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Appendix C: Optimisation
Optimisation of the System Configuration
(a) Connection/Supply Points (Including Embedded Generation Connections)
| Issue: |
Whether all existing points of supply are required, given the ELB's disclosed quality of supply criteria. |
| Approach: |
Location and supply voltage should be considered as fixed. All points of supply must be tested to determine whether a lower value network would result if the point of supply were eliminated and the load supplied from adjacent points of supply. |
(b) Transmission/Subtransmission Lines
| Issue: |
Whether the number of transmission/subtransmission lines exceeds the number required given the ELB's disclosed quality of supply criteria and allowed future load growth. |
| Approach: |
Location and voltage of both the point of supply and the zone substation should be considered to be fixed. Assess the number of transmission/subtransmission lines in relation to the ELB's disclosed quality of supply criteria and allowed future load growth. Optimise out those that are not required. |
(c) Substations/Zone Substations
| Issue: |
Whether the number and capacity of substations/zone substations exceeds that which is required to meet the ELB's disclosed quality of supply criteria and allowed future load growth. |
| Approach: |
The location of the substation/zone substation should be considered to be fixed. All substations/zone substations must be tested to determine whether a lower value network would result if the substation/zone substation were eliminated and the load supplied from an adjacent substation/zone substation. |
| Issue: |
Customer requirements vary |
| Approach: |
Identify customers with special requirements covered by specific non-standard contracts and optimise the substation/zone substation configuration to satisfy these customers without materially changing the security or customer service standard of other customers. |
(d) Distribution Lines
| Issue: |
Whether the number of distribution lines or feeders exceeds that which is required, given the ELB's quality of supply criteria and allowed future load growth. |
| Approach: |
Assess the number required in relation to the ELB's disclosed quality of supply criteria and allowed future load growth. Optimise out feeder connections and lines that are not required. Additional notional distribution lines will not result from this optimisation. |
| Issue: |
Whether the number of high voltage switches on the distribution network exceeds the number required given disclosed quality of supply criteria |
| Approach: |
Assess the number of switches on the network in relation to the ELB's disclosed quality of supply criteria. Optimise out any switches that are not required |
| Issue: |
Use of very low capacity or less than three phase distribution lines. |
| Approach: |
Where the existing distribution line or a part of it is of less than 3 phase construction, the line must be valued accordingly. Where the distribution line or a part of it is of 3 phase construction it will be valued at the lowest replacement cost of a line that meets the disclosed quality of supply criteria including allowance for future load growth. |
Transpower
| Security Guidelines for Transmission Planning |
| Load (MW) |
Basic Security |
Transmission Circuits |
Busbars |
Transformers |
| Less than 10 |
n |
One circuit |
One bus or bus section |
1 x 3-phase units. |
| (10 to 40, if more than 40km remote and local generation can limit load shed to 25%) |
n |
One circuit |
One bus or bus section |
4 x 1-phase or 1 x 3-phase unit, if backed up from alternative supply point. |
| From 10 to 300 |
n-1 |
Two circuits |
Two busbars or bus sections |
7 x 1-phase units or 2 x 3-phase units.
Firms supply of peak demand using any short term overload capability. |
| More than 300 |
n-2 |
Three circuits on at least two routes |
One redundant bus or bus section, such that supply is not lost after a single contingency while one bus is out of service for maintenance |
7 x 1-phase units or 2 x 3 phase units.
Firms supply of peak demand using any short term overload capacity |
| More than 600 |
Loss of station |
Supply into region should be diversified across more than one major terminal substation. |
Optimisation of Elements in the System
(a) Transmission/Subtransmission Lines and Cables
| Issue: |
Conductor and cable size. |
| Approach: |
Determine the required capacity, being the maximum demand and load cycle the line or cable will be exposed to during the planning period, given the disclosed quality of supply criteria and allowed future load growth.
Optimise down the size of the conductor or cable to the smallest standard size shown in Appendix B that meets the required capacity utilising the short term ratings of the conductors or cables and the disclosed quality of supply as appropriate. |
| Issue: |
Overhead / underground transmission. |
| Approach: |
If a distribution line consists of underground cables these must be valued as overhead lines of the required capacity unless there is specific evidence that the local authority would not, in normal circumstances, grant consent for overhead reticulation, or that a non-standard contract or a legal obligation requiring the installation of underground lines exists. |
| Issue: |
Underground Distribution Trenching |
| Approach: |
If more than one underground cable is laid in a trench only the cost of the additional cable(s) may be valued for those additional cables i.e. the trenching may only be valued for one cable. |
(b) Substations/Zone Substations
| Issue: |
Transmission/subtransmission substation design. |
| Approach: |
Optimise the overall design of the substation (including the need for more than a single bus bar) to the minimum required to meet the disclosed quality of supply criteria and allowed future load growth. |
| Issue: |
Under-utilised equipment is often installed at substations. |
| Approach: |
Optimise out substation equipment not required to meet the disclosed quality of supply criteria and allowed future load growth.
Optimise the size of the equipment used to nearest standard rating to not exceed the allowed future load growth. |
| Issue: |
Land and Buildings. |
| Approach: |
Optimise indoor substations to outdoor where land is available and this will result in a lower replacement value network unless there are clear technical reasons or local authority requirements that prevent this.
Optimise out any unutilised, or under utilised land so that the value of the land allowed reflects only the area of land required to meet the ELB's disclosed quality of supply criteria and allowed future load growth.
Reduce the replacement cost to that of a simple standard modern structure using pre-fabricated or other low cost designs. The size of the optimised design should not exceed that required to meet the essential functionality of the building. |
| Issue: |
Ancillary equipment. |
| Approach: |
Optimise out if not required to meet the ELB's disclosed quality of supply criteria. |
| Issue: |
Fire protection and oil retention facilities. |
| Approach: |
Include unless not required for Modern Equivalent Assets. |
(c) Substations/Zone Substations
| Issue: |
Numbers of Transformers. |
| Approach: |
Reduce the overall replacement cost of the transformers by optimising the number and arrangement of transformers that will meet and not exceed the ELB's disclosed quality of supply criteria. |
| Issue: |
Transformer rating. |
| Approach: |
Determine nominal, forced cooled, cyclic and short-term ratings. When optimising the equipment full use should be made of the forced cooled capacities and short-term ratings of the equipment in order to determine the transformer capacity to meet the ELB's disclosed quality of supply criteria. The transformer size should be optimised down so that it will require 100% of its capacity to meet the allowed future load. |
| Issue: |
Cable or circuit breaker or other equipment constraints. |
| Approach: |
Derate transformers to the lowest rating of any piece of equipment associated with the transformer. |
(d) High Voltage Distribution
| Issue: |
Conductor and cable size. |
| Approach: |
Examine thermal ratings, faults and current levels to determine minimum conductor size for each feeder, given the disclosed quality of supply criteria and allowed future load during the allowed planning period. Optimise down where necessary. |
| Issue: |
Overhead/Underground Distribution. |
| Approach: |
If a distribution line consists of underground cables these must be valued as overhead lines of the required capacity unless there is specific evidence that the local authority would not, in normal circumstances, grant consent for overhead reticulation, or that a non-standard contract or a legal obligation requiring the installation of underground lines exists. |
| Issue: |
Underground Distribution Trenching |
| Approach: |
If more than one underground cable is laid in a trench only the cost of the additional cable(s) may be valued for those additional cables i.e. the trenching may only be valued for one cable. |
| Issue: |
Achievement of satisfactory customer voltage. |
| Approach: |
The optimisation of each feeder must include consideration as to whether the existing customer service standard (in particular the voltage) is being achieved in the most cost-effective manner. A more cost-effective network may result from reducing the conductor size and utilising other means such as voltage regulators, and reactive compensators to maintain the disclosed quality of supply criteria throughout the length of the feeder. |
(e) Voltage Control Devices
| Issue: |
Degree of control.
Manual and on load tap changes.
Line regulators and line drop compensation.
Reactive compensation. |
| Approach: |
Test requirements for all items in the optimised network and optimise out where there is no clear justification for the equipment. |
(f) Distribution Transformers (Pole, Kiosk, Ground Types)
| Issue: |
Transformer rating. |
| Approach: |
Optimise out excess distribution transformer capacity so that the capacity utilisation (ratio of current peak load to total distribution transformer capacity) for the network is not less than 30% unless a lower utilisation is provided for in a specific customer non-standard contract. Transformer capacity optimised out shall be valued at the average DRC per kVA of the ELB's transformer equipment. |
(g) Low Voltage Distribution
| Issue: |
Overhead / underground reticulation. |
| Approach: |
If the low voltage distribution consists of underground cables these must be valued as overhead lines of the required capacity unless there is specific evidence that the local authority would not, in normal circumstances, grant consent for overhead reticulation, or that a non-standard contract or a legal obligation requiring the installation of underground lines exists. |
| Issue: |
Underground Distribution Trenching |
| Approach: |
If more than one underground cable is laid in a trench only the cost of the additional cable(s) may be valued for those additional cables i.e. the trenching may only be valued for one cable. |
| Issue: |
Services and load control equipment. |
| Approach: |
Standardise to minimum requirements to meet disclosed quality of supply criteria. |
| Issue: |
Whether the standard or quantity of low voltage distribution lines and equipment exceeds that which is required given the ELB's disclosed quality of supply criteria and allowed future load growth. |
| Approach: |
Identify any parts of the network where the standard of low voltage distribution is higher than required by the disclosed quality of supply criteria and optimise out excess assets. |
(h) System Control
| Issue: |
Degree of sophistication of SCADA equipment. |
| Approach: |
Determine whether equipment is appropriate on the basis of disclosed quality of supply criteria. |
| Issue: |
Need for load control system and degree of sophistication. |
| Approach: |
Determine whether equipment is appropriate on the basis of disclosed quality of supply criteria. |
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