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QUALITY OF SUPPLY REGULATIONS, DIRECTIVES AND STANDARDS: THE NEED TO CONSIDER VOLTAGE DIP PERFORMANCE IN THE SPECIFICATION OF 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: [email protected] Senior Lecturer, School of Electrical and Information Engineering, University of the Witwatersrand, Private Bag 3, Wits, 2050, South Africa; e-mail: [email protected] Abstract This paper describes the basis for defining plant and equipment specifications for the purpose of ensuring the compatibility of industrial production processes with the dip performance characteristics of South African supply networks. 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 to ISO 9001) and the revised dip assessment methods defined in the 2003 revision of NRS 048-2 (Edition 2). The paper also 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, as recently published in the IEC 61000-4-30 standard are also provided. The paper draws from extensive experience gained by Eskom in the monitoring of power quality at over 600 sites nationally, as well as testing undertaken at the Eskom/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 defined in NRS 048-2:1996 Edition 1 [2]. These standards were based on the worst performance seen by 95% of sites in the country, and were considered by many customers to be too lenient on utilities (particularly by customers operating plant in 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 Quality Advisory 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 Don’t 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 stakeholders address 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 of view, an important conclusion is that more needed to be done to address the manner in which end-use equipment is specified (equipment suppliers feel that customers do not sufficiently specify the power quality requirements, and customers feel that equipment suppliers do not design for, nor are able to quantify power quality limitations of their equipment). The above consultation process highlighted a specific need for a revised regulatory framework to include considerations related 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, and equipment 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 various licence requirements. The definition of a power quality management framework addresses this problem by defining 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 equipment supplier). 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 quality management based on ISO-9001 philosophies as opposed to the application of standards alone. This management system 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 various suppliers in South Africa. The standard categorises voltage dips according to both the expected frequency of occurrence, 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 Eskom dip measurements over a period of 4 years since then [4]. From this data it is clear that as far as large industrial customers are concerned, voltage dips of less than 30% in magnitude, and a duration shorter than 150ms, have a high probability of occurring in South African HV networks. Many customers are not affected by these events. This has resulted in the development 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 and customers on the most appropriate voltage dip categories, as well as a minimum level of dip immunity (represented by the shaded area). The categorisation method allows effective communication on basic network performance and mitigation requirements. (Eskom for example, uses the dip classifications in determining performance trends). A case study illustrating the effective use of the curve is discussed in the next section. Remaining Duration t Network Number of Dips per Year (95%) Voltage 20 t < 150 t < 0.6 t < 3 Voltage Dip W indow Category u % of Ud 150 600 (s) X1 X2 T1 S1 Z1 Z2 (ms) (ms) >44<=132kV 35 35 25 40 40 10 90> u  85 >132kV 30 30 20 20 10 5

85> u  80 Network Number of Dips per Year (50%) Z1 80> u  70 Voltage Dip W indow Category 70> u  60 X1 S1 X1 X2 T1 S1 Z1 Z2 60> u  40 X2 Z2 >44<=132kV 13 10 5 7 4 2 40> u  0 T1 >132kV 8 9 3 2 1 1

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 at 95% 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 Values of duration and depth Basis for definition Category

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

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

Duration > 20 ms to 150 ms Typical HV system Zone 1 clearance times (no pilot wire). Note that HV dips are reflected all the way down to LV. X1 Depth 30 % to 40 % Desired plant immunity - as this spans an additional large number of dips caused by remote faults on the licensee network.

Duration > 20 ms to 150 ms Typical HV system Zone 1 clearance times (no pilot wire). Note that HV dips are reflected all the way down to LV. X2 Depth 40 % to 60 % Dips potentially causing drives to trip, caused by remote faults on the licensee network.

Duration > 150 ms to 600 Typical Zone 2 clearance on HV networks and accelerated clearance ms S Also some distribution faults.

Depth 20 % to 60 % Plant compatibility (drives trip > 20 %) caused by remote faults on the licensee network

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

Depth 60 % to 100 % Plant compatibility (contactors trip > 60 %) Caused by close-up faults on the licensee network Duration > 600 ms to 3 s Back-up and thermal protection clearance or long recovery times (transient voltage stability) or both Z1 Depth 15 % to 30 % Remote faults Post-dip motor recovery without stalling

Duration > 600 ms to 3 s Back-up and thermal protection clearance Z2

Depth 30 % to 100 % Closer faults Potential motor stalling 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 and other industries in Port Elisabeth. To address this concern, Eskom undertook a study using statistical dip prediction simulations 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 to 2585 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 dip characterisation method in NRS 048-2 Ed2 (note that few customers knew the dip immunity of their plants) [4,8]. If the assumption is made that customer plants are able to meet the basic immunity criterion implied by the shaded (black) area in the figure below, the number of dips that actually affect these plants will improve from 37 to 28 events per year (i.e. as opposed 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 the country, based on the characteristic dip performance data for the country in NRS 048-2 Ed 2. Particularly the expected number 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 the country). Table 3. Dip performance statistics: present and (future) [4].

Remaining voltage Duration (t)

u % of Ud 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 NRS 048-2 Ed2 standard. Should the plant immunity be increased further to include X1 type dips (“desired” dip immunity defined in NRS 048-2 Ed2), the performance would improve from 29 events to only 16 events per year (i.e. the latter would 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 based on 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 is calculated 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 significantly from plant to plant. This impact is a strong function of the tolerance of individual equipment as well as the design of the plant. The sensitivity of equipment and plant to voltage dips has been addressed internationally in various ways. Some of these approaches (illustrated in Figure 4) are [7]:  ITIC/CBEMA Curves: These curves, originally developed for computers only, have been revised in the late 1990’s to address 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 per event), has resulted in the development of an immunity requirement for all equipment used in such plants. Since the implementation of the standard, many semiconductor wafer fabrication plants have implemented this requirement retrospectively.  IEC 61000-4-11: This standard specifies generic voltage dip test requirements for LV equipment < 16A, and recommends various classes of dip performance. A similar standard (IEC 61000-4-34) is being developed for equipment > 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 to the NRS 048-2 Ed2 “desired” immunity requirement (which includes immunity to X1-type dips).

0 . 9 NRS (minimum) ) 0 . 8 . 0.8 u .

p 0 . 7 (

IEC 61000-4-11 e ITIC/CBEMA (Class 2) g 0 . 6

a 0.6 t l

o 0 . 5 v SEMI F47 g 0 . 4 n

i 0.4 IEC 61000-4-11 (Class 3) n i 0 . 3 a m

e 0 . 2

0 0 1 2 3 101 0 20 1001 0 500 11s 0 3s Duration (ms)

Figure 4. Comparison of international tolerance and immunity test curves. Note that NRS 048-2 effectively has graded requirements - only the minimum requirement is shown. 1.0 1

) 0.8 . 0.8 u . PWM 3-phase dip 75% load

p 0.7 ( PC Motherboard e

t Contactor l

o 0.5 v PWM phase-phase dip 75% load g 0.4 n

i 0.4 n i 0.3 a m

0 0 1 2 3 10 20 10010 500 101s 3s Duration (ms) 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 PWM drives (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. below these, almost all equipment is affected). What is important to note is that in the case of variable speed drives, the tolerance curves may be significantly affected by factors such as the number of phases involved (note the difference between 3 phase and phase-phase tolerance curves), the phase shift that occurs with the on-set of the event, the loading of a variable speed drive etc. The test requirements of IEC 61000-4-11 do not cater for such complexities. What is also important to note is that, in the absence of information on the dip performance of a particular drive, the user may select a 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 (or even benchmarking such performance) [9]. The general trend, as in the case of NRS 048-2 Ed2, is therefore to define “characteristic” or “indicative” performance levels. Some examples of the communication of dip performance by utilities are [7]:  CENELEC EN 50160:1999: This standard is applied by European Regulators and wires companies as a default “minimum” 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 from “a few tens to up to one thousand”.  Electricité de France: EDF has contracts with about 10% of it’s larger customers which include dip performance. These contracts allow for consequential damages to be paid if the contracted number of dip events is exceeded. For this 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 considered normal and, because the dip is measured phase-to-phase, the majority of dips caused by single line faults are not considered). Note that, unlike EN 50160, NRS 048-2 Ed2 provides actual indicative values based on actual measurement at a large number of substations over several years [5]. Responsibilities Transmission and distributions companies are required to implement a quality management process. This process defines the manner in which the utility interacts with its customers: i.e. the communication of performance levels, rights and responsibilities, and how complaints and claims related to events that affect sensitive customers are managed. The latter 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 communication and record-keeping systems. In terms of the management process, customers may require licensees to focus on managing dip performance even in areas where this is performance is relatively good. Customers are specifically required to focus on how power quality considerations are taken into account in the design and 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 take reasonable measures to protect equipment and plant against common types of voltage dips. Even the minimum immunity requirements in NRS 048-2 will ensure a significant reduction in the number of plant trips due to voltage dips. The estimated dip performance that a customer can expect needs to be provided by the utility (the detail is relative to the size of the customer – for the general case indicative levels of dip performance are defined in NRS 048-2:2003 Ed 2). 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 dip immunity. In the long term, drive immunity classes need to be defined in national and international standards, together with the appropriate test protocols. Basic immunity testing is defined in IEC 61000-4-11, but this is not detailed enough for drive applications. This paper has highlighted some of the issues that need to be included, which are also discussed in 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 NRS 048-2 characterisation method. 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, August 2002. [4] R.G. Koch, P. Balgobind, P.A. Johnson, I. Sigwebela, R. McCurrach, D. Bhana and J. Wilson, “Power quality management in a regulated environment: the South African experience”, Proceedings of the 40th Cigré 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 PQA’97-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 Group CIGRE C4-07 and CIRED, “Recommending power quality indices and objectives in the context of an open electricity market”, 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 15 kW 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 variable speed drive at half rating”, Proceedings of the 2nd Southern African Power Quality Conference, Johannesburg, South Africa, 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.