With the increasing demand for electricity, the requirements for the distribution network are also getting higher and higher. It is not only necessary to ensure the reliability of power supply, but also to ensure the quality of power. However, in actual operation, due to various reasons, serious three-phase imbalance may occur in the distribution station area, threatening the safe and economical operation of the distribution network. Therefore, this paper expands the three-phase imbalance phenomenon in detail.
The current AC power systems in the world are generally L1, L2, and L3 three-phase, and the three-phase current imbalance is caused by zero-sequence current and negative-sequence current.
Concept:
The positive sequence, negative sequence and zero sequence components of the power system are determined according to the order of the three phases of L1, L2, L3.
Positive sequence: The L1 phase is 120 degrees ahead of the L2 phase, the L2 phase is 120 degrees ahead of the L3 phase, and the L3 phase is 120 degrees ahead of the L1 phase. Negative sequence: The L1 phase is 120 degrees behind the L2 phase, the L2 phase is 120 degrees behind the L3 phase, and the L3 phase is 120 degrees behind the L1 phase. Zero sequence: The three phases of L1L2L3 are the same, and neither phase is ahead nor behind.
Phenomenon:
If System is three-phase, three components above can be decomposed (a bit like the synthetic decomposition of forces, but in many cases one component has a value of zero)
For an ideal power system, due to the three-phase symmetry, the values of both negative and zero sequence components are zero (this is also why we often say that there are only positive sequence components in normal cases)
When the system fails, the three phases become asymmetric and can then be decomposed with magnitude negative and zero sequence components (sometimes only one)
When there is a single-phase ground fault, the system has positive sequence, negative sequence and zero sequence components.
When the two-phase short circuit fails, the system has positive and negative sequence components.
When the two-phase short-circuit ground fault occurs, the system has positive sequence negative sequence and zero sequence components.
Application:
Recently YTPQC ASVG equipment was used in the Swiss Photovoltaic (PV) Project (Photovoltaic, energy storage, charging pile).
The output of the on-site power system is 3-phase, but the load is one-phase, and the user wants to compensate for the three-phase imbalance. At the same time, only photovoltaic power supply, no mains power. At the same time, the on-site is a three-phase four-wire system. Note: A neutral line is required to compensate for the imbalance.
Power Quality Solution:
According the power quality analysis report, we calculated the unbalanced current that needs to be compensated on site through the on-site data. Combined with other governance requirements of users, the team configured 400kVAr YT Electric SVG on site.
Through the management of YTPQC SVG, the three-phase imbalance phenomenon on the customer's site has been well improved, and the imbalance has been reduced from the original 25% to less than 5% of the national standard GB requirements.
If any technical questions, please contact us: [email protected]
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