A Practical Case of Power Quality Study — Textile Factory Site Measurements

01 Context — Why This Paper Matters

Published in 1999 at the IEEE Transmission and Distribution Conference, this paper by Prof. José Carlos de Oliveira at the Federal University of Uberlândia is one of the landmark papers in industrial power quality assessment methodology from Latin America. It is not primarily a paper about a single unusual problem — it is a paper about methodology: how to conduct a comprehensive site PQ assessment at an industrial facility, what parameters to measure, how to structure the investigation, and how to derive actionable conclusions from the measurements.

This methodology framework — voltage profile, distorções harmônicas, unbalance, and feeder transfer risk — is precisely the framework that utility power quality engineers and industrial energy managers use today. The fact that the paper was written in 1999 does not reduce its relevance: the four-parameter framework is timeless, and the textile factory represents a load mix — motors, unidades, electronic equipment, power factor correction — that is still representative of medium-sized industrial facilities worldwide.

02 The Assessment Methodology Framework

The paper structures the PQ assessment around four investigative questions — each corresponding to a distinct PQ parameter category and a distinct engineering concern. This four-question structure is the framework that every industrial PQ assessment should follow:

The monitoring program was designed to capture all four parameter categories simultaneously — an important methodological point. Measuring parameters sequentially (one week for harmonics, another week for power factor) misses the correlations between parameters: harmonic distortion is higher at full production load, which is also when voltage is lowest and unbalance is most significant. Only simultaneous multi-parameter monitoring reveals the actual worst-case PQ environment that the facility’s equipment operates in.

03 Voltage Profile — The Starting Point

Voltage profile assessment — verifying that the supply voltage at all points in the facility’s distribution system remains within the acceptable range under all operating conditions — is the foundation of any industrial PQ assessment. Before harmonic distortion, unbalance, or any other PQ parameter can be meaningfully assessed, the fundamental voltage must be characterised.

For a textile factory, voltage profile assessment requires monitoring at multiple points in the distribution hierarchy:

• Point of common coupling (PCC) — the utility’s delivery point, where the factory’s connection to the distribution feeder is made. Voltage here reflects the utility supply quality plus the effect of the factory’s total load
• Main distribution switchboard — the incoming LV bus. Voltage here reflects the PCC voltage minus the drop through the main transformer and its protection devices
• Secondary distribution boards — the buses feeding individual production areas. Voltage here reflects the cumulative drop through all upstream impedances plus the local reactive demand of the production equipment
• Motor control centres — the terminal voltage available to the motors. This is the most critical measurement for process reliability — a motor that repeatedly operates at the lower end of its voltage tolerance range is at elevated risk of thermal overload during high ambient temperature periods

04 Harmonic Distortion Assessment

The harmonic assessment in the Oliveira paper covers both voltage harmonic distortion (THDv) at key busbar locations and current harmonic distortion (THDI and TDD) at the factory’s main supply point. This dual-measurement approach is important because voltage THD at the PCC is the compliance metric for utilities and large customers under IEEE 519, while current TDD at the PCC is the factory’s harmonic emission metric — what the factory is injecting into the network.

Sources in a typical textile factory

In a late-1990s textile factory, the primary harmonic sources are variable-speed drives (ASDs) on spinning and weaving machinery, electronic control systems, and power factor correction capacitor banks. The dominant harmonic orders from these sources are the 5th, 7ª, 11ª, and 13th — the characteristic harmonics of 6-pulse converter topologies. At high loading — simultaneous operation of many ASD-driven machines — the aggregate harmonic current at the main supply point can significantly exceed what any individual machine produces, because the harmonic currents add vectorially rather than cancelling.

05 Desequilíbrio de tensão

Voltage unbalance is an intrinsic risk in any industrial distribution system with significant single-phase load components — lighting, single-phase power supplies, single-phase welding equipment, and unevenly distributed three-phase loads where individual phase loads vary with production scheduling. In a textile factory, the mix of three-phase motors (balanced) and single-phase ancillary equipment (unbalanced) means that the phase balance at the motor control centres changes with the production schedule.

The critical consequence of voltage unbalance in a motor-dominated facility is the negative-sequence component of the unbalanced voltage. Negative-sequence voltage drives a reverse-rotating magnetic field in the motor, creating a braking torque that opposes the motor’s rotation. The motor compensates by drawing more current — increasing winding temperature. NEMA MG-1 quantifies this: um 3.5% desequilíbrio de tensão (PVUR definition) increases motor temperature rise by approximately 25%, requiring motor derating to 75% of nameplate capacity to maintain the same service life.

06 Feeder Transfer Risk — The Fourth Question

The investigation of the risk involved in transferring the supply from one distribution feeder to another is the most operationally specific element of the Oliveira assessment — and the one most likely to be neglected in a standard PQ survey that focuses only on compliance metrics. Feeder transfer capability is a supply reliability measure: the ability to switch from a primary feeder to a backup feeder when the primary fails.

The risk assessment addresses three distinct concerns:

• Voltage step on transfer — if the two feeders deliver different voltage levels at the factory’s PCC (due to different transformer tap settings, different feeder impedances, or different loading conditions on each feeder), transferring the load will cause a step change in supply voltage. A large step — more than 5–10% — can cause motor speed changes, drive trips, and control system upsets across the entire facility simultaneously
• Voltage sag during transfer — even a fast automatic transfer (sub-cycle to a few cycles) causes a brief voltage depression as the new feeder takes on the load. If the factory has sensitive equipment with tight voltage tolerance, this transfer sag may cause production disruptions indistinguishable from a grid-caused sag event
• Harmonic environment change — the two feeders may have different source impedances and different background harmonic levels, particularly if the alternate feeder serves different customers. The factory’s harmonic resonance conditions — determined by the interaction between transformer impedance, capacitor bank size, and source impedance — will change after transfer, potentially moving a resonant frequency from a non-problematic location to one that amplifies harmonic currents from the factory’s own drives

07 Power Quality Perspective

The Oliveira paper’s lasting contribution is methodological rather than technical: it demonstrates that a comprehensive industrial PQ assessment requires addressing multiple parameter categories simultaneously, at multiple measurement points, with the objective of answering specific engineering questions — not simply generating a compliance report.

This distinction between a compliance report and an engineering assessment is fundamental. A compliance report asks: does this facility’s PQ meet the applicable standard at the measurement point? An engineering assessment asks: what are the actual PQ conditions throughout the distribution system, how do they affect the production equipment, what reliability risks exist, and what options are available to improve the situation? The compliance report may be necessary for regulatory purposes; the engineering assessment is necessary for operational management.

The feeder transfer question — the fourth element of the Oliveira framework — illustrates this distinction clearly. IEEE 519 compliance at the PCC says nothing about feeder transfer risk. But feeder transfer risk is the most operationally significant reliability question the factory operator faces: can they maintain production if the primary feeder fails? Answering it requires the kind of integrated, multi-parameter, multi-point assessment that the Oliveira paper demonstrates.

Referências

1. Oliveira JC et al. “A Practical Case of Power Quality Study.” IEEE Transmission and Distribution Conference, 1999. DOI: 10.1109/T&D.1999.759917
2. IEEE Std 519-1992. Práticas IEEE recomendados e Requisitos para Controle de Harmônicos em Sistemas Elétricos de Potência. IEEE, Nova Iorque, Nova Iorque, 1992. (Standard applicable at time of publication.)
3. NEMA MG-1-2021. Motors and Generators. National Electrical Manufacturers Association, Rosslyn, VA. (Voltage unbalance derating guidelines.)
4. IEEE Std 1159-1995. IEEE Recommended Practice for Monitoring Electric Power Quality. IEEE, Nova Iorque, Nova Iorque, 1995. (Standard applicable at time of publication.)
5. Dugan RC, McGranaghan MF, Santoso S, Beaty HW. Electrical Power Quality Systems. 2nd ed. McGraw-Hill, 2002. (Comprehensive reference on industrial PQ assessment methodology.)