Evaluation of the Energy Quality of a 2 MWp Photovoltaic Power Plant Integrated into the Distribution Network: Revision of the EN50160 Standard on Total Harmonic Distortion and Voltage Variations

1. Introduction

The EN50160 standard, developed by the European Committee for Electrotechnical Standardization (CENELEC), defines the voltage characteristics supplied by public distribution networks. This standard, which primarily applies to alternating current (AC) distribution networks, regulates several key electricity quality parameters, such as frequency, voltage variations, sags and surges, as well as harmonic disturbances [1]. However, with the increasing integration of renewable energy sources and the growing complexity of modern power grids, limitations in the effectiveness of this standard are becoming increasingly apparent [2,3]. Recent research has highlighted voltage fluctuations in networks integrating renewable energy sources, emphasizing the challenges these configurations pose in terms of power quality [4,5].

This paper provides a critical analysis of the EN50160 standard, based on measurements taken at a 2 MWp photovoltaic power plant connected to a 22 kV medium-voltage network [6]. Three main aspects are addressed: Voltage Total Harmonic Distortion (THDU) [7], as well as voltage unbalances U0% and U2% [8,9,10]. Voltage THD, a crucial indicator of electricity quality, can lead to malfunctions, reduced equipment lifespan, and energy losses if excessively high [11,12]. Voltage unbalances, U0% and U2%, are also essential to ensure the stability and reliability of the power grid, particularly in a context where renewable energy integration complicates voltage management [13].

The measurements, conducted between April 24, 2022, at 14:17 and April 25, 2022, at 09:45, cover critical periods for photovoltaic production, such as sunrise and sunset. These periods directly influence energy production and the observed THD levels [14,15]. The results, based on aggregation periods of one second, ten seconds, one minute, and ten minutes, reveal significant deviations from the limits imposed by the EN50160 standard, particularly in terms of voltage THD values and voltage unbalances frequently exceeding the authorized thresholds [16,17]. This suggests that the standard, in its current form, may be inadequate for distribution networks integrating significant amounts of renewable energy [18,19].

These observations challenge the effectiveness of the EN50160 standard in ensuring an electricity quality assessment that meets the needs of modern networks. This paper delves into these discrepancies, highlighting the shortcomings of the standard while proposing revisions to better address the challenges associated with renewable energy integration [20,21]. The recommendations aim to improve electricity quality and ensure greater compatibility with present and future technologies. In conclusion, this study underscores the necessity of updating the EN50160 standard to meet the requirements of contemporary electrical grids [22,23].

The paper is structured to cover each aspect of the study: the first section presents the photovoltaic power plant and its technical specifications, followed by the methodology adopted for the measurements, including a description of the instrument used, such as the power quality analyzer, and the selected aggregation periods. A critical analysis of voltage THDU and voltage unbalances (U0% and U2%) is then conducted, comparing the results with the thresholds defined by the EN50160 standard. Finally, recommendations are provided to adapt the standard to modern grids while ensuring a reliable power supply quality..

2. Operating Procedure

2.1. Description of the 2 MWp Plant Installation

This 2 MWp photovoltaic power plant is connected to a 22 kV medium-voltage network. The energy produced is transmitted via an MV line to the 60/22 kV delivery substation located nearby. The plant's configuration includes 45 photovoltaic strings, each consisting of 160 panels. Each string is made up of 8 series-connected chains.

The inverters of the installation have 6 pairs of inputs, totaling 72 chains. Each input receives 12 chains, with 8 coming from one string and 4 from another, ensuring optimal energy management and its integration into the 22 kV network. Figure 1 illustrates the detailed configuration of the power plant.

The detailed technical specifications of the plant are as follows:

• Electrical Parameters: The maximum voltage is 710 V, and the maximum current is 568 A.
• Efficiency and Surface Area: The modules have an efficiency of 14.38% and cover a total area of 13,858 m².
• Production and Efficiency: The estimated annual production is 3050 MWh, with an overall capacity factor of 1.80% and total losses of 509 MWh per year.

The two inverter models used have capacities of 700 kW and 1050 kW. With the plant's estimated maximum power output of around 1619 kW, these inverters were selected accordingly. These specifications demonstrate the plant’s capability to optimize solar energy utilization in the region.

2.2. Qualimeter Measurements

The power quality meter (qualimeter) operates with a 10-cycle integration period (200 ms), making it compliant with the requirements of the EN61000-4-30 standard for measuring Total Harmonic Distortion (THD) and voltage unbalances. This qualimeter incorporates the NIDAQ 6009 card [24] and is inspired by its predecessor, which was based on an ARDUINO board [25].

It is connected to the electrical grid through connection devices that ensure both measurement accuracy and safety. These devices include current and voltage transformers, which adapt the grid's electrical levels to the qualimeter's inputs. This setup enables precise energy quality analysis while preventing potential damage to the device.Figure 2 illustrates the measurement location of the qualimeter. It is essential to adhere to electrical safety standards during these connections to protect both the operator and the equipment.

In the photo below (Figure 3), the qualimeter can be seen connected to the 22 kV MV feeder, where the electrical energy produced by the 2 MWp plant is injected.

Measurement results can be extracted over aggregation periods of 1 second, 10 seconds, 1 minute, and 10 minutes. The qualimeter is used to monitor and analyze the quality of electrical energy produced by the photovoltaic plant. It provides real-time visualization of essential data such as voltage, current, power, and harmonics. This display allows plant operators to quickly identify and diagnose any potential issues in the electrical network, ensuring efficient and reliable system management.

2.3. The EN50160 Standard: Characteristics and Implications

The EN50160 standard defines the voltage characteristics supplied by public electricity networks to ensure a stable and reliable power supply. Among the key quality criteria are Total Harmonic Distortion (THD) and voltage unbalance.

Total Harmonic Distortion (THD)

• Limit: 8% over a 10-minute aggregation period.
• Measurement: Proportion of harmonic components in the voltage relative to the fundamental component.

Voltage Unbalance

• Parameters: U0% (zero-sequence voltage unbalance) and U2% (negative-sequence voltage unbalance).
• Limits: U0% and U2% must not exceed 2% for 95% of the time (10-minute intervals).

Importance and Implications

• Equipment Performance: Excessive unbalance can lead to overheating and reduce the efficiency of motors and transformers.
• Energy Efficiency: Energy losses increase with unbalance, affecting overall system efficiency.
• Grid Reliability: Balanced voltages extend the lifespan of network components.

By following these recommendations and adhering to the specified integration periods, it is possible to ensure accurate and representative voltage quality measurements in a conventional distribution network.

3. Critical Analysis of Standard EN50160: Evaluation of Measurements in a 2MWp PV Power Plant

This study aims to identify the optimal aggregation period for these measurements in a 2MWp photovoltaic (PV) power plant. Given the intermittent nature of PV generation, choosing an appropriate aggregation period is crucial for accurate assessments. Our aim is to propose recommendations for improving the compliance and performance of PV power plants in relation to EN50160.

3.1. Harmonics (Harmonic Voltages)

Voltage harmonics are sinusoidal voltage components that exist at frequencies that are multiples of the fundamental frequency of the power system. In a power distribution system, the fundamental frequency is generally 50 Hz or 60 Hz. Harmonics can be caused by various non-linear equipment such as switch-mode power supplies, variable speed drives, power electronic converters, and other devices that distort the voltage's sinusoidal waveform.

Voltage harmonics

Total harmonic voltage distortion THD is a common measure used to quantify the presence and severity of harmonics in a voltage system. It is expressed as a percentage of the fundamental voltage. The formula for THD is :

$$\mathrm{T}\mathrm{H}\mathrm{D}={\displaystyle \frac{\sqrt{\sum _{\mathrm{n}=2}^{\mathrm{\infty }}{\mathrm{V}}_{\mathrm{n}}^2}}{{\mathrm{V}}_1}}$$

(1)

Where:

-

V₁ is the RMS value of the fundamental component of the signal.

-

V_n is the RMS value of the nth harmonic component of the signal.

3.2. Analysis of Measurements with Several THD Aggregation Periods Per Phase

Measurements were conducted from April 24, 2022, at 14:17 to April 25, 2022, at 09:45. This analysis helps to understand the variability of power grid quality over time.

3.2.1. Phase 1 THD Analysis

The document provided in Figure 4, appears to contain data concerning the total harmonic distortion (THD) of phase L1 over different time intervals (1 second, 10 seconds, one minute and 10 minutes).

Analysis of Total Harmonic Distortion (THD) Measurements of Phase L1

THD measurements for Phase 1 were conducted over four different time periods to analyze fluctuations in electrical signal quality:

• THD over 1 second: Significant instantaneous variations, with a maximum value of 2.8417 and a minimum of 1.3496, indicating rapid transient disturbances.
• THD over 10 seconds: Smoother view, with less extreme fluctuations (max: 2.7925, min: 1.4022).
• THD over 1 minute: More stable evaluation, reducing distortion peaks (max: 2.7163, min: 1.4422).
• THD over 10 minutes: A long-term representative average, showing increased stability (max: 2.4732, min: 1.6331).

The results indicate that variations are more pronounced over shorter periods and gradually diminish over longer periods, reflecting improved overall network stability over 10 minutes.

Data Interpretation

The observed variability in THD values for Phase L1 is normal and can be attributed to several factors, such as load variations, electrical disturbances, or other transient phenomena. Instantaneous measurements (1s and 10s) exhibit more pronounced fluctuations, whereas longer periods (1min and 10min) provide a smoother and more stable view of power quality.

3.2.2. Phase 2 THD Analysis

Figure 5 provides detailed data on the Total Harmonic Distortion (THD) of Phase L2, measured over different time periods: 1 second, 10 seconds, 1 minute, and 10 minutes. This analysis helps to understand the variability of power grid quality over time for Phase L2.

Analysis of Total Harmonic Distortion (THD) Measurements of Phase L2

THD measurements for Phase L2 were conducted over aggregation periods of 1 second, 10 seconds, 1 minute, and 10 minutes, revealing a progressive attenuation of fluctuations as the aggregation period increases.

• THD over 1 second: Significant variations, with a maximum value of 2.8250 and a minimum of 1.3842, reflecting rapid and transient disturbances.
• THD over 10 seconds: Fluctuations become more moderate (max: 2.7735, min: 1.4295), indicating the beginning of stabilization.
• THD over 1 minute: The variation range further decreases (max: 2.6925, min: 1.4759), providing better stability by filtering out brief disturbances.
• THD over 10 minutes: Measurements over this period show good overall stability, with a maximum value of 2.4608 and a minimum of 1.6291, reflecting more homogeneous grid conditions.

In summary, the significant fluctuations observed over shorter periods gradually diminish as the observation duration increases, ensuring greater stability over 10 minutes and allowing for a more reliable long-term assessment of power quality.

Data Interpretation

The variability observed in THD values for Phase 2 is normal and can be attributed to several factors, such as load variations, electrical disturbances, or other transient phenomena. Instantaneous measurements (1s and 10s) exhibit more pronounced fluctuations, whereas longer periods (1min and 10min) provide a smoother and more stable view of power quality.

3.2.3. Phase 3 THD Analysis

The document analyzed in Figure 6 provides detailed data on the Total Harmonic Distortion (THD) of phase L3, measured at different periods: 1 second, 10 seconds, 1 minute and 10 minutes.

Analysis of Total Harmonic Distortion (THD) Measurements of Phase L3

THD measurements for Phase L3 were conducted over four distinct periods, allowing for the observation of grid fluctuations at different time scales and providing a progressively detailed analysis.

• THD over 1 second: This measurement captures the fastest instantaneous variations, with a maximum value of 2.4945 and a minimum of 1.1551. These significant deviations highlight the presence of rapid transient disturbances.
• THD over 10 seconds: Over this period, fluctuations are slightly smoothed (max: 2.4525, min: 1.2212), reducing the impact of very brief variations and providing better stability.
• THD over 1 minute: The one-minute measurement offers a more stable average, attenuating transient anomalies (max: 2.3826, min: 1.3062). The range of values narrows, indicating a noticeable improvement in signal stability.
• THD over 10 minutes: This period provides a representative overview of general grid conditions (max: 2.2041, min: 1.4030). The reduction in extreme values shows a clear trend toward increased long-term stability.

These results highlight a progressive attenuation of fluctuations as the aggregation period increases, allowing for a more reliable assessment of overall power quality over extended periods.

Data Interpretation

The variability observed in THD values for Phase L3 is normal and can be attributed to several factors, such as load variations, electrical disturbances, or other transient phenomena. Instantaneous measurements (1s and 10s) exhibit more pronounced fluctuations, whereas longer periods (1min and 10min) provide a smoother and more stable view of power quality.

3.2.4. Conclusion of THD Analysis

The detailed analysis of THD data for phases L1, L2, and L3 shows that the electrical grid exhibits normal harmonic distortion variations, which should be closely monitored. The analysis of Total Harmonic Distortion (THD) over a one-minute aggregation period provides more meaningful and detailed insights for all three phases compared to the 10-minute period specified by the EN50160 standard.

Shorter aggregation periods, such as one minute, allow for a more precise capture of transient events and fluctuations, offering a better understanding of harmonic disturbances in the electrical system. Additionally, the THD curve remains similar to those observed for one-second and ten-second periods, demonstrating the consistency and accuracy of measurements at shorter aggregation intervals. This improved granularity is crucial for effectively monitoring and addressing power quality issues, ensuring a more reliable and efficient distribution network.

3.3. Voltage Unbalance

The Fortescue component method, developed by Charles L. Fortescue in 1918, is a technique used to analyze unbalanced three-phase systems. This method breaks down an unbalanced three-phase system into three balanced systems, called symmetrical components: direct component, inverse component and homopolar component. These components simplify the analysis of asymmetrical systems by treating them as symmetrical systems.

Symmetrical components

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Direct (or Positive, U1) component: Represents a balanced system with voltages rotating in the forward direction of the phase sequence (ABC).

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Reverse component (or Negative, U2): Represents a balanced system with voltages rotating in the reverse direction of the phase sequence (ACB).

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Homopolar component (or Zero, U0): Represents a system where all phases have the same voltage and are in phase.

Decomposition formulas

For a three-phase system with voltages Ua, Ub and Uc, the symmetrical components can be calculated using the following formulas:

$${\mathrm{U}}_1\mathrm{}={\displaystyle \frac{1}{3}}\mathrm{}\left({\mathrm{U}}_{\mathrm{a}}\mathrm{}+\mathrm{a}{\mathrm{U}}_{\mathrm{b}}\mathrm{}+{\mathrm{a}}^2{\mathrm{U}}_{\mathrm{c}}\mathrm{}\right)$$

(2)

$${\mathrm{U}}_2\mathrm{}={\displaystyle \frac{1}{3}}({\mathrm{U}}_{\mathrm{a}}\mathrm{}\mathrm{}+{\mathrm{a}}^2{\mathrm{U}}_{\mathrm{b}}+\mathrm{a}{\mathrm{U}}_{\mathrm{c}})$$

(3)

$${\mathrm{U}}_0\mathrm{}={\displaystyle \frac{1}{3}}\mathrm{}\left({\mathrm{U}}_{\mathrm{a}}\mathrm{}+{\mathrm{U}}_{\mathrm{b}}+{\mathrm{U}}_{\mathrm{c}}\right)$$

(4)

Where a is a complex phase operator defined by a=$e^{j120°}$, corresponding to a rotation of 120 degrees. In complex notation, a and a² are given by:

$$\mathrm{a}=-{\displaystyle \frac{1}{2}}\mathrm{}+\mathrm{j}{\displaystyle \frac{\sqrt{3}}{2}}$$

(5)

$$a^2=-{\displaystyle \frac{1}{2}}\mathrm{}-\mathrm{j}{\displaystyle \frac{\sqrt{3}}{2}}$$

(6)

Voltage Unbalance Calculations

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- Reverse component unbalance (U2)

Inverse component unbalance is calculated as follows:

$$\mathrm{\%}{\mathrm{U}}_2\mathrm{}=\left({\displaystyle \frac{\mid {\mathrm{U}}_2\mathrm{}\mid }{\mid {\mathrm{U}}_1\mathrm{}\mid }}\mathrm{}\right)\times 100$$

(7)

-

Zero sequence component unbalance (U0)

The imbalance of the zero sequence component is calculated as follows:

$$\mathrm{\%}{\mathrm{U}}_0=\left({\displaystyle \frac{\mid {\mathrm{U}}_0\mathrm{}\mid }{\mid {\mathrm{U}}_1\mathrm{}\mid }}\mathrm{}\right)\times 100$$

(8)

3.3.1. Homopolar Unbalance (U0)

Figure 7 provides detailed data on the Unbalance Voltage (U0%) measured at different periods: 1 second, 10 seconds, 1 minute and 10 minutes.

We will compare these results with the requirements of the EN50160 standard.

Analysis of U₀% Measurements

U₀% measurements were conducted over four distinct periods: 1 second, 10 seconds, 1 minute, and 10 minutes, to provide a detailed analysis of the variations in this key parameter and assess grid stability.

• U₀% over 1 second: This measurement provides a highly detailed view of instantaneous fluctuations, with a maximum value of 1.9995 and a minimum of 0.0995. The significant variations observed highlight rapid transient disturbances.
• U₀% over 10 seconds: Slightly extending the time scale, this period partially smooths the variations (max 1.9940, min 0.1032) while remaining close to the instantaneous measurements.
• U₀% over 1 minute: This period provides a short-term average (max 1.9583, min 0.1056), helping to mitigate transient fluctuations and stabilize the signal.
• U₀% over 10 minutes: Representing the overall grid conditions, this period shows a significant reduction in extreme values (max 1.6520, min 0.1066) and a strong improvement in overall stability.

The results indicate that the significant U₀% fluctuations observed over shorter periods gradually decrease as the aggregation period increases, ensuring better stability over 10 minutes and offering a representative view of the overall grid condition.

Comparison with the EN50160 Standard

The EN50160 standard states that voltage unbalance (U₀%) in low-voltage networks must not exceed 2% for 95% of the measurement time. The analysis of U₀% measurements shows that the recorded values comply with these requirements across all studied periods.

The maximum U₀% values measured over different periods remain below the 2% limit, ensuring compliance with regulatory requirements:

• 1 second: 1.9995
• 10 seconds: 1.9940
• 1 minute: 1.9583
• 10 minutes: 1.6520

These results indicate that, even over the shortest periods, the observed voltage unbalance levels do not exceed the permitted limit, ensuring good grid quality.

The data confirm that the observed U₀% values, both maximum and minimum, remain within the limits defined by the EN50160 standard. Voltage unbalance is generally compliant with the required quality criteria for low-voltage networks. There is no significant exceedance that would challenge the compliance of the studied grid with the standard.

3.3.2. Inverse Unbalance Voltage (U2%)

Figure 8 provides detailed data on the Voltage Unbalance (U₂%), measured over different time periods: 1 second, 10 seconds, 1 minute, and 10 minutes.

We will compare these results with the requirements of the EN50160 standard.

Analysis of U₂% Measurements

U₂% measurements were conducted over four distinct periods: 1 second, 10 seconds, 1 minute, and 10 minutes, allowing for a precise evaluation of fluctuations and grid stability.

• U₂% over 1 second: This period captures the fastest variations, with a maximum value of 1.775 and a minimum of 0.0001, revealing significant instantaneous fluctuations.
• U₂% over 10 seconds: Extending the time scale slightly, this measurement reduces the impact of very brief variations, with a maximum value of 1.762 and a minimum of 0.005.
• U₂% over 1 minute: This period averages transient fluctuations, providing better stability (max 1.724, min 0.006).
• U₂% over 10 minutes: Representing a long-term average, this measurement offers a more stable overview of grid conditions, with a maximum value of 1.439 and a minimum of 0.017.

The analysis of measurements shows a progressive attenuation of U₂% variations as the aggregation period increases. This allows for a better understanding of voltage unbalance dynamics and ensures a reliable assessment of overall grid stability under various conditions.

Comparison with the EN50160 Standard

The EN50160 standard defines the voltage characteristics supplied by public distribution networks, including voltage unbalance (U₂%). For medium-voltage networks, this standard requires that the unbalance level does not exceed 2% for 95% of the measurement time.

The maximum U₂% values recorded across all periods comply with the 2% limit imposed by the standard:

• 1 second: 1.775
• 10 seconds: 1.762
• 1 minute: 1.724
• 10 minutes: 1.439

The U₂% measurements demonstrate full compliance with the EN50160 standard, with unbalance levels significantly below the 2% threshold in all analyzed periods. These results highlight the good voltage quality and grid stability, even under instantaneous measurement conditions.

3.3.3. General Conclusion on the Analysis of Voltage Unbalance (U₀% and U₂%) Relative to the EN50160 Standard

The examination of voltage unbalance values (U₀% and U₂%) shows overall compliance with the EN50160 requirements, with maximum values respecting the 2% limit for 95% of the measurement period. However, significant fluctuations are observed, particularly over the shortest periods (1 second and 10 seconds), which could indicate the presence of transient disturbances that may affect the performance of sensitive equipment.

Importance of Shorter Aggregation Periods

Shorter aggregation periods, such as 1 second, 10 seconds, and 1 minute, provide better granularity in detecting rapid variations and transient unbalances. Compared to the 10-minute period specified in the EN50160 standard, these shorter periods allow for the more precise capture of instantaneous events that might not be visible over longer periods.

Consistency of Measurements across Different Periods

The comparison of U₀% and U₂% curves for aggregation periods of 1 second, 10 seconds, 1 minute, and 10 minutes shows a consistent pattern in the recorded values. This measurement stability across different periods highlights the accuracy of the measurement tools used and the reliability of the collected data. Although fluctuations observed over shorter periods are smoothed over 10 minutes, the variations remain significant enough to warrant attention.

The analysis of voltage unbalance values over shorter aggregation periods provides valuable insights for better understanding rapid grid fluctuations and transient unbalances. This improved granularity allows for better power quality management and a proactive response to potential disturbances.

To ensure stable power supply and compliance with regulatory requirements, it is recommended to:

• Maintain continuous monitoring of U₀% and U₂% parameters.
• Perform regular data visualizations.
• Conduct in-depth studies to identify the root causes of observed unbalances.

The implementation of power quality management systems with shorter measurement intervals could significantly enhance grid stability and reliability.

4. Conclusion

The analysis of power quality in the electrical grid highlights overall compliance with EN50160 standards, with some notable deviations requiring attention.

The Total Harmonic Distortion (THD) measurements across the three phases remain within the 8% limit, with maximum values recorded as:

• Phase L1: 2.8417
• Phase L2: 2.8250
• Phase L3: 2.4945

These results indicate good harmonic stability in the grid, in line with regulatory requirements.

However, regarding voltage unbalance (U₀% and U₂%), while the maximum values observed (1.9995 and 1.775, respectively) remain below the regulatory 2% limit, significant fluctuations are recorded over shorter periods.

These rapid variations, although they comply with the standard over a representative period, may pose challenges for sensitive equipment, leading to:

• Overheating risks
• Energy losses
• Operational disturbances

These results emphasize the importance of monitoring fluctuations at shorter intervals than those specified by the standard (10 minutes).

Shorter aggregation periods, such as 1 second, 10 seconds, or 1 minute, allow for the accurate detection of transient events and a better understanding of grid dynamics.

Recommendations for Improving Power Quality Management

To ensure continuous monitoring and improved compliance, it is recommended to:

• Deploy real-time monitoring systems and advanced diagnostic tools to track variations continuously.
• Conduct additional analyses, including graphical visualizations and detailed investigations into the causes of unbalances, to identify weak points in the grid and implement mitigation strategies.
• Integrate modern power quality meters capable of handling short aggregation periods with optimized storage capacity to ensure stable power quality that meets expectations.

Final Thoughts

In conclusion, this approach based on shorter aggregation periods offers enhanced granularity, essential for anticipating rapid disturbances and optimizing grid management. These advancements contribute to strengthening the reliability and overall performance of the electrical network, while ensuring compliance with international standards.