This paper first introduced the causes of three-phase unbalance. Several times, the three-phase current unbalance calculation formulas and three-phase current unbalance standards and applications were introduced. Finally, three-phase current imbalance solutions were described. Specific follow-up Compile it together to find out. Voltage fluctuations cause voltage imbalance and three-phase currents are unbalanced. Three-phase, single-phase load imbalance, resulting in three-phase current imbalance. A short circuit between the phases and the phase and a short circuit between the phase and the neutral lead to a three-phase voltage and current imbalance. The three-phase current varies in amplitude or its phase difference is not 120°, or both. In the low-voltage distribution network, when the three-phase voltage symmetrical three-phase load is also symmetrical, the three-phase current will also be symmetrical. The active power and reactive power of each phase load are not equal and the phasor of the three-phase current is not equal to zero, which is called three-phase current imbalance. Unbalanced three-phase loads will increase line power loss. The unbalance ratio is usually expressed as a percentage of the ratio of negative sequence or zero-sequence current to positive sequence current. Voltage fluctuations cause voltage imbalance and three-phase currents are unbalanced. Three-phase, single-phase load imbalance, resulting in three-phase current imbalance. A short circuit between the phases and the phase and a short circuit between the phase and the neutral lead to a three-phase voltage and current imbalance. The first formula is (maximum current - minimum current) / maximum current; The second formula is (MAX phase current - three-phase average current) / three-phase average current. For a simple example, the three-phase current is IA=9A IB=8A IC=4A, then the three-phase average current is 7A, the phase current-three-phase average current is 2A 1A 3A, and the difference is the largest, so MAX (Phase current - three-phase average current) = 3A, so three-phase current unbalance = 3/7. According to the relevant provisions of the People's Republic of China's power industry standard "Distribution Transformer Operation Regulations" (DL/T 1102-2009) and the national standard "Power Supply and Distribution System Design Specifications" (GB 50052-2009), when the three-phase distribution transformer When the load is unbalanced, monitor its maximum phase current. For distribution transformers with Yyn0 (or YNyn0) wiring, the permitted value of the neutral current is 25% of the rated current (phase current); for a distribution transformer with a Yznll (or YNznll) connection, the neutral current The allowable values ​​are 40% of the rated current (phase current), respectively, or as specified by the manufacturer. The distribution transformers used in public distribution networks mostly use Yyn0 connection group, which means that the high-voltage windings are star-shaped, the low-voltage windings are also star-shaped, and there is a neutral point and a "0" connection line. Distribution transformers. According to the above regulations, the permissible value of the neutral current for this type of transformer is: 100kVA transformer neutral current does not exceed 36A; 200kVA transformer neutral current does not exceed 72A; 315kVA transformer neutral current does not exceed 113A, etc. These are transformers Structurally determined. Since the increase of the neutral current will not only cause the increase of the transformer loss, but also cause the shift of the neutral point potential. When the three-phase current unbalance does not exceed 15%, the displacement of the neutral point voltage generally does not exceed 5%. Therefore, in the "Operation Regulations for Overhead Distribution Lines and Equipment" (SD 292-1988), it is stipulated that the three-phase load of the transformer should be balanced and the unbalance of the three-phase load should not exceed 15%, with only a small amount of single-phase load. For three-phase transformers, the neutral current should not exceed 25% of the rated current. Among them, "with only a small amount of single-phase load" means that the load of the transformer is very small or after the normal load of the special transformer is deactivated, there is only a single-phase lighting load. In order to reduce line investment and facilitate control, the neutral current is allowed to not exceed the rated current. % operating. Conclusion: The three-phase current unbalance standard is: a Three-phase current unbalance degree does not exceed 15%; b For the Yyn0 type distribution transformer, the allowable value of the neutral current is less than the rated current (phase current) 25%. In order to accurately determine the degree of unbalanced three-phase current exceeding the degree, scientific and rational arrangements for governance plans, and solid work well beyond the standard three-phase current imbalance control work, it is necessary through the distribution and operation monitoring system platform within one month three-phase current unbalanced degree exceeded The data is analyzed comprehensively, and the conditions beyond the standard are divided into the following three conditions, treated differently: 1 Allocating transformer station area with unbalance degree exceeding 30% and above for 10 consecutive days within one month, or accumulatively exceeding 30% for more than 15 days, and having neutral current greater than 25% of rated phase current, is classified as serious imbalance Super standard with variable zone. For such districts, we need to arrange weekly plans for governance. 2 Allocating over 15% of the unbalance over a period of 10 consecutive days in a month, or accumulatively exceeding 15% for more than 15 days, and the neutral current is greater than 25% of the rated phase current. Change the area. For such zones, monthly plans need to be planned for governance. (3) If the degree of unbalance exceeds the limit within one month, but it fails to meet the requirements of the above two standard areas, it is listed as the area of ​​focus where the imbalance exceeds the standard. For such a district, it is necessary to arrange the monthly plan for real-time treatment according to the specific conditions. 4 For the Taiwan area that does not have the real-time information collection and analysis of unbalancedness, manual measurement can be made manually during the peak hours of the peak load season or peak day. With reference to the above criteria, plans should be reasonably planned for governance. As long as the single-phase load consumers are evenly distributed to three phases, three-phase balance can be achieved. It must be noted, however, that evenly allocating users is not only a third of the total number of subscribers in each phase of the phase-by-phase load, but also a balanced allocation of users whose power load and leakage are at the same level. On three phases. For example, a single-phase user in a village, in which the average electricity consumption level, the load is small, the daily electricity consumption time is shorter, the quality of the line is poorer; the electricity consumption is higher, the load is larger, and the daily electricity consumption time is longer. The quality is better; when the buried households have large leakage currents, each phase should be connected to one-third of the three types of users. The specific implementation is: (1) The three-phase four-wire system shall be adopted as far as possible from the public transformer line to the low-voltage trunk line and branch line on the power supply side of the user's table, so as to reduce the bypass and avoid crossover. (2) Regardless of overhead or cable lines, phase conductors and neutral conductors shall be marked with A, B, C, O wires or markings of different colors and arranged in a certain order. (3) Before the low-voltage line shelf is well laid down and the individual meters are installed off the assembly line, the single-phase load consumers under the distribution transformer shall be planned in a unified manner and be evenly distributed to the three phases of the low-voltage line and recorded on the books. It is necessary to check the construction of the offline assembly table. The table box number should indicate the phase, such as "a certain line A phase number". (4) After the completion of the offline assembly table, it is necessary to look at whether the three-phase load of the low-voltage power grid is actually within the range of the balance, and if necessary, some adjustments can be made. (5) When developing users or changing users in the future, it is necessary to take into account the three-phase balance problem, form a normal mechanism in the actual work, and constantly improve and improve. In short, there is no absolute balance, but the relative balance must be limited to the balance index. In the actual work, the burden of investigation and analysis should be increased, and the maximum, average load, and development trend of all kinds of distribution transformers should be recorded on the record. The load current is tested to detect unbalanced conditions in a timely manner, and timely adjustments are made to fundamentally control the occurrence of unbalanced phenomena so as to avoid damages and accidents such as damage to electrical equipment. Wireless Sweatproof Earphone Bluetooth Headphones Bone Conduction Headset IP55 Waterproof
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