Voltage transformer

The voltage transformers (VTs) primary side is connected to the power network with the purpose to take the voltage down to a low voltage level where it safely can be connected to measurement devices. The most common VTs are magnetic (inductive) types and the typical connections for these VTs are (1) secondary phase-and-ground voltage with three double secondary VTs connected phase-to-ground and (2) secondary phase voltage with two single secondary VTs connected open delta.

Protective relays utilizing voltage are usually connected phase-to-phase, so the transformers are normally rated 120 V or 110 V line-to-line. Taps may be provided to obtain either 69.3/120 V or 63.5/110 V line-to-neutral. When available, double secondaries provide the means of obtaining zero-sequence voltage for ground relays. If only a single transformer secondary winding is available, an auxiliary wye ground-broken delta auxiliary VT can be connected to the secondary a, b, and c bus.

Voltage transformers Connection

Capacitor voltage transformers (CVT) or coupling capacitor voltage transformer (CCVT)

Another way to measure the primary voltage is to make it through a capacitor voltage divider — CVD, which contains capacitors connected in series where the secondary is measured over the last capacitor in the CVD. For high voltage CVTs, it is common that CCVTs are used where the last capacitor in the CVD is connected to an electromagnetic unit (EMU), which contains sufficient inductance for compensation of the capacitance in the CVD. The typical connection for this is secondary phase-and-ground voltage with three CCVTs connected phase-to-ground.

Low-power voltage transformers (LPVTs)

For voltage measurement in most medium-voltage low-power voltage transformers, is based on two principles: the resistive and the capacitive dividers. The output voltage of a voltage divider is directly proportional to the input voltage thus reproducing the actual waveform of the primary voltage signal. Due to its compactness, voltage sensors can be allocated into other equipment that are already placed in an existing switchgear (e.g., bushing, post insulator or insulating plug of cable connector). The requirements of the LPVTs were first described in the standard IEC 60044-7, released in 1999, that has been progressively replaced by a new set of standards under the IEC 61869 standard family. The new standard IEC 61869-11 covers the LPVTs without active electronics.

VTs and its circuits diagnosisRatio/phase analysis; an assessment of the VT’s accuracy

  • Saturation test; identification of core type; protection or meter
  • Short-Circuit Impedance test; a test to determine of the transformer impedance and losses
  • Winding resistance; used for calculations of saturation voltage and accuracy with specific burden
  • VT burden test; ensures that burden is within the VT’s VA limits
  • Fuse test; ensures that the cable and fuse to the relay has low resistance