Electrical Substation Equipment: Instrument Transformer and Its Functions

Fig-Instrument Transformer CT&PT

An instrument transformer is a type of electrical device used in power systems to facilitate the measurement, protection, and control of electrical quantities. It transforms high voltage and high current levels commonly found in power systems into standardized values suitable for measurement and monitoring by instruments and relays. There are two main types of instrument transformers: current transformers (CTs) and voltage transformers (VTs), also known as potential transformers (PTs).

Can you explain the workings of an instrument transformer in an electrical substation?

Instrument transformers are essential components in electrical substations that help in the accurate measurement and protection of high-voltage circuits. They are used to scale down the high voltages and currents present in power systems to levels that can be safely and conveniently measured by standard instruments, such as meters, relays, and protection devices. There are two main types of instrument transformers: current transformers (CTs) and voltage transformers (VTs), also known as potential transformers (PTs).

How do Current Transformers (CTs)Work in Substations?

Current transformers are used to measure or monitor the current flowing through a high-voltage conductor. They are connected in series with the circuit, and their secondary winding is connected to instruments or devices that require the current measurement, such as ammeters, wattmeters, and protective relays. Here's how they work:

• Step-Down Ratio: CTs have a large number of primary turns compared to their secondary turns, resulting in a step-down ratio. This step-down ratio is chosen based on the current range that needs to be measured accurately.
• Magnetic Induction: When current flows through the primary winding (the high-voltage conductor), it generates a magnetic field around the conductor. This magnetic field induces a current in the secondary winding, which is connected to the measurement or protection devices.
• Accuracy: The accuracy of CTs is critical, especially for protective relays, as they must accurately represent the current in the circuit. The accuracy class of a CT defines how closely the secondary current represents the primary current.

How do Voltage Transformers (VTs) or Potential Transformers (PTs) work in Substation?

Voltage transformers are used to measure or monitor the voltage across a high-voltage circuit. They are connected in parallel with the circuit, and their secondary winding is connected to instruments or devices that require voltage measurements, such as voltmeters, meters, and protective relays. Here's how they work:

• Step-Down Ratio: Similar to CTs, VTs also have a step-down ratio. The primary winding of a VT is connected across the high-voltage source, and the secondary winding is connected to measurement devices.
• Voltage Induction: The primary winding is exposed to the high voltage of the circuit. This voltage induces a lower voltage in the secondary winding, which is then connected to the measurement or protection devices.
• Accuracy: Just like CTs, the accuracy of VTs is crucial for reliable voltage measurements and protection operations. The accuracy class defines the precision of the voltage transformation.

Instrument transformers play a vital role in electrical substations by providing accurate measurements for monitoring and control purposes, as well as reliable inputs to protective relays that detect abnormal conditions and initiate protective actions. These transformers ensure the safety of equipment, personnel, and the overall power system by enabling accurate measurement and effective fault detection and protection.

Functions of Current Transformers (CTs)

Current transformers are used to step down high primary current levels to a secondary current level suitable for measurement by instruments and relays. They are primarily used for current measurement and protection purposes.

Functions of CT:

• Current Measurement: CTs convert the primary current flowing through a power line into a proportional secondary current that can be safely measured by ammeters and other measuring devices. This enables accurate monitoring of the current flow in the circuit without directly connecting instruments to the high-voltage primary circuit.
• Overcurrent Protection: CTs are essential components in protective relay schemes. They provide signals to relays that detect overcurrent conditions, helping to isolate faulty sections of the power system to prevent damage or outages.
• Ground Fault Protection: CTs are also used in ground fault protection systems to detect and mitigate ground faults by measuring the unbalanced current flow between the phase conductors and the ground.
• Energy Metering: CTs play a role in energy metering by providing accurate current measurements for billing and monitoring purposes in electrical substations and industrial facilities.

Functions of Voltage Transformers (VTs) / Potential Transformers (PTs)

Voltage transformers are used to step down high primary voltage levels to a secondary voltage level suitable for measurement by voltage meters, relays, and other devices. They are mainly used for voltage measurement and protection applications.

Functions of VT:

• Voltage Measurement: VTs reduce high primary voltages to a safe and manageable level for measuring voltage with voltmeters and other instruments. This allows accurate voltage monitoring without directly exposing measuring instruments to high voltages.
• Protective Relaying: VTs provide signals to protective relays that monitor voltage levels. These relays can detect abnormal voltage conditions, such as overvoltage or under voltage, and take appropriate actions to protect the power system.
• Metering and Monitoring: VTs are utilized for accurate voltage measurement and monitoring, essential for billing, control, and maintenance of power distribution systems.
• Control and Automation: VTs contribute to control and automation systems by providing reliable voltage signals that guide the operation of various control devices.

Instrument transformers are critical components of power systems, ensuring the safe and efficient operation of electrical networks while enabling accurate measurement and protection. They help protect equipment, prevent accidents, and ensure that power systems operate within safe limits.

Procedure for Connecting Instrument Transformers

The construction of current and potential transformers (CT and PT) will be discussed.

Instrument Transformers, specifically Current Transformers (CTs) and Potential Transformers (PTs), play a crucial role in power systems by providing accurate and scaled-down measurements of high currents and voltages, respectively. These measurements are essential for protection, control, and metering purposes. Here's an overview of the construction of CTs and PTs:

The construction of Current Transformers (CTs)

Current Transformers are used to step down high currents flowing through power lines to a level that can be easily measured by instruments. They are typically used for protective relaying, metering, and control applications. CTs are designed to maintain a high degree of accuracy, even under high fault conditions. Here's how they are constructed:

Core: CTs have a magnetic core made of high-permeability material, often laminated silicon steel. The core's purpose is to create a magnetic path for the current to flow through, which induces a proportionate voltage in the secondary winding.

Primary Winding: The primary winding consists of a few turns of heavy-gauge insulated wire. It is connected in series with the high-current line that needs to be measured. The primary winding carries the actual current that needs to be transformed.

Secondary Winding: The secondary winding has a much larger number of turns of finer gauge wire than the primary winding. It is wound around the same core as the primary winding. The secondary winding is responsible for producing a scaled-down output current that is proportional to the primary current.

Insulation: Since CTs are often used in high-voltage environments, careful insulation is essential to prevent electrical breakdown. Insulating materials such as paper, resin, or oil impregnation are used to isolate the windings and ensure safety.

The construction of Potential Transformers (PTs)

Potential Transformers, also known as Voltage Transformers, are used to provide a reduced and isolated voltage signal for metering and protective relaying purposes. They step down high voltages to a level suitable for instruments. Here's how PTs are constructed:

Core: Similar to CTs, PTs also have a magnetic core made of laminated silicon steel. The core provides a path for the magnetic flux generated by the primary voltage.

Primary Winding: The primary winding is connected across the high-voltage circuit whose voltage is to be measured. It typically consists of relatively fewer turns of wire compared to the secondary winding.

Secondary Winding: The secondary winding has a higher number of turns than the primary winding and is wound around the same core. The secondary winding produces a scaled-down output voltage that is proportional to the primary voltage.

Insulation: Just like with CTs, insulation is crucial in PTs to ensure safe operation in high-voltage environments. Adequate insulation materials are used to prevent breakdown and ensure reliable performance.

Both CTs and PTs are typically housed in protective casings to shield them from environmental factors and to provide additional insulation. Additionally, they might include features such as tap settings to adjust the transformation ratio slightly, if needed.

It's important to note that the construction details might vary slightly based on the specific design, application, and standards followed in different regions or industries.

CT and PT Terminals

CT (Current Transformer) and PT (Potential Transformer), also known as voltage transformers, are essential components in electrical power systems used for measurement, protection, and control purposes. They are both used to step down high currents and voltages respectively to levels that are safe and manageable for instrumentation and devices. These transformers are commonly used in various applications, such as metering, relaying, and monitoring in power systems.

Here are the terminals and their purposes for CTs and PTs:

Current Transformer (CT) Terminals

• Primary Terminals: These are the terminals where the high current from the power line flows through. The primary terminals are connected in series with the power circuit from which you want to measure the current. The current flowing through the primary terminals induces a proportional current in the secondary winding.
• Secondary Terminals: These are the terminals where the secondary winding of the CT is connected. The secondary winding produces a reduced current proportional to the primary current. This reduced current is used for measurement, protection, or control purposes.
• Polarity Terminals: In some CTs, you might find additional terminals referred to as "polarity terminals." These are used to ensure the correct phase relationship and directionality of the current between the primary and secondary windings. Proper polarity is crucial for accurate measurements and the proper functioning of protective relays.

Potential Transformer (PT) Terminals

• Primary Terminals: These are connected to the high-voltage source that you want to measure or monitor. The primary terminals are connected in parallel with the power circuit, allowing the PT to "tap" the voltage across the circuit.
• Secondary Terminals: The secondary winding of the PT is connected to these terminals. The secondary voltage produced is a scaled-down version of the primary voltage, suitable for measurement by instruments or for providing a reference voltage for protective relays.
• Polarity Terminals: Similar to CTs, some PTs might have polarity terminals. These ensure the correct phase relationship and directionality of the voltage between the primary and secondary windings. Proper polarity is essential for accurate voltage measurements and the proper functioning of protective relays.

It's important to note that both CTs and PTs should be installed and connected correctly to ensure accurate measurements and reliable protection in electrical systems. Improper installation or incorrect connection of terminals can lead to errors in measurements and potentially compromise the safety of the system. Always follow the manufacturer's guidelines and industry standards when installing and connecting CTs and PTs.

Can you 🙏 explain the distinction 🔍 between Instrument Transformers 🎛️ and Power Transformers ⚡? 

Of course! I'd be happy to explain the distinction between Instrument Transformers and Power Transformers.

Instrument Transformers

Instrument Transformers are devices used to measure and protect electrical systems by converting high currents and voltages into manageable levels that can be safely measured and processed by instruments and relays. They are primarily used in conjunction with measuring instruments, meters, relays, and protective devices in power systems. There are two main types of instrument transformers: Current Transformers (CTs) and Voltage Transformers (VTs), also known as Potential Transformers (PTs).

Current Transformers (CTs): CTs are used to step down the high currents present in power lines to a level that can be safely measured by current meters and instruments. They provide accurate current measurements for various purposes, such as load monitoring, overcurrent protection, and current-based relaying. CTs are commonly used in circuit breakers, protective relays, and energy meters.

Voltage Transformers (VTs) or Potential Transformers (PTs): VTs are designed to reduce high voltage levels to lower, manageable levels for measurement and protection purposes. They are commonly used to provide accurate voltage measurements to voltage meters, relays, and protective devices. VTs ensure that instruments and meters receive a representative voltage level without exposing them to the full system voltage.

Power Transformers:

Power Transformers, on the other hand, are devices used to transfer electrical energy between different voltage levels while maintaining the frequency. They are crucial components in power transmission and distribution systems, as well as in various industrial applications. Power transformers are responsible for stepping up the voltage at power generation plants to enable efficient long-distance transmission and stepping down the voltage at distribution substations to supply electricity to consumers.

Power transformers have two or more windings, usually referred to as primary and secondary windings, which are wound around a common core. When an alternating current flows through the primary winding, it creates a magnetic field that induces a voltage in the secondary winding according to the transformer's turn ratio. This enables the transformation of voltage levels while conserving power.

In summary, the main distinction between Instrument Transformers and Power Transformers lies in their functions and applications:

Instrument Transformers are used for measuring and protection purposes, converting high currents and voltages to manageable levels for instruments and relays.

Power Transformers are used to transfer electrical energy between different voltage levels while maintaining frequency, enabling efficient transmission and distribution of electricity in power systems.