Discover how instrument transformers play a crucial role in the accurate and safe measurement of voltage and current in electrical systems. Learn more about the types, functions, and importance of these essential electrical sensors.
Instrument Transformers for Precise Electrical Measurements
Instrument transformers, also known as voltage and current sensors, play a fundamental role in the accurate and safe measurement of electrical quantities such as voltage and current in electrical systems.
These devices are designed to replicate the real conditions of the electrical system, reducing the magnitude of the measured electrical quantities without altering their nature, shape, or frequency. Additionally, they provide galvanic isolation between the primary and secondary circuits, ensuring the safety of equipment and operators.
Function of Instrument Transformers:
The primary function of instrument transformers is to enable the precise measurement of electrical quantities in an electrical system. They reduce the voltage or current of the system to levels suitable for measurement, ensuring that measurement instruments receive safe and reliable signals.
Furthermore, instrument transformers play an important role in controlling and protecting electrical systems, providing essential information for monitoring and control devices.
There are several ways to classify instrument transformers. Two common classifications are:
1. Potential Transformers (PTs) and Current Transformers (CTs):
This classification is based on the electrical quantities being measured. PTs are used to measure voltage in an electrical system, while CTs are used to measure current.
2. Construction Type:
This classification refers to the physical construction of the instrument transformers. It may include inductive potential transformers (IPT), capacitive potential transformers (CPT), conventional current transformers, among others.
a) Potential Transformers (PTs):
Potential transformers, also known as voltage transformers, are designed to measure the voltage in an electrical system. They reduce the system voltage to a value suitable for measurement. There are two common types of PTs: inductive potential transformers (IPT) and capacitive potential transformers (CPT).
– Inductive Potential Transformers (IPT):
These PTs use a ferromagnetic core and copper windings to perform voltage reduction. They are widely used for measuring voltages between 600 V and 69 kV. IPTs are characterized by low resistances and leakage reactances, weak magnetization current, and moderate nominal induction.
– Capacitive Potential Transformers (CPT):
CPTs are employed for measuring voltages between 69 kV and 138 kV. They use capacitors to perform voltage reduction. When isolating the Object Near Live at High Voltage (ONLHV), the use of CPT is recommended. The choice between IPT and CPT depends on the voltage range and specific measurement requirements.
– Installation:
The installation of a potential transformer is performed in parallel with the circuit to be monitored. Fixing the connection claws to the high-voltage terminals must be done precisely, using appropriate equipment such as torque wrenches, to ensure proper contact between the measurement claws and the energized point. Additionally, the secondary terminals of a PT must never be short-circuited under any circumstances.
b) Current Transformers (CTs):
Current transformers are designed to measure the current in an electrical system. They are connected in series with the circuit through which the current flows, inducing a proportional current in the secondary winding.
They play an important role in the measurement and protection of currents in electrical systems. Here are some important concepts about CTs:
– Purpose:
The purpose of CTs is to replicate the primary current of the electrical system in its secondary, but with a reduced amplitude. Additionally, they ensure the decoupling of the primary and secondary signals, preventing interference between them.
– Operating Principle:
CTs follow the principle of electromagnetic transformation, where the primary current induces a proportional current in the secondary. However, CTs operate under two special conditions: (i) the primary current is completely independent of the transformer itself and (ii) the CT operates practically in short-circuit.
– Installation:
CTs can be installed externally (on brackets or poles) or internally, in a suitable shelter or box. They are connected in series with the circuit to be energized, and the connection of the equipment to the high-voltage terminals must be done precisely, using appropriate equipment such as torque wrenches, to ensure proper contact between the measurement claws and the energized point. The secondary terminals of a CT should always be short-circuited and should not be opened under any circumstances.
– Power Supply:
CTs supply measurement, protection, and control instruments, providing the reduced current necessary for these devices. It is important to note that CTs used in protection have a different specification from CTs used in measurement, especially regarding core saturation at higher currents.
– Representation of Transformation Ratio:
The transformation ratio of a CT is represented by a number, indicating how many times the primary current was reduced to obtain the secondary current. For example, a CT with a nominal ratio of 160:1 means that the secondary current is 1/160 (approximately 0.00625) of the primary current.
– Polarity:
The polarity of the CT is indicated in the same way as a conventional transformer, following the subtractive convention.
There are different types of CTs, depending on the current range to be measured and specific requirements:
– CTs have the primary in series with the circuit to be measured or protected. They follow the same principle of electromagnetic transformation, but with two special operating conditions: the primary current is independent of the transformer itself, and the CT operates practically in short circuit.
– CTs are classified according to their transformation ratio, represented by a number, such as RTC = 800 – 5A, indicating a nominal ratio of 160:1. The polarity of CTs is represented like a common transformer, following the subtractive convention.
Importance of Instrument Transformers:
Instrument transformers play a fundamental role in the accurate and reliable measurement of voltage and current in electrical systems. They enable the safe and efficient operation of electrical systems, providing essential information for monitoring, control, and protection. Additionally, their ability to isolate the primary circuit from the secondary circuit provides safety to equipment and operators, preventing the risk of electrical shock and damage to measuring devices.
Instrument transformers are widely used in sectors such as electrical power, industrial automation, telecommunications, among others, where precise and reliable measurement of electrical quantities is essential for the proper operation of systems.
In summary, instrument transformers play a vital role in the measurement and protection of electrical systems, ensuring accurate and reliable measurements of voltage and current. Their ability to reduce electrical quantities to suitable measurement levels, their decoupling between primary and secondary circuits, and their galvanic isolation are crucial characteristics for the safe and efficient operation of electrical systems.
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