Considerations in the Use of 796-Volt Power for Waterflood Plants
Authors: Roberts S.C. 
Published: December 2007

By using the higher voltage on dual-rated motors, initial cable and conduit costs can be cut and operating life can be extended, provided special precautions are taken.

Introduction

Waterflood plants make ideal locations for installing dual-rated 460/796-v electric motors on 796-v systems in place of the more common 460-v combinations. The higher operating voltage results in a 1.732 reduction in load current, which substantially reduces the initial cable and conduit costs. These savings, however, are contingent upon using standard 600-v-class insulation in the motors, controls, and capacitors installed in the plant. This is possible, since the 600-v rating refers to phase-to-ground insulation; and on a 796-v operation the phase-to-ground voltage will be only 460 v. Experience shows that if certain precautions are taken during installation a very reliable precautions are taken during installation a very reliable and safe system can result, since the requirement for a good neutral ground helps insure that the 600-v insulation will not be overranged because of power-system fluctuations. power-system fluctuations. Transformer Connections

The 796-v system is obtained by connecting the secondaries of 480-v distribution transformers in a wye configuration. The neutral of this low-voltage winding is then grounded through a low grounding resistance. The importance of obtaining a good ground at this point cannot be overemphasized! This is the only way to insure that the phase-to-ground potential on the control equipment does not exceed the 600-v insulation rating. Ground rods should be used with, but not instead of, a good ground to a well casing or a metal water line on waterflood plants. The resulting resistance to ground of the transformer neutral should be less than 3 ohms. Most underground piping systems and well casings will meet this criterion so that only periodic inspection of surface connectors is necessary to insure system integrity. However, the literature describes several methods of testing for ground resistances when there is reason for doubt.

Most power-system distribution voltages are in the 12- to 13-kv class, and transformers that are used in this range are normally connected wye on the primary side and delta on the secondary to give a nominally rated 480-v service. Fig. 1 shows how this connection looks on the transformer and Fig. 2 shows the electrical analogy for this connection. The wye secondary connection for 796-v service results in a wye-wye connected transformer bank, which requires that certain precautions be taken with it. The precautions are necessary because of the inherent neutral instability of this type of connection, which can cause the phaseto-ground potential to rise well above the 600-v rating phaseto-ground potential to rise well above the 600-v rating of the equipment being used. This instability generally results from third-harmonic magnetizing currents and line-to-neutral loads. The possible instability caused by line-to-neutral loads can be resolved by simply making sure that no appreciable single-phase loads are connected to the bank. The method of allowing for the third harmonic magnetizing currents is dependent upon the type of distribution system that is available.

Four-wire systems, in which the system neutral is carried along with the three-phase conductors, offer the easiest solution to the third-harmonic problem.

JPT

P. 1196