Voltage Unbalance in High-Voltage Networks — Oman Main Interconnected System
| Network | Oman Main Interconnected System (MIS) — 132 kV sub-transmission |
| Measurement points | Three HV grid stations supplying the three main industrial areas in Oman’s MIS |
| Parameters measured | Voltage and current unbalance — compared against international and Omani distribution code limits |
| Voltage unbalance result | Within limits — HV utility network well-balanced at transmission level |
| Standards applied | IEEE 519 · EN 50160 · Omani Distribution Code |
| Key value | Establishes a baseline: the utility HV supply is clean — any unbalance seen at equipment terminals originates downstream, not from the transmission system |
| Network context | Oman MIS serves industrial loads including aluminium smelting, steel, and cement — all significant contributors to PQ disturbances |
01 Context and Background
This case study presents the findings of voltage unbalance measurements conducted at the transmission and sub-transmission level in Oman’s Main Interconnected System (MIS) — the primary electricity network serving the Sultanate’s major industrial and urban load centres. The study by Albadi et al. (2015), presented at the IEEE International Conference on Industrial Technology, is one of the few published accounts of systematic voltage unbalance assessment at 132 kV HV level in a rapidly industrialising Middle Eastern grid.[1]
The Oman MIS is characterised by a load mix that presents significant PQ challenges: large industrial loads including aluminium smelters, steel plants, and cement factories — all of which are significant sources of harmonic distortion, flicker, and voltage unbalance — are connected to the same transmission network that serves residential and commercial customers. Quantifying the unbalance at the HV level is essential for understanding whether the source of unbalance seen at industrial equipment terminals is the utility transmission system or the industrial distribution network itself.
Most voltage unbalance studies focus on LV or MV distribution networks — where the effects on motors and equipment are most directly felt. But the unbalance at LV terminals is the sum of the transmission-level unbalance plus the distribution-level unbalance plus the internal facility unbalance. Measuring at the HV grid station level separates the utility transmission contribution from the distribution and facility contributions. If the HV level is balanced, the utility network is not the root cause — the investigation must look downstream.
02 Voltage Unbalance — Theory and Indices
Definition — what is voltage unbalance?
A three-phase power system operates ideally with three voltage phasors equal in magnitude and separated by exactly 120° in phase angle. Voltage unbalance occurs when either the magnitudes differ between phases, the phase angles between consecutive phases differ from 120°, or both conditions are present simultaneously.[1]
In practice, unbalance arises from a combination of network asymmetry (non-transposed transmission lines, unequal transformer impedances) and load asymmetry (single-phase loads, unbalanced three-phase loads, arc furnaces, traction systems). The resulting unbalanced three-phase system can be decomposed into three symmetrical sequence components using Fortescue’s theorem:
- Positive-sequence component — the balanced forward-rotating component (same rotation as the generator)
- Negative-sequence component — a balanced backward-rotating component (opposite rotation to the generator)
- Zero-sequence component — three equal in-phase phasors (no rotation, only present in systems with a neutral conductor)
Two definitions — IEC vs. NEMA
The IEC symmetrical components definition (VUF = V₂/V₁ × 100%) is the internationally preferred method and is used in EN 50160 and IEC 61000-2-2. It requires phasor measurement (both magnitude and angle) and is the most physically meaningful definition because negative-sequence voltage is directly responsible for the harmful effects in motors and other three-phase equipment.[2]
The NEMA definition (maximum deviation of any phase voltage from the mean, divided by the mean) requires only voltage magnitude measurements and is widely used in North America for field assessments. For small unbalances (below approximately 3%), both methods give numerically similar results. For larger unbalances or cases with significant angle asymmetry, the IEC method gives a more accurate characterisation.[3]
03 Measurement Methodology
Voltage and current unbalance measurements were conducted at three HV grid stations in the Oman MIS. Each grid station supplies one of the three major industrial areas in the system, making the measurement points representative of the PQ environment at the interface between the transmission system and the industrial sub-transmission/distribution network.[1]
The measurement methodology followed international standards for PQ assessment at high voltage. The key challenge at 132 kV is that direct measurement is not possible — voltage and current instrument transformers (VTs and CTs) are used to step down the signals to instrument-level voltages and currents, which requires verification of the instrument transformer accuracy class to ensure the measured unbalance values are not artefacts of transformer errors rather than real network asymmetry.
At 132 kV, a 1% voltage unbalance corresponds to a phase-to-phase voltage difference of approximately 760 V. Instrument transformers with accuracy class 0.2 or better are required to resolve this level of unbalance reliably. A class 0.5 VT introduces a measurement uncertainty of ±0.5% — potentially comparable to the unbalance being measured. This is why HV unbalance measurements require explicit documentation of instrument transformer accuracy class, and why apparent unbalance at the HV level below 0.5–1% should be interpreted with caution.
The measured unbalance data were compared against the limits specified in the Omani electricity distribution code and in the applicable international standards — EN 50160 (limit: VUF ≤ 2% for 95% of any one-week period) and IEEE 519-2014 (which addresses harmonic limits but references the same 2% unbalance threshold for planning purposes).[2][4]
04 Key Findings
Transmission-level unbalance — within limits
The voltage and current unbalance measurements at all three HV grid stations in the Oman MIS were within the limits specified by the Omani distribution code and the applicable international standards (EN 50160, IEEE 519). The transmission system, despite serving large and potentially unbalancing industrial loads, maintained its three-phase voltage symmetry within the 2% VUF threshold at the grid station measurement points.[1]
| Measurement point | Voltage unbalance (VUF) | EN 50160 limit | Omani code limit | Compliance |
|---|---|---|---|---|
| Grid Station A — Industrial Area 1 | Within limit — exact value not published | ≤ 2% (95th %ile) | ≤ 2% | COMPLIANT |
| Grid Station B — Industrial Area 2 | Within limit — exact value not published | ≤ 2% (95th %ile) | ≤ 2% | COMPLIANT |
| Grid Station C — Industrial Area 3 | Within limit — exact value not published | ≤ 2% (95th %ile) | ≤ 2% | COMPLIANT |
| Source: Albadi et al. (2015). Measurements at 132 kV grid stations in Oman MIS. Exact numerical values not published in publicly available abstract; compliance status confirmed. | ||||
The fact that the Oman MIS HV network is within unbalance limits at the grid station level is an important baseline finding. It means that if voltage unbalance problems are observed at industrial equipment terminals in these areas — motor overheating, protection relay misoperation, capacitor bank problems — the source is not the utility transmission system. It is the industrial distribution network between the grid station and the equipment: unequal single-phase loading, non-transposed feeders, blown capacitor fuses, or poorly balanced three-phase motor loads. The utility is delivering a balanced supply. This immediately redirects the engineering investigation from the utility to the facility.
Current unbalance — a separate indicator
Current unbalance was also measured alongside voltage unbalance. Current unbalance is a load-side quantity — it reflects the asymmetry of the connected loads rather than the asymmetry of the supply network. A balanced supply voltage with unbalanced load currents indicates that single-phase or unequal three-phase loads are creating asymmetrical current flows in the distribution system, which in turn produce small voltage unbalances through the network impedance.[1]
The relationship between current unbalance and voltage unbalance depends on the network impedance at the measurement point. At the HV grid station (high short-circuit level, low source impedance), even significant current unbalance from industrial loads produces only small voltage unbalance at the bus — which is why the HV measurements are within limits even though the downstream distribution network may show more significant unbalance at lower voltage levels.
05 Effects of Voltage Unbalance
The study provides a comprehensive review of the negative impacts of voltage unbalance, which form the engineering rationale for the 2% VUF limit in international standards:[1]
Induction motors — the most sensitive victim
Induction motors are the equipment type most severely affected by voltage unbalance. The negative-sequence voltage component (V₂) drives a rotating magnetic field in the opposite direction to the positive-sequence field. In the rotor reference frame, the negative-sequence field rotates at approximately twice the synchronous speed — the rotor offers very low impedance to this component, resulting in large negative-sequence rotor currents from a small negative-sequence voltage.
Other affected equipment and systems
- Three-phase rectifiers and drives — unbalanced supply voltage produces unequal conduction angles in rectifier diodes or thyristors, generating non-characteristic harmonic orders and increasing output ripple
- Power transformers — negative-sequence currents increase winding losses and core saturation. Transformer protection (differential relays) may produce spurious trips under severe unbalance conditions
- Power factor correction capacitors — unbalanced voltages produce unequal reactive current distribution across capacitor phases. A blown fuse on one phase of a capacitor bank is both a cause and an amplifier of voltage unbalance
- Protection systems — distance relays and differential protection schemes rely on balanced voltage assumptions. Persistent unbalance can cause relay misoperation or desensitisation
- Energy metering — unbalanced systems require true three-phase metering. Single-phase or two-element metering configurations introduce measurement errors under unbalanced conditions
06 Mitigation Techniques
The study reviews the principal mitigation approaches for voltage unbalance, which fall into three categories based on their point of application:[1]
| Technique | Mechanism | Applicable to | Cost range |
|---|---|---|---|
| Load balancing | Redistribution of single-phase loads across phases to equalise per-phase current draw | Commercial and industrial facilities; residential LV feeders | Low — operational measure |
| Network transposition | Systematic rotation of phase conductor positions along a line to equalise mutual impedances over the full length | HV transmission lines with inherent geometric asymmetry | Medium — construction cost |
| Static VAR Compensator (SVC) | Independently controllable reactive power injection on each phase to compensate asymmetrical reactive demand | Large single-phase loads (arc furnaces, traction, induction heating) | High — $1–5M USD |
| STATCOM | Voltage-source converter with per-phase control — faster response than SVC, better performance under dynamic unbalance | Industrial loads with rapidly varying unbalance | High — $2–8M USD |
| Motor derating | Operating motors below nameplate rating to maintain thermal margins under persistent unbalance — not mitigation but a protective measure | Existing motor installations where unbalance cannot be eliminated | Zero capital — production cost |
| Scott-T or Le Blanc transformer | Converts single-phase load (traction) to a balanced two-phase equivalent, reducing network unbalance from railway supply | Electric railway traction systems | Medium — transformer cost |
Before specifying any active compensation equipment for voltage unbalance, the first step is always a systematic load audit — identifying which single-phase loads are creating the imbalance, and whether rebalancing them across phases is feasible. In many industrial facilities, unbalance is simply the result of historical single-phase load additions to whichever phase happened to have spare capacity at the time of installation. A systematic rebalancing exercise costs nothing in capital and can reduce unbalance by 50–80% before any power electronics are considered.
07 Power Quality Perspective
This study occupies a specific and valuable position in the PQ case study literature: it is one of the few published accounts of systematic voltage unbalance measurement at the HV transmission level in a rapidly industrialising grid. The finding that the Oman MIS HV network is within international limits — despite serving large, potentially unbalancing industrial loads — provides an important baseline.
From a utility engineering perspective, the key insight is the impedance argument: the HV grid bus has high short-circuit capacity, meaning its voltage is stiff and resistant to distortion from unbalanced load currents. The same load current that produces a 2% VUF on a weak LV feeder might produce only 0.1–0.2% VUF at the 132 kV bus. This explains why the transmission system appears balanced while distribution-connected equipment experiences significant unbalance — the unbalance is created by distribution-level impedances and loads, not transmitted from the HV system.
Where you measure voltage unbalance determines what you find. Measure at the 132 kV grid station — you find a balanced supply. Measure at the 11 kV distribution bus — you may find 0.5–1.5% VUF from feeder asymmetry. Measure at the motor terminals in an industrial plant — you may find 2–4% VUF from internal load imbalance. All three measurements are correct — they are measuring different things. An engineering assessment that concludes “the utility supply is balanced” from a HV measurement, without measuring at the equipment terminals, misses the entire story.
The Albadi et al. study demonstrates exactly the kind of systematic, standards-referenced PQ measurement at the transmission level that is rarely published but critically important for utility planning. The Oman MIS baseline data confirms that the transmission network is not the source of the voltage unbalance problems reported in the industrial areas it serves — which is a finding with direct operational implications: the engineering effort should focus on the distribution network and facility load management, not on the transmission system. This is the utility perspective that most facility-side PQ studies miss.
References
- Albadi MH, Al Hinai AS, Al-Badi AH, Al Riyami MS, Al Hinai SM, Al Abri RS. “Unbalance in Power Systems — Review and Oman MIS Case Study.” Proceedings of the IEEE International Conference on Industrial Technology (ICIT 2015), Seville, Spain, pp. 1407–1411, March 2015. DOI: 10.1109/ICIT.2015.7125294
- EN 50160:2010+A3:2019. Voltage characteristics of electricity supplied by public electricity networks. CENELEC, Brussels.
- NEMA MG-1-2021. Motors and Generators. National Electrical Manufacturers Association, Rosslyn, VA.
- IEEE Std 519-2022. IEEE Standard for Harmonic Control in Electric Power Systems. IEEE, New York, NY, 2022.
- IEC 61000-2-2:2002+AMD1:2017. Electromagnetic compatibility (EMC) — Part 2-2: Compatibility levels for low-frequency conducted disturbances in public low-voltage supply systems. IEC, Geneva.
Albadi MH, Al Hinai AS, Al-Badi AH, Al Riyami MS, Al Hinai SM, Al Abri RS. “Unbalance in Power Systems — Review and Oman MIS Case Study.” IEEE ICIT 2015, pp. 1407–1411.
DOI: 10.1109/ICIT.2015.7125294 · View on Semantic Scholar →
This case study is presented in summary and commentary form for educational purposes. The original publication is an IEEE conference paper; copyright belongs to IEEE. The PQ Perspective section (Section 7) and SVG diagrams are original IPQDF editorial content by Denis Ruest, M.Sc. (Applied), P.Eng. (ret.). IPQDF does not claim authorship of the original research.