MV Metering Accuracy: CT/VT Selection for Billing vs. Protection

MV Metering Accuracy: CT/VT Selection for Billing vs. Protection

MV metering accuracy is a system outcome: CT/VT selection, burden, wiring, and commissioning discipline all affect the final measurement. Many projects lose accuracy because they use protection-grade cores for billing or underestimate wiring burden.

This guide explains how to select CTs and VTs for billing metering versus protection, and how to prevent common accuracy losses.

Billing Metering vs Protection: Different Objectives

• Billing / revenue metering: accuracy under normal operating ranges is the priority.
• Protection: dependable behavior under faults is the priority (CT saturation behavior, relay performance).

These objectives often require separate CT cores, and sometimes separate VT windings, depending on your standard and scheme.

CT Selection for Billing Accuracy

• Ratio: align with expected load range so normal currents are measured with good resolution.
• Metering class: match your billing requirements.
• Burden: select a burden rating that covers meter + wiring with margin.

VT Selection for Billing Accuracy

• Ratio/secondary voltage: match the metering system requirements.
• Accuracy and burden: match billing intent, not protection-only needs.
• Protection/fusing: maintain stable signals and safe maintenance isolation.

The Wiring Burden Problem

Secondary wiring adds resistance and increases burden. Practical controls include:

• minimize secondary run length
• use appropriate conductor sizes
• avoid adding extra devices to billing circuits without recalculating burden

Commissioning Checks That Protect Accuracy

• record CT/VT nameplate data and verify against design
• verify polarity, ratio, and phase association
• verify meter scaling and energy direction
• verify as-built wiring length and terminal tightness

Internal Links

CT products: Current Transformers.

VT products: Voltage Transformers.

Switchgear metering cubicles: Switchgear.

Conclusion

MV metering accuracy depends on matching CT/VT class and burden to billing intent and validating the as-built installation. Separate billing and protection requirements, plan burden realistically, and verify polarity/ratio and scaling during commissioning.

Image Suggestions

• Image: https://www.enweielectric.com/products/switchgear
• ALT: "MV metering cubicle integrating CTs, VTs, and billing meters"
• Image: https://www.enweielectric.com/products/current-transformers
• ALT: "Metering CT selection for revenue measurement accuracy"