Solving Low-Load Accuracy Issues: A Digital Meter Upgrade Guide for Oilfield Power Grids

2yrs + staff 1000+m² US$300,000,000+ China

I. Introduction and Background

Electrical measuring instruments are critical monitoring devices for the safe, stable, and economical operation of power grids. Traditionally, analog pointer-type electrical measurement indicating instruments have been widely used in oilfield grid substations. However, with grid development and increasing demands for measurement accuracy and reliability, pointer instruments have revealed numerous shortcomings in long-term application, such as significant reading errors, inaccurate indication under low loads, and difficulties in range matching.

To modernize substation operation monitoring and ensure the accuracy, intuitiveness, and reliability of data measurement, this proposal recommends a comprehensive upgrade from existing pointer instruments to digital electronic instruments. Digital instruments, with their high accuracy, ease of reading, strong anti-interference capability, and convenient installation and maintenance features, represent the ideal solution to current problems.

II. Current Situation and Problem Analysis (Limitations of Pointer Instruments)

The currently used pointer instruments primarily suffer from the following urgent issues:

Reading Errors: Reliance on manual visual reading easily introduces parallax errors. Improper reading methods also contribute to human error, compromising data accuracy.
Severe Inaccuracy under Low Loads: The actual load in oilfield substations often falls within the 5%-10% range of the instrument scale. However, the accurate indication range for pointer instruments is only 20%-80% of the scale. Under such low loads, readings can deviate from the actual value by tens or even hundreds of amperes, rendering monitoring meaningless.
Impractical Range Replacement: To bring the indication into the accurate range, changing the instrument range is necessary, but this must match the current transformer ratio. As measurement and protection transformers are often manufactured as an integrated unit, replacing transformers involves massive engineering work and high costs, making it impractical.

III. Solution: Advantages and Application of Digital Electronic Instruments

1. Measurement Principle

Digital instruments utilize advanced A/D (Analog-to-Digital) conversion technology. They first convert continuous analog electrical quantities (like voltage, current) into discrete digital quantities before measurement, processing, and display. This fundamentally differs from the direct analog driving mechanism of pointer instruments.

2. Core Advantages Comparison

Digital instruments hold overwhelming advantages over pointer instruments, as detailed in the table below:

Advantage Category	Specific Characteristics of Digital Instruments
Display & Reading	Direct digital display provides intuitive, clear results; completely eliminates viewing angle errors; enables fast and convenient reading.
Measurement Performance	High accuracy with small measurement errors; high sensitivity, maintaining precise indication especially under low-load conditions.
Ease of Use	High input impedance minimizes impact on the measured circuit; no installation angle restrictions allow flexible layout; simple operation with fast measurement response.
Energy Consumption & Durability	Low self-power consumption, energy-efficient and environmentally friendly; good overload protection capability, less prone to damage from overloads.

3. Application Positioning

Based on the above advantages, digital electrical measuring instruments are the preferred solution for instrument upgrades and intelligent operation and maintenance in oilfield grid substations. They effectively address the inherent drawbacks of pointer instruments, significantly enhancing operational monitoring levels and decision-making efficiency.

IV. Key Points for Implementation and Deployment

To ensure the smooth implementation and long-term stable operation of the digital instrument retrofit project, the following aspects require emphasis:

Auxiliary Power Supply Configuration:

Reliability Priority: It is recommended that the instrument's auxiliary power supply be fed from the DC power system, or from reliable sources like standby lighting circuits or circuits with backup power within the substation auxiliary power system. This prevents instrument power loss during a total substation power outage, which could lead to operator misjudgment.
Independent Protection: Each instrument's auxiliary power circuit should be equipped with a dedicated fuse or high-breaking-capacity miniature circuit breaker to ensure effective isolation in case of a fault.

Standardization and Aesthetics:

The type, panel color, cutout dimensions, etc., of the selected digital instruments should be standardized to maintain the overall aesthetics and consistency of the control panels/cabinets.

Anti-Interference Measures:

Given the complex electromagnetic environment within substations, select proven products that have passed tests for strong electric and magnetic field environments.
During the design and installation phases, pre-emptive measures like shielding and proper grounding must be implemented to ensure long-term stable operation of the instruments in harsh electromagnetic conditions.

Calibration and Maintenance Cycle:

All digital instruments should be included in a periodic calibration schedule, with a recommended calibration cycle of 1 year.
To ensure measurement accuracy, instruments should be powered on and preheated for 15 minutes before any important measurement or calibration.

Technical Support and Follow-up:

After the retrofit and commissioning, the supplier should conduct user follow-up visits, promptly address operational issues, and provide necessary technical explanations and training to operational personnel.

V. Calibration Methods for Key Digital Instruments

To guarantee measurement accuracy, all newly installed and periodically inspected digital instruments must be calibrated according to specifications. Below is an outline of the calibration process for main instrument types:

General Preliminary Steps: Connect the auxiliary power supply; check that the digital display or screen shows normally.
Ammeter Calibration: Connect wires according to the wiring diagram; apply a standard AC current (e.g., 5A); adjust the calibration potentiometer to meet specifications; then apply proportional currents (e.g., 2.5A, 1.25A) to verify linearity.
Voltmeter Calibration: First zero the instrument; then connect wires according to the wiring diagram corresponding to the voltage level (e.g., 35KV, 6KV); input a standard voltage (e.g., 100V); adjust the corresponding potentiometer for correct display; and verify linearity.
Active/Reactive Power Meter Calibration:

Use a standard source to output standard voltage and current, controlling their phase angle.
Active Power Meter: Zero the instrument at phase angle φ=90° (cosφ=0); adjust the full scale at φ=0° (cosφ=1); check linearity at points like φ=30°, 60°, etc.
Reactive Power Meter: Zero the instrument at phase angle φ=0° (sinφ=0); adjust the full scale at φ=90° (sinφ=1); and check linearity.

Power Factor Meter Calibration: Calibrate at a phase angle difference of 0° (Power Factor=1.00) and specific angles (e.g., 140°) to ensure accurate display values.