A factory rooftop is sized, the BOQ is priced, and then the DISCOM reply says the plant may not export. Sometimes the cause is that no net metering route fits the consumer. Sometimes the size is above the cap, or the transformer is full. The EPC then buys a “zero export device”, wires it to the inverters, and assumes the problem is closed.
It is not closed. Zero export is a control design with three numbers you can calculate. Where does the plant measure? How fast does it react? How much energy does it throw away? The device you pick matters less than those three.
Quick answer. Zero export means the plant never pushes power through the DISCOM meter. Build it in two layers. An inverter or site controller reads net power at the incomer and trims output. An ANSI 32 reverse power relay trips the solar feeder if the controller fails. Put the CT at the point of common coupling, set the relay slower than the controller, and size the array from the curtailment curve.
When does a C&I rooftop have to be designed for zero export?
Only when a rule, a connection agreement or a capacity limit says so. There is no single national trigger. We read the CEA connectivity regulations for plants below 33 kV in full. They contain no export limit and no mention of a reverse power relay. The condition comes from state rules and from the DISCOM’s connection agreement.
The table lists only what we could read in the source text. For any other state, ask the DISCOM in writing. The DISCOM approval guide covers the standard net metering route that zero export replaces.
| Situation | What the source says | Source and date |
|---|---|---|
| Consumer chooses a behind-the-meter (BTM) plant instead of net metering, Chhattisgarh | Grid-connected BTM solar is allowed with or without battery. A reverse power relay (RPR) and/or other grid-safety equipment is a must, verified by the licensee. Net metering and grid-connected BTM cannot operate simultaneously. Banking is not permitted for BTM | CSERC order on Petition No. 64 of 2025, paragraph 14, 5 February 2026 |
| Non-exporting plant under the state tariff order, Tamil Nadu | Existing parallel operation plants ran with a reverse power relay. The order converts them to net feed-in, after which the relay is removed. Parallel operation without approval can lead to disconnection on 3 days notice | TANGEDCO salient features of TNERC Order No. 8/2021, 22 October 2021 |
| Residential plant that feeds nothing to the grid and wants a subsidy | Eligible for central financial assistance subject to the state commission’s approval. The DISCOM inspects and checks that the reverse power relay protection works | MNRE guidelines for PM Surya Ghar, paragraph o, non-metered grid connected systems |
| Third-party or leased plant that does not export | Chhattisgarh says cross subsidy surcharge applies if ownership is with a third party or captive status is lost | CSERC order, paragraphs 11 and 12 |
Two points sit under that table.
First, captive status has a test. Rule 3 of the Electricity Rules, 2005, as reproduced in the CSERC order, needs two things. The captive users must hold at least 26 percent ownership. They must also consume at least 51 percent of the annual generation. A non-exporting plant on a third-party contract needs a lawyer’s view before it needs a relay.
Second, zero export is not automatic relief from a full transformer. Our transformer feasibility guide reports that the caps in four states count solar capacity against the transformer rating. We could not confirm that a non-exporting plant is excluded from the count. Ask before you rely on it.
Gulf utilities set their own export rules, and this article cites Indian rules only. Check the utility’s connection requirements, as in our UAE grid guide.
Reverse power relay or inverter export control: which one do you need?
Both, in most designs, because they do different jobs.
An inverter or site controller regulates. It reads net power at the incomer and lowers inverter output so that the net stays at or just above zero. A reverse power relay protects. It watches the same flow and trips a breaker if export persists. Relays use device number 32 in the IEEE C37.2 numbering, the directional power function.
| Feature | Inverter or site controller export limit | Reverse power relay (ANSI 32) |
|---|---|---|
| Job | Regulate output continuously | Trip when regulation has failed |
| Action | Lowers inverter power, plant keeps running | Opens the solar feeder breaker, generation stops |
| Typical response | Seconds, set by the control loop | Set by a time delay, usually longer than the control loop |
| Fails when | Meter link drops, CT is reversed, a setting is wrong | CT is reversed, trip circuit is open, setting is above the real leak |
| Curtailment cost | The curtailed energy in the example below | None until it trips, then the whole plant is off |
| What the DISCOM sees | A control feature | A protection device, often inspected |
The Chhattisgarh order asks for “Reverse Power Relay and/or any other equipment for the grid safety as per CEA regulations”. The DISCOM decides what that means for your plant. A controller alone is regulation, not protection.
The CEA regulations give a design reason for two layers. Regulation 11(7)(b) says a single device failure must not compromise the safety and reliability of the electricity system. If the controller and the relay share one CT, one failure disables both. Give the relay its own protection-class CT and its own trip path.
IEEE 1547-2018 supplies the control concept. Its subclause 4.6.2 covers the capability to limit active power. Interconnection guidance notes it can be applied as an export control for the whole DER system rather than at each inverter. That standard governs US interconnection, and India has not adopted it. We use it for the idea only. It also carries a warning that an export limit can impede other required inverter functions, so state in the design which function wins.
Where do the CTs go?
At the point of common coupling (PCC), on the consumer’s side of the DISCOM meter, measuring the net of all load and all generation. CEA regulation 2(1)(i) defines the interconnection point as the place where injected and drawn electricity can be measured unambiguously. Use that as the model.
Rules we apply when drawing the measurement point:
- Measure the incomer, not the solar feeder. A CT on the solar feeder measures generation. It cannot tell you what leaves the site.
- Do not tap the DISCOM’s metering CTs. Add your own CTs, or a meter, on the consumer side of the incomer. The HT billing CTs belong to the licensee.
- Check what sits upstream and downstream. A second solar feeder or DG feeder joining after the CT is invisible to it.
- Mark polarity on the SLD. A reversed CT turns “limit export” into “chase export”. Huawei’s export limitation guide has a meter power direction setting, Positive or Reverse, for exactly this reason. Test it on site, do not trust the label.
- Decide how phases are treated. SolaX documents two modes for three-phase inverters. Total control averages across phases, so one phase can still export when loads differ. Per-phase control references the phase with the smallest load. Check whether the DISCOM meter registers net or per-phase energy before you choose.
- Use one controller for many inverters. One loop per site. SolaX shows a Modbus master with up to 10 slaves and a data hub for up to 60 inverters. Two control loops regulating against one meter can interact, so give the site one controller.
Specify the CT ratio and accuracy class for the control decision, which is made near zero flow. A CT sized for the full incomer current reads coarsely at low flow, so check the class at that point.
How fast must the response be?
Define response time as the time from a load drop to net export back at or below the limit. Then measure it. Vendors publish little. SolaX’s zero injection page states no response time. SMA’s Sunny Home Manager manual says that because of control cycle times, a remaining quantity of energy might still be fed into the grid.
Huawei’s SmartLogger1000 export limitation guide gives numbers. Issue 01 of 14 November 2018 lists a default power lowering adjustment period of 0.5 s. It lists a default maximum protection time of 3.0 s and a default maximum grid feed-in power of 0 kW. Treat those as one maker’s defaults for one logger, not as a standard.
The energy at stake is small. A 200 kW load step that takes 3 s to correct leaks 200 kW for 3 s, which is 0.17 kWh. The risks are different:
- Relay nuisance trips. If the relay delay is shorter than the controller recovery time, every large load trip also trips the plant.
- Meter registers. A bidirectional DISCOM meter would record the leak. In Chhattisgarh banking is not permitted for BTM plants, so do not plan on credit for it.
- Communication loss. This is the dangerous case. SMA states that on a communication failure the inverter reverts to 0 W, for inverters that support the fallback function. Huawei’s guide asks you to enable communication interrupt shutdown on the inverters. Its parameter “Power limit for abnormal electric meter” has a default of 100 percent. Left alone, a dead meter link means full output. Set it to 0 percent and test it.
How does zero export solar work when a diesel generator is in the picture?
A diesel generator cannot absorb reverse power. Back-fed power motors the alternator and the engine, and DG panels carry their own reverse power protection. So the zero export limit must also hold on the DG, and the grid CT stops being useful when the grid breaker is open.
Three design decisions follow:
- Switch the reference. When the grid is connected, the controller regulates against net power at the PCC. When the DG carries the site, it must regulate against DG loading. Bring breaker status contacts into the controller so it knows which mode is active.
- Respect the DG minimum load. Take the minimum loading from the engine maker’s data sheet and cap solar so the DG stays above it. Our hybrid solar and diesel guide covers generator behaviour and dispatch.
- Prevent paralleling by accident. Above 650 V, CEA safety regulation 47(1)(iii) covers supplies not intended to run in parallel. Their breakers must be interlocked against inadvertent paralleling or backfeed. Apply the same logic on LT.
Inverter anti-islanding is a separate issue. CEA regulation 11(6)(e) requires the plant to stop energising within 2 s of an unintended island. Ask the inverter maker whether a DG-supplied bus counts as a grid for that logic and what its DG mode does to export limits. The DG side, including minimum load and controller sizing, is covered in our guide to DG synchronisation with solar.
How much generation does zero export solar curtail?
More than most EPCs expect, and it depends on load shape, not on the device. The example below is illustrative. Every number is an assumption, so replace them with the client’s interval data.
Assumptions (illustrative).
- 500 kWp DC with 400 kW AC inverters. Clear-day hourly output peaks at 400 kW.
- One-shift factory. Base load 110 kW. Running load 330 kW from 08:00 to 17:00, dropping to 220 kW in the 13:00 lunch hour.
- 300 working days and 65 non-working days a year. On non-working days only the 110 kW base load runs.
- Annual yield of 1,500 kWh per kWp. The clear-day shape stands in for an average day, so seasons are not modelled.
- Avoided grid energy valued at ₹8 per kWh.
A weekday. The plant makes 3,090 kWh. Load absorbs 2,720 kWh. The remaining 370 kWh, 12.0 percent, is curtailed, mostly around the 13:00 lunch dip when 175 kW would otherwise export.
A non-working day. The plant makes the same 3,090 kWh. Load absorbs 1,210 kWh. Curtailment is 1,880 kWh, or 60.8 percent.
Across the year, by plant size (same assumptions).
| Plant size (kWp DC) | Potential energy (kWh per year) | Used on site (kWh per year) | Curtailed (kWh per year) | Curtailed (percent) | Used per kWp (kWh) |
|---|---|---|---|---|---|
| 200 | 300,000 | 290,231 | 9,769 | 3.3 | 1,451 |
| 300 | 450,000 | 414,969 | 35,031 | 7.8 | 1,383 |
| 400 | 600,000 | 524,398 | 75,602 | 12.6 | 1,311 |
| 500 | 750,000 | 594,926 | 155,074 | 20.7 | 1,190 |
| 600 | 900,000 | 636,727 | 263,273 | 29.3 | 1,061 |
Read the last column. The first 200 kWp deliver about 1,451 kWh for each kWp. Going from 400 to 500 kWp adds 70,528 kWh of useful energy for 100 kWp, about 705 kWh per kWp. At the assumed ₹8 per kWh that is roughly ₹5.6 lakh a year, so the cost of the extra 100 kWp has to clear that bar.
Three levers shrink the number: shift flexible load into the solar window, add a battery for the lunch dip and the weekend, or cut the array. This is curtailment inside the plant, which differs from grid-ordered cuts covered in the curtailment glossary entry and from DC-to-AC inverter clipping. Model it in the yield report, not as a performance ratio loss.
If you need the measurement point, controller and relay drawn into the IFC set, our rooftop detailed engineering design service covers the drawing package. Scope and turnaround are agreed at scoping.
What goes on the SLD, and what settings does the relay need?
The single line diagram (SLD) is where zero export becomes checkable. CEA regulation 9 requires the applicant to prepare single line schematic diagrams and make them available to the licensee. Regulation 11(10) to 11(12) cover protection features and control diagrams. They must be agreed before first synchronisation and signed by both parties. They are amended whenever the plant changes. The SLD glossary entry shows the basic elements.
For a zero export plant, draw these on the SLD:
- The DISCOM meter and the PCC, with the CT and any VT locations marked, including ratio, class and burden.
- The solar feeder breaker, with trip signals from both the relay and the controller.
- The relay tagged 32, with a settings box beside it.
- The manual isolating switch. Regulation 11(9) lets the licensee require one with visible break, lockable open, and mounted at least 2.44 m above ground. CEA safety regulation 121(1)(viii) calls for a manual disconnection switch outside the AC combiner box.
- The DG changeover, its interlock and its breaker status contacts to the controller.
- The communication path from the meter to the controller to the inverters, with the fail-safe action written next to it.
The table gives illustrative settings for a 400 kW AC plant at an LT incomer. The DISCOM may prescribe its own values, and the relay maker’s manual governs.
| Item | Illustrative value | Reason |
|---|---|---|
| Controller target | Hold net import of at least 5 kW | A small bias absorbs measurement noise so the net does not tip into export |
| Relay pickup | 10 kW export, directional | Sits above the controller bias and measurement noise, so normal regulation never trips it |
| Relay time delay | 5 s | Longer than the assumed 3 s controller recovery |
| Relay trip target | Solar feeder breaker, not the plant incomer | The factory keeps running when the solar trips |
| Meter link lost | Inverters to 0 percent output | SMA and Huawei both describe this fallback or its setting |
| Voltage and frequency trips | Per CEA 11(6) or the DISCOM’s narrower values | 110 and 80 percent of nominal within 2 s, 50.5 and 47.5 Hz within 0.2 s as the outer limits |
| Reconnection | At least 60 s of stable voltage and frequency | CEA 11(6)(d) |
The layering rule is the point: target, then pickup, then delay, each one outside the last. If the order is wrong, the relay trips before the controller has a chance to work.
What do the DISCOM and CEIG expect?
Expect a protection file, not a device catalogue.
From the DISCOM:
- A connection agreement with the technical conditions, under CEA regulation 4(7).
- The protection and control diagrams, agreed before first synchronisation.
- Inspection of the relay and its function before commissioning. Chhattisgarh says the relay or other equipment must be verified or inspected by the licensee. The PM Surya Ghar guidelines say the same for residential non-metered plants.
- A clear statement of which mechanism you use. In Chhattisgarh the answer cannot be net metering and BTM at once.
From CEIG or the Electrical Inspector:
- The route depends on state notifications under CEA safety regulations 34 and 45. Our CEIG inspection checklist lists what we could confirm by state and the forms the inspector uses.
- Regulation 121(1)(ix) lists protections for overload, surge, short circuit, high temperature, voltage, frequency, reverse polarity and lightning. It does not name reverse power, so the relay is justified by the connection agreement and by regulation 11(7)(b), and should appear in the settings sheet.
- Test records. Carry the load-step test, the relay trip test, the meter-link-lost test and the CT polarity test, each signed and dated. Regulation 10 of the CEA connectivity regulations expects drawings, specifications and test records to be available to the licensee.
Do not tell a client that any DISCOM will accept the scheme. Acceptance is the DISCOM’s decision on the specific site. The CEIG glossary entry and the CEA connectivity guide cover the wider approval path. The relay coordination basics sit in our relay coordination study guide. Do not confuse the relay’s reverse power with reverse current in PV strings, which is a module-level fault issue.
FAQ
Is zero export allowed in India without net metering?
It depends on the state. The CEA connectivity regulations below 33 kV do not mention an export limit. Chhattisgarh has a written order that allows grid-connected BTM plants with a reverse power relay. Tamil Nadu had a parallel operation mechanism with a relay. For any other state, ask the SERC rules and the DISCOM in writing.
Can I net meter part of the roof and run zero export on the rest?
In Chhattisgarh the order does not permit net metering and grid-connected BTM at the same time. We did not find a rule from other states on this, so ask before you split a plant.
Does a battery remove the need for a relay?
No. A battery absorbs surplus until it is full, then export must still be prevented. The Chhattisgarh order applies the relay requirement to BTM projects with or without battery storage.
Is a CT-based zero export device enough for the DISCOM?
The Chhattisgarh order allows the relay “and/or any other equipment for the grid safety as per CEA regulations” and leaves verification to the licensee. A controller is regulation. A relay is protection. Design for both and let the DISCOM tell you what it will inspect.
How much generation will I lose?
In our illustrative factory the loss ran from 3.3 percent at 200 kWp to 29.3 percent at 600 kWp. Your number depends on interval load data, shutdown days and any battery. Run the same table with the client’s data before you fix the array size.
Before you finalise the design
- Confirm in writing which mechanism the DISCOM will use for this consumer.
- Fix the CT location at the PCC and mark polarity.
- Layer target, relay pickup and relay delay, in that order.
- Set meter-link-lost output to 0 percent and test it.
- Run the curtailment table on the client’s own load data.
- Put the settings sheet on the same drawing set as the SLD.
To have the control scheme drawn into a rooftop IFC package, send us the site details and load data and we will scope it with you. You can also message us on WhatsApp. If you want to see how our drawing sets are laid out first, request the design samples. We reply within 1 business day.
Source and review note
Sources read on 31 October 2026:
- CEA (Technical Standards for Connectivity of the Distributed Generation Resources) Regulations, 2013, as amended on 6 February 2019: CBIP consolidated copy.
- CEA (Measures relating to Safety and Electric Supply) Regulations, 2023, notified 8 June 2023: Internet Archive capture of 26 December 2024. The cea.nic.in link returns 404.
- CSERC order on Petition No. 64 of 2025, dated 5 February 2026.
- TANGEDCO salient features of TNERC Order No. 8/2021, a utility summary of the order dated 22 October 2021.
- MNRE guidelines for PM Surya Ghar, paragraph o.
- SMA Sunny Home Manager feed-in manual page and the SolaX zero injection page.
- A reseller-hosted copy of the Huawei SmartLogger1000 export limitation guide, Issue 01, 2018. It is an older issue, so check the current manual.
- The IEEE 1547-2018 reference comes from the BATRIES interconnection guidance, because the standard is paywalled.
The worked example is illustrative and uses only the stated assumptions. State rules change, so recheck the CSERC order, the TANGEDCO summary and your state’s Electrical Inspectorate notifications before quoting this article to a client. Next review: 31 January 2027.