6 PLANNING FRAMEWORK
6.4 Energex Distribution Network Planning – Assessing System Limitations
6.4.1 Overview of Methodology to Assess Limitations
The following methodology shown in Figure 41 is used to identify network limitations and determine future network development.
Figure 41 – Planning Process
Following the detailed assessment of emerging network limitations, including non-network assessments and proposals, all existing projects are reviewed. Network and non-network options are considered for addressing prevailing network limitations. These recommendations then become candidate projects for inclusion in the Energex Program of Work (PoW), and are allocated with a risk score based on the Energex network risk based assessment framework for prioritisation purposes. The PoW also undergoes ongoing assessment to determine if targeted area demand management activities can defer or remove the need for particular projects or groups of projects.
Load Forecast
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Remaining projects form the organisation’s PoW for the next five years. Detailed planning is also done for each PoW project to complete a RIT-D submission if required, and obtain project approvals for construction and implementation.
6.4.2 Bulk and Zone Substation Analysis Methodology
Energex uses a software tool to assess emerging capacity limitations for all bulk supply and zone substations taking into account information such as non-network, manual load transfers, remote and automated transfers, circuit breaker/secondary system ratings, generator support and reference to the current security standards. All reviews are performed annually with comprehensive results included in Volume 2 of the DAPR. All assessments are evaluated based on the current network security standards which are also included in Volume 2. All calculations are based on the latest load forecasts which align with the forecast information provided in Section 5.3.1.
6.4.3 Transmission Feeder Analysis Methodology
Based on the forecasting methodology described in Section 5.3.2 using the simulation tool, load flow studies are performed to identify system limitations on the transmission network under system normal or contingency conditions.
Contingency analysis is performed to identify all overloaded feeders for all credible contingency events. Contingency transfers are not included in this automated model, but are considered in subsequent analysis. The load flow results are then exported to Energex’s analysis tools and reporting systems.
The Energex 110 kV Feeder Analysis Tool is used to review and analyse load flow results.
The tool uses additional data which is not contained in the models itself. This includes information such as non-network alternatives, load transfer capacities (Manual, Remote and Automatic), circuit breaker/secondary system ratings, generator support and reference to the current security standards. The tool is also used in conjunction with other supporting tools to assist with the upload process and provides another level of error checking capability.
The outcome of the analysis would potentially trigger the creation of a new strategic project which indirectly may or may not trigger an update of the forecast and a re-run of the load flow models.
6.4.4 Sub-transmission Feeder Analysis Methodology
Based on the forecasting methodology described in Section 5.3.3 using the simulation tool, load flow studies are performed to identify limitations on the sub-transmission feeder network under system normal or contingency conditions.
Contingency analysis is performed to identify all overloaded feeders for all credible contingency events. Contingency transfers are not included in this automated model, but are considered in subsequent analysis. The load flow results are then exported to Energex’s analysis tools and reporting systems.
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The Energex 33 kV Feeder Analysis Tool is used to review and analyse the load flow results.
The tool uses additional data which is not contained in the models. This includes information such as non-network alternatives, load transfer capacities (Manual, Remote and Automatic), circuit breaker/secondary system ratings, generator support and reference to the current security standards.
The outcome of the analysis would potentially trigger the creation of new strategic projects which indirectly may or may not trigger an update of the forecast and a re-run of the load flow models.
6.4.5 Distribution Feeder Analysis Methodology
11 kV feeder analysis is performed to assess feeder loads relative to NCC to establish feeder utilisation. The Target Maximum Utilisation (TMU) of each feeder takes into account the ability of generally transferring loads from four feeders into three feeders with some use of mobile generation to restore all loads in the event of a fault on the 11 kV network. This is to allow for operational flexibility and load transfers to restore load during a contingency event. The TMU will vary for feeders that are a dedicated customer supply or CBD meshed or radial networks.
Analysis is conducted on feeder tie points to determine the feeder capacity to support loads of adjacent feeders during contingency situations. This involves determining the maximum transfer capacity using a block load at an open tie point with the following constraint categories:
The voltage drop at the tie point being analysed is no greater than 7%;
The voltage drop anywhere on the 11 kV feeder is less than 8%; and
The thermal limit of the conductors is not exceeded.
The contingencies can be categorised into two broad areas:
Ties to support the loss of an adjacent feeder (intra and inter substation transfers);
and
Ties to support sub transmission or substation capacity (inter substation transfers only) Refer to Sections 6.4.2, 6.4.3, and 6.4.4 for the application of manual load transfers for these asset classes.
Analysis for the first category is incorporated in the Target Maximum Utilisation (TMU) of each feeder. This takes into account the ability of transferring loads from adjacent feeders.
Analysis for the second category identifies limits transferring between substations on feeders, consistent with the COS standard.
6.4.6 Fault Level Analysis Methodology
Energex performs fault level analysis for switchgear at all 132 kV, 110 kV, 33 kV and 11 kV buses as well as 33 kV and 11 kV feeders. Both 3-phase and 1-phase to ground faults are
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simulated in the studies and the worst case is identified in accordance with AS 3851, (calculation of short-circuit currents in three-phase systems).
The network model used is based on a system normal configuration. This means all normally open feeders and transformers remain on standby.
All short circuit simulation results are stored in a database which is then validated and analysed. For meshed networks, additional analysis is carried out to identify the fault current contribution of individual circuits hence identifying the current which a breaker is subjected to under a fault condition. Equipment having a rated short circuit withstand below the observed fault level are then identified.
For 33 kV and 11 kV feeders, the analysis first identifies those feeders with fault current exceeding any conductor’s one second current carrying capacity. Additional analysis is then carried out on these feeders using protection setting data to determine the actual fault clearing time. Conductors having a fault current carrying capacity below the observed fault level are then identified.
Fault level studies are carried out based on the following assumptions:
Major network connected generators are assumed to be in operation;
All transformers are fixed at nominal tap; and
All loads and capacitors are switched out of service.
For the 11 kV feeder fault analysis, the additional assumptions are as follows:
The load flow analysis assumes only one source at the substation. Standby generation and solar PV fault level contributions are ignored (due to the characteristics of solar PV inverters);
Individual fault levels are calculated for feeders with significant co-generation; and
Fault levels are only calculated at nodes within the model with results showing individual line segments contribution to the fault.
With the proliferation of embedded generation and solar PV installations on the 11 kV and LV networks, investigations have commenced to identify appropriate software that will be able to model the fault level contributions to the network from these devices. The investigation also includes the identification of protection schemes to limit the fault levels from these devices.
The fault levels are calculated in accordance with the Australian Standard AS3851, except for a small variation in that a voltage factor (c) of 1.1 is used for all voltage levels.
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