the toroidal transformer. To assist accuracy, the primary conductor should be centered in the aperture. Current Ratio Errors — The current transformer is mainly due to the energy component of excitation current and is given as Where I p is the primary current.
The conductor carrying the primary current is passed through the ring. The burden load impedance should not exceed the specified maximum value to avoid the secondary voltage exceeding the limits for the current transformer. The disadvantage of making transformers with lower secondary current ratings is that they produce much higher voltage if they are ever accidentally left open circuited. They reduce the heavy current to lower values which are suitable for operation of Relays and other instruments connected to their secondary winding. The secondary current that is produced in these situations do not truly replicate the power system current waveform.
A current transformer provides a secondary current that is accurately proportional to the current flowing in its primary. 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. The current transformer is used in meters for measuring the current up to 100 amperes. Current transformers may be installed inside switchgear or in apparatus bushings, but very often free-standing outdoor current transformers are used. Therefore a current transformers secondary winding should never be operated into an open circuit, just as a voltage transformer should never be operated into a short circuit.
For this reason a current transformer should never be left open-circuited or operated with no-load attached when the main primary current is flowing through it just as a voltage transformer should never operate into a short circuit. In other words, the primary current is 20 times greater than the secondary current. Rated Ratio Accuracy Rating This number is simply the rated ratio accuracy expressed as a percent. Depending on their accuracy class, current transformers are divided into Metering Accuracy or Protection relay accuracy. Instrument transformers scale the large values of voltage or current to small, standardized values that are easy to handle for measuring instruments and. The secondary winding has the standard rating of 5A. With the advent of microprocessor meters and relays, the industry is seeing the 5-amp or 1-amp secondary being replaced with a mA secondary.
Low-voltage single ratio metering current transformers are either a ring type or plastic molded case. Because of their high degree of accuracy, these current transformers are typically used by utility companies for billing purposes. If the burden resistance is much less than inductive impedance of the secondary winding at the measurement frequency then the current in the secondary tracks the primary current and the transformer provides a current output that is proportional to the measured current. They are designed to perform with a reasonable degree of accuracy over a wider range of current. The conductor on which the transformer is mounted will act as primary windings of the current transformers.