Differences and selection between the secondary rated currents of 1A and 5A for current transformers

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This post was last edited by zhengpeichun on 2018-3-27 08:31. According to item 5.2 of GB1208-2016 \"Current Transformers\", the standard secondary currents for current transformers are 1A and 5A, with 5A being the preferred value; 1A should be chosen when the transmission distance is large. Compare the differences between 1A and 5A current transformers and their selection methods through case studies. Comparison of current transformers with secondary currents of 1A and 5A: 1. Reduced line power consumption. Line power consumption is proportional to the square of the current flowing through it; a current transformer with a secondary current of 1A results in power consumption that is 25 times lower than that of one with a 5A secondary current, meaning the power consumption at 1A is only 4% of that at 5A. Table 1 Power consumption of the current transformer measurement circuit. 2. As the transmission distance increases, under the same load, the transmission distance for a transformer with a secondary current of 1A is 25 times that of one with a secondary current of 5A; this eliminates the need for an intermediate transformer with a 5/1A ratio or allows for the use of a transformer with a higher capacity. Table 2: Transmission distance at different rated capacities. 3. Small wire cross-sectional area: In large and medium-sized factories, when the distance between the instruments and current transformers is considerable (for example, 45.5 m), it can be seen from Table 2 that when 5A or 10VA current transformers are used, the required wire cross-sectional area is 4 mm3 after calculation ； If 1A or 2.5VA current transformers are used, the wire cross-section only needs to be 1mm2. Currently, with the widespread use and development of computers and CNC instruments, the selection of current transformers with a rated secondary current of 1 A or less has become quite common. The selection of the secondary rated current for current transformers can be found at yunrun.com.cn/tech/1873.html. When the primary rated current and the rated output capacity are the same, current transformers use either 1A or 5A for their secondary current, and their structure and characteristics differ significantly in these cases. There are significant differences in structure and properties when using 1A compared to 5A. A current transformer using 1A has a turns ratio 5 times larger than one using 5A, its secondary winding has 5 times more turns, it exhibits a higher open-circuit voltage and greater internal resistance, requires less excitation current, is more difficult to manufacture, and is slightly more expensive. However, using 1A can significantly reduce the active power loss in the cable (to 1/25 of that with 5A), and under the same conditions, it allows for an increased allowable length of the current-carrying cable. The secondary rated current of current transformers is either 1A or 5A, and this must be determined through a technical and economic comparison. When 1A is used, the investment in the current transformer itself increases, while the investment in the control cables for the current circuit is lower ； On the contrary, when 5A is used, the investment in the current transformer itself decreases, while the investment in the secondary cables increases. When selecting the secondary rated current of the current protective conduit, it is also necessary to consider whether the rated currents of the protection devices and measuring instruments are compatible with it ； In a unified substation, factors such as ensuring that the secondary currents of the current transformers on each side are as consistent as possible. Generally, in substations with voltage levels of 220 kV and below, there are few 220 kV circuits, while there are more 10–60 kV circuits, and the cable lengths are shorter. It is economical to use a 5A secondary rated current for current transformers. In substations with voltage levels of 330 kV and above, there are a large number of circuits at 220 kV and above, and the current circuit cables are long; therefore, it is economical to use a secondary rating current of 1 A for current transformers. Once the primary and secondary rated currents of the current transformer are determined, its rated transformation ratio is also fixed. In practical engineering projects, the initial load condition is often low, resulting in a significant difference from the design load of the circuit, and the secondary current of the current transformer is very small. It is difficult to read the pointer ammeter, or it fails to meet the requirements for the minimum accurate operating current of the electromechanical protection device. This requires changing its current transformation ratio without replacing the current transformer. To change the turns ratio of a current transformer, the following methods are usually employed: ① Using a dual-turns current transformer. This type of current transformer has two primary windings, and their connection in series or parallel is used to modify its variation. For example, a current transformer with two primary windings, each having a rated current of 600A, has a transformation ratio of 1200/5A when the two windings are connected in parallel ； When the two windings are connected in series, the turns ratio is 600/5A. ②Taps are provided on the secondary winding of the power transformer. 【Classic Case】In a certain chemical plant, the environments in various production workshops are mostly explosive. The electrical control rooms in these workshops are not located within the workshops themselves, but rather in separate central electrical control rooms situated at a distance from the workshops, so as to enable centralized collection of system current data. The distance between the current transformers on-site and the control rooms is approximately 200 meters; in some cases, it even reaches 300 meters. The wires used for secondary transmission have a cross-sectional area of 2.5 square millimeters. The current transformers used include various models such as ALH-0.66/30I 200/5A, grade 0.5, 5VA, with one turn of wire passing through the core. The ammeters used are of the YR-GFI-9L1-2 type. This was a large-scale project, and during the trial operation of some of its components, it was found that the readings on all the ammeters were completely inaccurate compared to the actual current levels on site. 【Problem Analysis】The rated capacity of a current transformer is the rated secondary current I2e of the transformer, as well as the apparent power S2e consumed when a rated load Z2e is connected to the secondary circuit; that is, S2e = I2e²Z2e ； The apparent power consumed by the digital display meter is only 0.05 VA, which is very small; therefore it can be ignored. Z2e = ρ·2L/S = 0.0176 Ω·mm2/m × 2 × 200 m / 2.5 = 2.82 Ω. S2e = I2e²Z2e = 5 A² × 2.82 Ω = 70.5 VA, which is far greater than the rated capacity of 5 VA of the current transformer. Hence, a 200/1 A current transformer should be used in this case. In February 2010, all current transformers with a rating lower than 5 A were replaced in this project. Additionally, since the ammeter is a digital one, its transformation ratio can be reset to 200/1, allowing the entire system to return to normal operation. Instructions for selecting low-voltage current transformers: 1. Select the appropriate model based on parameters such as the primary current and busbar cross-section. A conductor passes through the transformer window. Open the flip cover, perform secondary wiring using the wire pressing piece, and reset the flip cover after the secondary wiring is completed. Measuring electrical energy can be directly achieved by sealing the small opening on the flip cover with a lead stamp to prevent electricity theft. 2. The operating current shall not exceed 1.1 times the rated value for an extended period; short-term use at 1.2 times the rated value is permitted, but only for a duration of no more than 1 hour ； 3. Determine the rated current ratio based on the magnitude of the current to be measured; generally, the current to be measured should be 2/3 of the rated current ； 4. The polarity of the product is indicated by the primary wiring marks P1, P2, and the corresponding secondary wiring marks S1, S2 ； S1 represents the homonymous endpoint of P1, and S2 represents the homonymous endpoint of P2 ； 5. The measuring instruments are connected to terminals S1 and S2; the total load of the circuit connected in this way should not exceed the rated load of the current transformer. When the location where the ammeter is installed is far from the current transformer or when the load of the circuit is high, it is preferable to use a current transformer with a secondary current of 1 A ； 6. Pay attention to selecting a transformer with a window size that matches the specifications and number of busbars. I guess you’ll like Wiring and Selection of Current Transformers, and Methods for Handling Open-Circuit Faults in AC Current Transformers