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IEC TS 62749-2015: Assessment of Power Quality & Characteristics of Electricity Supplied by Public Networks
This technical specification provides a comprehensive framework for assessing power quality (PQ) in public electricity networks. It defines recommended values for PQ indices and establishes standardized assessment methodologies applicable at low, medium, and high voltage levels.
1. Power Quality Indices and Recommended Values
IEC TS 62749-2015 defines a set of power quality indices covering both continuous phenomena (frequency deviation, voltage deviation, harmonics, unbalance, flicker) and discontinuous events (voltage dips, swells, interruptions, transient overvoltages). The recommended limits vary by voltage level, with LV systems generally having the most stringent requirements.
For supply voltage deviation, the specification recommends that under normal operating conditions, the 10-minute mean RMS voltage at LV supply terminals should remain within ±10 % of the declared voltage (Un) for 95 % of each week, and within ±15 % for 100 % of the time. For MV and HV systems, the recommended range is typically ±10 % of the declared voltage for 95 % of the week.
Frequency deviation is assessed using 10-second mean values. In interconnected systems, the frequency should remain within ±1 % (±50 mHz for 50 Hz systems) for 99.5 % of the year, and within ±6 % for 100 % of the time. Islanded systems have relaxed limits of ±2 % for 99.5 % of the year.
Frequency stability is a key indicator of the generation-load balance. When frequency deviations exceed recommended limits, it indicates a serious imbalance that may trigger under-frequency load shedding (UFLS) or over-frequency generation tripping schemes.
2. Harmonic Distortion and Flicker Management
The standard provides detailed recommended values for individual harmonic voltages up to the 50th order. For LV systems, total harmonic distortion (THD) should not exceed 8 % of the fundamental voltage, with individual odd harmonics typically limited to 6 % and even harmonics to 2 %. The table below summarizes the key individual harmonic limits for LV systems:
| Harmonic Order (n) | Odd Harmonics (% of U1) | Even Harmonics (% of U1) |
|---|---|---|
| 3 | 5.0 | — |
| 5 | 6.0 | — |
| 7 | 5.0 | — |
| 9 | 1.5 | — |
| 11 | 3.5 | — |
| 13 | 3.0 | — |
| 2 | — | 2.0 |
| 4 | — | 1.0 |
| 6–10 (even) | — | 0.5 |
Flicker severity is quantified using the Pst (short-term, 10-minute) and Plt (long-term, 2-hour) indices. The recommended Plt value at LV and MV supply terminals is Plt ≤ 1.0 for 95 % of the time. This limit ensures that voltage fluctuations from sources such as arc furnaces, welding equipment, and large motor starts remain acceptable to end users.
Engineering Insight: When designing compensation systems for flicker-prone loads, consider that the Pst and Plt indices follow a cube-law summation (∑Pst³), meaning that the dominant flicker source often determines the overall severity. A single large disturbance can mask multiple smaller ones.
3. Voltage Dip Assessment and SARFI Indices
For discontinuous phenomena, IEC TS 62749 introduces the SARFI (System Average RMS Variation Frequency) index family. SARFI-X quantifies the average number of voltage dips per year with a retained voltage below X % of the declared voltage. The standard recommends assessment over a one-year measurement period, with dips categorized by magnitude and duration in a standardized table format.
Voltage dips are characterized by both residual voltage and duration. The standard classifies dips into three duration ranges: 10 ms to 500 ms, 500 ms to 1 s, and 1 s to 60 s. The magnitude is categorized as 10 %–30 %, 30 %–90 %, and 90 %–99 % retained voltage.
| Duration ↓ / Magnitude → | 10–30 % Retained | 30–90 % Retained | 90–99 % Retained |
|---|---|---|---|
| 10 ms to 500 ms | Type D1 | Type D2 | Type D3 |
| 500 ms to 1 s | Type E1 | Type E2 | Type E3 |
| 1 s to 60 s | Type F1 | Type F2 | Type F3 |
Transient overvoltages are addressed separately for LV systems (typically caused by switching and lightning) and MV/HV systems. For LV systems, the standard notes that transient overvoltages between live conductors and earth should not exceed 6 kV peak, consistent with insulation coordination requirements in the IEC 60364 series.
A critical design consideration: Distributed generation (DG) and microgrids can significantly alter power quality characteristics. Inverters may inject DC currents causing magnetic bias in distribution transformers, and the bidirectional power flow can affect voltage profiles and protection coordination. Always conduct a thorough PQ study before integrating DG above 10 % of the local peak load.
4. Site vs. System Assessment Methodology
IEC TS 62749 distinguishes between site assessment (evaluating PQ at a single connection point) and system assessment (evaluating overall PQ performance across the network). For continuous phenomena at a site, the 95 % weekly probability level is the standard evaluation metric. For event-based phenomena, the assessment uses the SARFI index or event count per year.
For system assessment, the spec recommends using the 95th percentile of all site measurements for continuous phenomena, which provides a robust indicator of overall system PQ that is not unduly influenced by a single poor-performing site.
When performing PQ assessments, ensure measurement periods cover at least one full week for continuous phenomena and a full year for event-based phenomena to capture seasonal variations and statistical significance. Shorter measurement windows may miss critical events or misrepresent long-term trends.
5. FAQs
Q: What is the difference between Pst and Plt flicker severity indices?
A: Pst (short-term severity) is measured over a 10-minute interval and captures rapid voltage fluctuations. Plt (long-term severity) is calculated from 12 consecutive Pst values over 2 hours. Plt smooths out transient events and better represents the annoyance level for typical lamp flicker perception.
Q: Can IEC TS 62749 be used for contractual PQ specifications?
A: Yes, the standard provides recommended values that can be adopted into supply contracts. However, the spec emphasizes that regional variations, network characteristics, and economic factors must be considered when setting contractual PQ limits.
Q: How does the standard address power quality issues from renewable energy sources?
A: Annex D specifically covers PQ issues related to distributed generation and microgrids, including voltage deviation from reverse power flow, harmonic injection from inverters, DC current injection causing transformer saturation, and rapid voltage changes from variable generation.
Q: What measurement standards support the implementation of IEC TS 62749?
A: Key supporting standards include IEC 61000-4-30 (PQ measurement methods), IEC 61000-4-7 (harmonics measurement), IEC 61000-4-15 (flickermeter), and IEC 61000-4-11 (voltage dip immunity testing). Compliance with these measurement standards is essential for consistent PQ assessment.
