高压网络电压不平衡——阿曼主要互联系统
| 网络 | 阿曼主要互联系统 (管理信息系统) - 132 kV次输电 |
| 测量点 | 三个高压电网站为阿曼 MIS 的三个主要工业区供电 |
| 测量参数 | 电压和电流不平衡——与国际和阿曼配电规范限制相比 |
| 电压不平衡结果 | 在限制范围内 — 高压公用网络在输电层面均衡良好 |
| 适用标准 | IEEE 519 · 在 50160 · 阿曼分布代码 |
| 关键值 | 建立基线: 公用事业高压电源是干净的——设备终端出现的任何不平衡都源于下游, 不是来自传输系统 |
| 网络环境 | 阿曼 MIS 服务于包括铝冶炼在内的工业负荷, 钢, 和水泥——所有 PQ 干扰的重要因素 |
01 背景和背景
本案例研究介绍了在阿曼主互连系统的输电和次输电层面进行的电压不平衡测量的结果 (管理信息系统) — 服务于苏丹国主要工业和城市负荷中心的主要电力网络. 阿尔巴迪等人的研究. (2015), 在 IEEE 国际工业技术会议上发表, 是为数不多的关于系统电压不平衡评估的公开报道之一 132 快速工业化的中东电网中的 kV 高压水平.[1]
阿曼 MIS 的特点是负载混合带来了重大的 PQ 挑战: 大型工业负载，包括铝冶炼厂, 钢铁厂, and cement factories — all of which are significant sources of harmonic distortion, 闪烁, 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]
在实践中, 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, 电弧炉, 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. NO
The IEC symmetrical components definition (VUF = V₂/V₁ × 100%) is the internationally preferred method and is used in EN 50160 和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, 除以平均值) 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 测量方法
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.
在 132 千伏, 一 1% voltage unbalance corresponds to a phase-to-phase voltage difference of approximately 760 在. 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 (限制: VUF ≤ 2% 为 95% of any one-week period) 和IEEE 519-2014 (which addresses harmonic limits but references the same 2% unbalance threshold for planning purposes).[2][4]
04 主要发现
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 (IN 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]
| 测量点 | 电压不平衡 (真空超滤) | IN 50160 限制 | Omani code limit | 合规性 |
|---|---|---|---|---|
| Grid Station A — Industrial Area 1 | Within limit — exact value not published | ≤ 2% (95th %ile) | ≤ 2% | 合规 |
| Grid Station B — Industrial Area 2 | Within limit — exact value not published | ≤ 2% (95th %ile) | ≤ 2% | 合规 |
| Grid Station C — Industrial Area 3 | Within limit — exact value not published | ≤ 2% (95th %ile) | ≤ 2% | 合规 |
| 源: 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, 继电保护误动作, 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, 负序磁场以大约同步速度两倍的速度旋转 - 转子为此组件提供非常低的阻抗, 由小的负序电压产生大的负序转子电流.
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
- 电力变压器 — 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]
| 技术 | 机制 | Applicable to | Cost range |
|---|---|---|---|
| 负载均衡 | 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 | 大单相负载 (电弧炉, 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 电能质量视角
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.
从公用事业工程的角度来看, 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% 弱 LV 馈线上的 VUF 可能仅产生 0.1–0.2% VUF 132 kV总线. 这解释了为什么传输系统看起来是平衡的，而配电连接设备却经历了严重的不平衡——不平衡是由配电级阻抗和负载造成的, 不从高压系统传输.
测量电压不平衡的位置决定了您的发现. 测量于 132 kV 电网站 — 您会找到平衡的供电. 测量于 11 kV 配电母线 — 您可能会发现馈线不对称导致 0.5–1.5% VUF. 在工业厂房的电机端子处进行测量 — 您可能会发现内部负载不平衡导致 2–4% VUF. 所有三个测量值都是正确的——它们测量的是不同的东西. 工程评估结论 “公用事业供应平衡” 从 HV 测量, 无需在设备终端进行测量, 错过整个故事.
阿尔巴迪等人. 研究准确地证明了这种系统性, 输电层面参考标准的 PQ 测量很少发表，但对于公用事业规划至关重要. 阿曼 MIS 基线数据证实，输电网络并不是其所服务的工业区域报告的电压不平衡问题的根源，这一发现具有直接的运营影响: 工程工作应重点关注配电网络和设施负载管理, 不在传输系统上. 这是大多数设施方 PQ 研究忽略的公用事业视角.
参考文献
- 阿尔巴迪 MH, 阿尔希奈AS, 巴迪·AH, 阿尔·里亚米女士, 阿尔希奈 SM, 阿尔·阿布里·RS. “电力系统不平衡——回顾和阿曼 MIS 案例研究。” IEEE 国际工业技术会议论文集 (个人所得税 2015), 塞维利亚, 西班牙, PP. 1407–1411, 三月 2015. DOI: 10.1109/ICIT.2015.7125294
- IN 50160:2010+A3:2019. 公共电网供电的电压特性. CENELEC的, 布鲁塞尔.
- 否 MG-1-2021. 电机和发电机. 全国电气制造商协会, 罗斯林, VA.
- IEEE StD里 519-2022. 电力系统谐波控制 IEEE 标准. IEEE, 纽约, 纽约, 2022.
- 符合IEC 61000-2-2:2002+AMD1:2017. 电磁兼容性 (EMC公司) - 部分 2-2: 公共低压供电系统中低频传导骚扰的兼容性级别. 符合IEC, 日内瓦.
阿尔巴迪 MH, 阿尔希奈AS, 巴迪·AH, 阿尔·里亚米女士, 阿尔希奈 SM, 阿尔·阿布里·RS. “电力系统不平衡——回顾和阿曼 MIS 案例研究。” IEEE ICIT 2015, PP. 1407–1411.
DOI: 10.1109/ICIT.2015.7125294 · 查看语义学者 →
本案例研究以总结和评论的形式呈现，用于教育目的. 原始出版物是 IEEE 会议论文; 版权属于 IEEE. PQ 视角部分 (部分 7) 和 SVG 图表是 Denis Ruest 的原创 IPQDF 编辑内容, 硕士. (应用), P.Eng. (ret。). IPQDF 不声称原始研究的作者.