VOLTAGE MANAGEMENT BY THE APPORTIONMENT OF TOTAL VOLTAGE DROP IN THE PLANNING AND OPERATION OF COMBINED MEDIUM AND LOW VOLTAGE DISTRIBUTION SYSTEMS

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1 66 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS Vol.97(1) March 2006 VOLTAGE MANAGEMENT BY THE APPORTIONMENT OF TOTAL VOLTAGE DROP IN THE PLANNING AND OPERATION OF COMBINED MEDIUM AND LOW VOLTAGE DISTRIBUTION SYSTEMS C.G. Carter-Brown* and C.T. Gaunt** * Eskom Distribution, P O Box 5, Mkondeni 3212, South Africa, cartercg@eskom.co.za ** Department of Electrical Engineering, University of Cape Town, Private Bag 7701, Rondebosch, South Africa, ctg@eng.uct.ac.za Abstract: The maximum allowable voltage variation on the MV and LV networks is a major and often primary constraint in the planning and design of the distribution feeders, which cannot be considered in isolation from each other. Assumed or assigned volt drop limits affect the capital and lifetime costs of the networks. This paper presents a new approach, based on network classes and transformer tap zones, to: identify suitable maximum voltage drop limits for planning, designing and operating MV and LV networks, taking into account the characteristics of urban and rural networks, improve voltage regulation on existing networks, and reduce the costs of extending and strengthening MV and LV networks. Key words: Voltage regulation, voltage drop apportionment, voltage limits, distribution planning. 1. INTRODUCTION Voltage drop limits the design and operation of distribution networks with long feeders. In South Africa the apportionment of the voltage drop between the medium and low voltage (MV and LV) network levels is not standardised. Voltage apportionment limits depend on network and load characteristics, including distribution transformer tap settings. In another paper [1], a model was developed to calculate MV and LV voltage and voltage drop limits for different network and loads. The model was tested with sample network and load characteristics. In this paper, that model is used with South African networks and loads to identify transformer tap settings and voltage limits to adopt as a standard for application within the local utility, Eskom Distribution. 2. APPLICATION CONSIDERATIONS AND ALTERNATIVES FOR SOUTH AFRICA 2.1 Considerations The factors to be considered in establishing standards for voltage variation apportionment in South Africa include: National Regulatory Requirements: Regulations allow LV service voltage variation of ±10% of the nominal voltage at the service point (meter) [2]. The voltage drop within the customer s LV network (beyond the service point) is limited to 5% [3]. Customer Equipment and Appliances: The standard LV service voltage in South Africa is 230/400 V, but the earlier standard was 220/380 V and many customers still operate equipment with that rating. Some 220/380 V equipment may experience problems (failure to operate, reduced efficiency and/or life span) when operated at the higher voltage limit. Even equipment rated for the standard LV voltage of 230/400V may not be compatible with the voltage variations allowed by regulations. For example, most three-phase LV motors are designed for a continuous voltage varying within ±5% [4], and may need to be de-rated for operation at ±10%. Compatibility: The application of new apportionment limits to new and existing networks should increase, rather than reduce, their capacities, and not require network strengthening or modifications to achieve compliance with the regulations. The limits should be suitable for both urban and rural networks. Network and Load Data Constraints: Eskom Distribution s MV data is relatively well recorded, and simulations can be performed easily at the MV network level. However, LV network and load records are generally incomplete, so simulations of existing LV networks require considerable data capturing activities. Apportionment processes and limits that require detailed simulations of existing LV networks will have limited use. Resources and Processes: The same individual does not necessarily perform the planning and design, and in some cases the responsibility for the different network levels (MV and LV) is divided between different sections.

2 Vol.97(1) March 2006 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS Strategic alternatives There are two basic approaches to determining the maximum voltage drop and apportionment limits. Customised voltage apportionment: The optimum MV and LV voltage drop limits can be determined for every feeder using actual and planned network data, and load forecasts. Detailed MV and LV network studies would be performed for each network expansion and reinforcement project to establish the optimal voltage drop in the network within the overall limits. The optimum voltage drop apportionment is sensitive to assumptions of load and network parameters, which must be recorded for future use for every network. Standardised voltage apportionment: The allowable MV and LV voltage drop limits can be standardised for a limited set of broad network categories adequate for most applications. Customisation is reduced to identifying a suitable for a particular network or network section, and only the class needs to be recorded for future use. The standardised approach is preferred in the present situation because it: requires less data for the initial analysis, and less to be recorded for future use; allows the parameters needed for calculations to be established by identifying an appropriate class for each network; and requires a lower level of knowledge of feeder performance and modelling, so that staff training can be simplified to the application of standard limits. Customised apportionment may be feasible once resources, network data and systems evolve to the required levels, but this is not presently the case in most distribution utilities in Southern Africa. 3. ESKOM VOLTAGE VARIATION APPORTIONMENT LIMITS Voltage apportionment limits for typical South African network and load characteristics were calculated using the model described in [1]. The calculated limits were analysed and developed into a practical business solution to meet the requirements of network planning, design and operations. The methodology and results are described below. 3.1 es Each distribution feeder, or section of feeder, is classified as one of four es (C1, C2, C3 and C4, shown in figure 1 and applied in figure 2), representing the voltage drop apportionment between the MV and LV network levels. The es and their associated voltage apportionment were established iteratively using the apportionment model. The consideration of the different combinations of networks, distribution transformers, and target service voltage limits in Eskom Distribution resulted in four natural groupings (classes) of allowable maximum MV and LV network voltage drops, illustrated in figure 1. Maximum MV voltage drop Maximum LV voltage drop C1 C2 C3 C4 Urban Rural Figure 1: Apportionment of the maximum voltage drops in MV and LV for four es [6] Classes C1 and C2 will typically be used in urban networks where the voltage drop in the MV network is relatively small and the LV voltage drop can be large. The transfer capacity of most urban MV networks is thermally constrained. Classes C3 and C4 will typically be used in rural networks, where the LV network voltage drop is relatively small and a larger allowance is made for the MV voltage drop, which is usually the constraint on the capacity of a rural MV feeder. The voltages supplied to motors in relatively close proximity to distribution transformers (total LV voltage drop 5%) are summarised in table 1. Motor de-rating (due to voltage variation) should not be required in C1 networks if motors are relatively close to distribution transformers. Motor de-rating may be required in rural (C3 and C4) networks when the motors are at the extremities of the network. (The de-rating requirement depends on the LV network voltage drop, frequency and severity of low voltage problems and voltage unbalance and harmonic levels). Revised (2001) South African motor specifications [5] allow a +10% to -10% voltage variation, and newer motors should not require de-rating in all Classes. Newer transformers are specified with a secondary voltage of 240/415 V or 242/420 V to provide voltage boosting. Transformers complying with older specifications (nominal secondary voltages of 220/380V or 230/400V) constrain the allowable MV voltage drop due to the relatively low secondary LV voltage. Restrictions are placed on the use of certain distribution transformers in rural networks (C3 and C4), based on their nominal voltage, as indicated in table 2.

3 68 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS Vol.97(1) March 2006 MV source Rural MV distribution feeder supplying dispersed loads off the MV feeder backbone and laterals Voltage regulator installed for the urban area to keep the MV limits within the requirements of a C1 MV/LV transformer supplying single customer via short (<50m) utility owned LV network MV/LV transformer supplying numbers (>20) customers via long (>300m) utility owned LV network Rural area utilising C4 limits Very limited LV networks (small LV voltage drops) Urban area utilising C1 limits Extensive LV networks (large LV voltage drops) LV voltage during low load (greater than MV voltage due to tap boost on distribution transformer) Voltage pu MV backbone voltage drop during low load MV backbone voltage drop during peak load LV voltage during peak Distance from MV source Voltage regulator C1 MV C1 LV C4 MV C4 LV MV/LV apportionment for C1 and C4 Figure 2: Illustration of the concept of es [6] Table 1: Motor de-rating for voltage variations for three-phase motors designed for normal voltage variation of ±5%) [4]. Combined utility and customer LV voltage drop 5% Network Class Maximum variation of motor voltage Typical motor de-rating factor C1 ±5% 0% C2 +5% -7,5% 0% to 10% C3 & C4 +5% -10% 10% to 20% Table 2: Restrictions on use of distribution transformers according to voltage rating Distribution Transformer C1 C2 C3 C4 240/415V and 242/420V 230/400V N/A 220/380V N/A N/A 3.2 Distribution transformer secondary voltage and tap setting The characteristics and specifications of transformers vary significantly as a result of the changes to standard purchasing specifications in response to changes in electricity regulations. Eskom s distribution transformers have a wide range of nominal secondary voltages including 380V, 400V, 415V and 420V three phase, and 220V, 230V, 240V and 242V single phase. Furthermore the location of the tap switch (primary or secondary), the step-size (2.5%, 3% or 5%) and number of tap steps (2 or 4) vary depending on the nominal secondary voltage and technology (single, three and bi-phase). The following approach developed from the evaluation of many alternatives with the apportionment model. Customers are provided with similar maximum LV voltages regardless of local MV voltage variations. Small differences in the maximum LV voltages arise from differences in nominal secondary voltages, core flux limits, and DETS specifications. Standard tap settings are utilised as the default for all customers and networks (transformers/networks supply a mixture of different customers). The standard tap settings result in a maximum LV voltage of 400V +5% (loads with high load ratios) to 400V +7,5% (loads with low load ratios). This should result in acceptable maximum voltages for 380V equipment. Tap settings are applied when transformers are installed, and only adjusted for permanent network voltage-control changes or on receipt of a customer voltage complaint. An additional tap boost step used with distribution transformers supplying electrification networks results in a maximum voltage of 230V +10%. The standard tap settings depend on the local maximum MV voltage and transformer nominal secondary voltage. The DETS step size results in three Tap Zones (TZ1, TZ2 or TZ3), shown in table 3. Standard settings are also specified for single and bi-phase transformers. The Tap Zone represents the maximum MV voltage in the network (where a transformer is installed) during normal network operation.

4 Vol.97(1) March 2006 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS 69 Table 3: Standard tap settings (boost level) for 3-phase distribution transformers fitted with a 5 position DETS Tap Zone (maximum MV voltage) TZ1 TZ2 TZ3 Trfr 105% MV Max > 103% 103% MV Max > 100% MV Max 100% 420V 3ϕ 2,5% 0% +2,5% 415V 3ϕ 3% 0% +3% 400V 3ϕ 0% +2,5% +5% 380V 3ϕ +2,5% +5% +5% 3.3 MV voltage limits Table 5: Minimum MV voltage limits Tap Zone TZ1 TZ2 TZ3 C1 Normal 101,5% 99,5% 97,0% Abnormal 99,5% 97,0% 94,5% C2 Normal 98,0% 95,5% 93,5% Abnormal 95,5% 93,5% 91,0% C3 Normal 95,5% 93,0% 91,0% Abnormal 93,0% 91,0% 88,5% C4 Normal 92,5% 90,0% 87,5% Abnormal 90,0% 87,0% 85,0% Each has a set of upper and lower voltages within which the MV network must operate. The tap setting of a transformer then determines the limits of the MV voltage at that point in the network. Referring to table 3 and figure 3, (a) the maximum MV voltage at a particular point in the network dictates the Tap Zone (and hence tap setting) for the local distribution transformers; (b) this tap setting affects the minimum MV voltage limit at that point in the network (the MV voltage must be greater than this limit to provide acceptable minimum LV voltages); (c) the network must be operated such that the maximum MV voltages are within the limit for the Tap Zone, so that maximum LV voltage limits are met. Max MV voltage (Step 7) MV network, load and settings data Low-load study: Calculate max MV voltages Tap Zones: Check max MV limits. Apply Tap Zones Peak-load study: Calculate min MV voltages Assessment: Check min MV voltages for and Tap Zone (Step 1) (Step 2) (Step 3) (Step 4) (Step 5) (a) Standard MV/LV tap settings (Tap Zone) Network modifications: Implement network or settings changes (Step 6) (c) (b) Max MV voltage limit Min MV voltage limit Compliant network (Step 8) Figure 3: Tap settings and MV limits Table 4 shows the maximum MV voltage for the three Tap Zones for normal and abnormal conditions. Under normal conditions, all customers receive service voltages within the target limits, provided the LV networks are designed in accordance with requirements. The abnormal limits may result in isolated cases where service voltages do not comply with the regulations, and may be used only for shortterm emergency operating conditions. Table 4: Maximum MV voltage limits Tap Zone TZ1 TZ2 TZ3 Normal 105% 103% 100% Abnormal 106% 105% 102% Figure 4: MV variation assessment process The minimum MV voltage limit is shown in table 5 for the three Tap Zones and four es. The minimum MV voltage limit depends on the Tap Zone, which in turn depends on the maximum MV voltage. As illustrated in figure 4, low-load studies must be performed to optimise the tap settings (steps 2 and 3), after which the minimum MV voltage can be assessed (steps 4 and 5). Network and load changes influence both peak (minimum) and low-load (maximum) MV voltages, and require an iterative approach (step 7) to ensure that the network is compliant. The philosophy is flexible and can be applied with various voltage control philosophies and settings. Eskom Distribution adopted the following standards. TZ2 is the default Tap Zone for MV networks. The default MV voltage set-point is 103%, which is at the upper limit allowed for a TZ2. The minimum MV voltage (usually at MV feeder extremities) is maintained above the specified TZ2

5 70 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS Vol.97(1) March 2006 limit for the (the corresponding values are in bold in table 5). The Tap Zone is revised if the general network voltage rises, or network devices (such as voltage regulators or capacitors), distributed generation or voltage control settings result in maximum MV voltages outside of the limits for TZ2 (i.e. V= %). The minimum MV voltage limits change accordingly. In rural (C3 or C4) networks with significant MV voltage drop (typically >7%) and high load ratios (typically >30%), additional voltage boosting can be achieved at feeder extremities by adopting Class TZ3 (maximum MV voltage 100%). This reduces the minimum MV voltage limit, effectively increasing network capacity. 3.4 LV service voltage targets Service voltage targets (maximum and minimum voltages) vary for different customers due to the nature of the distribution networks (transformer size and MV and LV line lengths, which are related to load density) and loads (type of customer equipment and appliances). Target service voltages (table 6) were derived with the apportionment model, based on South African equipment specifications and practices. Many customers will receive voltages well within the regulatory limits of 230/400V ±10%. 3.5 LV voltage drop limits The LV voltage drop limits depend on, distribution transformer nominal secondary voltage and customer voltage-sensitivity: the specifies the LV voltage drop apportionment and restricts the distribution transformer nominal secondary voltage; the distribution transformer nominal secondary voltage influences the LV voltage drop limits due to maximum flux limits and boost capability restrictions; and the voltage-sensitivity of the customers types relate to meeting the target service voltages of table 6. Voltage-sensitive customers typically take three-phase supplies and operate motors >7,5kW. They include light industrial, large commercial (>25kVA) and rural non-domestic (e.g. agricultural pumping) supplies. Customers with three-phase supplies, but operating only single-phase appliances (e.g. domestic and small commercial), are excluded. Voltage-insensitive customers comprise domestic and small commercial (<25kVA) customers. Although LV load flow analysis is still required, detailed MV load flow studies are not needed to establish the allowable LV voltages. Instead, simple look-up tables (tables 7 and 8) are used to establish the maximum LV voltage drop limit for a particular network. The limits already allow for internal voltage drop in distribution transformers. Electrification customers are voltage-insensitive and the LV voltage drop limits can be increased further by 2.5% (add 2.5% to the values in table 8) with the use of an additional tap boost. The values in the lookup tables were obtained from inspection of the LV voltage drop limits calculated in the apportionment model for a wide range of network and load characteristics. The maximum error, compared with customised apportionment using detailed MV load flows, depend on the voltage limit differences between es. A maximum difference in MV voltage drop of 4% occurs between a C1 and C2. In certain applications where the network is bordering on C1, but a C2 is used, this may reduce the allowable LV voltage drop by a maximum of 4%. This error could be reduced by increasing the number of es, but is practically not justified considering the uncertainties in the network parameters, especially the future load. Category Urban, light industrial and large commercial Domestic and small commercial Rural, nondomestic Electrification (low income domestic) Table 6: LV service voltage targets (normal network operation) Maximum Explanation variation [%] +5% -2,5% Voltage sensitive customers in urban networks typically operate three-phase motors 7,5kW. If the voltage drop in the customer network is <2,5% (typically results in a voltage drop during motor starting <5%), the motor running voltages at the motor/load terminals will be within 400V ±5% +7,5% -10% Domestic and small commercial customers predominately use single-phase equipment/appliances. The minimum LV voltage is allowed to drop to the statutory limit of 90%. The same distribution transformer tap settings (standard tap settings) are used as for light industrial and large commercial loads, however the reduced load ratio results in a maximum voltage of 107,5%. +7,5% -7,5% Rural customers operating three-phase equipment are located in close proximity to the distribution transformer so that the voltage drop between the transformer and service point is small (typically <2,5%). The minimum voltage is 92,5%. A 2,5% voltage drop in the customer premise results in a minimum voltage at the appliance of 90%. The use of standard tap settings, and assuming a low load ratio, results in a maximum voltage of 107,5%. +10% -10% In domestic electrification (low income domestic customers) the maximum LV voltage drop is a primary capacity constraint. Taking into consideration the extent of the customer LV wiring (usually very short wiring to local plug and light fittings) and type of appliances (typically lights and a hot plate), these networks are designed for additional boosting of the distribution transformer taps (as compared with other networks/customers). The service voltages will reach the statutory limit of 230V ±10%. The wide range allowed results in considerable capital cost savings, thereby facilitating the electrification programme.

6 Vol.97(1) March 2006 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS 71 Table 7: Recommended maximum LV voltage drop for voltage-sensitive customers Transformer C1 C2 C3 C4 240/415V & 242/420V 5,0% 4,0% 2,5% 1,0% 230/400V 2,0% 1,0% 1,0% N/A 220/380V 1,0% 1,0% N/A N/A Table 8: Maximum LV voltage drop for voltageinsensitive customers Transformer C1 C2 C3 C4 240/415V & 242/420V 11,0% 7,5% 5,0% 3,5% 230/400V 8,5% 5,0% 2,5% N/A 220/380V 5,0% 2,5% N/A N/A The LV voltage drop limits for voltage sensitive customers are more restrictive (less voltage drop allowed) than for voltage insensitive customers, and the limits are usually achieved by locating distribution transformers closer to voltage-sensitive customers. If the voltage limits associated with voltage sensitive customers cannot be met within cost or practical constraints, the voltage-insensitive limits can be used. Service voltages will comply with statutory requirements, but the voltage-insensitive limits may result in customer complaints and require additional motor de-rating (typically between 10% and 20%). 3.6 General Maximum voltage drop limits: The standardised limits are maximum voltage limits within which the network must be planned, designed and operated. Many customers will experience voltage variation better than the minimum requirements. Tighter voltage ranges, not reaching the limits, may be implemented where it is economic to reduce the cost of technical losses. Additional constraints are placed on the MV voltage variation for distribution feeders supplying customers directly at MV (but are not included in this paper). Multiple Network classes: A single feeder may consist of several zones of different es, as was illustrated in figure 2. Impact of Distributed Generation (DG): MV networks inherently have relatively low X/R ratios (typically <1) and even if voltage control is performed by reactive power control, local generation increases MV voltage levels [7]. Non-dispatched DG (such as solar and wind) can generate during low-load periods and thereby violate maximum MV voltage limits. Changing the Tap Zone allows the maximum MV voltage to be increased (less distribution transformer tap boosting), but the minimum MV voltage limit is also increased. As the DG may not generate during peak loading the network capacity is actually reduced due to the addition of the DG and associated change in Tap Zone. The approach described in this paper provides for different Tap Zones and thereby facilitates the assessment of DG integration. 4. APPLICATION CONSIDERATIONS 4.1 Network planning and design Standard voltage drop allocation in four classes of network allows planning and design to be simplified, but is flexible enough to plan most networks close to their limits, reducing over-investment. Only the and not the Tap Zone is required for the LV design. The Tap Zone (and hence actual tap setting) depends on the MV voltage control settings and MV voltage variation. The Tap Zone may change due to operational changes, but the Network Class is fixed by the network planner/designer. 4.2 Network operation Active management of reticulation networks (voltage control methodologies and settings, including distribution transformer Tap Zones) achieves improved voltage management for existing customers, and increased supply capacity for many existing networks without further capital investment. The full-scale implementation of the new approach requires: enhancement of the network database to include es and transformer Tap Zones; staff training, resources and business processes for distribution network management and optimisation; and management emphasis on the importance of distribution network management (instead of only at the sub-transmission level). 5. IMPLEMENTATION Progress towards implementing these proposals has been made by preparing a technical application standard, including: selection of the most appropriate by considering the MV and LV network characteristics and costs; assistance with problem identification when investigating customer voltage complaints; selection of the most appropriate MV voltage control methodology and settings (line drop compensation, voltage compounding and fixed voltage control); accountability and process flowcharts for management and operations; worked examples; and implementing a pilot study. 5.1 Pilot study A pilot study was performed on an 11kV rural feeder to identify the benefits of standardised voltage control

7 72 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS Vol.97(1) March 2006 settings and Tap Zones. The feeder, Lidgetton NB16, is 35km long and supplies 115 distribution transformers. An on-load tap changing (OLTC) 88/11kV transformer regulates the 11kV source. An 11kV voltage regulator was installed along the line, but had failed and was bypassed, resulting in customer voltage complaints. The loads on the feeder NB16 in July 2002 were Maximum load: PF 0,98 Minimum load: PF 0,91 There are no 220/380V transformers on the feeder and the LV networks are limited. The feeder was classified as C Prior to optimisation The inspection of distribution transformer tap settings showed that the tap settings aligned with a Tap Zone TZ1. Referring to table 5, the minimum MV voltage limits for a C3 TZ1 are 95,5% and 93% for normal and abnormal conditions respectively. The OLTC was operated as fixed voltage control with a set-point of 102% of nominal voltage (11kV). The MV network was analysed using a commercial loadflow package (ReticMaster ). The simulated minimum MV voltage was 88,7%, which is below the abnormal minimum MV voltage limit of 93% and confirmed that LV voltages were unlikely to meet target limits (table 6). Figure 5 shows recorded LV voltages at an 11kV/415V distribution transformer at the end of the feeder prior to any optimisation. The mean LV voltage does not reach 230V, and the minimum voltage violates -10%. Before optimisation Voltage deviation from 230V nominal (%) Time of day Mean Maximum (+2 std dev) Minimum (-2 std dev) Figure 5: Recorded LV voltages (10 minute integration) in July 2002 prior to optimisation (line voltage regulator out of service) 5.3 Optimisation The network was analysed following the approach described in section 3. The OLTC voltage control set-point was increased from 102% to 104.5% (Table 4 shows the maximum normal network MV voltage for C3 is 105%). MV loadflow studies simulated a maximum end of line MV voltage of 100,5% (TZ2). Two Tap Zones (TZ1 close to the substation and TZ2 about halfway down the feeder) were defined, resulting in increased tap boosting (TZ2) on 87 distribution transformers. The minimum MV voltage limits for a C3 TZ2 are 93% and 91% (table 5) for normal and abnormal conditions respectively. MV load flow analysis simulated a minimum MV voltage of 91,8%, which is between the normal and abnormal MV voltage limits. The implication is that LV voltages should be acceptable, but will be close to exceeding the target service voltage limits (table 6). The maximum and minimum LV voltages of 98,2% and 89,0% are expected to increase to 103,9% and 94,7% respectively. The voltage control and tap setting changes were implemented, and the measured voltages after optimisation were significantly improved as illustrated in figure 6. The maximum and minimum voltages of 104,0% and 94,5% are comfortably within ±10% of the nominal voltage, and in good agreement with the predicted values. In both cases, the voltage recordings shown in figures 5 and 6 were made at the LV terminals of a 240/415 V transformer. The LV voltages at the customer points of supply will be lower due to LV network voltage drops. Voltages supplied by 230/400V transformers will be 1,25% lower. The minimum LV service voltage is expected to be between 92,5% and 90%, which is in agreement with the target service voltage limits (table 6). After optimisation Voltage deviation from 230V nominal (%) Time of day Mean Maximum (+2 std dev) Minimum (-2 std dev) Figure 6: Recorded LV voltages (10 minute integration) in July 2002 after optimisation (line voltage regulator out of service) No further customer voltage complaints have been received since the changes were implemented. The voltage regulator has not yet been returned to service.

8 Vol.97(1) March 2006 SOUTH AFRICAN INSTITUTE OF ELECTRICAL ENGINEERS Capacity implications The extent of network voltage variation affects network capacity. The results of network loadflow simulation for NB16 are plotted in figure 7 for four scenarios: (a) the network prior to any optimisation; (b) increasing the OLTC voltage control set-point; (c) (b) and applying multiple Tap Zones; and (d) (c) and applying voltage compounding on the OLTC voltage control. Network Capacity (Normal Limits) 2.0 MVA Lidgetton NB16 peak loading 1.8 MVA 1.7 MVA 1.4 MVA 0.9 MVA 0 MVA 0% +55% Figure 7: NB16 capacity limits (VR out of service) The capacity limits were calculated on the basis that the MV voltages drop to the minimum normal limits (table 5). The capacity increases derived from voltage management are significant and demonstrate that considerable cost savings may be achieved by optimising network voltage performance in this way. It is estimated that many of Eskom Distribution s MV networks are presently operated at the level of optimisation of (a) and (b). The new approach is estimated to save R25m per annum in Eskom Distribution by cancelling or deferring network strengthening. 6. CONCLUSION Voltage drop and the apportionment of limits are a key constraint in distribution network planning, design and operation. Formal management of voltage control and distribution transformer tap settings can improve the voltages at customers installations and reduce the investment in feeders. An approach, which is simple enough to apply in practical networks, has been developed. The approach is based on the following principles: there are different target service voltage limits for different customer and network types; the dictates the voltage drop apportionment; standardised distribution transformer Tap Zones are used to compensate for voltage variations and drops in the MV network; +85% +120% (a) (b) (c) (d) Extent of Network Optimisation the MV voltage limits depend on the network state (normal or abnormal), and Tap Zone; and the LV voltage drop limit depends on the Network Class, customer type (voltage-sensitive or - insensitive) and nominal secondary voltage of the distribution transformer. Research continues to include the effects of load loss costs and variations in revenue due to less than ideal voltage variation management in an optimisation model for MV/LV voltage drop apportionment, which may result in changes to the recommended maximum limits of voltage variation [8]. 7. ACKNOWLEDGEMENTS The authors acknowledge Eskom Resources and Strategy Division and Eskom Distribution Eastern Region for their sponsorship of this research. 8. REFERENCES [1] C.G. Carter-Brown and C.T. Gaunt: Model for the apportionment of the total voltage drop in combined medium and low voltage distribution feeders, Submitted for publication: Trans SAIEE - companion paper. [2] South Africa Department of Mineral and Energy Affairs: Electricity Act 1987, regulation 2665, 16 November 1990, [3] South African Bureau of Standards: SABS : The wiring of premises Part 1: Low-voltage installations, South African Bureau of Standards, South Africa, edition 1, pp. 51, [4] International Electrotechnical Commission: IEC : Rotating electrical machines Part 1 Rating and performance, International Electrotechnical Commission, edition 10.2, pp , [5] South African Bureau of Standards: SABS 1804: Induction Motors, South African Bureau of Standards, South Africa, edition 1.1, pp. 12, [6] C.G. Carter-Brown: Voltage drop apportionment in Eskom s distribution networks, Masters dissertation, University of Cape Town South Africa, pp , [7] S. Repo, A. Nikander, H. Laaksonen, P. Jarventausta: A method to increase the integration of distributed generation on weak distribution networks, CIRED th International Conference on Electricity Distribution, Session 3, Paper 60, [8] C.G. Carter-Brown and C.T. Gaunt: Model for voltage drop apportionment in MV/LV rural electricity distribution, IEEE Africon th Africon conference in Africa, ISBN X, Volume 2, pp , 2002.