---
title: "Functional block diagram of excitation control"
subject: "Power Plant Equipment"
chapter: 3
key: "semester_7/power_plant_equipment:excitation-control-block-diagram"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/excitation-control-block-diagram
site: BCT Engineering
---

# Functional block diagram of excitation control

Power Plant Equipment · Chapter 3: Var/Voltage Control in Hydrogenerating Systems

- Asked in: 2078 Bhadra, 8 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/excitation-control-block-diagram
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-3

## Question

What is excitation system? Draw a functional block diagram of a typical excitation control system and explain the role of each block and derive the complete transfer function of the system.

## Short answer

An excitation system supplies controlled DC field current to the generator to regulate terminal voltage and reactive power. The block diagram has a comparator, amplifier, exciter, generator, voltage sensor, and rate-feedback stabilizer. The complete transfer function is Vt/Vref = G_A G_E G_G/(1+G_A G_E H_F + G_A G_E G_G G_R).

## Answer

The **excitation system** is the set of equipment (exciter, regulator, control and protection) that supplies the DC field current to a synchronous generator and controls it to hold terminal voltage and reactive power at the set values.

#### Functional block diagram

```text
          +-----------+  +---------+  +-----------+
Vref ->(Σ)| Regulator |->| Exciter |->| Generator |-> Vt
       ^ ^|(amplifier)|  |         |  | + system  |
       | |+-----------+  +----+----+  +-----+-----+
       | |  +------------+    |             |
       | +--| Stabilizer |<---+             |
       |    +------------+                  |
       |    +--------------------------+    |
       +----| Voltage transducer and   |<---+
            | load compensator         |
            +--------------------------+
 Limiters/protection act on the regulator;
 PSS (optional) adds a signal at the summing point
```

#### Role of each block

| Block | Role |
|---|---|
| Terminal voltage transducer and load compensator | Senses, rectifies and filters $V_t$; may add $I$-dependent compensation for VAr sharing |
| Comparator (summing point) | Forms error $e = V_{ref} - V_t$ |
| Regulator / amplifier | Amplifies the error and produces control voltage $V_R$ for the exciter |
| Exciter | Power stage; gives field voltage $E_{fd}$ to the rotor |
| Generator | Converts field current into terminal voltage |
| Excitation stabilizer | Rate (derivative) feedback from $E_{fd}$ to damp oscillations |
| Power system stabilizer (PSS) | Adds damping to rotor (electromechanical) oscillations using speed/power signal |
| Limiters and protection | Keep field current, VAr and V/Hz within capability limits |

#### Transfer functions

$$
G_R = \frac{K_R}{1+sT_R},\quad G_A = \frac{K_A}{1+sT_A},\quad G_E = \frac{K_E}{1+sT_E},\quad G_G = \frac{K_G}{1+sT_G},\quad H_F = \frac{sK_F}{1+sT_F}
$$

($G_R$: sensor, $G_A$: amplifier, $G_E$: exciter, $G_G$: generator, $H_F$: stabilizer.)

The exciter TF comes from its field circuit: $v_R = R_e i_e + L_e\,di_e/dt$ and $E_{fd} = Ki_e$, so $E_{fd}/V_R = K_E/(1+sT_E)$ with $T_E = L_e/R_e$. The stabilizing transformer gives $V_F = M\,di_1/dt$ with $i_1 = E_{fd}/(R_1+sL_1)$, hence $H_F = sK_F/(1+sT_F)$, $K_F = M/R_1$.

**Minor (stabilizing) loop** around amplifier and exciter:

$$
G_1(s) = \frac{G_AG_E}{1+G_AG_EH_F}
$$

**Complete closed loop** (sensor in feedback path):

$$
\frac{V_t(s)}{V_{ref}(s)} = \frac{G_AG_EG_G}{1+G_AG_EH_F+G_AG_EG_GG_R}
$$

With unity sensor ($K_R=1$, $T_R \approx 0$):

$$
\frac{V_t}{V_{ref}} = \frac{K_AK_EK_G(1+sT_F)}{(1+sT_A)(1+sT_E)(1+sT_G)(1+sT_F)+sK_FK_AK_E(1+sT_G)+K_AK_EK_G(1+sT_F)}
$$

Steady-state ($s\to 0$): $\dfrac{V_t}{V_{ref}} = \dfrac{K}{1+K}$, with $K = K_AK_EK_G$, so the steady error is $\dfrac{1}{1+K}$ and falls as the loop gain rises.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does the excitation system do?
   A. Supplies and controls DC field current to hold terminal voltage and reactive power
   B. Drives the turbine
   C. Rectifies the stator output for the grid
   D. Regulates frequency
   Answer: A. It comprises exciter, regulator, control and protection supplying field current. (Key point: Definition)

2. What is the role of the power system stabilizer?
   A. Senses terminal voltage
   B. Adds damping to electromechanical rotor oscillations using speed or power signal
   C. Amplifies voltage error
   D. Limits V/Hz
   Answer: B. PSS provides positive damping to low-frequency rotor oscillations. (Key point: Role of PSS)

3. With the sensor in the feedback path, Vt/Vref is:
   A. GAGEGG/(1 + GAGEHF + GAGEGG GR)
   B. GAGEGG/(1 + GAGEGG)
   C. GA GE/(1 + HF)
   D. GG/(1 + GR)
   Answer: A. The minor loop gives GAGE/(1+GAGEHF), then the outer loop includes GG and sensor GR. (Key point: Closed-loop TF)

4. What is the steady-state Vt/Vref and the error?
   A. K/(1+K); error 1/(1+K)
   B. 1; error 0
   C. K; error K
   D. 1/(1+K); error K/(1+K)
   Answer: A. At s = 0 the stabilizer vanishes, giving K/(1+K), K = KA KE KG. (Key point: Steady-state)

## Related in Power Plant Equipment

- [Excitation system with stabilizing transformer: transfer function](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-stabilizing-transformer-model) — Power Plant Equipment, chapter 3, asked 13×
- [Function of excitation system and its transfer function](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-function-tf) — Power Plant Equipment, chapter 3, asked 2×
- [Brush excitation system](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Abrush-excitation-system) — Power Plant Equipment, chapter 3
- [Water turbine transfer function and characteristics](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Awater-turbine-tf-characteristics) — Power Plant Equipment, chapter 2, asked 2×
- [Dynamic response of excitation system](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aexcitation-dynamic-response) — Power Plant Equipment, chapter 3, asked 3×
- [Static excitation system](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Astatic-excitation-system) — Power Plant Equipment, chapter 3

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