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Questions and Answers
What is the primary cause of current or ratio error in a current transformer?
What is the primary cause of current or ratio error in a current transformer?
What factor is critical for a current transformer to function optimally?
What factor is critical for a current transformer to function optimally?
What is the purpose of the Instrument Security Factor (FS) in current transformers?
What is the purpose of the Instrument Security Factor (FS) in current transformers?
In metering contexts, what is required from the secondary output of a current transformer during fault conditions?
In metering contexts, what is required from the secondary output of a current transformer during fault conditions?
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Which of the following is a typical configuration of current transformer cores for various applications?
Which of the following is a typical configuration of current transformer cores for various applications?
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What are the two main types of instrument transformers mentioned?
What are the two main types of instrument transformers mentioned?
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What important factor is included in the design considerations of instrument transformers?
What important factor is included in the design considerations of instrument transformers?
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In the context of current transformers, what does an open secondary winding imply?
In the context of current transformers, what does an open secondary winding imply?
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Which of the following is a primary application of current transformers?
Which of the following is a primary application of current transformers?
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What is one of the key components used in the construction of voltage transformers?
What is one of the key components used in the construction of voltage transformers?
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Study Notes
Instrument Transformers
- Current and voltage transformers are collectively called instrument transformers.
- Instrument transformers provide reduced current and voltage signals to protect metering and protection equipment connected to high voltage power systems.
- The use of instrument transformers allows for the safe and accurate measurement of current and voltage in high voltage electrical systems.
Current Transformers (CT)
- The error in amplitude in a current transformer is called the current or ratio error (ε).
- Defined as: (I'1 − I2) / I'1
- Current transformers work best when connected to low-impedance burdens.
Metering and Protection Application
- Current transformers are utilized for both metering and protection purposes.
- Metering cores are used for metering/revenue billing purposes and should provide current signals that are very accurate representations of the primary line values.
- Protection cores are used to provide protection signals to relay circuits.
Construction and Components
- Oil-immersed current transformers are currently the most commonly used design.
- Oil-immersed current transformers can be categorized into two types: Tank type (hair-pin) and Top Core (inverted type).
- Tank type: cores situated in a tank close to the ground, the primary conductor is U shaped or coil shaped.
- Top core: cores situated at the top of the transformer, the primary conductor is shaped as a bar or coil shaped.
Design Considerations
- The accuracy class of the current transformer is specified according to its application.
- Typical accuracy classes (as per IEC) are 0.2 for precision metering and 5P for protective relaying.
Polarity Markings
- Polarity (or terminal) markings on a current transformer designate the relative directions of instantaneous currents.
- Polarity markings are crucial for ensuring that the currents measured by the current transformer are accurate and in phase.
- Incorrect application of polarity markings can lead to incorrect relay operation and potential damage to equipment.
Voltage Transformers (VT)
- Voltage transformers are connected in parallel between phase and ground.
- The external insulator housing the capacitor stacks should provide sufficient creepage distance to prevent excessive flow of leakage current.
- Capacitance Voltage Transformers (CVTs) are also used in PLC communication systems.
Basic Concepts of CVT
- CVTs are constructed with a high-voltage capacitor connected in series with a low-voltage transformer to step down the voltage to a safe level for metering and protection devices.
- The capacitance of the capacitor allows the voltage to be stepped down without significantly affecting the current flow.
Construction and Components
- CVT construction can vary depending on the voltage rating and application.
- Common design features include a capacitor stack made of multiple layers of capacitor plates, a low-voltage transformer, and a housing typically filled with oil or SF6 gas.
Design Considerations
- The voltage factor (Fv) is an important consideration in CVT design.
- Fv dictates the voltage output of the CVT during fault conditions.
- The required voltage factor depends on the system earthing conditions, with 1.5 for effectively earthed neutral systems, and 1.9 for non-effectively earthed neutral systems.
Accuracy and Burden Rating
- The rated burden of the CVT is the value of the impedance of the secondary circuit.
- Similar to current transformers, accuracy classes for CVTs are specified separately for metering and protection purposes.
- The accuracy classes of CVTs are dependent on the application with typical accuracy classes (as per IEC) of 0.2, 0.5, or 1.0 for metering and 3P or 6P for protection purposes.
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Description
This quiz covers the critical concepts of instrument transformers, focusing on current transformers (CT) and their applications in metering and protection. Learn about the significance of current ratio error, and how these devices ensure safe and accurate measurement in high voltage systems.
