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Voltage quality monitoring, dips classification and responsibility sharing

Jorge Esteves

Energy Services Regulatory Authority (ERSE) Lisboa, Portugal jesteves@erse.pt

Karstein Brekke Norwegian Water Resources and

Energy Directorate (NVE) Oslo, Norway kab@nve.no

Kevin Niall ESB Networks Dublin, Ireland

kevin.niall@esb.ie

Maurizio Delfanti Department of Energy, Politecnico di Milano

Milano, Italy maurizio.delfanti@polimi.it

Math Bollen Energy Markets Inspectorate

Eskilstuna, Sweden math.bollen@ei.se

Abstract — CEER and EURELECTRIC cooperation in the field of quality of electricity supply, involving joint meetings and the participation at the relevant CENELEC Technical Committee, contributed to the results attained in the recent publication of the EN 50160:2010 edition that includes a new voltage dips classification table allowing harmonisation at European level on voltage dips data collection. The generalisation of voltage quality monitoring data publication all over Europe will allow the definition of responsibility sharing between the different involved stakeholders and the evolution of voltage quality regulation applied at national level. Examples from Sweden and Italy are briefly presented in this paper.

Keywords – power quality, voltage quality, voltage dips, voltage quality monitoring, standards.

I. INTRODUCTION A high level of voltage quality is fundamental for some

European industries’ competiveness and assuring standardisation and harmonization at national level is essential for the European internal market development.

The Council of European Energy Regulators (CEER) and the Union of the European Electricity Industry (EURELECTRIC) have been cooperating for several years in the field of quality of electricity supply, involving joint meetings and the participation of the European Energy Regulators in the relevant CENELEC Technical Committee.

A workshop on voltage quality monitoring was organised jointly by CEER and EURELECTRIC during November 2009, in Brussels, involving more than 80 participants and achieving significant conclusions about the importance of this topic. Information is available at the “Events/2005-2009” folder of the CEER webpage www.energy-regulators.eu.

In parallel, one of the improvements agreed at CENELEC level for the EN 50160 standard revision is a comprehensive and effective treatment of voltage dips classification, including reference to standards (e.g. EN 61000-4-11 and

EN 61000-4-34) for testing of equipment against voltage disturbances and to the electromagnetic environment classes defined by the EN 61000-2-4. This reference can be seen as an important pre-condition to responsibility sharing between the stakeholders, as follows:

• The voltage quality at the customer’s bus, as defined in standards and regulations, is the network operator’s responsibility.

• It is the responsibility of the customers that the impact of the voltage disturbances that can be expected on their process, installation and equipment is limited.

• The quality for current withdrawals at the point of connection is the customer’s responsibility.

• Developing and supplying equipment with adequate tolerance to voltage quality and cost-effective power conditioning devices with appropriate technology is a continuing challenge for manufacturers.

• Ensuring an efficient balance between the views of different stakeholders – in order to reach the best optimum possible from the viewpoint of the society – is the role of the National Regulatory Authorities.

This paper intends to summarise the major conclusions on these topics attained at a Joint Round Table organized by CEER and EURELECTRIC at the CIRED 2011 held during June 2011, and representing the most recent cooperation activity developed by the two organisations, [1] - [4].

II. CEER OVERVIEW OF VOLTAGE QUALITY MONITORING The Council of European Energy Regulators (CEER) has

published several documents where the importance of quality of electricity supply and, in particular, voltage quality for the European Energy Regulators is revealed. Some examples available at www.energy-regulators.eu are:

978-1-4673-0378-1/11/$26.00 ©2011 IEEE

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• Third CEER Benchmarking Report on Quality of Electricity Supply [5].

• Towards Voltage Quality Regulation:

An ERGEG Public Consultation paper [6].

Evaluation of comments received [7].

An ERGEG Conclusions Paper [8].

• Fourth CEER Benchmarking Report on Quality of Electricity Supply [9].

• Energy Regulators’ pledge to ensuring good quality of electricity supply [10].

• Guidelines of Good Practices on Estimation of Costs due to Electricity Interruptions and Voltage Disturbances [11].

Another reference, [12], represents the European Energy Regulators vision about quality of service regulation, developed by a joint effort from CEER and the Florence School of Regulation.

The impact of voltage disturbances in national economies justified several surveys on costs that were carried out by different entities during last years in Norway, Sweden and Italy, and also the development of the Pan-European LPQI Power Quality Survey at European level. Table I summarises some of the conclusions from these survey.

In their collective positions, the European Energy Regulators assume the need of harmonised voltage quality regulation all over Europe.

CEER verified that at least in 18 European countries the distribution network operators are compelled to provide voltage quality measurements when requested by the customer or after complaints. However, there are some countries where this rule is not yet applied and CEER recommends its adoption by all the European countries, even in the absence of a former complaint by the requesting customer and in the absence of power quality contracts as well.

TABLE I. RESULTS FROM SURVEYS ON VOLTAGE DISTRUBANCES COSTS

Inhabitants Conclusions on estimated annual costs

Norway (2002) ca 5 million Estimated annual costs due to dips

for end-users to be between 120 and 440 million NOK

Sweden (2003) ca 9.5 million

Estimated annual costs for industrial customers due to dips and

interruptions at about 157 M€

Italy (2006) ca 60.6 million

Estimated annual costs due to dips and interruptions (< 1 s) for the whole production system to be

between 465 and 780 M€ PAN European

LPQI Power Quality Survey

(2005-2007)

Costs of PQ wastage EU-25 exceeds 150 billion € annually

Real voltage quality data is an important prerequisite for understanding voltage quality (VQ) and for regulation. According to the “4th CEER Benchmarking Report on Quality of Electricity Supply”, at least 11 European countries have VQ monitoring systems in operation; still, a complete lack of harmonisation has been observed concerning:

• Measurement instruments.

• Voltage levels and disturbances to be monitored.

• Number and location of instruments.

• Classification of results (e.g. dips/swells).

• Reporting and publication of results.

• Regulatory recommendations on the use of instruments and voltage transformers.

In this perspective, as explained in several of the earlier-mentioned papers and reports, CEER recommends that countries should consider monitoring VQ continuously and publish results regularly. Harmonisation should include accuracy for the whole measurement chain. It is further recommended that European countries disseminate their own national experience in order to consolidate the European view on VQ monitoring.

III. EURELECTRIC VIEWS ON POWER QUALITY MONITORING AND VOLTAGE DIPS

During 2008, EURELECTRIC carried out a survey on Power Quality (PQ) monitoring with the participation of 23 distribution system operators (DSO) serving 120 million customers and representing 16 European countries. Two national electric industry associations were also involved. The survey showed that 82% of the DSOs surveyed have PQ monitoring at some HV/MV substations. The majority of DSOs surveyed that have PQ monitoring at some HV/MV Substations, are monitoring less than 30% of MV busbars. It is EURELECTRIC view that it is not necessary to monitor all HV/MV substations to realise the benefits of PQ monitoring.

Costs on communication, storage, software, presentation and external access are the main VQ challenges in the EURELECTRIC perspective.

% MV Busbars with Fixed PQ Instrumnets

0

2

4

6

8

0% 0% - 10% 10% - 30% 30% - 50% 50%-99% 100%

% MV Busbars with Fixed PQ Monitoring

Num

ber

of R

espo

nden

ts

Figure 1. EURELECTRIC survey results about the number of PQ instruments on MV Busbars in HV/MV Substations

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EURELECTRIC point to new challenges and possibilities related to PQ monitoring:

• Recent developments affecting low voltage systems like the new loads (electric vehicles and heat pumps) and the distributed generation.

• Smart Metering implementation that could allow for recording of voltage level and voltage dips and swells.

In EURELECTRIC view there are shared responsibilities related to voltage quality with the overall objective being that the performance of customers’ equipment meets their need:

• Standardisation bodies are responsible for defining a coherent set of EMC compatibility levels, along with emission and immunity limits.

• DSOs are responsible for planning the system for adequate VQ, and for defining emission limits applicable to customers’ installations.

• Equipment providers being important for assuring acceptable levels of immunity and emission from their equipment.

• Customers assuring the protection of their sensitive equipment and processes against expected voltage disturbances and also for ensuring that the emission from their installation is within the allowed limits.

Focusing on voltage dips, EURELECTRIC view is that it is not possible to eliminate such events, with networks containing overhead lines being especially prone to faults due to external factors. Undergrounding of overhead networks and faster protection are possible solutions to reduce frequency and severity of voltage dips. However, these solutions impose substantial investments that can face difficulties on customers’ willingness to pay and long timescales to be implemented.

EURELECTRIC recognise that some of the customers most sensitive to voltage dips are also economically important customers. However, the most economic solution is often for the customers to make their installation and processes more robust. The report entitled “Voltage Dip Immunity of Equipment and Installations” [13], from the CIGRE, CIRED and UIE joint working group JWG C4.110, is a very good reference to be consulted.

DSOs view VQ monitoring as useful to ensure adequate voltage quality for their customers and most DSOs have some VQ monitoring in place already. EURELECTRIC emphasises that standardisation continues to make a key contribution and several challenges will be faced.

IV. EN 50160 VOLTAGE DIPS AND SWELLS CLASSIFICATION AS AN INSTRUMENT FOR RESPONSIBILITY SHARING

The European Standard EN 50160, entitled “Voltage characteristics of electricity supplied by public electricity networks” defines, describes and specifies the main characteristics of the voltage at a network user's supply terminals in public low, medium and high voltage electricity networks under normal operating conditions.

This standard describes the limits or values within which the voltage characteristics can be expected to remain at any supply terminal in public European electricity networks and does not describe a typical situation experienced by an individual network user.

The EN 50160 first edition has been published during November 1994 and two other editions were approved during 1999 and 2007. The most recent edition, the EN 50160:2010, was ratified as of 1 March 2010, and has therefore only recently been published. In this new edition some important improvements have been achieved.

In particular, a distinction is made between:

• Continuous phenomena, i.e. small deviations from the nominal value that occur continuously over time - such phenomena are mainly due to load patterns, changes of load or nonlinear loads.

• Voltage events, i.e. sudden and significant deviations from normal or desired wave shape like interruptions of the supply voltage, supply voltage dips and swells and transient overvoltages between live conductors and earth.

Limits are specified by the EN 50160: 2010 standard for some continuous phenomena; on the other hand, for voltage events, some indicative values are provided in an informative annex to the standard (Annex B).

Voltage events are typically due to unpredictable causes (e.g., faults) or to external causes (e.g., weather, third party actions). The standard states that more research is needed before setting European joint limits for voltage events.

Apart from interruptions, voltage dips are the events most affecting industrial customers. Defined as a temporary reduction of the voltage, a voltage dip is typically associated with the occurrence and termination of a short circuit or other extreme current increase in the system. According to the conventional view of the standard, a voltage dip is a two dimensional electromagnetic disturbance, the level of which is determined by both voltage (residual voltage) and time (duration).

In the perspective of the harmonisation on data collecting all over Europe, a major advance introduced by the new edition of EN 50160 is a new voltage dips classification table presented in Table II, allowing standardisation of voltage dips data presentation.

TABLE II. THE VOLTAGE DIPS CLASSIFICATION TABLE IN EN 50160:2010

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Being useful to describe the possible behaviour of the network with the same parameters used for testing appliances (Product Standards), the EN 50160 made reference to test levels given at EN 61000-4-11 (different test levels for different equipment classes: 1, 2, 3, X) using such levels as a base for the new classification table.

When compared with the original one published in the EN 50160:2010, Table II has been modified for this paper introducing a grey shading at a certain number of the cells. For a given class of appliances (e.g. class 3), the cells with grey colour (A1, B1, C1, A2, B2, A3 and A4) represent the immunity area to voltage dips (duration and residual voltage) that must be assured by these appliances. Classes are referred to different electromagnetic environment, as defined in EN 61000-2-4.

For the cells below the referred immunity curve, a regulation can eventually be enforced at a national level imposing voltage quality requirements to be fulfilled by the TSO or DSO.

V. RESPONSIBILITY OF STAKEHOLDERS FOR VOLTAGE DIPS The voltage dips classification table introduced in the

most-recent version of the EN 50160:2010 (Table II) allows for a more harmonized way of presenting these voltage quality events. Furthermore, it also gives the opportunity to apply the concept of “responsibility sharing” between customers, equipment manufacturers and network operators, as introduced in [5,6,7] and shown in Figure 2.

Some standards and national regulations (France, South Africa) already foresee setting up a boundary between the voltage dips for which appliances or installations have to be immune and the events that can be limited by the DSO/TSO and to be subject to voltage quality regulation. Other possible sharing curves would be based on the equipment immunity requirements in documents like SEMI F47 and ITIC.

More recently, the Swedish national regulatory authority (Energy Markets Inspectorate) has introduced voltage-quality regulation [14]. This regulation includes an evolution of the concept by introducing a second curve as shown in Figure 3.

In complement to the Area A (in terms of residual voltage and duration of the voltage dips) already defined where installations should be immune, the remaining area where the voltage quality regulation is applied has been divided in two.

Installations should tolerate these dips

Voltage-quality regulation

Duration of the dip

Res

idua

l vol

tage

Figure 2. The “responsibility sharing curve”

Installations should tolerate these dips

A reasonable number of dips is acceptable

Dips are unacceptable

Duration of the dip

Res

idua

l vol

tage

Figure 3. The evolution on the “responsibity sharing curve” according to the Swedish regulation.

Voltage dips with duration longer than certain values and overpassing a specified voltage reduction (Area C) are considered unacceptable and when such a voltage dip occurs the quality of supply is considered to be insufficient. Finally, a third intermediary area (Area B) has been introduced where a reasonable number of voltage dips is considered acceptable. Figure 4 and Figure 5 present the limits for Area A, Area B and Area C according to the Swedish regulation for networks above 45 kV and for networks with nominal voltage of 45 kV or less.

With this approach, the network operator must prevent dips in Area C and limit the number of voltage dips in Area B. On their side, customers should assure that their processes and equipment are immune to voltage dips in Area A and assure that the impact of dips in Area B is limited. Equipment manufacturers must assure that their equipment is immune to dips in Area A and must assure that different classes of equipment immunity are able to cover Area B.

0.2s 0.5s 1s 5s

40%

70%

90%

60s

80%

Figure 4. Limits according to the Swedish regulation for networks above 45 kV

0.2s 0.5s 1s 5s

40%

70%

90%

60s

80%

Figure 5. Limits according to the Swedish regulation for networks with 45 kV and less.

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This new approach opens a new challenge related to the answer to the question “What is a reasonable number of events to be specified in Area B?”

Different approaches can be adopted, like the one used in South Africa (related to the number monitored in a predefined percentage of locations, i.e. 95% of the sites) or the one used in France (resulting from the comparison with the last several years) or the one mentioned in the text of the Swedish regulation (comparing with similar networks elsewhere or defined in contracts signed between customers and the DSO/TSO). A further approach could also consist in evaluating (for each individual case) the costs for end-users due to the number of voltage dips in area B, and comparing them with the costs associated with the measures to reduce the number and duration of the specific dips.

VI. A NATIONAL EXAMPLE: TOWARDS VQ REGULATION IN ITALY

Installing a reliable VQ monitoring system in each country is important in order to be able to inform customers about the voltage quality to be expected in each location.

In Italy a VQ monitoring system, QuEEN (Quality of Electric ENergy), is spread over the country involving 400 monitoring units installed on medium voltage (MV) bus-bars of high voltage/medium voltage (HV/MV) substations, Figure 6.

The 400 VQ monitoring units installed monitor about 10% of the MV bus-bars of the MV distribution network, being considered that this sample is representative of the network characteristics in terms of:

• Number of HV/MV substations in each region.

• Length of the MV lines.

• Type of MV lines: cable, overhead, mixed.

• Neutral compensation or isolated neutral.

• Number of MV customers.

• Density of low voltage customers.

Tables III, IV and V present the QuEEN report on voltage dips collected annually during 2008, 2009, and 2010, respectively. QUEEn reports all the poly-phase events (given that neutral is isolated/compensated, no dip originates from single-phase to ground faults), regardless of any time aggregation (i.e., if two dips occur even in a few seconds, they are both reported). The same applies to adverse weather condition: no exclusion is foreseen, all events are duly reported, and contribute to the figures of the tables. For more details on the methods used to calculate the values in the table see reference [15].

Figure 6. Location of the 400 VQ monitoring units installed in Italy by QuEEN

TABLE III. QUEEN REPORT ON VOLTAGE DIPS: YEAR 2008

Duration t [ms]

Range of res. voltage Ur (in % of Un or Ud) 10 < t 200 200 < t 500 500 < t 5000

90 > Ur 80 29,2 5,6 2,080 > Ur 70 18,6 4,3 0,670 > Ur 40 40,0 6,8 0,740 > Ur 5 15,4 2,6 0,3

TABLE IV. QUEEN REPORT ON VOLTAGE DIPS: YEAR 2009

Duration t [ms]

Range of res. voltage Ur (in % of Un or Ud) 10 < t 200 200 < t 500 500 < t 5000

90 > Ur 80 34,9 7,5 2,680 > Ur 70 17,1 5,3 0,870 > Ur 40 28,2 5,3 0,740 > Ur 5 9,9 1,7 0,2

TABLE V. QUEEN REPORT ON VOLTAGE DIPS: YEAR 2010

Duration t [ms]

Range of res. voltage Ur (in % of Un or Ud) 10 < t 200 200 < t 500 500 < t 5000

90 > Ur 80 31,5 6,4 2,080 > Ur 70 15,5 4,4 0,670 > Ur 40 22,6 4,8 0,540 > Ur 5 8,5 1,3 0,2

Data collected on 10% of MV main busbars show:

• An average number of dips per bus reducing from 126.1 in 2008 till 98.3 in 2010.

• An evolution from 99.7 voltage dips that would be overcome by class 3 equipment in 2008 till 82.4 in 2010.

• An evolution from 26.4 voltage dips that might impair the operation of class 3 equipment in 2008 till 15.9 in 2010.

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Based on the EN 50160:2010, on the data collected by QuEEN and on an economic analysis of the impact of transient interruptions and voltage dips on the Italian economy, AEEG, the Italian Energy Regulatory Authority, published two Consultation Papers (DCO 42/10; DCO 15/11) to be considered for the Italian regulatory period from 2012 till 2015 [16].

Some proposals regard:

• Extension of VQ monitoring to all MV busbars of HV/MV stations (about 4000 VQ meters needed).

• Publication of expected/registered values of transient interruptions and voltage dips by the DSO for each MV main busbar (publication of long and short interruptions is already in force).

The availability of such data will allow a sensitive user to evaluate on a sound technical base if further immunization or mitigation is needed for his specific industrial process. Comparisons between VQ levels provided by different DSOs, or by different technical solutions put in place along the entire national MV systems, will also be possible.

VII. CONCLUSIONS Cooperation between CEER and EURELECTRIC in the

field of quality of electricity supply involved joint meetings and the participation of the European Energy Regulators in the relevant CENELEC Technical Committee contributing to EN 50160:2010. This new edition of the standard includes a new voltage dips classification table allowing harmonisation at European level on voltage dips data collection in line with immunity testing standards like EN 61000-4-11 (and EN 61000-4-34).

This European standardised approach gives the opportunity to apply the concept of “responsibility sharing” between customers (end-users), equipment manufacturers and network operators. This concept defines a boundary between the voltage-dip events for which appliances or installations have to be immune and the events that can be limited by the DSO/TSO and be subject to voltage quality regulation.

More recently, the voltage-quality regulation in force in Sweden introduced an evolution in the concept defining an intermediate area where a reasonable number of voltage dips is considered acceptable.

All this requires the installation of reliable VQ monitoring systems. The majority of DSOs have already developed efforts in VQ monitoring through which the importance of harmonisation and standardisation has been identified.

It is also important that European countries disseminate their own national experience in order to consolidate the European view on VQ monitoring. The Italian example has been presented with their last 3 years results obtained from a

voltage dips monitoring system covering a significant share of MV main busbars ( 10%) that is compliant to the data collection rules specified by EN 50160:2010.

The systematic monitoring all over Europe and the implementation at national level of this standardised classification will result in an improved knowledge on the voltage dip phenomenon. It will also contribute to the benchmarking of their impacts for networks and customers, and to supporting voltage quality regulation, definition of equipment and processes immunity, and emission limits for voltage dips. All these efforts will contribute to mitigate the costs related to VQ events in Europe, in the general perspective of increasing the European productivity.

REFERENCES [1] Karstein Brekke, “Overview of Voltage Quality Monitoring in Europe,”

in Proc. 21st CIRED, International Conference and Exhibition on Electrictity Distribution, June 2011.

[2] Kevin Niall, “Voltage Quality Monitoring, Dip Classification and Responsibility Sharing”, in Proc. 21st CIRED, International Conference and Exhibition on Electrictity Distribution, June 2011.

[3] Maurizio Delfanti, “EN 50160 voltage dips and swells classification as an instrument for responsibility sharing”, in Proc. 21st CIRED, International Conference and Exhibition on Electrictity Distribution, June 2011.

[4] Math Bollen, “Responsibility of stakeholders for voltage dips”, in Proc. 21st CIRED, International Conference and Exhibition on Electrictity Distribution, June 2011.

[5] CEER, Third Benchmarking Report on Quality of Electricity Supply 2005. Brussels, Belgium, 2005.

[6] ERGEG, Towards Voltage Quality Regulatio In Europe: An ERGEG Public Consultation Paper. Brussels, Belgium, 2006.

[7] ERGEG, ERGEG Public Consultation Towards Voltage Quality Regulation in Europe: Evaluation of the Comments Received. Brussels, Belgium, 2007.

[8] ERGEG, Towards Voltage Quality Regulation in Europe - An ERGEG Conclusions Paper. Brussels, Belgium, 2007.

[9] CEER, 4th Benchmarking Report on Quality of Electricity Supply 2008. Brussels, Belgium, 2008.

[10] CEER, Energy Regulators’ pledge to ensuring good quality of electricity supply, Brussels, Belgium, 2009.

[11] CEER, Guidelines of Good Practice on Estimation of Costs due to Electricity Interruptions and Voltage Disturbances, Brussels, Belgium, 2010.

[12] E. Fumagalli, L. Lo Schiavo and F. Delestre, Service Quality Regulation in Electricity Distributiona and Retail. Springer Berlin Heidelberg, 2007.

[13] CIGRE/CIRED/UIE Joint Working Group C4.110, Voltage Dip Immunity of Equipment and Installations. 2010.

[14] L. Ström, M. Bollen, R. Kolessar, Voltage quality regulation in Sweden, Int. Conf. Electricity Distribution (CIRED), Frankfurt, June 2011.

[15] L. Tenti, R. Chiumeo, M. De Nigris, C. Gandolfi, L. Garbero, The voltage dip performance assessment of the Italian MV network through global indices, Int. Conf. Electricity Distribution (CIRED), Frankfurt, June 2011.

[16] R. Vailati, F. Villa, Prospettive di regolazione della qualità della tensione, AEIT 2011.