Low Power Voltage Transformers (LPVT) for Medium Voltage Measurement

The Challenge

Conventional voltage transformers have served MV substations reliably for decades, but their physical size, iron core construction, and installation requirements create real constraints in modern projects. In GIS installations and urban substations where space and field costs are high, a dedicated measurement cubicle represents a significant and often avoidable cost. Low power voltage transformers address these constraints directly, but selecting and specifying an LPVT system is not a straightforward component substitution. The measurement chain from transformer to output device must be engineered as a complete system. Accuracy class, burden values, and amplifier compatibility must be validated together, component-level specifications alone are not sufficient.

Our Approach

How It Works

Two system paths exist depending on installation type. In AIS installations, a support type LPVT is mounted on the busbar, connected by cable to a voltage amplifier, which then feeds the IED, PQ analyzer, or meter. In GIS installations, a cone type LPVT connects to a T-connector, either symmetric or asymmetric, followed by the same cable, amplifier, and IED chain. The voltage amplifier is the common element in both paths. It conditions the low-power signal from the sensor before it reaches the measuring device.

Applications

• EDAS distribution substations — primary market in Turkey
• GIS installations in urban areas where eliminating a dedicated measurement cubicle produces a direct cost saving
• 36kV systems, the most common operating voltage in Turkey
• Conventional VT replacement projects
• SCADA integrator measurement point expansion

IEC 61869-1 and IEC 61869-11 Compliance

LPVT is governed by IEC 61869-1, which covers general requirements for instrument transformers, and IEC 61869-11, which adds specific requirements for low-power passive voltage transformers. No Turkish regulatory mandate exists yet. Standard accuracy classes for measuring applications are 0.1, 0.2, 0.5, 1.0, and 3.0. Protection classes are 3P and 6P. Accuracy class is a system property. The LPVT and amplifier must be tested and certified together as a complete unit.

Selection Guide and Common Mistakes

Two critical mistakes appear repeatedly in LPVT specifications. The first is a GIS sensor and T-connector mismatch. Cone type LPVTs must match the specific T-connector geometry of the GIS installation, whether symmetric or asymmetric. This must be verified with the manufacturer before any order is placed. The second mistake is specifying the LPVT and voltage amplifier separately. The correct replacement for a conventional VT is the LPVT and amplifier as a matched and validated system. Burden values must be approved by the manufacturer for that specific combination. On accuracy class stacking: a 0.5 class LPVT paired with a 0.5 class amplifier does not deliver 0.5 class at the output. Measurement errors compound across the chain. The system must be tested and certified as a complete unit.

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