Draft Guide for Harmonic Limits for Single-Phase Equipment

Differences in distribution design

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7.1 Differences in distribution design

A typical distribution system found in the U.S. (and in North America in general) operates at 12.5 kV (phase-to-phase). It is radial in that the loads are connected to only one source of supply. Main feeders are 4-wire (3 phases and a neutral) and branches are usually single-phase with a limited amount of three-phase used for branches. Transformers are connected wye wye. The large majority of transformers are single-phase connected from phase-to-neutral at 7.2 kV, and stepping the voltage down to 120/240 V (3-wire) for residential and most single-phase commercial loads. A typical transformer rating for residential application is 25 kVA or 50 kVA with services run to between 6 and 10 homes.

The European style of distribution system operates at 11 or 12 kV (phase-to-phase), but does not have a neutral. Thus, the main feeders are 3-wire circuits. Transformers are three-phase connected delta wye; they are more like a miniature substations with 500, 750 or 1000 kVA ratings being popular. Secondaries operating at 400/230 V are usually tapered (i.e., 2 or 3 conductor sizes are used, with the conductor size close to the transformer is considerably larger than that found near the end of the secondary) and cover extensive distances to provide service to 60 or 100 customers. Individual 230 V (2-wire) services are tapped off the secondary system.

A much less common system in the U.S. is secondary network distribution, in the form of either a secondary grid network or a spot network. Grid networks are found in the downtown areas of many older cities to supply office buildings and other commercial loads. With secondary grid networks, 6 or more feeders and network transformers in parallel supply a low voltage grid (208/120 V or so). Transformers are large (e.g., 500 or 1000 kVA) compared to radial distribution and are connected delta wye, similar to the European distribution systems, making the third harmonic of little concern. Due to high rise construction, spot networks have replaced grid networks in popularity. With spot networks, each major load is served from a combination of 3 or more circuits and transformers in parallel (at 480/277 V typically). With the grid or spot networks, the equivalent source impedance to the point of common coupling is an order of magnitude smaller than that found in a radial system. Thus, the ability of a secondary network system to handle harmonics is much greater than a radial system. However, it should be remembered that the secondary network is not the typical distribution system; most utilities utilize radial distribution as their standard.

Table 6-2 summarizes the important differences between the two systems. The U.S. distribution system is more susceptible to problems brought about by the third harmonic. Third harmonics readily pass through the wye wye connected transformers in the U.S., while the European delta wye transformers provide a closed path for third harmonics in the delta winding, thus providing third harmonic isolation between the secondary and the primary. In addition, stray voltage can also result due to the presence of third harmonics on the neutral of U.S. systems [Conrad et al].

The equivalent impedance to the customer is greater with the European style of distribution largely due to the extensive amount of secondary used. However, the service transformer is usually the point of common coupling for U.S. distribution systems. Because of the smaller transformers ratings used in the U.S., the equivalent system impedance to the point of common coupling is higher with the U.S. type of distribution, suggesting that the U.S. harmonic limits ought to be lower than those for the European distribution. One could argue that the point of common coupling for the European type of distribution would be beyond the transformer, i.e., including some amount of secondary. Nonetheless, the equivalent system impedance at

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