A power generation plant is more than a turbine and a generator. Between the turbine shaft and the high-voltage grid bus sits a chain of equipment — breakers, a step-up transformer, a neutral grounding resistor and an excitation system — and each item has a specific job, especially when something goes wrong. This post walks through the plant one-line diagram and explains the objective of each of the three components the exam asks about, plus the standard roles of the other items in the chain.
Figure 7.1: Equivalent one-line diagram of a synchronous power generation plant. Power flows from the turbine through the generator, the generator circuit breaker and the step-up transformer to the HV grid bus; the NGR grounds the generator neutral and the excitation system feeds the rotor field.
The main power chain
Power flows from left to right along the one-line diagram: the turbine drives the synchronous generator (~), whose output passes through the generator circuit breaker (GCB) into the step-up transformer (GSU, Δ–Y), which raises the generator voltage to grid voltage, and then through the HV circuit breaker to the HV grid bus and on to the grid.
The standard roles of the three switching and transformation items in this chain are: the GCB connects and disconnects the generator from the rest of the plant; the GSU step-up transformer raises the generator terminal voltage to the high grid voltage; and the HV circuit breaker connects the step-up transformer to the HV grid bus and isolates it on a fault.
1. Turbine valve — power, frequency and overspeed
The turbine stop/control valve regulates the mechanical steam (or water) admitted to the turbine, and through it controls the active power output (MW) and the system frequency (50 Hz): more admission means more power and a rising frequency, less admission means the opposite.
Its protection duty is just as important. On an internal electrical fault, or on a sudden load rejection such as a generator circuit breaker trip, the emergency stop valve closes simultaneously with the trip, cutting off the prime-mover input energy. That prevents a dangerous rotor overspeed (runaway) — with the electrical load suddenly gone, the turbine would otherwise accelerate violently — and it prevents the generator from motoring, that is, drawing power from the grid and running as a motor.
2. Neutral grounding resistor (NGR) — limiting the earth-fault current
The NGR (Rn) is connected between the generator stator neutral and earth. Its job is to limit a single-line-to-ground (L–G) fault on the stator to a safe value:
L–G stator fault current limited to 5 A to 200 A
Without it, a stator earth fault would drive a huge current through the stator iron, burning and melting the core laminations — damage that is often beyond repair. The resistor keeps the fault current small enough to avoid core burning, and it also limits the transient arcing overvoltages that an arcing earth fault would otherwise produce.
3. Excitation system with AVR and field breaker — voltage, MVAR and stability
The excitation system, with its automatic voltage regulator (AVR) and field breaker (device 41), supplies regulated DC field current to the generator rotor. It has three duties: to maintain the terminal voltage (kV), to control the reactive power output (MVAR), and to maintain synchronous stability — keeping the generator locked in step with the rest of the system when the network is disturbed.
Key points to remember
- Main chain: turbine → synchronous generator → GCB → step-up GSU (Δ–Y) → HV CB → HV grid bus.
- ① Turbine valve: controls MW and 50 Hz frequency; on a fault or load rejection the stop valve closes to prevent rotor overspeed (runaway) and motoring from the grid.
- ② NGR: between stator neutral and earth; limits L–G stator fault current to 5–200 A, preventing stator core burning and limiting arcing overvoltages.
- ③ Excitation + AVR & field breaker (41): supplies regulated DC field current to maintain terminal voltage (kV), control reactive power (MVAR) and maintain synchronous stability.
Based on Power System Protection & Switchgear, Paper II, Q7(c), Exam 11 June 2026.