Deferral

DERs can be utilized to defer investment in additional distribution or transmission capacity. In DER-VET, the “overloaded asset” is the piece of equipment whose power limits would be violated without the DERs and load modeled in DER-VET. The N-0 reliability case is implemented explicitly, although readiness to cover N-1 contingencies can be modeled using the user service. The N-0 case is modeled by assuming the DERs and load are at the same location as the overloaded asset, neglecting any power losses in between the two. Along with a time series deferral load profile, the user inputs a deferral load growth rate, which will escalate the load until the modeled DERs are no longer capable of keeping the power in the overloaded asset within bounds. At this point, the deferral has failed and the upgrade is assumed to have happened, but hopefully the DERs pushed this date back long enough to justify their cost. In years after the deferral failure, the DERs are not constrained to meet the deferral objectives and can operate for economic benefit depending on the other services selected.

In addition to the simple deferral case modeled in DER-VET, a connection with the software tool DRIVE has been developed. This connection allows the user to perform detailed distribution analysis in an external tool and export the results directly to DER-VET and vice versa. This enables the quick and easy workflow already used at EPRI to perform multi-model analysis involving detailed DER and grid models respectively.

1. Inputs

1. 1. Planned Load Limit

Tag	Key	Description
Deferral	planned_load_limit	This input sets the maximum power flow through the overloaded asset in in the direction from the transmission system to the loads.

1. 1. Reverse Power Flow Limit

Tag	Key	Description
Deferral	reverse_power_flow_limit	This input sets the maximum power flow through the overloaded asset in in the direction from the loads back toward the transmission system.

1. 1. Deferral Load Growth Rate

Tag	Key	Description
Deferral	growth	The annual non-compounding growth rate of the deferral load input. This input, when positive, will grow the deferral load every year. Eventually, this will result in the local DERs being unable to shave the forward or reverse power flow peaks, triggering the traditional asset upgrade.

1. 1. Deferral Value

Tag	Key	Description
Deferral	price	This is the economic carrying cost (nominal $/year) of the traditional upgrade that is being deferred by the DERs. This value is applied as a benefit in the benefit-cost analysis in every year for which the DERs successfully defer the traditional upgrade.

1. 1. Deferral Value

Tag	Key	Description
Deferral	price	This is the economic carrying cost (nominal $/year) of the traditional upgrade that is being deferred by the DERs. This value is applied as a benefit in the benefit-cost analysis in every year for which the DERs successfully defer the traditional upgrade.

1. Size Optimization

The deferral service is not compatible with size optimization.

1. Optimization Formulation

Deferral is a constraint (pre-dispatch) service, though this is handled differently from other constraint services. The deferral service pre-calculates the power and energy requirements to meet the deferral in every year of the analysis and then finds the first year where the DERs' capabilities do not meet or exceed the deferral requirements. Time series power and state of energy constraints are applied only for years before the deferral fails.

The deferral service time series constraints simply ensure that the DERs collectively discharge/generate enough power to keep the net load through the overloaded asset below the planned load limit and that the DERs and loads collectively charge/consume enough power to keep the net load through the overloaded asset above the reverse power flow limit.

• Reverse Power Flow Limit <= Total Load - ESS Power - Generator Power - Intermittent Generation (note: the reverse power flow limit is usually negative)
• Planned Load Limit >= -Total Load + ESS Power + Generator Power + Intermittent Generation

The deferral service will trigger an optimization year both before and after the deferral fails, which are in addition to user-input opt_years.