Quick Facts
What CTU and STU are
In India’s electricity sector, two categories of high-voltage transmission utilities exist:
CTU (Central Transmission Utility): Power Grid Corporation of India Limited (PGCIL). Operates the inter-state transmission system (ISTS) connecting state grids across India. Regulated by CERC.
STU (State Transmission Utility): Each state has its own STU operating the intra-state high-voltage network. Regulated by the state’s SERC.
The two transmission networks interconnect at major substations, enabling power flow between states. Together they form the integrated national grid.
For solar projects, the choice between connecting to CTU or STU determines the project’s regulatory category (inter-state or intra-state) and affects tariffs, approvals, and operations.
Power Grid Corporation of India (CTU)
Power Grid Corporation of India Limited (PGCIL) is India’s central transmission utility, established in 1989 as a public sector enterprise. It is now one of the world’s largest power transmission utilities.
PGCIL’s responsibilities:
Operate India’s inter-state transmission system (ISTS).
Plan, construct, and maintain inter-state transmission infrastructure.
Coordinate inter-state power flow with state utilities.
Operate national load dispatch centre (NLDC) and regional load dispatch centres (RLDCs).
Manage real-time grid balancing.
Set transmission tariffs (under CERC oversight).
PGCIL’s network includes:
100,000+ kilometres of transmission lines.
Hundreds of substations.
Network voltages from 132 kV to 765 kV.
Integration with neighbouring country networks (Bangladesh, Bhutan, Nepal).
For utility-scale solar:
Solar plants above 33 kV are typically connected to either CTU or STU.
SECI-procured solar uses CTU for inter-state delivery.
Major IPPs interconnect at CTU substations for inter-state PPAs.
State Transmission Utilities
Each Indian state has its own State Transmission Utility:
| State | STU Name |
|---|---|
| Gujarat | GETCO (Gujarat Energy Transmission Corporation) |
| Maharashtra | MSETCL (Maharashtra State Electricity Transmission Co. Ltd.) |
| Karnataka | KPTCL (Karnataka Power Transmission Corp Ltd.) |
| Tamil Nadu | TANTRANSCO (Tamil Nadu Transmission Corporation) |
| Andhra Pradesh | APTRANSCO |
| Telangana | TSTRANSCO |
| Rajasthan | RVPN (Rajasthan Rajya Vidyut Prasaran Nigam) |
| Madhya Pradesh | MPPTCL (MP Power Transmission Co Ltd.) |
| UP | UPPTCL (UP Power Transmission Co Ltd.) |
| Punjab | PSTCL (Punjab State Transmission Corp Ltd.) |
| Haryana | HVPNL (Haryana Vidyut Prasaran Nigam Ltd.) |
| Delhi | DTL (Delhi Transco Limited) |
Each STU:
Operates the intra-state high-voltage network.
Plans state-level transmission infrastructure.
Coordinates intra-state power flow.
Connects to CTU’s network at interconnection substations.
Submits transmission tariff orders to the state SERC.
For utility-scale solar plants selling intra-state:
The plant connects to the STU’s network.
Tariff and charges follow the state SERC framework.
State DISCOMs typically purchase the power.
CTU vs STU comparison
| Aspect | CTU (Power Grid) | STU (state-specific) |
|---|---|---|
| Coverage | National (inter-state) | Single state (intra-state) |
| Voltage range | 132 to 765 kV | 132 to 400 kV typically |
| Regulator | CERC | State SERC |
| Tariff | ISTS framework | State-specific |
| Number of entities | 1 (Power Grid) | 28+ (one per state) |
| Network length | 100,000+ km | Variable by state |
| Operational scale | National | State |
The complementary roles enable India’s integrated grid: STUs handle within-state transmission; CTU handles between-state.
Solar project connection decisions
For utility-scale solar projects, the choice of CTU or STU connection depends on:
PPA structure: Inter-state PPA requires CTU connection. Intra-state PPA needs STU connection.
Customer location: If the consumer is in another state, CTU. If in the same state, STU.
Tender source: SECI procurement typically uses CTU. State DISCOM tenders use STU.
Economics: ISTS waiver makes CTU economically attractive for renewables. Without waiver, STU may be cheaper.
Approval timeline: CTU coordination through Power Grid; STU through state utility. Timelines vary.
For Rajasthan or Gujarat solar plants targeting consumers in Maharashtra or Karnataka, CTU connection (with ISTS waiver) is standard. For state-specific procurement, STU connection is typical.
Major STU operational issues
Indian STUs face various operational and financial challenges:
Aging infrastructure: Some STUs have aged equipment requiring upgrades.
Right-of-way issues: New transmission corridors face land acquisition challenges.
Funding constraints: State governments may delay transmission investments.
Inter-utility coordination: STU-CTU coordination requires effort.
Capacity additions: STU expansion has lagged generation additions in some states.
For renewable integration, STU capacity is sometimes a constraint. Some Indian states have curtailed renewable output due to STU congestion. The ISTS waiver was partly motivated by enabling renewable evacuation when STU has limited capacity.
Open access through CTU and STU
For open-access solar transactions:
Intra-state open access: Uses STU network. Regulated by state SERC.
Inter-state open access: Uses CTU network. Regulated by CERC.
Mixed open access: Some transactions use both STU (intra-state portion) and CTU (inter-state portion).
For corporate open-access deals across states, the structure typically involves:
Generator connected to STU in source state.
Power transmitted to CTU through interconnection.
Power transmitted via CTU to destination state.
Power received by STU in destination state.
Final delivery to consumer through DISCOM wheeling.
The multi-utility involvement requires coordination, with each entity managing its segment of the transmission chain.
Common CTU vs STU mistakes
Confusing the roles. CTU is national (inter-state); STU is state-level (intra-state).
Mismatching procurement and connection. SECI procurement requires CTU connection.
Ignoring STU capacity constraints. Some states have STU congestion that limits new solar.
Forgetting STU charges. Even with ISTS waiver, STU charges still apply at the source state.
Underestimating cross-utility coordination. Inter-state transactions require multiple utility approvals.
Best practices
For utility-scale solar developers, evaluate both CTU and STU connection options.
For SECI tenders, plan for CTU interconnection and ABT metering.
For state DISCOM tenders, plan for STU connection.
For cross-state corporate deals, coordinate with both source STU and destination STU plus CTU.
For long-term operations, build relationships with the relevant transmission utility for smooth operation.
Monitor STU capacity expansion plans to anticipate constraints.
Standards and references
CTU and STU operations follow the Electricity Act 2003. CTU is regulated by CERC. STUs are regulated by state SERCs. The Forum of Regulators coordinates across both. CEA Connectivity Regulations 2019 cover interconnection standards.
Related glossary terms
Key takeaways
CTU (Central Transmission Utility) is Power Grid Corporation of India, operating the inter-state transmission system. STU (State Transmission Utility) is each state’s transmission company, operating intra-state high-voltage networks. Both are involved in solar power evacuation: CTU for inter-state flows (governed by CERC), STU for intra-state flows (governed by state SERCs). For utility-scale solar projects, the choice of CTU or STU connection depends on PPA structure, customer location, and economics. The ISTS waiver for renewables has made CTU-connected inter-state solar economically attractive for cross-state corporate deals.