Plant Islanding Operation: House Load vs. Site Islanding

Suppose a Japanese specification for a cogeneration or in-house power plant asks for "自立運転" (jiritsu unten) capability, and another document on the same project says "所内単独運転" (shonai tandoku unten). Both describe continued operation on the plant's own generation after separation from the utility grid, but they can mean very different islands.

Plant islanding operation means the generating plant separates from the external grid and keeps supplying a chosen set of loads from its own generators. If only the power plant's own auxiliaries stay energized, English usually calls it house load operation. If the whole factory stays on, it is site (or facility) islanding, and the generation, control, and steam handling needed are quite different.

This guide uses a steam power plant with a synchronous generator as the example. It maps the Japanese terms to English, then covers the power balance, frequency and voltage control, load rejection, excess steam, and resynchronization. It ends with a checklist and questions to send to the Japanese side.

Contents

1. Japanese Terms and Their English Equivalents
2. House Load Operation vs. Site Islanding
3. Frequency and Voltage Control in Island Mode
4. Why Plants Want Islanding Capability
5. Engineering Challenges: Load Rejection, Excess Steam, and Resync
6. Checklist: Reviewing an Islanding Requirement
7. Limits of This Guide
8. FAQ
9. Sources

Japanese Terms and Their English Equivalents

Japanese documents use several overlapping terms. The table gives the closest common English wording, but usage varies by company, so always confirm the meaning against the single-line diagram.

Japanese term	Closest English wording	What it means here
自立運転 (jiritsu unten)	islanding operation, island mode	Running on your own generation after separating from the grid. The term alone does not say which loads are included.
所内単独運転 (shonai tandoku unten)	house load operation (HLO)	The power plant supplies only its own auxiliaries (house load) and stays in operation.
Whole-site islanding (no standard Japanese term; sometimes written as 工場単独運転, kojo tandoku unten)	site islanding, facility islanding	The plant supplies its auxiliaries plus production loads in the factory.
単独運転 (tandoku unten) in grid-protection documents	(unintentional) islanding	A generator unintentionally keeps energizing part of the utility network after the network is disconnected. Protection must prevent this.
所内負荷 (shonai fuka)	house load, station auxiliary load	Feedwater pumps, fuel supply, fans, cooling equipment, and similar loads.
系統 (keito) / 系統連系 (keito renkei)	grid / grid-connected (parallel) operation	The external transmission and distribution network, and running in parallel with it.
解列 (kairetsu) / 並列 (heiretsu)	separation (disconnection) / paralleling	Opening the connection to the grid / connecting back to the same grid.

The key distinction is intentional versus unintentional. A planned island inside your fence is a design feature. An island that leaves a de-energized utility line powered by your generator is a hazard that interconnection rules require you to prevent.

House Load Operation vs. Site Islanding

In normal grid-connected operation, the generator and the utility network are electrically tied together. The plant powers its own pumps and fans, sends the rest to the factory, and may export any surplus. When generation falls short, the site imports from the grid.

The real question is not whether a generator exists, but which breaker opens and which buses and equipment stay energized. Figure 1 uses a fictional example to show two possible boundaries.

Fictional steady-state example with losses ignored: generation 100 kW, house load 10 kW, factory load 50 kW, export 40 kW. ① and ② are labels for this explanation only.

Thick lines are electrical connections; orange arrows show active power (kW) in normal operation. This is not a breaker numbering scheme or switching sequence for a real plant.

In this example, breaker ① is the boundary between the power plant and the factory, and breaker ② is the boundary between the factory and the grid. Opening ① separates the house bus from the factory bus. Opening ② separates the whole site from the grid.

Real plants also have bus sections, standby supplies, transformers, and protection zones. Opening the breaker nearest the generator can cut off loads you meant to keep. An islanding study starts by tracing, load by load, where each one gets its power.

Read Breaker Status and Power Balance Together

With 100 kW generated, 10 kW of house load, and 50 kW of factory load, normal export is 40 kW. The example sets house load at 10% of the 100 kW rating. That figure is an assumption for following the power flow, not a typical station service ratio or minimum stable load.

With ① open and ② closed, only the power plant islands, and generation drops to match the 10 kW house load. The 50 kW factory load is then fed from the grid, provided the grid is healthy, so the two zones have different sources.

If the grid itself is down, the factory gets nothing even with ② closed. A diagram showing house load operation is not a promise that production keeps running, so evaluate "keeping the plant online" and "keeping production running" separately.

With ① closed and ② open, the plant and the factory form one island. Generation must settle at 60 kW (10 kW house load plus 50 kW factory load). If the generator kept producing 100 kW after losing its export path, the 40 kW mismatch would have nowhere to go.

Figure 2 compares the three states side by side.

Steady-state balance for the fictional example in Figure 1. Check the grid connection and the source of each load at the same time.

State	① / ②	Generation and house load	Factory load (50 kW)	Power exchanged with grid
Normal grid-connected	Closed / Closed	Generate 100 kW; house load 10 kW	Fed by the generator	40 kW exported
House load operation (plant only)	Open / Closed	Generate 10 kW; house load 10 kW	Fed from the grid, if the grid is healthy	50 kW imported to the factory side; zero from the island
Site islanding (whole site)	Closed / Open	Generate 60 kW; house load 10 kW	Fed by the generator	Separated at ②; no import or export

This compares end states, not a switching procedure for ① and ②. Losses, load swings, and switching transients need separate study.

In steady state with losses ignored, generation equals consumption inside the island. A real island must also supply transformer and cable losses, and its load changes over time. Matching the average is not enough; the plant needs margin to follow sudden changes.

With more than one generator, you also have to decide how the units share load. The control that holds frequency on a single machine is not simply applied the same way to every unit.

This is where droop and isochronous control come in.

Frequency and Voltage Control in Island Mode

While a generator is synchronized to a large grid, the grid fixes its frequency. Changing turbine input mainly changes the active power sent to the grid. After separation, any mismatch between supply and demand in the island speeds up or slows down the rotating mass, and inertia affects how fast speed changes.

For a synchronous generator, frequency is set by rotational speed and the number of poles. A 4-pole generator produces two electrical cycles per revolution, so at 50 Hz it turns 25 revolutions per second, or 1,500 rpm. At 60 Hz the same generator turns at 1,800 rpm.

Figure 3 shows why a 4-pole rotor produces two cycles per turn.

Schematic showing only the relationship between pole count and speed. Actual winding construction is omitted.

n is the generator speed. Where a gearbox sits between turbine and generator, check the turbine speed separately.

If load drops suddenly during island operation, the prime mover has surplus input, so speed and frequency rise. If load rises, frequency falls until input catches up.

The turbine governor adjusts input, for example through the steam control (governor) valves, to hold the required speed. Steam turbine control must distinguish output control while grid-connected from speed control while islanded, and the design must cover the response and protection at the changeover.

Speed alone does not hold voltage. On a synchronous generator, the automatic voltage regulator (AVR) monitors terminal voltage and adjusts excitation. Reactive power demand and motor starting also affect voltage, so the governor (frequency) and the excitation system (voltage) have to be studied together.

Even when grid-connected, your generator affects bus voltage and reactive power; a large grid does not set your voltage for you entirely. Taking lagging and leading power factor into account, keep two adjustments separate: prime-mover input controls active power, and excitation controls voltage and reactive power.

Why Plants Want Islanding Capability

The main benefit is that a grid-side fault is less likely to trip the whole generating plant. If the plant keeps its house load and stays online, it is ready to export again once the grid recovers, with less effort than a restart from shutdown.

Stopping and restarting a steam plant involves boiler and turbine temperatures, steam conditions, and checks on the auxiliaries. Getting power back does not mean the plant returns to rated output immediately. Staying in operation is valuable because it shortens the time to resume.

If the island also feeds critical factory loads, it helps keep production running or lets processes shift to a safe state. The island does not have to carry every production machine; it can be limited to the processes and auxiliaries you most need. Decide which loss each retained load is meant to avoid.

Decide What Stays On and What Can Be Shed

Loads in a typical review fall into three groups:

• Auxiliaries the generating plant itself needs
• Critical production loads
• General loads that can be stopped

Do not rank loads by name alone; check what happens when each one stops and what it needs to restart. Also decide in advance which loads are shed first when generation falls short.

A total load below the generator rating does not guarantee the island survives. Motor starting current and large load steps cause temporary voltage and frequency dips, so the load list needs swing size, rate of change, and starting conditions, not just steady-state kW.

A brief voltage dip during the transfer can drop out control power supplies and contactors. The generator may keep turning while production equipment still trips. Define the continuity you need, including ride-through of control power, continued auxiliaries, and restart conditions.

Islanding is not the same as a no-break transfer or a black start. Islanding keeps running equipment going after separation, while black start brings a plant up from a shutdown with no external power. For each function, including emergency power, define which situations it covers and the conditions under which it can be used.

Islanding therefore cannot guarantee zero losses from grid faults. The result depends on the fault type, operating point, retained loads, and acceptable transients, so evaluate it together with equipment availability and the recovery plan.

Keep the Island Separate From the Utility Grid

The islanded zone must be properly separated from the external grid. A private generator that unintentionally energizes a de-energized utility line creates a safety hazard and complicates grid restoration. Treat a deliberate island inside the site and an unintentional island left on the utility side as two different things.

Design protective relays, separation points, and transfer conditions to the plant's interconnection requirements. In Japan, the grid interconnection technical requirements of TEPCO Power Grid, for example, set different measures by voltage class and by whether power flows back to the grid. In the US, IEEE 1547 covers interconnection of distributed energy resources and requires them not to keep energizing the utility system in an unintentional island, while IEEE 1547.4 is a separate guide for intentional islands (currently inactive-reserved); confirm with the local utility and a qualified protection engineer which rules apply to your site.

Breakers ① and ② in Figure 1 are not a protection design. Verification links grid fault detection, confirmation of separation, stable control of the island, and holding the target loads. Actual switching follows approved procedures and protection coordination, never a workaround that disables required detection or protection.

Engineering Challenges: Load Rejection, Excess Steam, and Resync

Islanding needs more than a way to change output. Load shedding, voltage control, protection, and steam handling all have to work together. More equipment means more design, testing, and maintenance, so weigh the value of staying online against that burden.

Load Rejection and the Move to Low Load

When the plant is cut off from the grid while exporting, the generator sees a sudden load drop. In the house-load case of Figure 1, output has to fall from 100 kW to 10 kW. Beyond the final balance, the plant must limit overspeed and steam pressure rise during the transition.

Reducing steam flow to the turbine does not necessarily reduce the boiler's steam generation at the same rate. Heat stored in the furnace and heat-transfer surfaces, along with fuel supply and combustion response, slows it down. The mechanism that absorbs the gap between steam produced and steam used must be studied from both the boiler side and the turbine side.

Whether the plant can run stably at low load also depends on the equipment. Check combustion stability, steam temperature, the turbine's allowable operating range, and condensate and cooling conditions. A 10% house load does not mean every plant can run continuously at 10%.

When load increases, the plant needs margin until generation catches up. If demand exceeds available output, frequency keeps falling unless loads are shed. To protect critical loads, plan staged load pickup and a load-shedding priority.

Where the Excess Steam Goes

When turbine input drops, excess steam goes through a path that bypasses the turbine or a designed steam vent path. The method depends on the boiler, the steam users, and the condenser arrangement. Safety valves protect against overpressure; they are not a control device for absorbing normal load mismatches.

In a condensing plant, the turbine bypass valve routes steam around the turbine and reduces its pressure and temperature before it reaches the condenser. Valve flow capacity is not enough on its own: the receiving side must also be able to take the steam and its heat.

Simplified condensing steam plant. Reheat, extraction, and most valves are omitted. The bypass (pressure reduction and desuperheating) must match what the condenser can accept.

Concept diagram, not a construction drawing. Check condenser heat load, cooling, vacuum, feedwater, and continued operation of auxiliaries.

Steam sent to the bypass is at high pressure and temperature. If spray water flow or pressure, or cooling capacity, falls short, the receiving side cannot stay within its limits.

The condenser must handle the heat load and hold vacuum, and the condensate and feedwater systems must handle the flow while maintaining level and feedwater supply.

In plants that also supply process steam to the factory, the need to cut electrical output may not match steam demand. Depending on whether the turbine is an extraction or back-pressure type, check the steam balance as well as the electrical balance.

The basic Rankine cycle is a useful frame for reading this.

A Hitachi technical paper also explains that the required bypass capacity depends on the purpose of the system and on fuel response. There is no universal "X% of rated flow is enough." Size the steam handling from the size and duration of the mismatch and the boiler's response.

More Control Integration, Testing, and Maintenance

The transfer to island mode coordinates several functions:

• Detection of grid separation
• Turbine speed control
• Generator excitation
• Boiler load control
• Load shedding

Changing one setting does not create this function. The design must include each device's response and the protective action if the island cannot be held.

If a distributed control system (DCS) handles monitoring and control, check the command and the actual equipment state separately. Operators need breaker open/closed status, frequency, voltage, output, and steam pressure to judge the state.

Issuing a command does not prove the transfer succeeded.

Testing confirms the operating point at separation, load swings, auxiliary continuity, and protection coordination. Tests on real equipment are planned with the plant owner, the manufacturers, and other parties involved. Removing key protection for a test is never treated as normal practice.

Added valves, actuators, instruments, and ride-through power supplies all need maintenance. If actuators run on instrument air, check that the air supply continues too.

An electrical island alone is not enough if utilities needed for control and actuation are lost.

Resynchronizing After the Grid Returns

When external power returns, the island cannot simply be connected back. Voltage, frequency, phase angle, and other conditions on both sides must be checked, and the permissive conditions met, before paralleling. The time the breaker takes to actually close also matters to the closing decision.

Concept diagram of two voltage phasors. It does not show allowable angles or closing settings.

Reconnection follows the applicable interconnection requirements and approved procedures. Never connect without protection and synchronism checks.

After closing, the generator returns to load sharing and to the control mode used when grid-connected. Treat the transfer into island mode and the stable return to the grid as one continuous operating sequence.

Assign roles for recovery: who confirms that the grid side is restored, in what state closing is permitted, and what gets recorded. After any plant modification or added load, recheck that the assumptions for islanding and reconnection still hold.

Checklist: Reviewing an Islanding Requirement

Use this list when a Japanese specification, proposal, or test plan mentions 自立運転 or 所内単独運転.

• Island boundary: which breaker opens, shown on the single-line diagram
• Island scope: house load only, or house load plus named factory loads
• Load list: steady-state kW, largest motor start, load steps, and shedding priority
• Minimum stable load of the boiler and turbine, and how long it can be held
• Load rejection: expected overspeed and steam pressure rise, and how they are limited
• Excess steam path: bypass or vent capacity, spray water, condenser and feedwater limits
• Process steam balance (for cogeneration with extraction or back-pressure turbines)
• Governor mode in island (isochronous or droop) and load sharing between units
• AVR and excitation response to motor starting and reactive load
• Ride-through of control power, contactors, and instrument air
• Anti-islanding protection toward the utility and the applicable interconnection rules
• Resynchronization: sync check, permissives, breaker closing time, who authorizes
• Test plan: operating point, witnesses, and confirmation that no required protection is bypassed

Questions to Send to the Japanese Side

• In this document, does 自立運転 mean house load operation (所内単独運転) or islanding of the whole site? Which breaker defines the boundary?
• Which loads are included in the island, and what is their total kW and largest motor start?
• What house load and minimum stable load have you assumed, and on what basis?
• How is excess steam handled after load rejection, and what bypass capacity is provided?
• Which governor and AVR modes are used in island operation and on return to the grid?
• Has the transfer been tested on a comparable plant? What were the test conditions?
• Is black start capability included, or only islanding from a running state?

If you need to ask by email, you can adapt the short inquiry below.

Subject: Clarification of islanding scope (自立運転 / 所内単独運転)

Dear [Name],

Thank you for the specification for [project]. Before we finalize our review, could you clarify the islanding requirement?

1. Is the island limited to the plant's house load, or does it include factory loads? Please mark the separation breaker on the single-line diagram.
2. Please share the load list for island operation, including the largest motor start.
3. How is excess steam handled after load rejection (bypass capacity, condenser limits)?
4. Is black start capability within your scope?

Best regards,
[Your name]

Limits of This Guide

• The 100/10/50 kW figures are fictional. Real house load and minimum stable load differ by plant; confirm them with the plant designer or manufacturer.
• The example is a steam plant with a synchronous generator. Gas turbines, engines, and inverter-based generation behave differently.
• Japanese terms are not standardized across companies. Always confirm the meaning against the single-line diagram.
• Protection, interconnection, and switching requirements depend on the local utility and codes. This article is not a procedure; decisions belong to qualified protection engineers, the utility, and the equipment manufacturers.

FAQ

Is Islanding the Same as House Load Operation?

Not always. House load operation (所内単独運転) means the power plant stays in operation while supplying only its own auxiliary loads. Islanding can cover a larger area, and in grid-protection documents "islanding" (単独運転) often means an unintentional island that protection must prevent, so state which zone is separated and which keeps running.

If We Have a Generator, Will the Factory Keep Running During a Blackout?

Only if the wiring can feed those loads and the plant has the control and protection to transfer to island mode. A design that supplies only the house load does not keep production running. Check the required loads, capacity, motor starting, and tolerance to voltage dips.

Is House Load Always 10% of Rated Output?

No. The 10% in this article is an assumption for the example.

Actual house load depends on the auxiliaries and the operating point, and minimum stable load is set for each plant. A site island must also cover factory loads and losses.

What Sets the Frequency in Island Mode?

For a synchronous generator, rotational speed and the number of poles. The governor adjusts prime-mover input to hold speed as load changes. Holding voltage needs the excitation system as well, so frequency and voltage are controlled separately.

Does Islanding Capability Mean the Plant Can Black Start?

No, they are different functions. Islanding keeps a running plant going after separating from the grid. Starting from a shutdown with no external power requires a separate design, including power for the auxiliaries needed to start.

Sources

• Steam conditioning and condenser acceptance (Ch. 7), condensate and feedwater (Ch. 18): Emerson, "Control Valve Sourcebook — Pulp & Paper" (2011)
• House load operation definition (glossary p. 7, "所内単独運転"): NEDO, interim evaluation report on the coal gasification fuel cell combined cycle demonstration project (January 2021, Japanese)
• Turbine bypass: absorbing load mismatch and sizing: Hitachi Hyoron, "Planning and Operating Results of the Turbine Bypass Startup System" (June 1990, Japanese)
• Intentional island systems: IEEE Std 1547.4-2011, "Guide for Design, Operation, and Integration of Distributed Resource Island Systems with Electric Power Systems" (inactive-reserved since 2022; a related project, P1547.4, is active)
• Interconnection of distributed energy resources, including unintentional islanding (Clause 8.1): IEEE Std 1547-2018
• Safety valves for overpressure protection: Spirax Sarco, "Safety Valves"
• Voltage and frequency dips during motor starting: STAMFORD AvK, "AGN 090 — Motor Starting Fundamentals"
• AVR voltage sensing and excitation control: STAMFORD AvK, "Automatic Voltage Regulators"
• Grid interconnection requirements in Japan (protection and synchronism check, high-voltage example): TEPCO Power Grid, "Grid Interconnection Technical Requirements" (effective April 1, 2026, Japanese)
• Single unit, grid-connected, and isolated bus operation: Woodward, "Speed Droop and Power Generation," Application Note 01302 (1991)
• Synchronizing conditions (pp. 21–22): Woodward, "SPM-D2-10B/PSY5 Synchronizing Unit," manual 37616C