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Current transformers step down high primary currents to reduced secondary currents for measurement and protection, typically using a single-turn primary winding. Voltage transformers reduce high primary voltages to lower secondary voltages using multiple primary windings. Current transformers connect their secondary in series to low-impedance devices, while voltage transformers connect in parallel to high-impedance voltage-sensing devices.

A current transformer's primary winding often consists of a single turn by threading the power system's primary conductor directly through the transformer's core. This design allows the high current flowing through the conductor to induce a reduced current in the multiple secondary windings wound around the core, enabling accurate current measurement without additional winding complexity.

Connecting a current transformer's secondary to a low-impedance device ensures the entire secondary current flows through the sensing device rather than being diverted. This maximizes current flow and measurement accuracy. High impedance would cause current to divide between the sensing device and the transformer's shunt excitation impedance, introducing measurement errors.

Practical voltage transformers experience small voltage drops and phase-angle shifts between primary and secondary sides, deviating from the ideal scaled-down, in-phase representation. These errors occur due to transformer impedance and core losses. Despite these imperfections, voltage transformers remain essential for providing accurate voltage replicas suitable for protective relays and measurement devices in power systems.

Instrument transformers provide isolated, scaled-down replicas of system voltage and current for protective relays and measurement devices. By reducing high voltages and currents to manageable ranges, they enable accurate monitoring and control of electrical parameters. This scaling ensures relays can operate safely and reliably while maintaining system stability and enabling effective line protection with impedance relays.

Current transformer performance depends critically on accurately replicating primary current without saturation errors. Saturation occurs when the core reaches magnetic capacity, preventing further current induction and causing measurement inaccuracy. Excessive shunt excitation impedance can also divert current away from the secondary winding, reducing the transformer's ability to faithfully reproduce the primary current.

Voltage transformers connect in parallel to high-impedance devices to measure voltage across the device without drawing significant current. Current transformers connect in series to low-impedance devices to measure current flowing through the circuit. These opposite configurations reflect their different measurement purposes: voltage transformers sense potential difference, while current transformers sense current flow.