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QUALITY OF SUPPLY REGULATIONS, DIRECTIVES AND STANDARDS:THE NEED TO CONSIDER VOLTAGE DIP PERFORMANCE IN THE SPECIFICATIONOF VARIABLE SPEED DRIVES

R G Koch, J M van Coller, Corporate Consultant: Power Quality Private Bag 40175, Eskom Holdings (Pty) Ltd,Cleveland, 2022, South Africa; e-mail: robert.koch@eskom.co.za

Senior Lecturer, School of Electrical and Information Engineering, University of the Witwatersrand, Private Bag 3,Wits, 2050, South Africa; e-mail: j.vancoller@ee.wits.ac.za

Abstract

This paper describes the basis for defining plant and equipment specifications for the purpose of ensuring thecompatibility of industrial production processes with the dip performance characteristics of South African supplynetworks. The information is based on the framework for power quality management defined by a directive of the

National Electricity Regulator (NER) in 2002. This framework is based on quality management principles (similar toISO 9001) and the revised dip assessment methods defined in the 2003 revision of NRS 048-2 (Edition 2). The paperalso contextualises the NER framework by discussing comparative international approaches to defining voltage dip

performance requirements. Important fundamentals in the definition of voltage dip performance measurement, asrecently published in the IEC 61000-4-30 standard are also provided. The paper draws from extensive experiencegained by Eskom in the monitoring of power quality at over 600 sites nationally, as well as testing undertaken at theEskom/Wits Dip Test Facility at the University of the Witwatersrand.

Introduction

The revised regulatory framework for power quality management, issued by the National Electricity Regulator (NER)in 2002 [1], has important implications for the users and suppliers of LV electrical equipment and control devices. In

particular, it defines the responsibilities of customers and end-use equipment suppliers.

The previous regulatory framework (1996 to 2002) simply required licensees to meet minimum dip standards as definedin NRS 048-2:1996 Edition 1 [2]. These standards were based on the worst performance seen by 95% of sites in thecountry, and were considered by many customers to be too lenient on utilities (particularly by customers operating plantin better-performing parts of the country, for example where lightning ground-flash densities are comparatively low)[3].The NER addressed this concern through an extensive process of consultation, coordinated by the NER Power QualityAdvisory Committee, which ensured balanced representation form the various stakeholders: customers, licensees(utilities), suppliers of end-use equipment, standards bodies, and the service industry (consultants and academics) [3].

Table 1. A summary of stakeholder opinions [4].

Opinion of Level of attention given to power quality by

Licensee Customer Supplier

Licensee Some Some Dont know

Customer Some Significant Insufficient

Supplier Some Insufficient Insufficient

The table above summarises the opinions of each stakeholder category of the manner in which other stakeholdersaddress power quality concerns [4]. Voltage dips were generally considered by customers to be the most significant

power quality problem, and interruptions considered as the biggest threat in the future. From a voltage dip point ofview, an important conclusion is that more needed to be done to address the manner in which end-use equipment isspecified (equipment suppliers feel that customers do not sufficiently specify the power quality requirements, andcustomers feel that equipment suppliers do not design for, nor are able to quantify power quality limitations of theirequipment).

The above consultation process highlighted a specific need for a revised regulatory framework to include considerationsrelated to the interface between the equipment supplier and the customer (user of the equipment). The new framework

therefore recognises the need for each of the parties (transmission and distribution companies, customers, andequipment suppliers) to appropriately address power quality issues in the design and operation of their plant /equipment. A practical problem was that the NER only has direct influence over licensee behaviour through its variouslicence requirements. The definition of a power quality management framework addresses this problem by defining

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the conditions under which the NER is able meaningfully to intervene where power quality problems are experienced by any of the stakeholders (thereby indirectly influencing the relationship between the customer and the equipmentsupplier). The new framework acknowledges that dip and interruption performance varies from location to location,and that customer power quality requirements vary. This is achieved by defining licensee requirements for qualitymanagement based on ISO-9001 philosophies as opposed to the application of standards alone. This managementsystem includes non-technical aspects such as communication requirements and a formal complaints management

process (see section 4).Voltage dip specifications in nrs 048-2

NRS 048-2 is the power quality standard applied by the NER [1] as a licence condition in the licences of the varioussuppliers in South Africa. The standard categorises voltage dips according to both the expected frequency ofoccurrence, and the impact on customer plant. The original NRS-048:1996 (Edition 1) dip characterisation method was

based on theoretical considerations [2]. Figures 1 and 2 show actual measured dip density plots based on national Eskomdip measurements over a period of 4 years since then [4]. From this data it is clear that as far as large industrial customersare concerned, voltage dips of less than 30% in magnitude, and a duration shorter than 150ms, have a high probability ofoccurring in South African HV networks. Many customers are not affected by these events. This has resulted in thedevelopment of a revised dip categorisation method, published in NRS 048-2:2003 Edition 2 [5].

Figure 1. Dip density plot: networks with nominal voltage > 132 kV.

Figure 2. Dip density plot: networks with nominal voltage >44 kV and 132 kV.

The revised dip categorisation method, summarised in the table below, represents the consensus reached by utilities andcustomers on the most appropriate voltage dip categories, as well as a minimum level of dip immunity (represented bythe shaded area). The categorisation method allows effective communication on basic network performance andmitigation requirements. (Eskom for example, uses the dip classifications in determining performance trends). A casestudy illustrating the effective use of the curve is discussed in the next section.

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Figure 3. Graded dip categorization in NRS 048-2:2003 Edition 2 based on extensive measurements of network performance characteristics and experience with customer plant immunity. The number of dips not exceeded at95% of sites and 50% of sites is shown for HV and EHV networks based on several years of data at the majority

of HV sites supplied by Eskom.

The basis for determining the various dip categories is based on the considerations defined in Table 2.

Table 2. Basis for the definition of dip categories [5].

Dip

Category

Values of duration and depth Basis for definition

(shaded)

Duration > 20 ms to 3 s Dip definition (20 ms to 3 s).

Depth 30 %, 20 %, 15 % Minimum plant compatibility requirement (this covers a significantnumber of short duration dips that occur at any given supply point)

X1

Duration > 20 ms to 150 ms Typical HV system Zone 1 clearance times (no pilot wire). Notethat HV dips are reflected all the way down to LV.

Depth 30 % to 40 % Desired plant immunity - as this spans an additional large number ofdips caused by remote faults on the licensee network.

X2

Duration > 20 ms to 150 ms Typical HV system Zone 1 clearance times (no pilot wire). Notethat HV dips are reflected all the way down to LV.

Depth 40 % to 60 % Dips potentially causing drives to trip, caused by remote faults onthe licensee network.

S

Duration > 150 ms to 600ms

Typical Zone 2 clearance on HV networks and accelerated clearance

Also some distribution faults.

Depth 20 % to 60 % Plant compatibility (drives trip > 20 %)

caused by remote faults on the licensee network

T

Duration > 20 ms to 600 ms Zone 1 and zone 2 clearance times

Depth 60 % to 100 % Plant compatibility (contactors trip > 60 %)

Caused by close-up faults on the licensee network

Duration tRemainingVoltage

u % of U d 20 t u 8080> u 70 Z170> u 60 X160> u 40 X2

S1

40> u 0 T1Z2

Number of Dips per Year (95%)Dip Window Category

NetworkVoltage

X1 X2 T1 S1 Z1 Z2

>44132kV 30 30 20 20 10 5

Number of Dips per Year (50%)Dip Window Category

NetworkVoltage

X1 X2 T1 S1 Z1 Z2>44132kV 8 9 3 2 1 1

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Z1

Duration > 600 ms to 3 s Back-up and thermal protection clearance or long recovery times(transient voltage stability) or both

Depth 15 % to 30 % Remote faults

Post-dip motor recovery without stalling

Z2

Duration > 600 ms to 3 s Back-up and thermal protection clearance

Depth 30 % to 100 % Closer faults

Potential motor stalling

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Case study: application of the nrs 048-2 dip categorisation method

With the proposed expansion of the transmission system in the Eastern Cape to supply the Coega development area,concerns were raised about an expected increase in the number voltage dips seen by vehicle manufacturing plants andother industries in Port Elisabeth. To address this concern, Eskom undertook a study using statistical dip predictionsimulations to determine the real impact of the increase in transmission line exposure to faults on the dip performance seen

by existing customers [6]. The results showed that the proposed increase in transmission lines length from 1807 km to2585 km in the zone of sensitivity (i.e. lines where faults on the network would give rise to dips in the Port Elisabeth area)would significantly increase the number of transmission-caused dips seen by customers i.e. from 38/yr to 50/yr.

The actual impact on customers was communicated at a power quality forum for key customers in the region, using the dipcharacterisation method in NRS 048-2 Ed2 (note that few customers knew the dip immunity of their plants) [4,8]. If theassumption is made that customer plants are able to meet the basic immunity criterion implied by the shaded (black) areain the figure below, the number of dips that actually affect these plants will improve from 37 to 28 events per year (i.e. asopposed to 50/yr if a plant is affected by all dips of magnitude greater than 10% in depth).

The future performance will also generally improve to be more comparable with that for 50% of 132 kV sites in thecountry, based on the characteristic dip performance data for the country in NRS 048-2 Ed 2. Particularly the expectednumber of future T-type dips is significantly improved over the present performance, which is significantly worse than that

seen by 50% of HV sites in the country (i.e. the 21 for the existing system is close to the 25 for 95% of sites in thecountry).

Table 3. Dip performance statistics: present and ( future ) [4].

Remaining voltage

u % of U d

Duration (t)

20 t < 150 (ms) 150 t < 600 (ms) 0.6 t < 3 (s)

90> u 85

85> u 80 1 (22)

0 (0)80> u 70

1 (1)70> u 60 8 (12)

0 (0)60> u 40 7 (6)

40> u 0 21 (9)

The above example clearly shows the benefit of implementing the basic level of immunity required in the present NRS048-2 Ed2 standard. Should the plant immunity be increased further to include X1 type dips (desired dip immunitydefined in NRS 048-2 Ed2), the performance would improve from 29 events to only 16 events per year (i.e. the latterwould be the effect of applying the Swedish immunity requirement for industrial installations as discussed below).

Different international approaches to defining minimum immunity requirements are described in the next section.International approaches to dip standardisation and characterisation

Measurement method

The measurement of voltage dips has recently been standardized in IEC 61000-4-30. The description of a dip is basedon two parameters: dip magnitude (a percentage difference between the lowest r.m.s. voltage and the reference voltage

which is the declared normal voltage) and the dip duration (for a multi-phase dip the duration from when the first phase of the supply drops below a dip threshold to when the last phase voltage recovers). The r.m.s. voltage iscalculated over a full cycle period at half-cycle intervals. This method is applied by NRS 048-2 Edition 2.

Equipment/plant tolerance curves and immunity test standards

What makes the definition of a single international categorization method difficult is that, in addition to dip

performance varying from network to network, the actual impact of dips on customer plant also varies significantlyfrom plant to plant. This impact is a strong function of the tolerance of individual equipment as well as the design ofthe plant.

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The sensitivity of equipment and plant to voltage dips has been addressed internationally in various ways. Some ofthese approaches (illustrated in Figure 4) are [7]:

ITIC/CBEMA Curves: These curves, originally developed for computers only, have been revised in the late 1990s toaddress general LV equipment. They are used extensively as a reference for LV immunity requirements in the USA.

SEMI 47 : The significant cost of a dip to a semiconductor wafer fabrication plant (which can be in excess of $1m perevent), has resulted in the development of an immunity requirement for all equipment used in such plants. Since theimplementation of the standard, many semiconductor wafer fabrication plants have implemented this requirementretrospectively.

IEC 61000-4-11 : This standard specifies generic voltage dip test requirements for LV equipment < 16A, andrecommends various classes of dip performance. A similar standard (IEC 61000-4-34) is being developed forequipment > 16A. The generic nature of the standard makes it difficult to apply to variable speed drives.

Swedish Industry : A basic immunity requirement for Swedish plants has been published in 2004. This is very close tothe NRS 048- 2 Ed2 desired immunity requirement (which includes immunity to X1-type dips).

Figure 4. Comparison of international tolerance and immunity test curves. Note that NRS 048-2effectively has graded requirements - only the minimum requirement is shown.

Figure 5. Examples of measurements of actual equipment tolerance curves.

The figure above provides a summary of data collected by Eskom over the past 5 years on tests conducted on PWMdrives (only curves for 75% loading are shown lighter loading results in improved ride-through), motor contactors,and computer motherboards. The shaded areas show the range in which most equipment tolerance curves lie (i.e. belowthese, almost all equipment is affected). What is important to note is that in the case of variable speed drives, thetolerance curves may be significantly affected by factors such as the number of phases involved (note the difference

101

102

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0

0.1

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PC Motherboard

10 20 100 500 1s 3sDuration (ms)

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between 3 phase and phase-phase tolerance curves), the phase shift that occurs with the on-set of the event, the loadingof a variable speed drive etc. The test requirements of IEC 61000-4-11 do not cater for such complexities. What is alsoimportant to note is that, in the absence of information on the dip performance of a particular drive, the user may selecta drive with either a very good or a very poor performance.

Utility dip performance standards

Recent international work has highlighted the difficulty of setting international dip standards for utility performance (oreven benchmarking such performance) [9]. The general trend, as in the case of NRS 048-2 Ed2, is therefore to definecharacteristic or indicative performance levels. Some examples of the communication of dip performance byutilities are [7]:

CENELEC EN 50160:1999: This standard is applied by European Regulators and wires companies as a defaultminimum standard (i.e. in the absence of specific contracts). It was developed to describe the electrical product,and is specifically aimed at providing customers with a clear indication of levels of quality they could experience.Given the large variations in dip statistics from one network to another, the number of dips is indicated as being froma few tens to up to one thousand.

Electricit de France : EDF has contracts with about 10% of its larger customers which include dip performance.These contracts allow for consequential damages to be paid if the contracted number of dip events is exceeded. Forthis reason, dips are characterized as only events of duration longer than 600ms and deeper than 30% in magnitude.A dip is therefore only defined if it can be specifically managed (i.e. the occurrence of shorter events are considerednormal and, because the dip is measured phase-to-phase, the majority of dips caused by single line faults are notconsidered).

Note that, unlike EN 50160, NRS 048-2 Ed2 provides actual indicative values based on actual measurement at a largenumber of substations over several years [5].

Responsibilities

Transmission and distributions companies are required to implement a quality management process. This processdefines the manner in which the utility interacts with its customers: i.e. the communication of performance levels, rightsand responsibilities, and how complaints and claims related to events that affect sensitive customers are managed. Thelatter increases the focus on power quality issues in network planning, operation, and maintenance practices. Licensees

have submitted to the NER a program of timeframes and actions to be taken to implement the relevant communicationand record-keeping systems.

In terms of the management process, customers may require licensees to focus on managing dip performance even inareas where this is performance is relatively good.

Customers are specifically required to focus on how power quality considerations are taken into account in the designand operation of their plant. Equipment suppliers will increasingly be required by their customers to provide the dip

performance characteristics (and mitigation options) of their equipment.

The NER may mediate/arbitrate on disputes that are already technically well defined by the time they reach the NER.The NER role is focussed on ensuring that utility quality management systems are in place, and that utilities report on

power quality trends on an annual basis.

Conclusions

The revised regulatory framework for power quality management requires plant designers and operators to takereasonable measures to protect equipment and plant against common types of voltage dips. Even the minimumimmunity requirements in NRS 048-2 will ensure a significant reduction in the number of plant trips due to voltagedips. The estimated dip performance that a customer can expect needs to be provided by the utility (the detail is relativeto the size of the customer for the general case indicative levels of dip performance are defined in NRS 048-2:2003 Ed2). Equipment and plant specifications should be based on these indicative levels.

The framework further requires that equipment suppliers need to be able to provide more information on drive dipimmunity. In the long term, drive immunity classes need to be defined in national and international standards, togetherwith the appropriate test protocols. Basic immunity testing is defined in IEC 61000-4-11, but this is not detailed enoughfor drive applications. This paper has highlighted some of the issues that need to be included, which are also discussedin a parallel paper [12].

The achievement of basic immunity levels of customer plant and equipment has a dramatic impact on how the plant performance responds to network changes. This paper has illustrated this through a case study application of the NRS048-2 characterisation method.

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References

[1] National Electricity Regulator, Power Quality Directive, April 2002.

[2] NRS 048-2 Edition 1, 1996.

[3] R.G. Koch, P. Balgobind and E. Tshwele, New developments in the management of power quality and performance in a regulatory environment, Proceeding of the IEEE Africon Conference , George, South Africa, August2002.

[4] R.G. Koch, P. Balgobind, P.A. Johnson, I. Sigwebela, R. McCurrach, D. Bhana and J. Wilson, Power qualitymanagement in a regulated environment: the South African experience, Proceedings of the 40 th Cigr Paris Session ,Paris, France September 2004.

[5] NRS 048-2 Edition 2, 2003.

[6] R.G. Koch and A. Petroianu, The practical implementation of voltage dip considerations in power system planning, Proceedings of the Power Quality Applications Conference PQA97 -North America , Columbus, Ohio,March 1997.

[7] R.G. Koch, Power quality management initiatives: standards, regulatory frameworks and incentives, monitoring,

contracting, and compatibility guidelines, 15th Conference on Electric Power Supply Industry (CEPSI) , Shanghai,China, October 2004.

[8] P. Byrne, D. Pillay and RG Koch, The value of power quality customer forums in a regulated environment, Proceedings of the 17th International Conference on Electricity Distribution , Barcelona, Spain, 12-15 May 2003.

[9] G. Beaulieu, G. Borloo, M. Bollen, R.G. Koch, S. Malgarotti and X. Mamo on behalf of Joint Working GroupCIGRE C4- 07 and CIRED, Recommending power quality indices and objectives in the context of an open electricitymarket, Proceedings of the IEEE / Cigre Symposium Quality and Security of Electric Power Delivery Systems ,Montreal, Canada, 7-10 October 2003.

[10] A.K. Keus, R. Abrahams, J.M. Van Coller and R.G. Koch, Analysis of voltage dips (sag) testing results of a 15kW PWM variable speed drive, Proceedings of the IEEE International Electric Machines and Drives Conference ,Seattle, USA, May 1999.

[11] R. Abrahams, A. Keus, R.G. Koch and J. Van Coller, The results of comprehensive testing of a 120 kW variablespeed drive at half rating, Proceedings of the 2nd Southern African Power Quality Conference , Johannesburg, SouthAfrica, 7-9 September 1999.

[12] J.M. van Coller and R.G. Koch, Dip problems and solutions in practical variable speed drive installations", Proceedings of the First Independent LV Switchgear and Drives & Control Conference and Exposition , Johannesburg,27-29 September 2004.