6.1
Estimation of Load Demand
Maximum demand is the all important parameter in system design because this value directly determines component sizes (e.g. cables, transformers), voltage drops, line currents and ultimately the cost of servicing the loads.
The fluctuating nature of electrical loads, particularly that of residential peaks, makes the measurement of instantaneous demand difficult, and sometimes, undesirable. System components are rated in terms of their thermal (overload) capacity and thus their “average demand ” over a period of, say, 15 minutes is far more meaningful than the moment by moment fluctuations which actually occur. For this reason the demand on electrical equipment is often obtained by the use of special instruments (e.g. load data-loggers) which can provide an average reading for a certain period. The information provided by this type of meter is often employed in system design.
Subject to predefined conditions, maximum demands can be measured, adjusted and projected to become the basis of design for new systems. While easily understood in principle, maximum demand can be expressed in various terms and measured in various ways. Unless these aspects are fully understood and appreciated, confusion and inaccurate design may result.
6.2
Effect of Load Diversity on Maximum Demand
The peak load of any installation is characterised by the demand fluctuations from the switching in and out of appliances within the installation. It is improbable that every appliance will impose its maximum demand at the same instant. As such, the maximum demand of the installation is generally less than the sum of the individual maximum demands of all the appliances within that installation. Similarly, the maximum demand of a LV feeder is characterised by the demand fluctuations from the varying load demands of all the loads on the feeder. The maximum demand of the feeder will generally be less than the sum of the individual maximum demands due to the “diversity ” between the loads.
It is conceptually possible that if the “average maximum demand” of a “typical” load in a group is known, then the maximum demand for the whole group can be obtained by simply multiplying the average maximum demand of this typical load by the number of loads and also by an appropriate “multiplication factor” chosen for that particular number of loads.
This multiplication factor is commonly referred to as the “diversity factor ”. Used in conjunction with the number of loads, the diversity factor “scales” the “average demand” of a “typical” load within a group, to the maximum demand for that group of loads.
6.3
Residential Load ADMDs
ADMD values for residential loads are provided in Horizon Power document HPC-3DC-07-0001-2012 (Information – Electrical Design for Distribution Networks: After Diversity Maximum Demand). While the ADMD values are applicable only to standard sized lots, there may be cases where the actual ADMDs could be even higher than these values (e.g. for larger lots, beach front
houses, riverside lots, canal developments, etc.). Similarly, it may be necessary to reduce the recommended ADMD values. Changes to recommended ADMD values must, at all times, be made in consultation with the technical staff in the relevant Regional Area office prior to the design being carried out.
Since these ADMD values are averaged, they must be “scaled up” to obtain the maximum demand for a group of loads before the LV feeder can be designed. The “scaling” of the ADMD values is automatically taken into account in Horizon Power’s Voltage Drop and Line Current formulae.
6.3.1 Determination of ADMD when standard values are not used
The maximum demand on a residential substation, when divided by the number of loads supplied, provides a value which is in essence the “average contribution per customer” to that maximum demand, or simply the “average demand” for a “typical” customer. The larger the number of customers involved, the nearer to its ultimate value will be this “average demand”.
For practical purposes, groups of 60 or more loads are considered to produce a figure sufficiently close to the ultimate for it to be considered as the “After Diversity Maximum Demand” or ADMD.
Because the load ADMD is the all important basis of residential distribution design, this matter must receive full and careful consideration, concerning its value at the initial loading of the system, the provision for future growth and the repercussions of having to alter the system as a result of a poor choice of design ADMDs
Among the factors influencing the choice of the ultimate design ADMD values are:
1) Limited capital resources;
2) Apprehension concerning the future;
3) Penetration of natural gas in traditionally all-electric areas; 4) Climatic, socio-economic and/or geographic influences; 5) Load growth, changing standard of living;
6) Trend towards more efficient appliances/equipment; and 7) Tariff structure.
Whatever the ultimate design ADMD figures are, the designer must endeavour to ensure that the system is not under/over designed for the reasons given in clause 5.3.
Optimum design requires optimum choice of ADMD. In most cases, a designer has to make a value-judgement as to what value of ADMD is most appropriate for the particular distribution system, after having considered all relevant issues. For most instances, the load demand can be estimated based simply on the designer’s previous experience with similar developments. However, careful thought must still be given to this crucial design parameter for each residential development, rather than simply using highly conservative “standard” values. It is not uncommon for a designer to find himself/herself in the position of having to be a mixture of an engineer, an economist and even a prophet at the same time!
6.3.2 Non-Residential Load Demands
As mentioned earlier, maximum demand values are expressed in a variety of ways, e.g. amps, kVA, kVA/hectare, kW etc. The following load demand values for non-residential loads are a mixture of “average demand” type figures (kVA/hectare figures) as well as “maximum demand” type figures (kVA, kW, hp etc. figures).
Typical design load demand values for non-residential loads are as follows: 1) High Schools: 220 kVA;
2) Primary Schools: 82 kVA;
3) Neighbourhood Shopping Centres: obtain the load kVA based on an average load density 200 kVA/hectare. (Alternatively, enquire from consultant or measure maximum demand);
4) Large Shops/Business Centres: enquire from consultant;
5) Pumps and other large 3-phase fixed equipment: obtain full load kVA from equipment name-plate or specifications;
6) Small Shop Groups: 200 kVA/hectare; 7) Light Industrial Lots: 100 kVA/hectare.
More information is available in Horizon Power document HPC-5DC-07-003- 2012 (Distribution Design Manual Volume 3 – Supply to Large Customer Installations).
6.3.3 Residential Lot Classification
Some lots have an “Rn” classification (e.g. R25, R30). This classification relates to the “density” of houses on the lot. The “n” index refers to the Number of Units/hectare, so that an R25 lot classification refers to 25 units per hectare. Since 1 hectare = 10 000 m2, each unit on a R25 lot would occupy approximately (10 000 ÷ 25) m2 = 400 m2.
The number of units in a given “Rn” lot of area, A (m2), can then be calculated as follows:
No. of Units = A (m2) × n ÷ 10 000
For example, if an R25 lot has an area of, say, 4898 m2, the number of units in the lot would be 4898 × 25 ÷ 10 000 = 12 units.