Calidad de la energía en el sistema de distribución eléctrica de una institución académica - Universidad Sultán Qaboos
| Instalación | Sultan Qaboos University (SQU), Oman — full campus electrical distribution system |
| Voltage levels measured | 33 kV/11 kV main substations · 11 kV/415 V building substations |
| Key measurement points | College of Engineering · Centre of Information Systems · Two 33/11 kV main substations |
| Non-linear loads identified | PV converters · UPS systems · Chillers with variable-speed motors (VFD) · Computer labs · Server rooms |
| THDI range measured | 2% a 10% depending on location and loading |
| TDD range measured | 2% a 8% depending on loading — within IEEE 519 limits at most points |
| IEEE 519 voltage THD limit | 5% at the PCC (33 kV/11 kV interface) — generally compliant |
| Future direction | SQU plans large-scale PV integration and smart grid upgrade — PQ assessment establishes the pre-DER baseline |
01 Context — The Campus as a PQ Microcosm
University campuses represent one of the most complex and instructive environments for power quality assessment. They combine, within a single distribution system, virtually every category of non-linear load found in modern buildings: computer laboratories with hundreds of switch-mode power supplies, data centres and server rooms with large UPS systems and rectifier loads, research facilities with variable-speed drives and precision laboratory equipment, air conditioning systems with VFD-controlled chillers, and increasingly, rooftop PV generation with grid-connected inverters.
La Universidad Sultán Qaboos en Omán es un campus grande y moderno que atiende a miles de estudiantes y personal de las facultades de ingeniería., ciencia, medicamento, e informática, todo ello conectado a un 33 Sistema de distribución de tres niveles kV/11 kV/415 V. La 2024 Un estudio realizado por investigadores de SQU realizó una auditoría PQ integral en múltiples puntos de este sistema., desde 33 Subestaciones de toma de kV hasta el nivel de entrada del edificio, Establecer una línea de base armónica sistemática para el campus antes de la integración fotovoltaica a gran escala planificada..
Las evaluaciones de PQ industrial normalmente se centran en uno o dos tipos de carga no lineal dominantes: los hornos de arco., VFD, rectificadores y uno o dos puntos de medición. Campus PQ se caracteriza por un gran número de pequeños, diverse non-linear loads distributed across many buildings, connected to a shared distribution system. The aggregate harmonic distortion at the campus substation is the statistical result of hundreds of individual switch-mode power supplies, Sistemas UPS, VFD, and PV inverters — each with its own harmonic spectrum, each partially cancelling or reinforcing the others depending on the phase relationships of their switching frequencies. This statistical aggregation behaviour makes campus PQ both more tractable (no single dominant source) and more difficult to attribute (many sources, complex interactions).
02 The Campus Non-Linear Load Mix
The study identified four primary categories of non-linear loads contributing to harmonic distortion at SQU:
- inversores fotovoltaicos — rooftop solar installations with grid-connected inverters producing both classical harmonics (from PWM modulation) and supraharmonic emissions (from high-frequency switching). The PV contribution is time-varying — it is zero at night and peaks at midday solar irradiance, creating a time-varying harmonic background that changes the harmonic environment across the day
- Sistemas UPS — large centralised UPS systems for data centres and server rooms, and smaller distributed UPS units for individual laboratories. UPS systems are among the most prolific harmonic sources in institutional environments — a typical double-conversion UPS at 50% load draws current with 25–35% THDI, dominado por los armónicos 5º y 7º
- Chillers with variable-speed drives — air conditioning systems are the dominant electrical load on a Middle Eastern university campus, where outdoor temperatures regularly exceed 40°C. VFD-controlled chillers provide significant energy savings compared to fixed-speed equivalents but introduce harmonic currents at 5th, 7ª, 11ª, and 13th orders that are proportional to the chiller’s operating power
- Computer laboratories and server rooms — hundreds of desktop computers, monitors, and servers, each drawing current through switch-mode power supplies that produce dominant 3rd harmonic (triplen) corrientes. The aggregate triplen harmonic from computer loads is the primary driver of neutral conductor loading in the 415 V building distribution system
03 Measurement Results Across the Distribution Hierarchy
The study measured harmonic content at multiple points in the SQU distribution system, desde 33 kV main intake down to individual 415 V building entrances. This hierarchical measurement approach reveals how harmonic distortion varies across voltage levels and how the aggregate substation distortion relates to the individual building-level distortion.
| Measurement location | Nivel de voltaje | THDI range | TDD range | IEEE 519 THDv limit | Conformidad |
|---|---|---|---|---|---|
| Main substations A & B | 33 kV / 11 kV | 2–5% | 2–5% | 5% THDv | Compliant |
| College of Engineering substation | 11 kV / 415 En | 4–8% | 3–6% | 8% THDv | Compliant |
| Centre of Information Systems | 11 kV / 415 En | 5–10% | 4–8% | 8% THDv | Borderline at peaks |
| Individual building entrances (LV) | 415 En | 8–15% | varies | 8% THDv | Exceeds at high load |
The THDI at main substations (2–5%) is significantly lower than at individual buildings (8–15%). This is not because the substation supply is cleaner — it is because the harmonic currents from many different building loads partially cancel at the common bus. UPS systems produce dominant 5th harmonics with a given phase angle. VFD chillers produce 5th harmonics with a different phase angle depending on their switching pattern. Computer labs produce 3rd harmonics. When all these currents flow back to the common 11 kV bus, their vector sum is smaller than their arithmetic sum — partial cancellation reduces the aggregate distortion. The substation measurement is correctly compliant with IEEE 519 (which is assessed at the PCC with the utility), but this compliance tells nothing about the distortion experienced by sensitive equipment within individual buildings.
04 THD vs. TDD — Why the Distinction Matters
The SQU study correctly applied Total Demand Distortion (TDD) rather than Total Harmonic Distortion of Current (THDI) when assessing IEEE 519 compliance — a distinction that is frequently misunderstood in campus and commercial building PQ assessments.
The critical difference
THDI expresses harmonic current content as a percentage of the fundamental current at the moment of measurement. At light load — 20% of rated load — a UPS that draws 30% THDI at full load may draw 60% THDI because the harmonic currents are relatively constant while the fundamental decreases. This makes THDI a misleading metric for compliance assessment at variable-load installations.
TDD expresses harmonic current content as a percentage of the maximum demand current — the maximum average current drawn over a 15-minute period in the past 12 meses. A UPS drawing 30% THDI at 20% load may show TDD of only 6% — well within the IEEE 519 limit — because the harmonic currents are a small fraction of the maximum demand the system was designed for.
When a campus facilities engineer sees a power quality analyser reporting 35% THDI on the UPS feeder, the instinctive reaction is “we have a serious harmonic problem.” When the same engineer applies the TDD calculation using 12 months of maximum demand data, the TDD is typically 6–8% — within the IEEE 519 límite. The harmonic currents are real and cause real heating, but the system is designed to handle the maximum demand current — and the harmonic content is a modest fraction of that design current. Understanding the difference between THDI and TDD prevents both unnecessary alarm and unnecessary expenditure on active harmonic filters that are not required for standards compliance.
05 PV Integration — Establishing the Baseline
One of the key objectives of the SQU PQ audit was to establish a harmonic baseline before the planned large-scale PV integration — a sensible engineering practice that is rarely executed in advance of DER deployment. By characterising the existing harmonic environment at each measurement point before PV panels are added, the study creates a before/after comparison framework that will allow the harmonic contribution of the PV inverters to be separated from the background distortion already present in the network.
This pre-DER baseline approach addresses a fundamental problem in post-hoc PQ assessments: without a baseline, it is impossible to determine whether an observed compliance exceedance was caused by the newly installed PV system or was already present before the installation. The SQU study’s systematic multi-point measurement approach — covering all voltage levels from 33 kV to 415 V — provides exactly the baseline that future post-installation assessments will need.
SQU’s plan to move toward a green smart campus with large-scale PV integration is consistent with the broader trend in Middle Eastern university campus electrification. The PQ assessment provides the engineering foundation for this transition — identifying which parts of the distribution system have harmonic headroom for additional non-linear loads (inversores fotovoltaicos) and which are already approaching limits. The Centre of Information Systems, already showing borderline TDD at peak loads, will require harmonic management if significant PV capacity is added to its supply feeder. The main 33 kV substations, with TDD of 2–5%, have substantial headroom.
06 Perspectiva de la calidad de la energía
The SQU case study is valuable not for the scale of its PQ problems — the campus is largely compliant with IEEE 519 — but for the systematic methodology it demonstrates. A hierarchical PQ measurement campaign covering all voltage levels from the utility interface to individual building entrances, applied to a complex mixed-load environment before a planned major change (PV integration), is textbook engineering practice. The fact that it is rarely executed in this form is the more important observation.
The aggregation effect finding has direct implications for how utilities and campus operators interpret PQ compliance. A campus that is compliant at the 33 kV utility interface — where IEEE 519 compliance is assessed — may simultaneously have individual buildings with significantly higher harmonic distortion that causes equipment problems, shortens transformer and UPS life, and increases losses. Compliance at the PCC does not imply acceptability throughout the distribution system. The internal distribution system is the campus operator’s responsibility — and the SQU methodology, extended to building-level monitoring, would identify which buildings require active harmonic mitigation and which do not.
Large university campuses — with their own 33 kV or 11 kV distribution systems, their own substations, and their own generation — function as mini-utilities. The PQ engineering discipline that applies to a utility distribution system applies equally to the campus distribution system: harmonic limits at internal PCCs, voltage regulation across the feeder, reactive power management for VFD-heavy building loads, and now DER integration planning. Most campus facilities engineers do not have utility distribution engineering backgrounds. The SQU study is an example of what happens when that gap is bridged — systematic, standards-referenced, multi-point PQ assessment that provides an actionable engineering baseline rather than a collection of isolated measurements.
Referencias
- Al-Badi A et al. “Investigation and Analysis of the Power Quality in an Academic Institution’s Electrical Distribution System.” Energías, 17(16), 3998, 2024. DOI: 10.3390/en17163998. Acceso abierto CC BY 4.0.
- IEEE Std 519-2022. Estándar IEEE para control de armónicos en sistemas de energía eléctrica. IEEE, Nueva York, Nueva York, 2022.
- IEC 61000-3-2:2018. Límites para las emisiones de corriente armónica (corriente de entrada del equipo ≤ 16 A por phase). IEC, Ginebra.
- IEC 61727:2004. Photovoltaic (PV) systems — Characteristics of the utility interface. IEC, Ginebra.
- EN 50160:2010+A3:2019. Características de tensión de la electricidad suministrada por las redes eléctricas públicas.. CENELEC, Bruselas.
Al-Badi A et al. “Investigation and Analysis of the Power Quality in an Academic Institution’s Electrical Distribution System.” Energías (MDPI), vuelo. 17, no. 16, p. 3998, Agosto 2024. DOI: 10.3390/en17163998. Acceso abierto CC BY 4.0 — Sultan Qaboos University, Omán.
Este estudio de caso se presenta en forma de resumen y comentario con fines educativos.. Diagramas SVG y la sección Perspectiva PQ (Sección 6) son contenido editorial original de IPQDF de Denis Ruest, Maestría en Ciencias. (Aplicado), P.Eng. (retirado.). IPQDF no reivindica la autoría de la investigación original.