A **Capacitor Voltage Transformer (CVT)**, also known as a **Capacitor Coupled Voltage Transformer (CCVT)**, is a device used in high-voltage power transmission systems to step down high voltages to a lower level for measurement, protection, and control purposes. CVTs are commonly used in substations and power plants to measure voltages, often in combination with protective relays for system protection.
The key principle behind a CVT is the use of capacitors to divide the voltage and reduce the overall size and cost compared to traditional electromagnetic voltage transformers. Below is a detailed explanation of how a CVT works.
### Basic Components of a CVT
1. **Capacitor Stack**: A series of capacitors that divides the high voltage (HV) from the transmission line into smaller, more manageable levels.
2. **Inductive Transformer**: A step-down transformer that further reduces the voltage and provides isolation between the high-voltage line and the low-voltage equipment.
3. **Tuning Reactor (Inductor)**: This component is used to tune the CVT for proper operation by compensating for the capacitive reactance of the capacitor stack, ensuring proper voltage regulation.
4. **Voltage Regulating Circuit**: This includes various components that help in stabilizing and filtering the output signal to ensure that the voltage measured is accurate and reliable.
5. **Secondary Winding**: Provides the low-voltage output for measurement and protection equipment.
6. **Coupling with Communication**: Sometimes, a CVT is used not only for voltage measurement but also as a coupling device for power-line carrier communication (PLC) over high-voltage lines.
### Working Principle of a CVT
The working of a Capacitor Voltage Transformer can be broken down into a few steps:
#### 1. Voltage Division through Capacitors
The high-voltage signal from the transmission line is first fed into a series of high-voltage capacitors, forming a **capacitor divider**. Capacitors have the property of dividing voltage in proportion to their capacitance, so by using a series of capacitors with the right values, the voltage can be divided into much smaller segments. This allows a significant reduction in the voltage while avoiding the need for very large and costly transformers.
For example, if the input voltage is 400 kV, a portion of that voltage, say 5 kV, is dropped across the capacitor stack. The key point is that the capacitor stack handles the bulk of the high voltage, so the size and insulation requirements of the transformer are much smaller compared to a traditional voltage transformer.
#### 2. Further Step-Down Using an Inductive Transformer
The reduced voltage from the capacitor divider is then applied to an inductive transformer. This transformer provides two key functions:
- **Voltage Reduction**: It further reduces the voltage to a level that can be safely measured (typically 110V or 120V).
- **Isolation**: It electrically isolates the high-voltage transmission line from the measuring and protection equipment for safety reasons.
This transformer operates on the same principles as a traditional step-down transformer, utilizing electromagnetic induction to reduce voltage proportionally based on the turns ratio between its primary and secondary windings.
#### 3. Tuning and Compensation with a Reactor
The combination of the capacitors introduces a capacitive reactance into the system. To balance this effect, a **tuning reactor** (inductor) is used. The tuning reactor compensates for the phase shift and voltage drop caused by the capacitive reactance of the capacitor stack. This ensures that the voltage being measured is accurate and that the CVT operates effectively over a range of frequencies.
#### 4. Voltage Regulation and Filtering
The output from the secondary side of the transformer is passed through a voltage regulating circuit that may include filters and other electronic components. This step smooths out any remaining irregularities in the signal, such as high-frequency noise, and ensures a clean, stable output voltage for accurate measurement.
#### 5. Output for Metering and Protection
Finally, the output is provided to low-voltage metering devices, protective relays, and other control equipment. These devices use the reduced voltage to monitor the system for faults, measure the voltage, and control equipment such as circuit breakers.
### Advantages of CVTs over Electromagnetic Transformers
- **Cost-Effective**: Since the bulk of the voltage reduction is done by capacitors, the size of the inductive transformer can be smaller, making the CVT less expensive and easier to maintain.
- **Compact Design**: CVTs are smaller in size compared to traditional electromagnetic transformers, especially for very high voltages like 400 kV or 765 kV.
- **High-Voltage Handling**: CVTs are ideal for handling very high voltages in transmission lines.
- **Capacitor Coupling for Communication**: In addition to voltage measurement, CVTs can serve a dual purpose by allowing communication signals to be superimposed onto the power lines (for power-line carrier communication or PLC).
### Application Areas of CVTs
- **High-voltage transmission systems**: CVTs are commonly used to measure voltages in substations, transmission lines, and large industrial systems.
- **Protection systems**: They work with protective relays to detect faults like over-voltages, under-voltages, or imbalances in the system.
- **Power line communication**: CVTs help in coupling communication signals onto the high-voltage lines for communication between substations.
### Summary of CVT Working Steps:
1. **High voltage input** is applied to a capacitor stack.
2. **Voltage division** occurs through the capacitor stack.
3. The reduced voltage is further **stepped down** by an inductive transformer.
4. A **tuning reactor** compensates for capacitive reactance.
5. **Voltage regulation** and filtering ensure a stable output.
6. **Low-voltage output** is used for metering and protection.
In essence, the CVT uses the principle of capacitive voltage division combined with a smaller inductive transformer, making it a practical and cost-effective solution for high-voltage measurement and protection in modern power systems.