By Steve Eckles, El Paso Electric Company
Understanding basic principles and rules can go a long way toward effectively mitigating distribution voltage unbalance and keeping polyphase motors cool. For example, shunt capacitors are usually installed to improve distribution feeder voltage. However as discussed later in this article, shunt capacitors may increase voltage unbalance-especially on long overhead feeders.
Polyphase motors are susceptible to overheating and increased vibration due to voltage unbalance-the most extreme condition being single-phasing. A brief explanation of this phenomenon touches upon symmetrical components with negative sequence components rotating in reverse of positive sequence components. Polyphase motors, driven by positive sequences, rotate in the same direction. Negative sequence rotation opposes motor rotor rotation. Increasing voltage unbalance increases negative-sequence voltage components, thereby increasing negative-sequence currents circulating in the motor. This results in increased power loss in the rotor, increased motor temperature, increased vibration and decreased efficiency.
When possible, customers should be urged to derate three-phase motors and apply unbalanced equipment protection. NEMA standard MG1 discusses derating motors where voltage unbalance occurs. If voltage unbalance at the motor terminals does not exceed 1 percent, a three-phase motor may operate at its rated load. Load should be decreased to 90 percent when operating at 3 percent voltage unbalance and 75 percent loading at 5 percent unbalance.
In addition, other devices besides motors are susceptible to voltage unbalance, including electronic equipment such as computers. Ripple may increase in some three-phase AC/DC power supplies, raising capacitor temperature.
Diversity of Guidelines, Formulas and Terms
When reviewing literature, there appear to be inconsistencies in guidelines and formulas used to calculate voltage unbalance. (Occasionally, the term “voltage imbalance” is used also.) Electric utilities in the United States follow ANSI C84.1, which calls for voltage at the customer meter to be within plus or minus 5 percent of nominal for those receiving 120- to 600-volt service. ANSI C84.1-1995 (pages 12-13) recommends that supply systems be designed and operated to limit voltage unbalance to 3 percent when measured at the utility revenue meter under no-load conditions. It and NEMA MG1 define voltage unbalance as the maximum deviation from the average phase-to-phase voltage divided by the average voltage.
ANSI/IEEE Std 141-1986 (IEEE Red Book, pages 90 and 91) quantifies a voltage-unbalance factor in symmetrical components as the ratio of negative-sequence voltage magnitude divided by the positive-sequence voltage magnitude. However, it also provides the same phase-voltage unbalance calculation as in ANSI C84.1 and NEMA MG1.
ANSI/IEEE Std 446-1987 (IEEE Orange Book, pages 73 and 77) offers another phase-voltage unbalance equation determined by the quantity of the maximum voltage minus the minimum voltage divided by the average voltage:
No matter which formula is used, keep in mind that load variations may cause more voltage unbalance at various times of day and during different seasons.
An example of transposition to counteract mutual coupling effects on long distribution lines.
Utility troubleshooters should measure voltage at more than one transformer in the area to determine if voltage unbalance is occurring due to secondary effects (unbalance transformation and/or unbalanced customer load) or on the primary distribution system. Despite using phase-to-phase voltages in calculating unbalance, troubleshooters typically favor measuring phase-to-ground voltage at grounded wye-grounded wye transformers (e.g., padmount transformers) with balanced load to determine which phases need higher or lower voltage.
One or a combination of the following factors may cause voltage unbalance:
“- feeder load unbalance,
“- unsymmetrical feeder spacing and/or conductor sizes, or
“- mutual coupling between feeder conductors.
Feeder Load Unbalance
Most commonly, feeder load unbalance occurs when single-phase load is connected phase-to-neutral on grounded four-wire feeders. Typically, phase load should fairly well balance at the substation, but there is no guarantee that phase load is distributed evenly throughout the feeder length. A blown capacitor fuse also fits into this category but may not be apparent by measuring total feeder current. The further the capacitors are on the feeder, the more impact they have.
While performing voltage drop calculations on distribution feeders, engineers have discovered that leading current actually raises voltage across conductor inductance. Therefore, shunt capacitors are installed to reduce voltage drop, thereby improving voltage profile. Small unbalances of capacitive current may also develop from balanced shunt capacitor banks. Capacitors have constant impedance; increasing capacitor voltage increases capacitor current proportionally, raising voltage across conductor inductance. Capacitors on a low-voltage phase generate less leading current, producing less voltage rise-in effect, slightly amplifying voltage unbalance.
Uneven capacitive current may also be generated from long runs of single-phase underground cable. Companies should avoid serving underground residential subdivisions consistently from the same one or two phases.
Mutual Coupling Between Conductors
If “no man is an island,” then it is definitely true that no conductor is an island-especially on overhead three-phase feeders. Magnetic fields from current flowing in each phase (and the neutral) are coupled to the other conductors. Advanced distribution modeling software accounts for these mutual couplings by using 4-by-4 impedance matrices with off-diagonal elements incorporating mutual coupling impedances. Modeling shows that symmetrical feeders with uniformly balanced three-phase load still have different voltage drops in the two outside phases due to the different voltage-drop phase angles from mutual coupling impedances.
Several tools in the engineer’s bag can reduce feeder voltage unbalance. They are:
“- balancing feeder load,
“- phase transpositions on long feeders,
“- installing unequal feeder capacitance,
“- voltage regulators, and
“- feeder voltage regulators with downstream capacitors.
Balancing Feeder Load
Balanced feeder current along a feeder’s entire length-not just at the substation-is the engineer’s goal, but highly loaded single phase laterals act as an obstacle to that goal. Residential distribution fed from one or two phases is usually the worst offender. Extensive underground residential distribution should be balanced among the three feeder phases to evenly distribute capacitive current (hence, power factor) generated by underground cable as well as customer load. Proper design and installation upfront is much easier than “swapping” phases later.
If the current cannot be evenly balanced throughout the feeder length, then monitoring three-phase voltage near the end of the feeder (or upstream of voltage regulators) will indicate which phase (or phases) is consistently higher than the others and able to support more load. Care should be taken to match the voltage unbalance profile with customer class load profile. If voltage unbalance occurs during typical peak residential load, then residential load should be shifted to the phase with the higher voltage.
Phase transpositions basically involve interchanging the physical positions of conductors along the line while keeping the same basic structure configuration downstream of the transposition (see photograph, page 74). The principle of conductor transpositions is more aptly understood by considering a transmission line. To counteract mutual coupling and unsymmetrical effects, two transposition points are usually installed at one-third and two-thirds the line’s length. This allows each phase conductor to occupy each physical space for one-third of the length. For simplicity, transmission line current magnitude is considered uniform throughout the line’s length.
Capacitor banks downstream of feeder voltage regulators alleviate voltage unbalance upstream of regulators.
This is typically not the case with distribution feeders unless they are feeding a dedicated large balanced load. Therefore, transposition placement is not as intuitive on a distribution system but may be aided by modeling software and accurate data. Here again, seasonal and daily load variations should be considered. It is usually not practical to retrofit an established feeder with transpositions and still maintain rotation of three-phase loads already in place.
Some utilities alternate outside phases at corners while keeping the middle phase conductor in the same location. This is not considered a full transposition but will help reduce voltage unbalance from mutual coupling and/or unsymmetrical construction.
Installing Phase Capacitance
Balanced shunt capacitor banks are installed to improve feeder voltage profile and improve power factor. In some situations, unequal phase capacitance will increase voltage unbalance. In other situations, it may counteract unbalance when other corrective measures are too costly, too inconvenient or would take too long to implement. Capacitors may be placed quickly and temporarily on low-voltage phases until long-term solutions are investigated. In some cases, it may be easier to remove capacitor cans from the highest-voltage phase. Using hot-line clamps, an existing capacitor bank can easily be reconfigured by disconnecting a capacitor can from the high-voltage phase and reconnecting it to the low-voltage phase-in essence, doubling-up capacitor cans to raise voltage on one phase and drop voltage on another.
Single-phase voltage regulators at the substation may cost more than using a load tap changing (LTC) transformer, but they can mitigate voltage unbalance through their compensation circuits. If all three phase voltage regulators have the same resistance (R) and reactance (X) settings, then feeder load unbalance may cause the highest loaded phase to step voltage up to allow for more voltage drop along the feeder length. If this is not occurring, regulator resistance and/or reactance settings may need to be increased on the lower voltage feeder phase.
There is a limit to the amount of feeder voltage balancing a substation voltage regulator may achieve. The closest customers to the substation should not suffer high voltage unbalance to accommodate customers near the end of the feeder. In these cases, installing voltage regulators out on the feeder may become necessary. In addition, advances in control panels allow some regulators to automatically regulate voltage properly when backfed (i.e., reverse power).
Feeder Voltage Regulators with Downstream Capacitors
When installing voltage regulators out on the feeder, engineers should keep in mind the positive and negative use of capacitors to balance voltage. Adding shunt capacitors downstream of feeder voltage regulators achieves better voltage balance than placing capacitors upstream of the regulators (see Figures 1 and 2).
Utility customers upstream of voltage regulators will see more benefit when capacitors are placed downstream of the regulators. Likewise, voltage unbalance is exacerbated if shunt capacitors are placed upstream of feeder regulators. This phenomenon is due to the effect of voltage on capacitors and the fact that regulators are adjustable autotransformers.
Increasing capacitor voltage increases capacitor current proportionally to raise voltage across conductor inductance. Placing capacitors downstream of feeder regulators will have a positive effect on capacitive current and voltage rise on upstream conductors. Further benefit comes from conservation of kVA in voltage regulators. Neglecting losses, input kVA equals output kVA in transformers (including voltage regulators). As voltage regulators raise output voltage, input current rises proportionally. For regulators raising voltage 8 percent, (upstream) input current is 8 percent higher than output current. Due to capacitor constant impedance and kVA conservation through transformers, regulators increasing voltage by 8 percent to downstream capacitors will actually increase upstream capacitive current by approximately 16 percent, thus raising upstream voltage approximately 16 percent more than a capacitor immediately upstream of the regulator.
Three-phase voltage unbalance occurring at source-side of voltage regulators causes them to raise (or lower) voltage differently. The low-voltage phase regulator raises the most; hence, downstream capacitor current increases the most by the downstream voltage increase and is approximately doubled again through the voltage regulator. This “doubled” increase in capacitive current raises voltage across upstream conductor inductance. Hence, voltage rises most on the lowest voltage phase upstream of the regulators when adding capacitors downstream of the regulators. All this helps to balance upstream voltage.
Keeping distribution voltage balanced will help keep customer polyphase motors running cooler. Voltage unbalance is more prevalent on long, heavily loaded distribution feeders and may vary with time of year and time of day. Single-phase voltage regulators, balancing load along the feeder length and/or transpositions can help mitigate voltage unbalance.
When voltage unbalance is suspected, utility troubleshooters should confirm it by taking voltage measurements at more than one customer transformer. A check should be made for blown capacitor fuses before other solutions are investigated. Adding and/or removing capacitor cans from individual phases may be a quick fix before more permanent solution(s) are incorporated.
Installing voltage regulators out on the feeder is the brute-force solution for correcting voltage unbalance throughout daily and seasonal load variations. In these cases, shunt capacitors downstream will tend to help balance voltage upstream of the regulators.à¯£à¯£
Steve Eckles has been a distribution engineer for 13 years and is a licensed P.E. in New Mexico and Texas. He obtained a BSEE from San Diego State University and a MSEE from going through New Mexico State University’s Electrical Utility Management Program (EUMP). He has previously authored technical papers in electrochemistry, photovoltaics, electrical utility distribution and power quality.