Generator Load Management: Reviewing Load Add/Shed and Start/Stop Sequences
Generator load management coordinates which loads receive power and which generators remain connected to supply them. Reviewing the sequence means checking both decisions against the plant’s actual state, the next load change, and the required recovery conditions.
A screen says another generator is running. Can the waiting motor group start? That question exposes the gap between a status label and usable capacity. An ambiguous sequence leaves the project team guessing about what happens between an engine start and permission to add load.
This guide gives buyers, facility engineers and integrators a document-review framework for an islanded multi-generator system. Use it alongside the wider generator paralleling specification.
Key Takeaways
- Separate load add/shed decisions from generator start/stop decisions.
- Confirm the rating basis and connected-breaker state before counting capacity.
- Describe recovery and anti-cycling conditions alongside shedding triggers.
- Request time-stamped command, feedback and power records for each claimed sequence.
What Generator Load Management Must Coordinate
Separate load admission from generator commitment
Load admission decides which load blocks may draw power. Generator commitment decides which machines supply the bus. Load sharing divides demand among the connected machines. Give each function an owner, a command and a way to confirm the result.
| Function | Decision to document | Feedback to identify |
|---|---|---|
| Load add | Permit a defined block to connect | Switching state and resulting demand |
| Load shed | Disconnect a selected block | Device response and demand reduction |
| Start another set | Bring additional generation onto the bus | Readiness, breaker state and loading |
| Stop a set | Remove capacity after evaluating the remaining plant | Unloading, breaker opening and stopped state |
Cummins separates load control from load-demand capacity control in sections 5.4–5.5 of its June 2014 application manual. Use those functional explanations within the project’s approved operating boundary. Cummins T-016
Define the operating boundary and capacity basis
State whether the sequence covers initial islanded pickup, stable operation or recovery after a trip. Identify the load blocks, generator duty ratings, site limits and controller versions used in the review. A percentage needs a named denominator and units. Capacity calculations also need a rule for excluding unavailable or disconnected machines.
Illustrative scenario A. A motor group requests permission while a second generator starts. The reviewer sees an engine-running indication, but the sequence has not identified the generator-breaker feedback used to count that machine. The open question is when its capacity becomes available to the load-admission function.
For a proposed ZC Power supply, send the load schedule and sequence questions through the contact page and request confirmation of the offered scope. Contact ZC Power
Review Load Add Priorities Before the First Pickup
Match every priority to a device and load block
A priority label needs a physical destination. Record the feeder, transfer switch, contactor or load-control interface associated with each block. Identify who operates it and the feedback that confirms its state. A priority schedule becomes reviewable when the sequence names what actually receives or loses power.
Review initial pickup against a short checklist.
- Name the loads held off until the first eligible source connects.
- Identify the condition that permits each later block.
- State whether admission uses connected-generator count, available capacity or both.
- Define the handover if startup and stable operation use different methods.
Ask what happens to a waiting request during that handover. Its priority and permission need a defined state in both modes.
Count-based admission requires special attention with unequal machines. In scenario A, suppose the available machines have agreed capacity bases of 300 kW and 900 kW. A rule saying that one generator permits the motor group leaves two different supply conditions under the same label. Ask which machine combinations are eligible and what evidence supports the proposed block in each condition.
Separate command, permission, feedback and timeout
A request expresses demand for a load. Permission indicates that the controller’s stated conditions have been met. The switching command and observed device response are later events. A useful sequence describes all four, including what happens if the requested device fails to respond.
Ask whether a permission remains valid after admission. DEIF’s legacy AGC option manual describes an available-power relay that can drop out after the connected load consumes the spare capacity. That implementation needs a holding circuit. The review question is how the selected design maintains and resets admission after pickup. DEIF AGC power-management manual, section 7.3
Check starting duty and readiness
Static spare kW alone does not establish whether a motor step meets the project’s voltage and frequency criteria. Request the load’s starting method, expected demand and model-specific load-acceptance evidence. Include kVA, current and power-factor considerations where relevant to the selected equipment.
The admission decision also needs an observation period for loads that ramp after connection. If another priority is admitted while the previous block is still increasing demand, the measured headroom may describe an incomplete step. Ask how the sequence recognizes that condition before releasing the next block.
For scenario A, close the document question by identifying the required connected state and the permitted motor-start condition. Review the source’s connection prerequisites separately. Synchronization requirements before connection
Review Load Shed Triggers and Remaining Capacity
Identify measured inputs and event inputs
A generator disconnection, a sustained overload and a low-frequency condition can activate different shedding functions. For each one, identify the signal, measurement location, units, threshold basis and delay. Establish which function takes precedence if more than one becomes active during the same disturbance.
The name of a function may conceal its input basis. Rehlko explains that certain systems described as kW shedding actually use current and can respond differently to reactive demand during motor starting. Its examples show the tradeoff between avoiding unwanted inrush-related shedding and responding quickly to a genuine overload. The selected equipment and approved response criteria must determine the settings. Rehlko load-management applications paper, pages 7–9
Keep pickup and shedding orders explicit
Write down both orders. Cummins explains that pickup and shedding priorities can differ and that shed blocks may be smaller than pickup blocks. The designer should identify protected services and the permissible interruption of each controllable group. A simple instruction to reverse the startup order leaves those decisions unanswered.
Check whether the shed block is drawing power when the event occurs. An intermittent machine that is already idle may release little immediate demand when disconnected. Ask how the controller determines whether the action achieved enough relief, when it evaluates another block and what feedback it uses.
Evaluate the credible remaining plant state
Illustrative scenario B. Assume three connected generators have usable capacity bases of 400, 600 and 1,000 kW and supply 1,100 kW. For this arithmetic example, ignore losses and assume unchanged demand. Losing the 1,000 kW machine leaves 1,000 kW of capacity against 1,100 kW of demand, a 100 kW shortfall.
If a selected block actually removes 350 kW, remaining demand becomes 750 kW. Static headroom becomes 250 kW. These assumed values describe capacity accounting. They establish neither an acceptable transient nor a recommended shed block or delay.
Now ask the integrator to evaluate the event with a machine already stopped by demand control. Rehlko warns that a demand-stopped generator may be treated differently from a failed generator by engine-failure shed logic. Request the actual connected-capacity calculation for that state and the corresponding cause-and-effect row.
Assign an approver to that row. During testing, record the assumed block demand, the demand just before shedding and the demand that remains. If the selected block was idle, record that state and check the next action. This comparison exposes the difference between expected relief and the relief actually obtained.
Review Generator Load Management Start and Stop Logic
Allow time for new capacity to become usable
A start request begins a sequence. The proposed logic should show source selection, start delay, readiness, synchronization where required, breaker closure and loading. Name the state at which the capacity calculation includes the added set. While that sequence is incomplete, identify which existing loads remain supplied and which new requests wait.
Ask what happens if the preferred machine is unavailable, fails to start or fails to connect. The sequence should identify the alternative action, any continued load inhibition and the alarm or lockout presented to the operator. Equipment-specific documentation must establish which actions the selected system supports.
Calculate the post-stop state
Evaluate the proposed stop after subtracting the capacity of the machine being removed. Record the expected remaining demand and the required reserve on that new basis. DEIF’s available-power method explicitly accounts for the size of the set to be stopped, helping prevent a stop that immediately recreates a start condition.
Illustrative scenario C. Two connected 750 kW sets supply 650 kW.
With the same simplified assumptions as scenario B, static spare capacity is 1,500 − 650 = 850 kW. Stopping one set leaves 750 − 650 = 100 kW. The much smaller remaining margin requires review against the next load request and the approved reserve requirement.
The present loading looks low. The remaining machine has much less spare capacity after the stop. Ask whether the approved criteria permit this state, including the next credible demand change. Neither the original 850 kW margin nor the later 100 kW margin establishes the machine’s transient response.
Distinguish anti-cycling, unloading and cooldown
Hysteresis separates start and stop conditions. Delays establish how long the relevant condition must persist. Initial stabilization inhibition, minimum run or off times, and cooldown address different parts of the sequence. List the functions actually supported by the selected controller and the party approving their application.
Cummins describes demand-based stopping through unloading, opening the paralleling breaker, cooldown and engine stop. It also describes starting, synchronization, connection and load ramping as demand rises. Ask for the selected design’s ordered states and the behavior if unloading or breaker opening fails.
Check the stop sequence against three questions.
- When does the capacity calculation exclude the departing machine?
- What happens to a waiting load request during unloading?
- Which state applies if the stop is canceled before the breaker opens?
Then compare the start and stop thresholds on the remaining-capacity basis. Check their observation periods through a normal load rise and fall to identify a proposed cycle of repeated starts and stops.
Also identify when demand-based stopping is inhibited to preserve required online capacity. The operating policy, load steps and redundancy requirement belong in the approval record. Copying a manual’s example percentage or delay leaves those project decisions unresolved.
Specify Restoration and Recovery Conditions
Define permission to re-add a shed block
After shedding, the sequence needs a separate restoration decision. State which connected-capacity, loading and stability conditions permit re-admission. Identify any hold period, priority order and required operator acknowledgment. A cleared alarm alone does not describe the capacity available for the next step.
Return to scenario B. The hypothetical shed leaves 750 kW on 1,000 kW of remaining generation. Restoring the entire 350 kW block without another change returns demand to 1,100 kW. A recovery row should therefore evaluate the present plant state, the block to be added and any additional source that has genuinely become available.
Make the re-add order explicit even if it matches pickup priority. Describe how the design avoids releasing multiple queued requests together when capacity returns. Confirm how maintained permissions, reset signals and load-side interlocks interact so the document shows whether a permission can cause an automatic restart.
Define failure handling and return to service
If commanded and observed states disagree, identify the consequence for the affected load or generator. Does the system hold the request, inhibit later additions or require an operator action? Ask the integrator to specify the available behavior and the designer to approve it for the required operating mode.
A generator returning from maintenance needs a defined availability state before the system selects it. The same applies to a shed load whose local process conditions have changed. Include the owner of each permission and the indication used to show that the block can rejoin the sequence.
Keep normal demand-control recovery, recovery from a trip and return from a manual lockout distinguishable in the matrix. This lets reviewers see which actions are automatic and which require acknowledgment. Detailed communication-failure recovery requires its own equipment-specific review, with clear interfaces to these operating states.
Request a Traceable Sequence Review Package
Build the event-to-evidence matrix
For each event, connect the approved requirement to the input, command, observed response and acceptance record. Use the same device names across the one-line diagram, load schedule, controller configuration and test report. Resolve aliases before comparing records from different suppliers.
| Review field | What to request |
|---|---|
| Event and initial state | Load request, generator loss or proposed stop, with connected-source states |
| Load and device | Block identity, expected demand, priority and actual switching interface |
| Decision basis | Measurement location, units, capacity denominator, permissions and inhibit states |
| Command and feedback | Time-stamped output and independent device-state indication |
| Delay or failed action | Timer start/reset conditions, timeout and escalation |
| Recovery | Re-add criteria, order, hold conditions and acknowledgment |
| Acceptance | Approved criterion, observed result, pass/fail or pending disposition, configuration revision and responsible reviewer |
A controller settings screenshot can identify configured thresholds. To assess execution, request a time-aligned trace covering those thresholds, source states, commands and feedback. Ask the commissioning lead whether the recording resolution can distinguish the specified events and whether values from separate devices share a reliable time basis.
For scenario C, the evidence row should connect the 750 kW capacity withdrawal to the actual breaker state and remaining demand. The reviewer can then compare the observed sequence with the approved post-stop criteria. Keep the arithmetic assumptions attached to the row so later reviewers know what was assumed.
Separate factory checks from installed response
Identify which interfaces were simulated and which devices operated physically. A factory test with simulated load feedback supports a different claim from an installed motor-start test. Record untested interfaces, deviations and the party responsible for closing them in the site acceptance plan.
A steady load-bank run can document behavior under its stated test conditions. Assessment of this sequence also needs the relevant load transitions, priority actions, source changes and recovery states. Organize the supporting records within a traceable dossier. Factory acceptance dossier
When comparing a ZC Power offer, submit the one-line diagram, load blocks, generator ratings and proposed controller models. Ask which sequence functions and verification activities are included, who owns external interfaces and which exclusions remain. Discuss the project scope with ZC Power
Generator Load Management FAQ
Is load management the same as load sharing?
Load management decides which loads are admitted and which generators remain connected. Load sharing distributes real and reactive demand among connected machines. Review them together at their interfaces, but give each a separate requirement and evidence record. A correct sharing result alone leaves the load-admission sequence untested.
Does another running generator mean more load can be added?
A running engine still needs the required connection and loading states before its capacity is counted. Check breaker feedback, availability, the controller’s calculation basis and the proposed load step. Static spare power also needs separate review against starting duty and the project’s approved voltage and frequency response criteria.
Should shed loads always return in reverse order?
Restoration order should follow the approved priority and recovery rules. Pickup and shedding orders can differ, and re-addition needs sufficient capacity for the returning block. Ask the designer to identify protected services, delays, interlocks and acknowledgment requirements rather than assuming that reversing the last shed action is acceptable.
What start/stop percentages and delays should I specify?
Specify the evidence and approval responsibility before selecting values. The required inputs include connected capacity, remaining capacity after a stop, load-step demand, equipment limits and the operating policy. Require the integrator to document the denominator, timer conditions and hysteresis for the selected controller, then assess them against project criteria.
Does a load-bank pass prove the load-management sequence?
A load-bank result supports only the conditions and functions actually exercised and recorded. Request evidence of relevant source changes, device commands, feedback, shedding and recovery. Identify simulated interfaces and outstanding site tests. Use the separate guide to define the supporting test scope. Load-bank test scope
Prepare the sequence handoff
Choose one ambiguous event row and resolve its initial state, decision basis, command, feedback and recovery condition before approving the next revision. In scenario A, the practical handoff is the exact permission that allows the waiting motor group to proceed after the added generator becomes usable capacity.
Bring the one-line diagram, load blocks, ratings, controller models and required operating states to the review. Attach open questions and assign an owner for each answer. For a proposed supply, request confirmation of scope and exclusions through the contact page. That package gives the next generator load management review a defined starting point. Contact ZC Power
