Design Of Electrical Infrastructure

The Nigerian Institution Of Electrical & Electronics Engineers A division of the Nigerian Society of Engineers Kaduna Branch

DEVELOPMENT AND DESIGN OF ELECTRICAL INFRASTRUCTURES IN MODERN CITIES By Engr. Paul I. Audu

- 08028451593, 08054143288, 08030723801

Developing Electrical Infrastructures in a City setup involves the following components: Load growth and load demand of the area to be served (load forecast).  This depends on Historical Data, Geographical factors, Land use, City plans, Industrial plans, Community Development Plans, Alternative Energy Sources, Load Density, and Population Growth.  Injection Substation sites election depends on Load Forecast , proximity to existing 33KV lines, Land availability, Land acquisition, Land use regulations, Feeder Limitations, coverage of nearest 33KV injection substation, proximity to load centers and load density.

A typical Map of a new City Development showing Land use Distribution and requiring Bulk Power Supply is shown below:

Based on the concept behind the city layout, our electrical facility shall be solely based on four main objectives as summarized below:  To comply with the applicable codes and standards;  To meet the specific power and communications requirements for the different zones/plots;  To achieve reliability and durability of systems and components;  To implement safety measures for the protection and safety of people and equipment. Consideration

While achieving the design objectives, other important factors shall have to be considered carefully:  Cost effectiveness;  Efficiency and energy conservation;  Co-ordination with other design elements;  Simplifying installation operation and maintenance; Two considerations shall be adopted as follows: 1. Primary Supply Consideration 2. Secondary Feeder Consideration

Design Consideration for Primary Feeder  This is the portion between the injection substation and the Distribution Transformers.  It consists of main and Lateral (or Tee-offs) as indicated in figure 1  Due to growing emphasis on service Reliability, the Protection Scheme will be more sophisticated, incorporating remotely controlled automatic devices based on supervisory controlled or computer controlled system.  The Congested and high density location in the metropolitan area shall be served with underground primary feeders.

FIGURE 1B: TYPICAL LOOPED-FEEDER UNDERGROUND CIRCUITSUSING TWO SOURCES

FIGURE 1A: TYPICAL LOOPED-FEEDER UNDERGROUND CIRCUITS

Design Consideration for Primary Feeder The various and interrelated factors affecting the selection of the primary feeder shall include:  The nature of the load to be connected (Residential, Commercial or Industrial)  The load density of the area to be served  The growth rate of the load The need for spare capacity on emergency operation The type and cost of circuit construction employed The design and capacity of the substations involved The type of regulating equipment to be used The quality of service required The continuity of service required

Design Consideration for Secondary Feeders  The part of the electric utility system between the primary system and the customers’ property shall be considered as the secondary system. This includes the step-down distribution transformers, secondary circuits (secondary mains), consumer services (service drops) and metering facilities.  In this design, it shall be borne in mind that the customer shall install all types of energy-consuming devices that can be connected in every conceivable combination and times of customers’ choice.  Consequent upon the consideration above, our concept of distribution shall start with the individual customers and loads, and proceed through several gathering stages to include various groups of increasing number of customers and their loads.

Design Consideration for Secondary Feeders  To minimize secondary-circuit lengths, we shall locate the distribution transformers close to the load centres and will try to have secondary service drops to the individual customers as short as possible.  Most of the secondary systems shall be radial-designed except for some specific areas like central business districts, security installations, hospitals etc. where reliability and service-continuity considerations are far more important than the cost and economic considerations.  In the special considerations mentioned above, the design shall be grid- or mesh-type network configurations.  The one-line diagram of a small segment of a typical secondary network to be utilized is shown in figure 2.

FIGURE 2: ONE-LINE DIAGRAM OF A TYPICAL SECONDARY NETWORK

Based on the concept behind the city layout, our electrical facility shall be solely based on factors that enhance adequate power capability, system flexibility, power reliability and availability, as well as network optimization. In this regard, the power system components shall be considered as follows: 1 Network Configuration The basis for design of the primary feeder in this particular instance is service continuity and near-unity reliability, and the loop-type primary feeder best suites this purpose. For the looped network to be effective, packaged substations shall be used and the network shall be essentially underground system best suited for looped ring-main operation as indicated in figure 1a above.

2 Service Delivery With emphasis on reliability of service, the definite trend in this design shall be greater use of protection and sectionalizing equipment in the primary system so as to minimize the number of outage to customer as well as outage time on the feeder. High-voltage Distribution Systems (HVDS) shall be essentially utilized to reduce voltage-drop and extend major equipment to the city load centres. We shall explore the need for optimization of distribution voltage and substation size in planning the primary distribution network using the tract area served, area shape (ratio of length and breadth) and the anticipated load density in MVA/square kilometer based on nature of load.

The length of the primary trunk feeders and tee-offs shall be based on the following relationship (assuming a square area). Length of primary circuits (Km)  N, A Length of Tee-offs (Km)  N, A and X Where,

N  number of primary circuits A  tract area in Km2 X  Unit Distribution Transformer area in Km2

3 Load Centres Load centre consideration shall be based on two premises Substation For a cost-effective load centres, the substation locations shall be based on:  Minimum Losses and better voltage regulation with proximity of the substation to loads of its service area as related by d  l,  Si Where,

d  optimum distance for minimum losses l  respective distances of load from source Si  loads of service area Location that allows proper access to incoming and other outgoing primary feeders as well as giving room for future growth.

3 Load Centres Distribution Transformers For location of distribution transformers, location points (X,Y) shall be established by moments equated round the reference point of origin x- and y-axis, using loads of a service area as masses (see figure 3).

FIGURE 3: TYPICAL DISTRIBUTION TRANSFORMER LOAD CENTRE LOCATION

The LT line lengths and load positions shall be plotted on graph/survey sheet to scale as per geographical orientation. The load centres on X and Y axis are chosen as follows: (Li)X  Lixi (Li)Y  Liyi Where,

Li = loads on x- and y-axis xi, yi = distances of loads on x- and y-axis

4 Primary Feeder Loading Factors that shall be taken to consideration in primary feeder loading are:

4 Primary Feeder Loading Factors that shall be taken to consideration in primary feeder loading are:  The density of feeder load  Nature of feeder load  Expected growth rate  The reserve-capacity requirements for emergency  Service-continuity requirements  Service-reliability requirements  The quality of service  Primary-feeder voltage level  Type and cost of construction Location and capacity of the distribution substation The voltage regulation requirements

5 Development of Distribution Feeder Exit This shall be done with a view to providing uniform development plan to minimize circuitry changes associated with distribution extension. Rectangular type of development shall be used to determine primary feeder loads. The main thrust is to maximize flow and minimize losses. Network flow augmentation shall be used to establish maximum flow and minimum-cut for optimum feeder exit as illustrated in N2 figure 4. R1

N1

R3 R2

R6

R4 N4

Ni = Service Nodes Ri = Service Routes

N3

R5

R7 N5

FIGURE 4: FLOW DIAGRAM FOR DISTRIBUTION EXIT DESIGN

5.1 Rectangular Development Strategy This shall be adopted with a view to locating distribution transformers in their optimal service outlets and providing options for additional service extension. Figures 5 and 6 are illustrations of strategies to be adopted for high-and low-density service areas. In general, adjacent service areas are served from different transformer banks in order to provide for transfer to adjacent circuits in the event of transformer outages.

FIGURE 5: RECTANGULAR DEVELOPMENT STRATEGY FOR HIGH-DENSITY AREAS In service areas with high-load density, the adjacent substations are developed to provide for adequate load-transfer capability and service continuity. Sufficient circuit ties must be available to support the loss of large transformer unit. The 1-2-4-8-12 feeder method is especially desirable.

FIGURE 6 : RECTANGULAR DEVELOPMENT STRATEGY FOR LOW-DENSITY AREAS In low-load density areas, where the adjacent substations are not adequately developed and circuit ties are not available due to excessive distances between substations , the 1-2-4-6-8-12 circuit-developing substation scheme is more suitable as shown in figure 6.

6 33/11KV Injection Substations The basic philosophy behind the design of the injection substations is hinged on the following criteria:  Sufficient capacity for the supply of the entire city loads  Short primary feeder lengths to avoid unnecessary voltage drops  High network flexibility to forestall unnecessary power outage in the event of failures  Near-unity Reliability and availability to ensure continuity of supply at all times as well as adequate maintainability provisions  Effective substation location to establish optimal service load-centres

7 Bulk Power Supply It may be obvious that there is the need to locate a 132KV bulkpower supply source within the proximity of the development areas because of the load requirements and the location of the presently available sources. Power system stability cannot be guaranteed from 33KV sources that may be close to the development area for loads beyond 30MVA capacity, using PHCN present configurations. The source and substation location shall be guided by:  Proximity of available 132KV source  Optimal load-centre location for the entire nine districts of the development area  Most economic feeder-route that will not interfere with the city master-plan  Provisions for future developments in the entire development area

LOAD ALLOCATION FOR L.V. CUSTOMER FEEDER UNITS Load allocation per plot is dependent on the size and type of housing units in a plot. For houses that qualify for single-phase supply, such as medium density townhouses, the expectation is that load demand can range between 15A – 45A (or 3.3KVA – 9.9KVA) and so an average of about 7KVA is allocated to each of these. Houses designed to take loads of 12KW (15KVA) and above are normally supplied on three phase. Allowance is made for load range of 15 – 25KVA baring load and diversity factors. These include houses low density and medium density areas. Commercial establishments and as are found in blocks of flats for mixed use in high density areas institutions like secondary schools and health-care centres are given allocations suitable for 50A wholecurrent MD meters. This places the demand on a minimum of 36KVA load allocation.

LOAD ALLOCATION FOR L.V. CUSTOMER FEEDER UNITS Light Industrial locations, large commercial areas, large institutions, hospitals e.t.c. are expected to take minimum of 0.0279KVA allocation per m2 by our original estimate. Therefore, a transformer of 1,500KVA shall be taken for an industrial land area of 53,763.4m2. BASED ON THE OUTLINED LOAD REQUIREMENTS, THE TYPICAL CITY LAND-USE DISTRIBUTION MAP IS DIVIDED INTO VARIOUS RINGED LOOPS FOR EFFECTIVE SUPPLY AS INDICATED IN FIGURE 7 BELOW. THE LOOPS ARE ALSO COMBINED TO ESTABLISH THE INJECTION SUBSTATION REQUIREMENTS AS SHOWN FIGURE 8.

FIG. 7 PARTITIONING OF CITY INTO ELECTRICAL LOOP NETWORK

2 x 15MVA,33/11KV

33KV LINE SUBSTATION

2 x 15MVA,33/11KV

2 x 7.5MVA,33/11KV

2 x 15MVA,33/11KV

FIG. 8 33KV PRIMARY NETWORK INTER-CONNECTIONS ON MODIFIED RING-LOOPS

The perceived network configuration is shown in figure 9 The substation locations are made to take care of interconnections to all looped ring-circuits in such a way that power quality is not compromised.

The substations are expected to be custom-built, complete with stateof-the-art SCADA facilities such that each field components and activities can be coordinated from a remote central control via computerized detection schemes and ultramodern communication facilities. By this arrangement, the status of field equipment can be monitored and controlled from a remote base station. The inter-connections are such that the field equipment in each of the loops are supervised and controlled in real time at the substation level with the information gathered from the intelligent field ringmain-units or pole-mounted RTUs and also translated to the remote control centre for information or active control. Real-time protection and control is also achieved through the coordinated substation’s computerized control and monitoring systems.

11KV RING NETWORK As indicated earlier in this report, the 11KV Ring network shall be fed from various loops. The loops in this option shall be fed from a dual-network overhead lines using the interface model indicated in fig. 10.

The loops in-feed to the 11KV network is shown in fig. 10.

Based on desires to optimise and economise system configurations using 11KV overhead ring network, the components for the HV network is largely based on overhead materials with accompanying cable interface coupled to transformers substations via intelligent Ring-main-units which is SCADA compliant. A typical 11KV intelligent Ring-main-unit proposed is the modern LUCY type with the automated features. The development of the GEMINIRTU2 has enabled Lucy Switchgear to provide automated solutions for its range of RMU and pole-mounted switchgear. This systems approach has also been possible by the development of a number of ‘building block’ products that allows a secondary automation system to be devised.

The features include: Local control & indication of switch positions (open/close), RS232 Control Port, Compact G2RTU, Padlockable control selector (OFF/LOCAL/REMOTE), RS232 Configuration Port, Plant Interface Connections, The substation outlets to the ring network is shown in the single-line diagram of figure 11 based on the configuration of figure 10.

FIGURE 11: 11KV RING CIRCUIT OUTLETS

11/0.415KV TRANSFORMER LOAD ALLOCATION Based on the concept earlier discussed in previous sections, our distribution transformer configuration and loading shall be based strictly on standard PHCN specifications, optimal load centre and service delivery requirements as well load demand specification of the original plan. Modern practice requires that transformer loads be split to manageable economic level that will be near enough to the customer load-centre in order to enhance effective power quality. In line with this requirements, all loads within a loop shall be reconfigured to accommodate a maximum transformer load of 500KVA. Based on the original planned load configurations, the following design data are used for distribution transformer location: 1.SERVICE AREA LOCATION

Based on the original planned load configurations, the following design data are used for distribution transformer location: 1.SERVICE AREA LOCATION To effectively locate transformers in a service area, the following parameters were utilized in our design calculations: Number of Primary Circuits Total Tract Area Load Density (MVA/Km2) Length of Primary Circuit (Km) Length of Tee-offs (Km) Unit Distribution Transformer Area

Example of a typical design data for two selected loops is tabulated below: LOAD LENGTH LENGTH UNIT DIST MV NO OF TRACT LOOP PRIMARY AREA DENSITY OF PRY. OF TEE- TRF. AREA CCTS. (Km2) (MVA/Km2) CCT. (Km) OFFS (Km) (Km2) NO L-1-01 2 0.5762 7.7751 3.50 0.7596 0.0640 L-1-02 2 0.2756 14.1509 2.42 0.44 0.0344

2. LOAD CENTRE IDENTIFICATION Optimal load centres were determined for the injection substations as well as distribution substations based on the following parameters: Optimum Distance for minimum losses Distances of Load from Source Loads of Service Area

LT load positions on x-and y-axis LT load distances on x-and y-axis Detailed data are given on table 2 SUPPLY SOURCE

CONNECTED LOOP

AVG. DISTANCE FROM SOURCE (Km)

LOADS OF SERVICE AREAS (MVA)

SUB. 1 (33/11KV)

Loop 1 Loop 2 Loop 3

0.300 0.450 0.555

12.78 3.89 3.54

3.834 1.750 1.965

SUB. 2 (33/11KV)

Loop 1 Loop 2 Loop 3

0.469 0.300 -

14.11 4.27 -

6.618 1.281 -

Loop 1 Loop 2 Loop 3

0.750 0.302

3.87 18.54

2.902 5.601

Loop 1 Loop 2 Loop 3

0.750 0.525

10.79 3.75

8.093 1.969

SUB. 3 (33/11KV)

SUB. 4 (33/11KV)

LT LOAD POSITIONS (Km)

RESPECTIVE LT LOAD DISTANCES (Km)

OPTIMUM SUBSTATION DISTANCES (Km)

3. TRANSFORMER ALLOCATION DETAILS To appropriately allocate the number of transformers that will adequately serve the loops, the total loop loads and load densities are shared among several transformers of standard commercial ratings, to a maximum of 500KVA. The typical allocation details for three loops are shown on table 3 MV LOOP NO L-1-01 L-1-02 L-1-03

TOTAL DEMAND (MVA) 4.48 3.90 4.76

TRACT AREA (Km2) 0.5762 0.2756 0.1917

LOAD TRANSF. DENSITY RATINGS (MVA/Km2) (KVA) 7.7751 9x500 14.1509 8x500 24.8305 9x500, 1x300

NUMBER REQ’D. 9 8 10

UNIT DIST TRF. AREA (Km2) 0.0640 0.0344 0.01917

SYSTEM LOADING AND INSTALLED CAPACITY Based on the final parcellation drawings, the installed capacity on the given load and substation outlets are evaluated as in the table below for two Substations: SUBST. NO

SUB. 1

SUB. 2

TOTAL LOOP NO INSTALLED (MVA) L-1-01 2 x 15 =30 L-1-02 2 x 15 =30 L-1-03 2 x 15 =30 L-1-05 2 x 7.5 =15 L-1-06 L-2-03 L-3-07 L-1-04 L-1-07 L-1-08 L-1-09 L-2-01 L-2-07

LOAD DEMAND (MVA) 4.48 3.90 4.76 5.00 5.00 3.89 3.75

TRANSF. RATINGS (KVA) 9x500 8x500 9x500, 1x300 10x500 10x500 5x500 7x500, 1x300

4.35 2.5 4.00 4.00 4.27 4.00

8x500,2x200 5x500 8x500 8x500 7x500, 1x200 8x500

% LOADING

REMARKS

103%

These estimates are based on installed capacity

77.1%

Load factors of 60-80% and diversity factor of 85% shall be applied

PRACTICAL INTELLIGENT RING-MAIN-UNITS

OVER-HEAD RING CONTROL FACILITIES

G2-RTU for ground mounted applications include:  Controls and monitoring for up to 4 motorised functions (ring switches or circuit breakers) 

Inbuilt earth and phase fault passage detection



Supports up to 4 external fault passage indicators



Current monitoring from analogue inputs and voltage sensing Embedded automation sequences such as Autochangeover or Auto-sectionalising

 

RTU mounting options include wall, floor or switchgear

G2-RTU for pole mounted applications include:  Inbuilt low power actuator to drive air break switch disconnectors (RAPIER RX-RC) 

Supports retrofit fault passage indicator/current sensor (RAPIER RX-RC)



Voltage sensing (RAPIER GX-RC/S)



Sectionaliser functionality (RAPIER GX-RC/S)



Pole mounted RTU

The benefits to the network operator of implementing a programme of remote control and automation on their secondary distribution networks are significant and include:  Reduced time in diagnosing system faults, locate and isolate the faulty section of network 

Faster response time reconfiguration resulting in customer minutes lost (CML)



Optimisation of asset management through the implementation of customized automation schemes



and network a reduction in

Reduced operational costs associated with routine network switching Increased operator safety

SINGLE-LINE DIAGRAM OF A TYPICAL DISTRIBUTION SUBSTATION

SUMMARY OF LOAD ALLOCATION Feeder Unit Load Allocation is based on the fact that the outgoing units from a normal conventional feeder-pillar do not exceed four (4), operating on a regime of N + 1 concept with three units active and the fourth as a spare. As earlier proposed, the distribution transformer shall be limited to 500KVA maximum per distribution substation and the number of transformers per loop have already been determined in our preliminary designs. Therefore the transformer unit loads per substation relative to capacity are given as follows: 500KVA 300KVA 200KVA 100KVA

= = = =

500/3 300/3 200/2 100/2

= = = =

167KVA per unit 100KVA per unit 100KVA per unit 50KVA per unit

SUMMARY OF LOAD ALLOCATION Load density has already been taken into consideration in our earlier determination of number of transformers per ring-loop. What we set out to do in this detailed design is to adequately distribute these transformers for effective utilization by the consumers. The first step was to adequately locate the optimum load centres for each of the transformers so that we can effectively radiate the L.V. feeders. All allocations and supplies from individual distribution transformers are as indicated in the detailed distribution geographical maps. The allocation detail is tabulated below:

PLOT TYPE

BUILDING TYPE

Low Density

Villas or Bungalows

Medium Density

AVERAGE POWER NUMBER IN A SIZE REQUIREMENT UNIT FEEDER REMARKS (m2) (KVA) 80% Load/ Diversity factor 75% Diversity

600

15 – 25KVA

8 – 14

Townhouses/R ow houses

250

3.3 – 10KVA

21 – 27

Medium Density

Flats/apartment buildings

1,000

25 – 30KVA

6–8

“

High Density/ Mixed use

Commercial Large Flats

750 750

36 – 50KVA 25 – 35KVA

3–6 5–8

80% / 75% Diversity

Varied

0.0279KVA/m2

Dedicated

65 – 100%

Light Industry/ Industrial Hospitals, and Institutions

- 08028451593, 08054143288, 08030723801