Precision Power Monitoring: High-Accuracy Dual-Ratio Outdoor Digital Current Transformer Solution

ROCKWILL

07/14/2025

Solution Summary:
Our advanced Outdoor Current Transformer (CT) solution integrates Multi-Tap Dual-Ratio Sensing with a Digital Merging Unit (MU) to deliver adaptive, high-precision current measurement for dynamic power environments like renewable integration sites. This IEC 61850-compliant system eliminates traditional accuracy/reliability trade-offs by enabling real-time ratio switching and digital data output.

Core Technology Breakdown

Dual-Ratio Sensing Core

Innovation: Embedded multi-tap windings supporting 200:5 and 1200:5 ratios within a single CT body.
Adaptability: Software-controlled ratio selection via the MU.
Problem Solved: Automatically adjusts measurement range to accommodate drastic load shifts (e.g., solar farm daylight fluctuations or wind farm cut-in/cut-out) without physical intervention or overdesigned CTs.

High-Precision Digital Merging Unit (MU)

Accuracy: Exceeds <0.2S Class (per IEC 60044-1/IEC 61869) for both revenue metering (ANSI C12.1) and protection functions (IEEE C37.90).
Output: Native IEC 61850-9-2 LE sampled values over Ethernet.
Calibration: Digital signal processing compensates for phase errors & non-linearity.

Key Features & Advantages

Dual-Ratio On Demand: Switch seamlessly between 200:5 (low load/high sensitivity) and 1200:5 (high fault current) via software commands – no rewiring needed.
Substation-Ready: Native digital output (9-2 LE) integrates directly into IEC 61850- based protection relays, meters, and control systems.
Future-Proof Accuracy: <0.2S accuracy ensures compliance with strictest revenue metering standards and enables high-impedance differential protection.
Robust Outdoor Design: IP67-rated enclosure, UV-resistant polymer housing, and corrosion-resistant materials ensure reliability in harsh environments (-40°C to +70°C).
Reduced Installation Cost: Single CT installation replaces need for multiple conventional CTs or burdensome auxiliary units.
Enhanced Grid Stability: Precise, real-time data enables faster protection response for fluctuating fault currents (e.g., inverter-based resource faults).

Implementation: Renewable Energy Use Case

Scenario: 200 MW Solar/Wind Farm connection to 138kV substation. Fault currents range from near-zero (offline inverters) to 30kA+ (full output grid fault).
Challenge: Conventional CTs compromise accuracy at low current (200:5 needed) but saturate at high fault currents (1200:5 needed).
Our Solution:

Install dual-ratio CTs at point of interconnection (HV bus/circuit breaker).
During normal operation (<50% load), MU selects 200:5 ratio – capturing granular data for precise energy accounting and flicker monitoring.
Upon fault detection (rapid current rise), MU auto-switches to 1200:5 ratio in <5ms – preventing saturation, maintaining accuracy, and enabling relays to clear faults reliably.
Sampled values transmitted digitally via 9-2 LE to protection relays, meters, and SCADA – eliminating analog wiring errors and converter delays.

Why This Solution Wins

Eliminates Saturation Risk: Maintains accuracy across 100:1 dynamic range (e.g., 30A to 3000A primary).
OPEX Savings: Remote configuration/ratio switching reduces site visits.
Safety Boost: Digital isolation replaces dangerous CT secondary opens.
Data Integrity: Digital output avoids signal degradation from EMI/RFI.
Compliance Ready: Designed to meet IEEE 1547-2018 fault ride-through requirements for renewables.