In electrical power distribution systems, instrument transformers are responsible for converting high voltages and currents into lower, manageable levels that can be safely and easily measured by instruments, relays, and other devices.
Instrument transformers are found extensively in various applications, including circuit breakers, switchgear, UPS systems, and more. These important devices allow for accurate monitoring, control, and protection of the power system.
There are several different types of instrument transformer an electrical test technician will encounter in the field, each with their own purpose and unique design:
1. Current Transformer (CT)
Current transformers are used to measure electrical currents. They are primarily used to step down high currents to a level that can be safely measured by instruments.
CTs are installed around an electrical conductor. If this current is passed through the primary winding of a CT, the iron core inside becomes magnetized, which then induces a voltage in the secondary coils.
CTs are commonly used in power systems for protection and metering purposes. There are four typical types of current transformers: window, bushing, bar, and wound.
2. Potential Transformer (PT)
Voltage (Potential) Instrument Transformers
Potential transformers, also known as voltage transformers, are simple devices used to measure electrical voltages. They provide a reduced voltage output that is proportional to the primary voltage input, allowing for accurate monitoring and control of the power system.
This helps ensure that the protective relays can detect abnormal operating conditions, such as overvoltage or undervoltage, and initiate appropriate protective actions to prevent damage to the equipment and ensure system reliability.
PTs are usually installed in switchyards or substations close to the equipment they are monitoring. Polarity marking on PTs is essential for proper connection to protection relays.
3. Control Power Transformer (CPT)
Control power transformers are specifically designed to provide power to control circuits in electrical systems. They typically have lower voltages and power ratings compared to power transformers but higher than that of potential transformers used for metering.
Control power transformers are commonly used in motor control centers, switchgear, and control panels. It provides isolation between the control circuitry and the higher voltage levels present in the main power distribution system.
4. Capacitor Coupled Voltage Transformer (CCVT)
Capacitor Coupled Voltage Transformer (CCVT)
CCVTs are a type of instrument transformer used for measuring high voltages, typically in the range of 69 kV and above. Like other voltage transformers, they are primarily used to step down high voltage levels to a lower level that can be safely and accurately measured by meters and protective relays.
A capacitance voltage divider is used to take the line to ground voltage applied to the capacitor and reduce it to an intermediate voltage typically 5 to 20 kV. The electromagnetic unit (EMU) steps the voltage further down to the required output level, which is usually below 120 V.
Instrument Transformer Testing
Over time, instrument transformers may experience drift or degradation, leading to inaccurate measurements. Periodic testing ensures that the instrument transformer is still operating within acceptable accuracy limits, allowing for reliable and precise measurement of voltage and current levels.
Current Transformer Test Equipment Example Photo: Megger
Testing helps assess the performance of the instrument transformer under various operating conditions. It can identify issues such as saturation, ratio errors, phase angle errors, insulation breakdown, or any other discrepancies that may affect its performance.
A neglected instrument transformer poses a safety risk as it may fail to accurately detect fault conditions, jeopardizing the safety of personnel and equipment. This can result in delayed or inadequate protection, leading to potential equipment damage, electrical hazards, or even catastrophic failures.
Many electrical industry standards and regulations, such as NFPA and NETA, require periodic testing and maintenance of instrument transformers. Regular testing and maintenance help ensure the reliable and accurate operation of instrument transformers, contributing to the overall performance and safety of the electrical power system.
