ISTS Explained: How India Builds Transmission Lines
to Evacuate Solar Power
A complete guide to India's Inter-State Transmission System — from RE zone planning to grid dispatch — and why ~50 GW of renewable capacity remains stranded across the country.
Why This Guide Matters
India added 38 GW of solar capacity in 2025 alone — more than most countries have installed in total. The country has crossed 250 GW of non-fossil fuel installed capacity and is racing towards its 500 GW renewable energy target by 2030. But here's the problem nobody talks about enough: approximately 50 GW of India's renewable energy capacity is currently stranded — unable to deliver power to the grid because the transmission lines needed to evacuate it don't yet exist, or are delayed, or are congested.
If you're a solar developer, EPC contractor, infrastructure investor, procurement professional, or policy researcher, this guide will give you a complete understanding of how India's Inter-State Transmission System works, where the bottlenecks are, and who the key institutional players are. We've built this to be the resource we wished existed when we started covering solar procurement at Headsup B2B.
What Is ISTS? The Backbone of India's Green Grid
The Inter-State Transmission System (ISTS) is the high-voltage electricity highway that carries power across state boundaries in India.
Think of it as the national highway network — but for electrons. While intra-state transmission handles power movement within a state, ISTS handles the long-distance, bulk transfer of electricity from generation-rich states to consumption-heavy ones.
For solar energy, ISTS is existentially important. India's best solar irradiance zones are concentrated in Rajasthan, Gujarat, and parts of Karnataka and Andhra Pradesh. But the biggest consumption centres — Delhi-NCR, Mumbai, Chennai, Bengaluru, Kolkata — are hundreds or thousands of kilometres away. Without ISTS, solar power generated in Bhadla, Rajasthan, cannot reach a factory in Noida or a household in Mumbai.
ISTS vs. Intra-State Transmission
ISTS operates at 400 kV and 765 kV (and HVDC for ultra-long distances) and is regulated by the Central Electricity Regulatory Commission (CERC). Intra-state transmission operates at lower voltages (66 kV to 220 kV) and falls under State Electricity Regulatory Commissions. For large solar parks and RE zones, ISTS connectivity is the critical link between generation and the national grid.
The key principle: renewable energy has "must-run" status under the Indian Electricity Grid Code 2010. This means solar and wind power must be dispatched before thermal generation, subject only to grid security constraints. This policy was designed to accelerate RE adoption — but when the transmission lines aren't ready, the power simply has nowhere to go.
The Complete 7-Stage ISTS Process
From the moment a solar zone is identified to the point when a unit of electricity flows through a newly commissioned transmission line, here is the complete lifecycle of an ISTS project in India:
Total timeline: 3–7 years from planning to commissioning.
The Institutional Ecosystem — Who Does What
India's ISTS ecosystem involves eight major central institutions, each with a distinct mandate. Understanding who does what is critical for anyone operating in the solar infrastructure space.
The policy architect. Sets national RE targets (500 GW by 2030), identifies RE zones, administers ISTS charge waivers, manages ALMM and PM-KUSUM, and drives the National Green Hydrogen Mission. MNRE constituted the sub-committee that originally identified 66.5 GW of solar and wind potential across India for ISTS planning.
The national transmission planning body. Prepares the National Electricity Plan and the transmission expansion roadmap. CEA has shifted to "potential-based planning" — revising plans every six months to reflect evolving ground realities rather than static five-year cycles. The ₹2.44 lakh crore ISTS investment estimate comes from CEA.
Carved out of PGCIL to ensure planning independence. Grants connectivity (GNA) to RE developers. Plans ISTS corridors and coordinates with NLDC, state transcos, and developers. The single-window portal for RE injection access. CTU has planned transmission systems to evacuate power from 172.5 GW of RE capacity.
India's largest transmission utility and Maharatna CPSE. Wins 40–50% of all TBCB schemes. Executes mega ISTS projects — it won all 6 SPVs for Khavda (Gujarat) evacuation, secured the 8.1 GW Rajasthan Phase-2 projects, and recently bid for the 5 GW Visakhapatnam green hydrogen ISTS. PGCIL also operates as a developer, not just a contractor.
The Bid Process Coordinator (BPC) for TBCB tenders. A subsidiary of REC Limited (NBFC Maharatna under Ministry of Power). Creates SPVs, manages the entire tender process — from qualification to e-Reverse Auction to LoI — and transfers winning SPVs to developers. RECPDCL has coordinated the bidding for the majority of India's RE evacuation transmission projects.
The alternative BPC alongside RECPDCL. Also creates SPVs and manages TBCB bidding for select ISTS schemes. PFC Consulting handled the Bikaner-III Neemrana-II transmission scheme, among others.
The regulator. Approves transmission schemes, sets tariff norms, adjudicates disputes. ISTS charges sharing regulations, the GNA framework, and curtailment compensation rules all fall under CERC's jurisdiction. Every ISTS scheme must receive CERC's regulatory approval before tendering can begin.
The real-time grid operator. Schedules RE dispatch, manages grid frequency, activates TRAS (emergency curtailment), and publishes daily VRE reports. NLDC is the entity that ultimately decides if solar power flows or gets curtailed. It operates five Regional Load Dispatch Centres (RLDCs) across India.
India's ISTS Bottleneck Map — State-Wise Analysis
Not all states face equal transmission stress. The bottleneck geography is heavily concentrated in states with high RE generation but insufficient evacuation infrastructure. Here is a state-by-state analysis of where India's ISTS is most critically strained:
"The improvement in transmission has been limited and insufficient to offset the scale and speed of new RE commissioning. The core issue remains asynchronous planning — renewable generation is commissioned in 12–24 months, whereas ISTS transmission takes three to five years."
— Solar Power Developers Association (SPDA)
The Numbers That Matter — ISTS by Data
| Metric | Value | Source / Context |
|---|---|---|
| Total RE capacity (Nov 2025) | 262.74 GW | 51.5% of India's total installed capacity (509.64 GW) |
| Solar capacity (2025) | ~132.85 GW | 41% year-on-year increase |
| Solar added in 2025 alone | 38 GW | Highest-ever annual addition |
| RE capacity stranded (nationwide) | ~50 GW | Due to transmission bottlenecks (IEEFA estimate) |
| Rajasthan stranded capacity | ~8 GW | Out of 22 GW ISTS-connected RE |
| Unsigned PSAs (Sep 2025) | 44 GW | Awarded but unable to reach financial closure |
| ISTS investment target (by 2030) | ₹2,44,200 Cr | ~$29 billion; CEA estimate |
| New transmission lines planned | 50,890 ckm | For integration of additional solar + wind by 2030 |
| Substation capacity planned | 4,33,575 MVA | 765/400/220 kV transformation capacity |
| Transformer cost inflation | ₹14 Cr → ₹30 Cr | More than doubled; lead times exceed 18 months |
| GIS bay cost inflation | ₹6 Cr → ₹15 Cr | 2.5x increase in recent years |
| ISTS transmission lag vs. target | ~50% behind | SBI Caps report, late FY26 |
| Curtailment across major RE states | 15–20% average | Rajasthan, Gujarat, Karnataka; up to 100% at midday |
| Energy storage needed by 2032 | 411 GWh | BESS + pumped hydro; current deployment severely inadequate |
Why Solar Gets Curtailed — The Mismatch Problem
Curtailment is the term for when a solar or wind plant is forced to reduce or stop generating power, even though it can produce. Under India's grid code, RE has "must-run" status — meaning it should be evacuated before thermal power. But in practice, curtailment has become routine in 2025–26, driven by two primary mechanisms:
When grid frequency rises above safe limits — typically because supply exceeds demand — NLDC activates TRAS-down, requiring generators to reduce output. Solar plants, being operationally flexible (they can ramp down instantly), are disproportionately called upon. On 12 October 2025, approximately 93.3 GWh of ISTS solar output was curtailed through this mechanism — up from 45 GWh just five months earlier.
When the physical transmission corridor cannot carry the generation being produced, NLDC reduces schedules of generators holding T-GNA (Temporary General Network Access). Unlike TRAS, this type of curtailment is not always financially compensated, making it a critical risk for RE developers. In December 2025, approximately 4 GW of T-GNA solar capacity faced complete curtailment between 11 AM and 2 PM on certain days.
Why It Matters Commercially
For solar developers, curtailment directly reduces the Plant Load Factor (PLF), lowering revenue below projections. For investors, it introduces a structural risk that can't be hedged. For DISCOMs, it means paying for must-run solar they can't receive. And for India's climate targets, it means clean energy is being wasted while coal continues to burn.
ISTS Charge Waiver — The Policy That Changed Everything
One of the most impactful policy instruments in India's RE journey has been the ISTS charges waiver — a government-mandated exemption from paying interstate transmission charges for renewable energy projects. The waiver was first introduced to make RE cost-competitive with thermal power by reducing the effective cost of delivering solar/wind power across state boundaries.
What Changed in 2025
In June 2025, the government announced a phased withdrawal of the ISTS charges waiver:
| Commissioning Window | Waiver Level | Impact |
|---|---|---|
| Before June 30, 2025 | 100% waiver | Full benefit; most projects rushed to commission |
| July 2025 – June 2026 | 75% waiver | Still attractive; partial commissioning strategy used |
| July 2026 – June 2027 | 50% waiver | Landed cost increases significantly |
| July 2027 – June 2028 | 25% waiver | Further cost pressure on RE projects |
| After June 30, 2028 | 0% waiver | Full ISTS charges apply; RE must compete on total cost |
The Unintended Consequence
The 100% waiver, while commercially beneficial, created a geographic concentration problem. Developers flooded into Rajasthan and Gujarat — where irradiance is highest — without regard for transmission capacity. The waiver reduced the cost of moving power interstate, eliminating the natural incentive to build closer to consumption centres. The result: massive generation clusters in western India with insufficient evacuation infrastructure.
Private Players in Transmission — The TBCB Revolution
India's ISTS development was historically a PGCIL monopoly. The introduction of TBCB (Tariff Based Competitive Bidding) opened the door for private players to build, own, and operate interstate transmission assets. This has been transformative — bringing competition, driving down tariffs, and accelerating capacity addition.
How TBCB Works
RECPDCL or PFC Consulting creates an SPV for each transmission project and conducts a competitive bidding process. Bidders compete on the annual transmission charge they're willing to accept. The lowest bidder wins, receives the SPV, and must build and operate the asset for the license period (typically 35 years). The revenue model is a regulated, fixed annual charge — making it attractive for infrastructure-focused investors.
Key TBCB Players (as of FY26)
| Company | Profile | Notable Wins |
|---|---|---|
| PGCIL | Dominant; wins 40–50% of TBCB schemes | Khavda (6 SPVs), Rajasthan 8.1 GW (Phase-2), Davangere, Sonbhadra |
| Adani Energy Solutions | Largest private transmission player | WRNES Talegaon PSP scheme; multiple Rajasthan corridors |
| Sterlite / Resonia | Private transmission pioneer | NER Expansion scheme (Jul 2025) |
| GR Infraprojects | New entrant from highways | Rajgarh-Neemuch (MP) scheme (Aug 2025) |
| HG Infra Engineering | Newest debutant in TBCB | Angul Sundargarh scheme (Jul 2025) |
| Tata Power | Integrated utility | Multiple qualifications; Bikaner-III Neemrana-II LoI |
| IndiGrid (Enerica) | Infrastructure InvIT | Qualified for Sonbhadra, Rajgarh-Neemuch |
| Reliance Industries | New entrant | L1 for Kandla GHA scheme (Apr 2025) |
| Dineshchandra Agrawal | EPC-turned-developer | Raghanesda RE scheme (Jul 2025) |
What Needs to Change — The Road Ahead
The structural challenge is clear: India's RE generation capacity is growing at 2–3x the pace of transmission infrastructure. Solving this requires coordinated action across several dimensions:
Currently, generation projects are planned and commissioned before transmission infrastructure is ready. Flipping this — building transmission corridors ahead of generation — would prevent stranding. This requires longer planning horizons and tolerance for initial underutilisation of transmission assets, but it is the approach adopted by countries like Germany and China.
Advanced composite-core conductors can double the power-carrying capacity of existing transmission lines without building new towers. This is the fastest way to debottleneck existing corridors — particularly in Rajasthan, where RoW issues delay new line construction.
India needs an estimated 411 GWh of energy storage capacity by 2032 to bridge the gap between when solar generates (daytime) and when demand peaks (evening). BESS costs are declining, but deployment is far behind targets. Pumped hydro storage (like the Sonbhadra and Talegaon projects) can provide longer-duration storage, but takes 4–6 years to build.
Moving from static five-year planning cycles to dynamic, semi-annual revision (as CEA has started doing) allows transmission planning to respond to the reality of RE deployment patterns rather than outdated projections.
Establishing clear priority corridors — ranked by stranded capacity and economic impact — and assigning accountability for timely execution with penalties for delays would introduce the urgency the system currently lacks.
RoW disputes are the single largest cause of construction delays. Streamlining land acquisition processes, standardising compensation norms, and creating a dedicated fast-track mechanism for green energy corridors would have an outsized impact.
Glossary of Key Terms
| Term | Full Form | What It Means |
|---|---|---|
| ISTS | Inter-State Transmission System | The high-voltage grid that carries power across state boundaries |
| GNA | General Network Access | Permission to inject/draw power from the ISTS grid; replaces old LTA/MTOA system |
| T-GNA | Temporary GNA | Short-term grid access; first to be curtailed when corridors are congested |
| TBCB | Tariff Based Competitive Bidding | The bidding process through which private players win ISTS projects |
| SPV | Special Purpose Vehicle | A company created for each ISTS project; transferred to the winning developer |
| BPC | Bid Process Coordinator | RECPDCL or PFC Consulting — the entity that runs the tender |
| BOOT | Build-Own-Operate-Transfer | The business model for ISTS projects; developer builds, owns, operates for ~35 years |
| TRAS | Tertiary Reserve Ancillary Services | Grid emergency mechanism used by NLDC to curtail generation |
| GIS | Gas Insulated Switchgear | Compact switchgear used in substations; costs have risen sharply |
| STATCOM | Static Synchronous Compensator | Reactive power compensation device; stabilises voltage on long transmission lines |
| RoW | Right of Way | Legal permission to build transmission lines across land; major cause of delays |
| DPR | Detailed Project Report | Technical and financial plan prepared by the TSP before construction begins |
| GIB | Great Indian Bustard | Critically endangered bird; habitat restrictions mandate underground cabling in Rajasthan |
| ALMM | Approved List of Models & Manufacturers | MNRE's mandatory list for solar modules/cells used in government-supported projects |
Frequently Asked Questions
ISTS (Inter-State Transmission System) is the high-voltage transmission network that carries electricity across state boundaries. For solar energy, ISTS is essential because India's best solar zones (Rajasthan, Gujarat) are far from the major consumption centres (Delhi, Mumbai, Bengaluru). Without ISTS, solar power cannot reach the markets that need it.
The typical construction timeline is 24–36 months from the start of physical work. However, the full lifecycle — from RE zone identification to grid commissioning — takes 3–7 years when planning, regulatory approvals, tendering, and clearances are included. By comparison, a solar project can be commissioned in 12–24 months.
Two primary reasons: (1) Grid emergency curtailment through TRAS, where NLDC reduces generation to maintain frequency stability, and (2) transmission corridor congestion, where the physical lines cannot carry all the power being generated. The root cause is that RE generation capacity has been added much faster than transmission infrastructure.
No. Since the introduction of TBCB, private companies can bid for and build ISTS projects. PGCIL remains the largest player (winning 40–50% of schemes), but Adani Energy Solutions, Tata Power, Sterlite/Resonia, GR Infraprojects, HG Infra, Reliance Industries, and IndiGrid are all active in the space. The market is open and competitive.
The ISTS charge waiver exempted RE projects from paying interstate transmission charges. It's being phased out: projects commissioned before June 2025 got 100% waiver, declining to 75%, 50%, 25% in successive years, and reaching 0% after June 2028. This means future RE projects will need to factor transmission charges into their cost calculations.
GNA (General Network Access) is the permission granted by CTU to a generator or consumer to use the ISTS grid. It replaced the earlier LTA (Long Term Access) and MTOA (Medium Term Open Access) system. Generators with GNA have a right to inject power into the grid, while T-GNA (Temporary GNA) holders have lower priority and are the first to face curtailment.
Monitor RECPDCL's official website (recpdcl.in) and PFC Consulting for TBCB tender announcements. CTU (ctuil.in) publishes upcoming ISTS plans and connectivity grant information. Headsup B2B's Research Desk tracks and analyses major RE-related ISTS tenders as part of our solar procurement intelligence coverage.
Headsup B2B Research Desk
Headsup B2B Pvt. Ltd. · New Delhi
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