Combined-Cycle Power Plants
A Combined-Cycle Power Plant (CCPP) is a high-efficiency power-generation facility that combines gas turbine (Brayton cycle) and steam turbine (Rankine cycle) technologies. The key principle is to use the hot exhaust gases from the gas turbine to generate steam, which is then used to produce additional electricity.
This allows the plant to generate significantly more electricity from the same fuel compared with a simple-cycle gas turbine plant.
Basic Operating Principle
The process can be summarized as:
Natural Gas → Gas Turbine → Generator → Electricity
The hot exhaust from the gas turbine is then directed to:
Gas Turbine Exhaust → HRSG → Steam → Steam Turbine → Generator → Additional Electricity
The main stages are:
- Gas Turbine: Natural gas is burned with compressed air. The resulting hot gases expand through the turbine, driving an electrical generator.
- HRSG – Heat Recovery Steam Generator: Instead of releasing the hot exhaust directly to the atmosphere, the heat is recovered to produce high-pressure steam.
- Steam Turbine: The generated steam expands through a steam turbine, producing additional mechanical energy and electricity.
- Condenser: Exhaust steam from the steam turbine is condensed back into water.
- Feedwater System: Pumps return the water to the HRSG to repeat the cycle.
Main Equipment
A typical CCPP consists of:
- Gas turbines
- Gas turbine generators
- HRSGs
- Steam turbine
- Steam turbine generator
- Condenser
- Cooling system
- Feedwater pumps
- Transformers
- MV/HV switchgear
- Auxiliary transformers
- MCCs and VFDs
- Generator protection systems
- Control and instrumentation systems
- Fuel-gas system
- UPS and DC systems
- Emergency generators
Electrical Engineering Perspective
From an electrical engineering perspective, CCPPs are complex generation facilities requiring highly reliable generation, distribution, protection, and control systems.
Typical electrical engineering responsibilities include:
- Generator and transformer selection
- Generator step-up transformer (GSU) design
- MV/HV switchgear design
- Auxiliary power distribution
- Generator synchronization
- Grid interconnection
- Load-flow studies
- Short-circuit studies
- Protection coordination
- Generator protection
- Transformer protection
- Motor starting studies
- Grounding and lightning protection
- Harmonic studies where required
- Emergency power systems
- UPS and DC systems
- Electrical equipment specifications
- Cable sizing and voltage-drop calculations
- Review of vendor and EPC contractor documents
- Testing and commissioning
Generator and Grid Connection
The generators typically produce electricity at a medium voltage, which is then stepped up through a Generator Step-Up Transformer (GSU) to the transmission voltage.
For example:
Generator → GSU Transformer → HV Switchyard → Transmission Grid
The electrical engineer must ensure that the plant can safely synchronize with and operate in parallel with the grid.
Important considerations include:
- Voltage matching
- Frequency matching
- Phase-angle matching
- Synchronizing systems
- Generator excitation and AVR
- Governor control
- Grid protection
- Anti-islanding and loss-of-grid protection where applicable
- Load shedding
- Under/over-voltage protection
- Under/over-frequency protection
Why CCPPs Have High Efficiency
The major advantage is heat recovery.
In a simple-cycle gas turbine, much of the energy leaves the turbine as hot exhaust gas. In a combined-cycle plant, this thermal energy is recovered through the HRSG and converted into additional electrical output.
Therefore:
Fuel → Gas Turbine Electricity + Recovered Exhaust Heat → Steam Turbine Electricity
Modern CCPPs can achieve very high thermal efficiencies, particularly compared with conventional simple-cycle generation.