978-1-4799-5022-5/14/$31.00 ©2014 IEEE

Abstract—The implementation of a Power Quality monitor-

ing system can be a complex task. Number of instruments, selection of sites and monitored parameters as well as the final presentation of results strongly depend on the objective of the monitoring. Six major monitoring objectives are identified by the CIGRE/CIRED joint working group C4.112 “Guidelines for Power quality monitoring – measurement locations, pro-cessing and presentation of data”: compliance verification, performance assessment, site characterization, troubleshoot-ing, advanced applications and studies and active PQ manage-ment. Based on the intensive work and discussion within the working group the paper describes possible implementation processes for selected objectives by particular case studies. For each case the key aspects for a successful and efficient imple-mentation are summarized. The main intention of the paper is to provide some guidelines to network operators with less experience in the field of Power Quality monitoring.

Index Terms—Power quality case study, power quality

measurements, power quality measurement equipment, power quality monitoring.

I. INTRODUCTION Virtually all aspects of a PQ monitoring deployment are

influenced by the objective(s) that the utility is seeking to address. These aspects include the monitoring technology selected, the number of monitors deployed and their locations, the parameters that are measured and how often they are measured. Some PQ monitoring objectives can be met with simple RMS voltage measurements at a handful of locations, while other objective(s) may require waveform captures with high resolution. As such, the single most important step in deployment of a PQ monitoring system is clear identification of that system’s objective(s).

Utilities currently monitor power quality for several important reasons. The primary motivation underpinning all others is economic. A number of objectives are provided in IEC 61000-4-30 Annex B [1] and in the CEER/ECRB Guideline [2]. The working group C4.112 has identified during its work six major objectives for PQ monitoring, which partly cover the objectives in the above mentioned documents.

After a detailed overview of the objectives, some general recommendations regarding measurement accuracy are discussed. The major part of the paper is focused on four

J. Meyer is with the Institute of Electrical Power Systems and High

Voltage Engineering, Technische Universität Dresden, 01062 Dresden, Germany (e-mail: jan.meyer@tu-dresden.de).

J. Kilter is with the Department of Electrical Power Engineering, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia (e-mail: jako.kilter@ttu.ee).

particular case studies for some of the mentioned objectives, namely performance benchmarking, troubleshooting, ad-vanced applications and studies and site characterization.

II. OBJECTIVES FOR POWER QUALITY MONITORING

A. Compliance Verification Compliance verification compares a defined set of PQ

parameters with limits given by standards, rules or regulatory specifications. In most cases a minimum of two stakeholders is involved and at least some results are reported externally. Compliance verification is usually done for individual sites and provides qualitative results (PASS/FAIL). It can also be applied to multiple sites by using appropriate aggregation.

Typical tasks are compliance assessment at a specific site according to a given standard (e.g. EN 50160), the assess-ment of compliance of a connected installation with given standards, like IEC 61000-3-6 or measurements based on regulatory requirements or bilateral contracts.

B. Performance Analysis/Benchmarking Performance analysis is mainly an issue for a network

operator and results are used primarily for internal purposes (e.g. strategic planning, asset management, etc.). In some cases a reporting to external parties can be required. Performance analysis is usually done for multiple sites, but can be applied to single sites as well. It provides quantitative results (indices), which can be flexible adapted to a prede-fined objective for the analysis and must not mandatorily follow specific standards. Benchmarking compares one or more indices for different sets of sites and belongs to performance analysis.

Typical tasks are the assessment of average quality for a specified region or the analysis of long-term trends for a specific set of sites.

C. Site Characterization Site characterization is used to describe PQ at a specific

site in a detailed way. Usually it is initiated by request. The selected monitoring parameters and measurement methods are strongly linked to a pre-defined objective and can be even beyond the typical used indices given by standards.

Typical tasks are measurements in order to answer pre-connection questions on PQ of a specific customer, the specification of constraints on new customers or the verification of performance of existing customers. Espe-cially for industrial customers it is also important that they know their realistic minimum requirements on PQ, which is not always the case.

Case Studies for Power Quality Monitoring in Public Distribution Grids –

Some Results of Working Group CIGRE/CIRED C4.112

Jan Meyer, Member, IEEE, and Jako Kilter, Member, IEEE

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D. Troubleshooting Troubleshooting measurements are always based on a PQ

problem (e.g. exceeding PQ limits, malfunction or damage on equipment). A troubleshooting measurement may follow a compliance verification measurement, if limits are not met. Usually there is a specific initiating event for a trouble-shooting measurement.

Typical tasks are the response to customer complaints, the analysis of specific network phenomena, like resonances or the investigation of equipment damages.

E. Advanced Applications and Studies Advanced applications and studies are growing in

popularity due to the higher resolution and complexity of the data and its more timely communication. Advanced applications cover new, highly sophisticated methods to improve the efficiency of network operation. Advanced studies include more specific measurements and analyses that are often not part of the daily business of a network operator.

Typical tasks include fault location techniques, methods for automated analysis of PQ signatures or the study of harmonic current phase angles in public LV grids.

F. Active PQ Management Active PQ-Management includes all applications where

any kind of network operation control is derived from the measurement results. This may be offline or real-time control.

Typical task are the control of harmonic levels in LV-grids, e.g. by using active in-feed converters as active filters, the introduction of additional tariff component based on metering distortion energy or the reactive power manage-ment for microgenerators.

III. MEASUREMENT EQUIPMENT AND ACCURACY

A. Measurement Equipment The selection of suitable measurement equipment as well

as the consideration of the overall accuracy of the whole measurement chain is of crucial importance for a successful realization of a PQ measurement. The required features and characteristics of the measurement equipment strongly depend on the objective of the measurement.

Troubleshooting and site characterization usually require only a few monitors, but with powerful measurement performance (like trigger or higher sampling rates) and flexible interactive analysis functionality.

In case of a larger number of monitors (e.g. for perfor-mance analysis) more focus has to be set on the features of the monitoring system, like reliable data transfer links, usage of open format interfaces in order to ensure an easy integration of different types of monitors or the capability of analysis software to handle large data amounts and to generate clear and easy-to-interpret result reports for many sites at different aggregation levels. The level of automation of the whole measurement and analysis process mainly determines the overall costs of the PQ measurement system.

Advanced applications and studies usually require features beyond the standardized measurement methods. It is highly recommended to check the availability of the necessary features as well as their expected implementation in advance of buying any equipment. Experiences have shown that in many cases only the additional verification of

the expected functionality by the user itself ensures a correct implementation (e.g. measurement and aggregation of harmonic phase angle).

B. Measurement Accuracy In terms of measurement accuracy it has to be distin-

guished between the accuracy of the measurement device itself and the accuracy of the required sensors to adapt the primary signal to the input ranges of the measurement equipment.

As long as the measurement equipment complies with IEC 61000-4-30 class A or class S, the accuracy is clearly specified in the standard and is usually sufficient for accurate measurements. Care should be taken for class B equipment, because in this case the manufacturer can specify the accuracy himself.

Only the measurement of voltages in LV networks can be directly connected to the input of the measurement equipment. In any other case additional sensor equipment, like instrument transformers or Rogowski coils are required. These can have a significant impact on the overall accuracy of the measurements. It is highly recommended to perform an assessment of measurement accuracy in advance of any measurement in order to determine the threshold below which measurement results are not reliable anymore. This can be done by theoretical calculations, but selected test measurements using the particular setup, which is intended to be used in the measurements, should be preferred. As example of such test measurements, Fig. 1 presents a test signal with varying magnitude and phase angle of a voltage harmonic which is generated by a high accuracy calibra-tion source (variable i corresponds to individual test points). The response of a particular PQ measurement device is presented in Fig. 2. It shows that in order to ensure suffi-cient magnitude and phase angle accuracy for this particular measurement device, the measured harmonic voltages should be higher than 400 mV. It should be noted that these tests does not replace manufacturer type testing.

Fig. 1. Test signal for accuracy verification for voltage harmonics.

Fig. 2. Results of accuracy test for a particular PQ measurement device.

The accuracy of current measurements with respective instrument transformers (ICT) is highly sensitive to induc-tive parts in the burden. Therefore this has to be kept as low as possible or new technologies, like ICTs with voltage out-put should be used. Rogowski coils do not show nonlineari-ties or saturation effects due to its coreless design. Its accuracy is affected by the performance of the used integra-tor and the placement of the conductor within the coil.

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Especially close to the coil lock errors of up to 10% may occur due to the always existing gap between both ends of the coil. Therefore the phase conductor should be always placed as far as possible away from the lock.

In case of voltage measurements the frequency response of the instrument transformers (IVTs) has significant impact on measurement accuracy. Classical IVTs show distinctive resonance points, which in general decrease in frequency with increasing rated primary voltage (Fig. 3).

0 1 2 3 4 5-100

0

100

200

300

Frequency in kHz

Rat

io e

rror

in %

Fig. 3. Ratio error for 66-kV-IVT (dashed), 110-kV-combined ICT/IVT, transformer (dotted) and 220-kV-IVT (solid).

The location of the resonances depends on many factors, like temperature or manufacturing tolerances. It is not possible to calibrate the measurements based on the full frequency response. As also recommended by IEC TC38 a bandwidth should be requested from the manufacturer for which the IVT has a ratio error below a defined value. As example Fig. 4 shows the 1-%-accuracy bandwidths of about 100 different IVTs. Table I summarizes the results and provides indicative information on the suitability of the analyzed IVTs for harmonic measurements. More infor-mation can be found e.g. in [3].

Fig. 4. 1-%-accuracy-bandwidth of different IVTs.

TABLE I. INDICATIVE VALUES FOR THE SUITABILITY OF VOLTAGE INSTRUMENT TRANSFORMERS FOR HARMONIC MEASUREMENTS

Voltage Level

2nd – 7thHarmonic

8th – 20thHarmonic

21st – 50thHarmonic

MV 10 kV Yes Yes Yes 20 kV Yes Yes Uncertain 30 kV Yes No No

HV 60 kV Yes Yes Uncertain 110 kV Yes Uncertain No

EHV 220 kV Uncertain No No

Classical IVTs are in general not suitable for high accuracy measurements for wider frequency ranges. If such measurements are required, different types of sensors, like RC-dividers or IVTs with extended frequency range must be used.

IV. CASE STUDIES This section provides a summary of several case studies

and shall illustrate the realization of PQ measurements in terms of selected PQ monitoring objectives. The case studies are intended to give some helpful suggestions and ideas for users planning to do PQ measurements.

A. Case Study for Troubleshooting In an urban grid several customers complained about

electrical device malfunction to their network operator. In a household (I) a TV set generated audible noise and a fully automated coffee maker did not operate properly. In a hairdresser's shop (II) the hair dryers autonomously turned on and off 30–40 times a day and in a cogwheel factory (III) the control of a new CNC milling machine showed periodic malfunctions. The network schema of the grid and the locations of the complaining customers are shown in Fig. 5.

Fig. 5. Network diagram with measurement sites and locations of complaining customers.

By default the network operator performed measurements according to EN 50160 at each customer supply terminal. No parameter exceeded the limits, but the problems were still present. This is observed in many troubleshooting cases. Therefore measurements according to EN 50160 are usually not suitable to solve customer complaints.

Based on the character of the described interferences the cause for the complaints is most likely related to emission at frequencies higher than 2 kHz. Therefore in the second step additional measurements, but using another measurement device with extended frequency range up to 40 kHz were carried out. The measurement data was pre-processed according to IEC 61000-4-7, which recommends for the emission above 2 kHz the use of 200-Hz-bands. First measurements during the day time at site c (Fig. 5) showed a significant emission at 8 kHz (Fig. 6), which is most likely the cause of the complaints.

Fig. 6. Measured voltage spectrum at site c (200-Hz-bands above 2 kHz).

In order to identify the emission source, a full day measurement was performed and selected customers were contacted in order to ask about possible new or replaced equipment.

Fig. 7 shows a spectrogram of the full day measurement. Spectrograms are a suitable way to present frequency and

Customers

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emission magnitude simultaneously over time, which is easier to interpret. However, not all standard software packages that are delivered with PQ monitors support this way of presentation. In case of buying a measurement device for troubleshooting, functionality and performance of analysis software should be assessed carefully in advance.

Fig. 7. Voltage spectrogram at site c (200-Hz-bands above 2 kHz).

Based on the time behavior of emission (Fig. 7), which clearly shows a two-shift working system (6am to 10pm), the responsible customer was easily identified. The cause of the emission was a newly bought CNC mill (site b) with an active in-feed converter, which was commissioned without any EMC filter.

For mitigation in a first step a generic EMC filter was installed. This reduced the emission at site c to about 1/3 of the emission without filter, but the interferences still existed for some customers. Only the installation of a specific EMC filter which reduced the emission at 8 kHz well below 1 V could solve the complaints (Fig. 8).

Fig. 8. Voltage level at 8 kHz at site c with different EMC filters (boxes indicate 5% and 95% quantiles, line marks median).

In order to quantify the effect of the EMC filter, the emission level at 8 kHz has to be compared for the different cases: without filter, with generic filter, with specific filter. Fig. 6 shows multiple spectral lines around 8 kHz, which indicates that the total signal energy at 8 kHz is not covered by one single 200-Hz-band alone. Therefore an additional grouping into 800-Hz-bands was applied and typical percentiles (5%, 95%, and 50%) were calculated for the time series of the 800-Hz-band containing the switching frequency. This kind of post-processing requires in most cases individual software tools. Therefore flexible data export capabilities in the standard software are important.

Especially in case of higher frequencies the emission does not propagate very far. Major parts of the signal will be short-circuited by shunt capacitors (DC-link and input filters) of other electronic equipment, which is connected close to the emission source. Fig. 9 shows that the emission cannot be detected anymore at the transformer bus bar (site a). Therefore troubleshooting measurements, especially in case of higher frequency emission must be carried out as close as possible at the source or the customers with complaints. Further details can be found in [4].

Fig. 9. Voltage level at 8 kHz at different sites.

B. Case Study for Performance Benchmarking The assessment of average PQ performance of a grid

usually requires a larger number of PQ monitors and therefore a very careful planning in order to keep the whole system cost-efficient. The general design process, possible methods of site selection as well as the infrastructure for transfer and storage of measurement data are detailed described in [5]. System aspects like open interfaces for integration of different types of monitors or reliable and automated functions for data transfer and high scalability of data storage should be carefully considered during the design process. Especially the very large amount of data requires analysis software that is device-independent and capable to visualize and report the data at different aggregation levels. Relying only on weekly EN 50160 reports per site is definitely not sufficient and gets quickly inefficient with increasing number of sites and measurement weeks.

One idea for a flexible data aggregation is based on a system of PQ indices. For each site, week and PQ parameter a normalized single index is calculated, which represents the remaining reserve to a specified limit (e.g. according to EN 50160). If e.g. the index of 5th voltage harmonic in phase L1 for a particular site and week amounts r = 35, it means that the current level is 35% below the specified limit. If a single index value is below zero, the respective PQ parameter exceeds the limit. Based on this normalization it is possible to aggregate multiple parameters, multiple sites and weeks and to calculate one “top-level” index, the so-called network index, independent of the amount of data. The calculation of one index for each site presents an inter-mediate aggregation level, which is especially suitable for integration into a Geographical Information System (GIS). More details about the assessment concept can be found in [6]. This case study focuses on the aspect of graphical visualization for a PQ measurement system based on energy meters with PQ functionality, which was setup by a German distribution network operator in his MV grid.

The network operator replaced step by step his energy meters in the MV network. Due to the very low additional costs of about 30 €, it was decided to buy meters, which are capable to measure some PQ parameters. Unfortunately the implementation does not comply with IEC 61000-4-30 neither class A nor class S, but due to the already existing communication infrastructure the additional costs for PQ measurement and storage of data is very low. It is recom-mended to check with the meter manufacturer already during the planning stage of a PQ measurement system based on energy meters, if the implementation of PQ measurement meets at least IEC 61000-4-30 class S.

Until mid of 2013 about 120 new meters had been installed. The measurement sites were determined by the location of the meters, which are usually placed at the

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interface between customers or upstream grid and the net-work operator. This constraint has to be considered for the evaluation of representativeness of the measurement sites for the whole MV grid of the network operator.

The data is transferred once a week via the existing communication links to a central storage folder. User-specific software transfers the data from the files into an Oracle database, which stores the data independent from the type of measurement device and provides access from every computer connected to the Intranet. For each week the aggregated index of each site, which corresponds to the minimum remaining reserve, is calculated. Each measure-ment site is represented by a dot in a stylized map of the distribution area (Fig. 10). Depending on the minimum reserve all site are divided into four performance groups, which are identified by a color code. E.g. green means more than 50% reserve left and red means an exceeding of the limits. This way it is very easy to identify areas with PQ problems in a single diagram.

Fig. 10. Measurement sites in GIS view colored depending on the mini-mum remaining reserve (green: >50%, yellow: between 25% and 50%; orange: between 0% and 25%; red: below 0%).

Another view shows the minimum remaining reserve for different network regions separately for the best and worst of the performance groups (Fig. 11). If all sites in a region are allocated to a single group, only one value is shown. The number of aggregated sites per network region is shown in the white box. The box in the center of the GIS map in Fig. 11 e.g. means that for all 21 sites the remaining reserve for each considered PQ parameter is better than 55%.

Fig. 11. Minimum remaining reserve for different network regions.

In the particular case the PQ reserve in most regions is higher than 50%. Only one region, in which a large steel plant is connected, exceeds the limits. A deeper analysis at a level with more details has shown that the long-term flicker value exceeds the limit, which results in the negative value.

In general, large amounts of data collected during PQ monitoring campaigns contain a lot of information. The sole analysis of compliance with standards like EN 50160 does only use a very small part of this information. It is highly recommended to perform additional deeper analysis in order to gain more knowledge about the behaviour of specific PQ phenomena. As utilities often do not have the capacities for

such additional analyses, e.g. the collaboration with an university can be beneficial.

C. Case Study for Advanced Applications and Studies Limits for the harmonic emission of electronic mass-

market equipment are defined in IEC 61000-3-2 [7]. The limits are set to promote a cancellation between different types of equipment, mainly for the 5th harmonic. During the discussion in several working groups the question arose, how effective the standard is and which prevailing levels and phase angles exist in public LV grids, especially for 5th but also for 3rd harmonic. This information would be important e.g. in order to set limits for new equipment like car chargers or to evaluate the efficiency of the standard in terms of phase cancellation between different equipment. Furthermore the prevailing equipment mix or the domina-tion of a particular type of equipment (e.g. compact fluores-cent lamps) could be obtained. More information on the equipment emission and the phase cancellation can be found in e.g. [8]. This case study describes some design aspects and results of a one-year project, which had the goal to identify the prevailing phase angle and magnitude of 3rd and 5th harmonic current in German public LV grids.

In order to obtain as reliable as possible results, a representative sample of LV grids had to be selected for the measurements. Therefore a schema for the classification of public LV grids was developed. The schema distinguishes between qualitative and quantitative parameters of

Consumer characteristic (e.g. type of consumers or number of households),

Generation characteristic (e.g. total power of distributed generation), and

Network characteristic (e.g. length and type of lines).

Thanks to the support of more than 30 utilities, finally 130 LV grids distributed all over Germany were available for measurements and the parameters mentioned above were collected for each of the grids. In order to identify obvious mistakes in the grid data, some basic plausibility checks were performed. In general it should be considered for any PQ study, if additional non-electrical data, like in this case the parameters of grid characteristic, can be beneficial for further analysis.

Consumer characteristics Generation characteristics

A1: Single family houses A2: Multi family houses A3: Shopping A4: Office

E1: No generation E2: Small share of generation E3: Medium share of generation E4: High share of generation

Fig. 12. Qualitative analysis of consumer and generation characteristic.

Some results of the qualitative analysis of consumer and generation characteristic are shown in Fig. 12. E.g. 35 of the grids supply multifamily houses. More than 50% of the grids contain distributed generation, while almost 10% of the grids have already a high share of Photovoltaic

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installations (more than 50% of the rated power of the supplying MV/LV transformer.

The additional collection of quantiative data (like number of households, number and size of generating installations or total length of lines) usually allows a more detailed analysis of the grids. Fig. 13 exemplarily shows the distribu-tion of the number of single family houses for the 68 grids of type A1, which shows an equal coverage of sizes between 30 and 300 houses. The analysis of the total size of generating installations per grid shows e.g. that significantly more generation is installed in grids with single family houses (A1) than multifamily houses (A2). 20% of the grids already contain generating installations (almost exclusively Photovoltaic installations) with a total power of more than 150 kWp.

Fig. 13. Example for quantitative analysis of selected parameters of grid characteristics.

In order to ensure a comparability of measurement data, a measurement guide was developed that specifies in detail which measurement device (including firmware version) and sensors have to be used as well as how the device has to be setup (e.g. measurement interval, measurement duration). Especially if no clear or multiple definitions exist for a particular measurement parameter, like harmonic phase angle it is essential to test the proper implementation in advance. Just relying on the statements of the manufacturer is often not enough. Especially in case of the harmonic phase angle not only the relation to voltage fundamental (like required for this study), but also the correct aggrega-tion (vectorial addition, no arithmetic mean) had to be verified.

A prefered way of graphical presentation of the measurement data of a single site is the complex plain or a polar plot. Fig. 14 exemplarily presents the 5th harmonic current vectors (1-minute-values for 2 weeks) for two differ-ent grids. While the left plot shows a small cloud with a prevailing direction in the 4th quadrant (indicated by the arrow), the measurements in the right plot are distributed over the whole angle range and does not show any prevail-ing direction at all.

Fig. 14. Example of measurment data at 2 different sites (with/without clear prevailing direction for 5th harmonic current).

Hence it is possible to calculate a prevailing vector (magnitude: RMS average of all measurement points; phase angle: resulting angle of vectorial sum of all measurement

points) for each site, this makes only sense if the measure-ment data shows a prevailing direction. In this study sites whitout a clear prevailing direction of the measurement points at a site were identified by an index and excluded from the further analysis. Finally the prevailing vectors for all remaining sites were calculated and presented seperately for each consumer type. Fig. 15 exemplarily shows the results for the grids with single family houses (A1) and offices (A4). While almost all of the grids with single family houses are located in the 4th quadrant between 300° and 330°, many grids with office buildings show a prevailing phase angle in the 2nd quadrant at about 120°. One possible reason for this behaviour could be a dominating share of compact flourescent lamps in office grids compared to residential ones with single family houses [8]. Again this analysis is only possible, because additional information about the grid characteristics was collected.

Grids with single family houses (A1) Grids with office buldings (A4)

Fig. 15. Prevailing vectors of 5th current harmonic depending on type of connected consumers.

Another useful tool is the analysis of possible dependen-cies by correlation analysis or scatter plots. It supports the identification of interesting relationsships between different parameters. Fig. 16 exemplarily shows the scatter plot of 95-%-percentile versus average value for the 3rd harmonic current. The different consumer types are identified by different colors.

0 5 10 15 20 25 30 35 40 450

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IV3 in A

I 953

in A

Fig. 16. Scatter plot of 95-%-percentiles vs. average of 3rd harmonic current for different consumer types (A1: blue; A2: green; A3: red; A4: yellow).

The plot shows a strong linear relationship that is almost independent of consumer type. The average ratio is 1.5, which means that the 95-%-percentile is usually about 1.5 times higher than the average. This index can be used as indicator for the variation characteristic of 3rd harmonic current and used e.g. for planning purposes.

Number of single family houses

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D. Case Study for Site Characterization One wind power plant (WPP) developer is planning to

connect its plant to one of the utility network 110 kV substation. In order to specify its equipment and perform preliminary studies, existing PQ levels in the connection point are requested. From the network operator perspective this information is also important and needed for charac-terizing the specific site and to determine the background levels of different PQ characteristics existent in the network and consequently define the possible allowed level of PQ for new planned WPP.

This WPP is planned to be connected to a substation where already two other WPP are connected. This means that coordination for allocating additional connections should be carefully done without exceeding the general planning limits. For clarity it should be mentioned that in the considered network there are regulations determining the general planning level for PQ parameters and specific PQ levels for each generation plant. It is also specified that the planning levels are not allowed to be exceeded. Conse-quently this means that when there are many connection points in the same substation and the allowed PQ limits are exceeded then the last plant that was connected is responsible for fixing the situation.

In order to determine the background levels separate PQ monitoring was performed in the connection point. A portable PQ meter corresponding to the requirements of IEC 61000-4-30 Class A device was used. The monitor was connected to the available metering circuits considering the requirements for PQ measurements.

For characterizing the connection point parameters in particular interest were the level of different individual harmonics and voltage THD, voltage flicker (Pst, Plt) and unbalance. The duration of measurements was one week and 10 minute averaging intervals were used.

Results of the measurements are presented in Fig. 17 to Fig. 20, representing voltage THD weekly measurements, individual voltage harmonic distortion levels, short term and long term voltage flicker severity and voltage unbalance, respectively.

For site characterization also a specific table including numeric PQ parameters (Pst, Plt, THDU and unbalance) for observable connection point were composed and provided to the WPP developer (Tab. II). It should be noted that in addition to the usually used 95th percentile also the not so common 99th percentile and maximum value are reported. Based on the performed PQ measurements and after site specific analysis comprehensive calculations for the purpose of determining site specific contractual PQ limits were performed by the network operator.

TABLE II. PQ MEASUREMENT RESULTS FOR SITE CHARACTERIZATION

Measurement point Source Phase Characteristic Maximum CP 99 CP 95

SS1 Voltage L1-L2 Flicker Plt 0.3007 0.3006 0.1148

SS1 Voltage L2-L3 Flicker Plt 0.154 0.1496 0.1112

SS1 Voltage L3-L1 Flicker Plt 0.1686 0.1288 0.1132

SS1 Voltage L1-L2 Flicker Pst 0.6871 0.1695 0.1031

SS1 Voltage L2-L3 Flicker Pst 0.318 0.1789 0.1043

SS1 Voltage L3-L1 Flicker Pst 0.2913 0.1707 0.1045

SS1 Voltage L1-N THD 0.8096 0.7795 0.7426

SS1 Voltage L2-N THD 0.7483 0.7152 0.6894

SS1 Voltage L3-N THD 0.6577 0.6236 0.5905

SS1 Voltage L1L2L3 Unbalance 0.5445 0.5316 0.5065

Fig. 17. Voltage THD weekly measurements, connection point substation measurments.

Fig. 18. Individual voltage harmonic distortion levels in the connection point (blue colums maximum values, red colums, 95th percentile value).

Fig. 19. Short term and long term voltage flicker severity.

Fig. 20. Voltage unbalance measurements.

Based on the provided information, specific PQ studies and assessment of suitability of planned equipment shall be done by the WPP developer. The results of the studies shall provide information about any need for changes in the project, e.g. adding additional filters or any other site

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specific devices. For filter parameter calculations and performance assessment separate network impedance characteristics are needed. In this particular project the preliminary studies indicated that there may be some problems with the harmonics. Therefore a detailed calculation of network impedance characteristics for various network configurations was performed by the network operator. Fig. 21 shows the impedance curves for all net-work configurations summarized in separate plots for R, X, and Z. The plot for X shows small potential of resonance at 13th and 25th harmonic.

Fig. 21. Connection point harmonic impedance curves.

Such characteristics can be considered as helpful addi-tional information in order to determine any possible adverse interactions between the WPP and network as well as for further harmonic studies.

V. CONCLUSION The clear definition of objectives is essential for a

successful and efficient PQ assessment. The objective mainly determines the selection of measurement sites, number and required functionality of PQ monitoring devices, the parameters that have to be measured and finally the ways of result presentation. Moreover the accuracy of the whole measurement chain including the sensors has to be carefully considered in order to ensure sufficient exact measurement results.

The paper discusses selected aspects of a proper implementation of PQ assessment for different objectives by real case studies. Namely these are troubleshooting of interferences by higher frequency emission, the graphical visualization of network performance, an advanced study of the prevailing harmonic phase angle in public LV grids and site characterization for Wind Power Plant connection. For each case the specific challenges are discussed and general recommendations are derived. Even if not every issue could be explained in detail for each case study, the paper can hopefully provide some helpful tips and advices for everybody dealing with PQ measurement.

The whole work was carried out in the context of CIGRE/CIRED working group C4.112. Beside a compre-hensive survey of current PQ monitoring practice [9] the group discusses the future trends in PQ monitoring and provides guidelines on how to challenge it in an efficient way. The final report of the working group is expected to be published in the second half of 2014.

CIGRE/CIRED JWG C4.112 MEMBERS This paper is based on the result of the intensive work of

all members of the working group, namely: J. (UK, Convenor), J. Kilter (EST, Secretary), B. Howe (USA), F. Zavoda (CAN), J. M. Romero Gordon (ESP), L. Tenti (ITA), J. Meyer (GER), S. Bahramirad (USA), R. Ball (UK), V. Barrera (ESP), D. Correia (BRA), N. Cukalevski (SRB), A. Dabin (BEL), N. Trinchant (FRA), P. Doyle (IRL), J. Höglund (SWE), R. Neumann (ITA), P. F. Ribeiro (NED), B. Parent (CAN), M. Bollen (SWE), S. Elphick (AUS), G. Paulillo (BRA), J. Schaug-Pettersen (NOR).

REFERENCES [1] Standard IEC 61000-4-30: Testing and measurement techniques –

Power quality measurement methods. Ed. 3, 2012. [2] CEER/ECRB: Guideline of Good Practice on the Implementation and

Use of Voltage Quality Monitoring Systems for regulatory Purposes, 2012.

[3] R. Stiegler, J. Meyer, M. Elst, E. Sperling: "Accuracy of harmonic Voltage Measurements in the Frequency Range up to 5 kHz using Conventional Instrument Transformers," 21st International Confer-ence on Electricity Distribution (CIRED), Frankfurt, Germany, 2011.

[4] M. Klatt, J. Meyer, P. Schegner, A. Koch, J. Myrzik, C. Körner, et.al., “Emission Levels above 2 kHz – Laboratory Results and survey Measurements in public low Voltage Grids,” 22nd International Conference and Exhibition on Electricity Distribution (CIRED 2013), Stockholm, Paper 0999, June 2013

[5] J. Meyer et al.: “Contemporary and Future Aspects of Cost Effective Power quality monitoring – Position paper of CIGRE WG C4.112”, 8th International Conference on Electrical Power Quality and Supply Reliability 2012, Tartu, Estonia.

[6] J. M. Romero Gordon, J. Meyer, P. Schegner, "Design Aspects for large PQ Monitoring Systems in future Smart Grids," presented at IEEE PES General Meeting 2011, Detroit, USA.

[7] Electromagnetic compatibility (EMC) – Part 3-2: Limits – Limits for harmonic cur r phase), IEC Standard.

[8] J. Meyer, P. Schegner, K. Heidenreich. “Harmonic summation effects of modern lamp technologies and small electronic household equipment,” in Proc. 21st International. Conf. on Electricity Distribu-tion (CIRED), 2011.

[9] H. J. Bollen, “International Industry Practice on Power Quality Monitor-ing”, IEEE Transaction on Power Delivery, Vol. 29, Issue 2, April 2014, page 934–941.

BIOGRAPHIES Jan Meyer studied Electrical Power Engineering at the Technische

Universität Dresden (Germany). He received the Ph.D. degree with a thesis on the statistical assessment of power quality in distribution networks.

Now he is a senior academic assistant at the Technische Universitaet Dresden, Germany and team leader of the Power Quality research group. His research interests include network disturbances and their assessment, especially for harmonics below and above 2 kHz. Further aspects of his research are statistical methods to assess Power Quality in distribution networks and all aspects of designing large measurement campaigns. He is member of several national and international EMC working groups. Furthermore he gives regular presentations on current topics in the field of Power Quality and is organizer of several seminars in the field of network disturbances and its assessment.

Jako Kilter (Student Member ’08, M’10) was born in Tallinn, Estonia,

1981. He received the B.Sc., M.Sc. and Ph.D. degrees in electrical engineer-ing from Tallinn University of Technology (TUT), Tallinn, Estonia, in 2003, 2005, and 2009, respectively. Currently he is working at the Department of Electrical Power Engineering of TUT as an associate professor and at the Estonian Transmission System Operator as a power system expert. He has published various papers on load modelling, system control and power quality and is the author of three textbooks. He is a Chartered Engineer, Chairman of Estonian Centre for Standardization Committee of High Voltage Engineering, and member of different ENTSO-E and CIGRE working groups.

His special field of interest include electrical network state estimation and analysis, power system monitoring and control, modelling of loads, power system stability, power quality, HVDC and FACTS.

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