The Electrical Distribution Architecture – Part Eight

In the previous Topic “ The Electrical Distribution Architecture – Part One “, I listed the Tasks required for application of Electrical Distribution architecture design process, they were: 

• Assigning of electrical installation characteristics, 
• Assigning of Technological characteristics, 
• Using Architecture assessment criteria, 
• Step (1): Choice of distribution architecture fundamentals, 
• Step (2): choice of architecture details, 
• Step (3): choice of equipment, 
• Recommendations for architecture optimization. 

And I explained the first three tasks: Assigning of electrical installation characteristics, Assigning of Technological characteristics & Using Architecture assessment criteria in the following previous topics:

• The Electrical Distribution Architecture – Part Two

• The Electrical Distribution Architecture – Part Three

• The Electrical Distribution Architecture – Part Four

• The Electrical Distribution Architecture – Part Five

• The Electrical Distribution Architecture – Part Six

• The Electrical Distribution Architecture – Part Seven

Today, I will explain Step (1): Choice of distribution architecture fundamentals as follows.



Forth: Step (1): Choice of distribution architecture fundamentals


The single-line diagram can be broken down into different key parts, which are determined throughout a process in 2 successive steps, which are

• Step (1): Choice of distribution architecture fundamentals, 

• Step (2): choice of architecture details. 

During Step (1): Choice of distribution architecture fundamentals, we make the following choices: 

1. Connection to the utilities network,
2. Configuration of MV circuits, 
3. Number and distribution of transformation substations, 
4. Number of power transformers, 
5. MV back-up generator. 

1- Connection to the utilities network

The main configurations for possible connection to the utilities network with the most probable and usual set of Characteristics are summarized in the following table and fig (1):

Characteristic To Consider	Configuration Of Electrical Distribution Architecture
	LV	MV
		Single-Line	Ring-Main	Duplicate Supply	Duplicate Supply With Double Busbars
Activity	Any	Any	Any	Hi-Tech, Sensitive Office, Health-Care	Any
Site Topology	Single Building	Single Building	Single Building	Single Building	Several Buildings
Power Demand	< 630kva	≤ 1250kva	≤ 2500kva	> 2500kva	> 2500kva
Service Reliability	Minimal	Minimal	Standard	Enhanced	Enhanced
Other connection constraints	Any	Isolated site	Low density urban area	High density urban area	Urban area with utility constraint

Fig (1): MV Connection to the utilities network 

Notes:

• Metering, protection, disconnection devices, located in the delivery substations are not represented on the following diagrams. They are often specific to each utilities company and do not have an influence on the choice of installation architecture. 

• For each connection, one single transformer is shown for simplification purposes, but in the practice, several transformers can be connected. 

2- Configuration of MV circuits

The main configurations for possible connection of MV Circuits with the most probable and usual set of Characteristics are summarized in the following table and fig (2):

Characteristic To Consider			Configuration of Electrical Distribution Architecture ( Configuration of MV circuits )
			Single Feeder	Open Ring 1 MV Substation	Open Ring 2 MV Substations
Activity			Any	Hi-tech, sensitive Office, Health-Care	Any
Site Topology			Single Building	Building With One Level or Several Buildings	Several Buildings
Power Demand			≤ 1250kva	> 2500kva	> 2500kva
Maintainability			Minimal or Standard	Enhanced	Enhanced
	Disturbance sensitivity	Long interruption acceptable		Short interruption acceptable	Short interruption acceptable

Fig (2): Configuration of MV circuits 

Notes:

• The basic configuration is radial single-feeder architecture, with one single transformer. 

• In the case of using several transformers, no ring is realized unless all of the transformers are located in a same substation. 

• Closed-ring configuration is not taken into account. 

3- Number and Distribution of Transformation Substations

The choice Of Electrical Distribution Architecture Configuration based on the number and Distribution of Transformation Substations are summarized in the following table that shows the Characteristics affecting that choice.

Characteristic To Consider	Configuration Of Electrical Distribution Architecture
	1 Substation With N Transformers	N Substations N Transformers (Identical Substations)	N Substations M Transformers (Different Powers)
Building Configuration and Surface Area	< 25000m²	≥ 25000m² (1) building with several floors	≥ 25000m² several buildings
Power Demand	< 2500kVA	≥ 2500kVA	≥ 2500kVA
Load Distribution	Localized loads	Uniform distribution	Medium density

Notes:

• Building Power demand to be compared with standardized transformer power before determining the Number and Distribution of Transformation Substations. 

• The preferred basic configuration comprises one single substation. Certain factors contribute to increasing the number of substations (> 1): 

1. A large surface area (> 25000m²), 
2. The site configuration: several buildings, 
3. Total power > 2500kVA, 
4. Sensitivity to interruption: need for redundancy in the case of a fire. 

4- Number of power transformers

Main characteristics to consider determining the number of transformers:

• Surface of building or site,
• Total power of the installed loads,
• Sensitivity of circuits to power interruptions,
• Sensitivity of circuits to disturbances,
• Installation scalability.

The basic preferred configuration comprises a single transformer supplying the total power of the installed loads. Certain factors contribute to increasing the number of transformers (> 1), preferably of equal power: 

• A high total installed power (> 1250kVA): practical limit of unit power (standardization, ease of replacement, space requirement, etc), 

• A large surface area (> 5000m²): the setting up of several transformers as close as possible to the distributed loads allows the length of LV trunking to be reduced, 

• A need for partial redundancy (down-graded operation possible in the case of a transformer failure) or total redundancy (normal operation ensured in the case a transformer failure), 

• Separating of sensitive and disturbing loads (e.g.: IT, motors) 

5- MV back-up generator

Main characteristics to consider for the implementation of an MV back-up generator:

• Site activity,
• Total power of the installed loads,
• Sensitivity of circuits to power interruptions,
• Availability of the public distribution network.

The preferred basic configuration does not include an MV generator. Certain factors contribute to installing an MV generator:

• Site activity: process with co-generation, optimizing the energy bill, 
• Low availability of the public distribution network. 

Note:

• Installation of a back-up generator can also be carried out at LV level. 

In the next topic, I will explain Step (2): choice of architecture details. So, please keep following.