Load Balancing
Introduction
In North American distribution systems, four-wire distribution feeders are made up of three-phase and single-phase sections, sometimes with limited two-phase sections. Customers are supplied three-phase or single-phase, either from the primary feeder of from a spur. As a consequence, the currents in the three-phase sections are never completely balanced and, in many cases, can be significantly out of balance. It is not uncommon to have as much as 50% difference in magnitude between the highest and lowest loaded phases. Moreover, the degree of imbalance varies along the length of each feeder. Balancing is accomplished by selecting the phase of the supply for each load so that the total load is distributed as evenly as possible between the phases for each section of feeder.
If a single-phase load is supplied from a three-phase node, there are three possibilities for connecting the load. If there are two single-phase loads present, there are 6 possible combinations. Three-phase loads are not considered since most are balanced and changing the phasing order can affect three-phase loads such as motors. If such loads are unbalanced, any corrective balancing action is usually taken at the load itself.
Since single-phase spurs are common in residential areas, the total load associated with such spurs can be significant when considered at the point of connection to the three-phase feeder section. In cases where such spurs are distributed along a feeder and spur loads differ considerably, the three-phase current imbalances between spur connections may be substantial. This is particularly so if the phase selected at such points is not carefully chosen. Figure 1 illustrates a situation where achieving balanced currents along the complete length of feeder is not possible, although the balance is perfect at the feeder start. The spurs could just as well be single-phase loads on a three-phase line.
Figure 1 - Unbalanced Feeder
Balancing Procedure
The procedure must consider all possible combinations of phase load changes at each 3-phase connection point for either single-phase spurs or loads. Consideration must be given to the order in which loads are considered so as not to exclude the best combinations of load phase connections when all selections have been made. The best set of connections will minimize the imbalance as far as possible for each 3-phase section of feeder between spur/load connections.
When planning a new feeder, the spur/load phasing selection is an important factor in producing the best design. A practical consideration when balancing an existing feeder is the need to accomplish the requisite degree of balance with the minimum number of changes to load phasing. To achieve this manually is a difficult exercise and the adoption of optimization techniques is necessary to produce a satisfactory solution. The optimized load balancing procedure ensures that the best balance is achieved along the feeder length, not just at the feeder supply point. It may also be necessary to consider the effect on multiple feeders, as a single-phase subdivision may be fed from more than one feeder, and it is undesirable to have mismatched phasing across open points in the single-phase section.
Benefits
There are a number of benefits that make efficient load phase balancing a worthwhile objective. These are discussed below.
Loading on a feeder section is synonymous with the most heavily loaded phase and, in the case of significant imbalance, feeder capacity is used inefficiently. Balancing between phases tends to equalize the phase loading by reducing the largest phase peak while increasing the load on the other phases. This equates to releasing feeder capacity that can be used for future load increases without reinforcing feeder conductors.
Balancing reduces feeder losses because any phase peak reduction affects the losses for the phases as the square of the current magnitude. A feeder section with 1-ohm resistance that has phase currents of 50A/100A/150A will have 35kW in losses. When balanced at 100A/100A/100A, the loss reduces to 30kW. The same effect is even more evident in the reduction of reactive power losses because the X/R ratio of most feeder sections is greater than 1.
Balancing improves voltage on a feeder by equalizing the voltage drops in each phase along the feeder.
Released feeder capacity provides more reserve loading capacity for emergency loading conditions.
It is realistic to assume that the benefits in improved use of feeder capacity and improved voltage quality are of more significance than the value of loss reduction except when loading is already high.
Overcoming Limitations
Where single-phase spur load is substantial, it may be difficult to improve upon the overall feeder load balance because spur phasing changes simply transfer the imbalances to different phases with no overall improvement. One solution is to increase the number of phases on the spur. If additional conductors are to be strung, the added flexibility of upgrading to three rather than two phases, with the potential for additional future loading and load balancing between feeders, is more attractive. Where a large single-phase load is supplied from a single-phase transformer on a 3-phase feeder, a possible solution is to replace the transformer by two of half the size supplied from different phases.
In certain types of feeder design, all spurs are single-phase and each spur can have substantial loading, as often happens in underground supplied residential areas. Great care must be taken to ensure that significant imbalances do not occur between spur connections on the main feeder, especially if the distances between connection points are significant. This situation is avoided if the spur connection is three-phase and the phases then separate to supply the individual spurs. Where spurs are designed as open loops, the other loop supply is arranged in the same way to form a 3-phase connection at the feeder.
Balancing over a range of loading levels is not a practical proposition because the load/spur connections are not switchable between phases. Consequently, balancing is targeted for what is considered the feeder mean peak loading pattern over all seasons of loading. A practical approach is to identify the conditions that give rise to the most severe imbalances between phases and endeavor to achieve the best balance for that loading condition.
The use of switched capacitors affects the feeder reactive power flow and hence the total phase current. Since capacitor switching is primarily affected by loading level, such capacitors are considered connected for phase balancing purposes.
©2009 Essex Energy Inc.