Question

What is CT ratio and VT ratio?

Answer 1

**CT ratio** and **VT ratio** refer to the transformation ratios of **Current Transformers (CTs)** and **Voltage Transformers (VTs)**—also known as **Potential Transformers (PTs)**—used in electrical power systems for measurement and protection.

---

###  CT Ratio (Current Transformer Ratio)

The **CT ratio** is the ratio of the **primary current** (the actual current in the power line) to the **secondary current** (the scaled-down current used for meters or protection relays).

**Formula:**
\[
\text{CT Ratio} = \frac{\text{Primary Current}}{\text{Secondary Current}}
\]

**Example:**
A CT rated 1000/5 means:
- Primary current = 1000 A
- Secondary current = 5 A
- Ratio = 1000:5 = 200:1

---

###  VT Ratio (Voltage Transformer Ratio)

The **VT ratio** is the ratio of the **primary voltage** (line voltage) to the **secondary voltage** (scaled-down voltage for meters or relays).

**Formula:**
\[
\text{VT Ratio} = \frac{\text{Primary Voltage}}{\text{Secondary Voltage}}
\]

**Example:**
A VT rated 11,000/110 means:
- Primary voltage = 11 kV
- Secondary voltage = 110 V
- Ratio = 100:1

---

### Purpose of These Ratios

- **Safety**: They isolate measurement devices from high voltages/currents.
- **Standardization**: Most meters and relays are designed for 1A, 5A (CT) or 110V (VT).
- **Accuracy**: Ensures consistent, scaled measurements across different system voltages and currents.

Would you like an illustration showing how CTs and VTs are connected in a power system?