N-1 Contingency Analysis: How Much Load Can You Serve?

N-1 contingency analysis evaluates system performance after a specified single contingency, commonly the loss of a line, transformer or generator. For a utility considering new load, it helps establish whether the remaining network can carry that demand within the applicable operating limits.

Installed capacity is only the starting information. The assessment must account for existing demand, the equipment that remains connected and the corrective actions permitted after the event. Two large transformers can leave surprisingly little room for another customer when either unit must carry the load alone.

PJM’s September 2, 2026 operating update described maintenance-deferral considerations during a Maximum Generation Alert. It is a timely reminder that equipment availability is part of the operating condition. A capacity assessment needs equally clear assumptions about what is in service when the contingency occurs. PJM September 2 update.

Define what passing the assessment means

Before calculating capacity, establish the applicable criteria. Identify the contingencies to assess, permitted thermal and voltage limits, allowed corrective actions and any restrictions on interrupting supply.

A transmission-planning assessment and a customer’s contractual service commitment have different boundaries. NERC’s TPL-001-5.1 addresses planning performance for applicable Transmission Planners and Planning Coordinators on the Bulk Electric System. It should not be presented as a universal customer-level guarantee of uninterrupted service. NERC TPL-001-5.1.

Terms such as firm capacity need a stated definition. For a particular utility, the capacity credited after an outage may depend on approved emergency ratings and specific transfer arrangements. The customer should understand those conditions before using the number in development plans.

Where a project depends on continuous supply, document the acceptable interruption and recovery behavior separately. A network that reaches an acceptable post-contingency condition may still experience a transfer interruption that matters to the facility.

A worked N-1 contingency analysis example

Consider a hypothetical substation with two identical 75 MVA transformers. Existing demand is 60 MW, and a developer requests another 30 MW.

Assume equal loading in normal operation, a constant aggregate power factor of 0.95 and a permitted switching arrangement that allows the surviving transformer to supply the connected load. For this example, each transformer has an applicable 75 MVA loading limit in both the normal and studied outage condition. No higher emergency rating is credited. Ignore losses and other network constraints for this initial thermal screen.

With the proposed development, total demand becomes 90 MW:

90 MW ÷ 0.95 = approximately 94.7 MVA.

Under normal conditions, each transformer carries approximately 47.4 MVA. Both remain below the assumed 75 MVA limit.

After either transformer is lost, the surviving unit would have to carry the full 94.7 MVA. That exceeds the limit used in the example.

On those assumptions, the transformer-limited increment is 11.25 MW. The requested 30 MW addition cannot be supported after the specified outage without changing the arrangement, accepted operating conditions or infrastructure.

This is a simplified screen, not an approved connection capacity. A full study may identify a lower limit elsewhere. It may also establish a different permissible operating arrangement, but that conclusion needs evidence rather than an assumed overload allowance.

Follow the power through the surviving network

Transformer loading may be the first constraint found, but other assets can set the limit.

Trace the entire remaining supply path. An incoming line, bus section, cable or transfer connection may carry additional current after the outage. In a meshed network, power redistributes across multiple paths, so the governing asset can be some distance from the proposed load.

Review voltage performance at the same time. A thermal solution can leave an unacceptable voltage condition. Transformer tap controls, reactive-power resources and generator reactive limits need to be represented consistently with the study method and time frame.

The assessment should identify which constraint governs each case. A report that lists every overload without explaining the limiting mechanism gives decision-makers little basis for comparing reinforcement options.

Where a solved power-flow case leaves concerns about voltage margin or the disturbance response, scope the additional analysis needed. A converged steady-state solution alone cannot establish acceptable transient behavior.

Put corrective actions on a realistic clock

An operating action can support a capacity conclusion only if it is available, permitted and effective within the required time.

Automatic controls and manual switching have different response times. Access to equipment, communications, interlocks and operating procedures affect what personnel can accomplish. The model should not credit an operator action before it could physically occur.

Distinguish the immediate post-contingency condition from the condition after allowed adjustments. PJM’s planning manual specifies different treatment of controls and adjustments for different tests. Those distinctions illustrate why the study time frame must be explicit. PJM Manual 14B, Section 2.3.

Emergency ratings also need a defined basis and duration. Confirm the applicable asset-owner rating and the action that returns loading to an acceptable level before that duration expires. Do not insert an assumed percentage above nameplate simply to make the case pass.

For a proposed battery or controlled-load response, verify the available power, duration and operating authority. An action that depends on stored energy needs an acceptable starting state of charge and a plan for the period after that energy is exhausted.

Examine what the two supplies share

Two feeders can provide useful redundancy while retaining a shared upstream exposure. Review their connection to buses, transformers and transmission circuits. Physical routing can introduce further common exposure, such as circuits carried on the same structure.

Some shared exposures fall outside a simple one-element-at-a-time screen. Identify them explicitly and assess the relevant common-mode, bus-fault or other events under the applicable planning and customer requirements. Do not assume the N-1 label covers every way multiple connections could be lost.

Protection clearing also determines what remains connected. One initiating fault can lead to the disconnection of several elements. NERC’s planning standard requires contingency analysis to reflect the elements that protection and other automatic controls are expected to remove. NERC TPL-001-5.1, Requirement R3.3.1.

Review planned maintenance conditions separately where relevant. A normal-system assessment does not describe operation with a major asset already unavailable. Likewise, required sequential-contingency assessments need their own event sequence and permitted adjustments.

Give the utility a capacity decision it can use

The final deliverable should connect each proposed load phase to its governing contingency and limiting asset. Record the maximum supported demand, required operating actions and infrastructure that must be available before the next phase proceeds.

Include a case register identifying the study year, loading, generation pattern, network configuration, equipment ratings and treatment of controls. Then explain which conclusions change if a planned upgrade is delayed or an assumed operating action is unavailable.

For the hypothetical substation, the decision is specific: the proposed 30 MW addition exceeds the surviving transformer’s assumed capability. The next assessment can compare reinforcement or other accepted service arrangements against that defined deficiency. It should also show whether the preferred option creates a new limiting condition elsewhere.

PowerTek supports N-1 contingency analysis, power-flow studies and transmission and distribution reinforcement planning. The useful outcome is a documented connection between the load a utility can serve, the conditions under which it can serve it and the investment or operating commitment needed to support growth.