Case Study: Continuous Thermal Monitoring at 110 kV Hoi Hop and 22/0.4 kV Ao Hai Substations (EVNNPC)

Key Project Parameters

Operating Utility

EVN Vinh Phuc — Northern Power Corporation (EVNNPC), Vietnam

Substation Sites

110 kV Hoi Hop Substation; 22/0.4 kV Ao Hai Distribution Substation

Commissioning Date

November 2023

Monitoring Scope

20+ direct contact monitoring points, recording load current, voltage, and phase imbalance

Equipment Monitored

Current transformers, disconnectors, circuit breakers, 110 kV & LV transformer bushings, tank flanges, and 0.4 kV incoming/outgoing MCCBs

Thermal Camera Tracking

0.4–3.2 °C variance relative to calibrated handheld infrared cameras

Defects Detected

2 confirmed anomalies — 110 kV bushing on Transformer T2; Phase C contact on 0.4 kV branch MCCB. Both resolved proactively

Primary Scholarly Citation

Electrical Engineering Portal, January 5, 2026

Why Do Substation Electrical Connections Require Continuous Monitoring?

The vast majority of thermal electrical failures originate at conductive joints. Bolted busbar connections loosen under cyclic thermal expansion and mechanical vibration. Crimped cable lugs degrade over time. Switchgear contacts suffer surface oxidation and contact wear. As contact resistance increases, local heating accelerates exponentially according to Joule's law ($I^2R$). CIGRE TB 999 (§ 2.2.1) outlines the degradation chain: contact loosening → resistance rise → local overheating → surface oxidation → dielectric flashover.

Periodic surveys with handheld thermal cameras remain widespread maintenance practice. However, they carry three critical limitations:

• Single snapshot in time: A connection may overheat severely during evening peak loading, yet appear normal during a daytime inspection when current is low.
• Lack of optical line of sight: Connections inside enclosed switchgear cubicles, cable boxes, or behind silicone insulation boots cannot be viewed by external thermal cameras.
• Sparse inspection intervals: Surveys are typically performed monthly, quarterly, or annually; rapid degradation can culminate in flashover between scheduled rounds.

A global utility survey in CIGRE TB 999 (p. 197) reveals that 94.34% of grid operators still rely on manual inspections, with 75.47% using periodic thermography for HV-MV assets. Continuous 24/7 monitoring (CTM) eliminates blind spots between scheduled maintenance rounds.

Deployed Equipment and Monitored Points

110 kV Hoi Hop Substation

Wireless direct-contact sensors were installed at 11 positions across the 110 kV outdoor yard, including current transformers, disconnectors, and circuit breakers. On 63 MVA Power Transformer T2 (115/38.5/23 kV), sensors monitored primary high-voltage (HV), medium-voltage (MV), and low-voltage (LV) bushing terminals, along with tank flange transitions. A fixed infrared camera was also deployed for overall thermal tracking.

22/0.4 kV Ao Hai Distribution Substation

At Ao Hai distribution substation, sensors monitored transformer low-voltage terminals, incoming MCCB busbar connections, and outgoing 0.4 kV feeder lines. Integrated current sensors recorded primary line currents, voltage levels, and phase balance synchronously with temperature readings.

Monitoring Parameters and Alarm Logic

The platform aggregates real-time data across more than 20 points: bushing temperatures, contact temperatures, transformer oil, and ambient air. Data is presented via web dashboard and mobile interface. Four automated alarm triggers are configured:

• Thermal Overheating Alarm: Triggered by absolute temperature threshold or inter-phase delta-T between symmetrical phases of the same feeder;
• Current Overload Alarm: Triggered when measured primary current exceeds equipment continuous rated capacity;
• Voltage Tolerance Alarm: Triggered when operating voltage deviates beyond utility tolerance limits;
• Phase Imbalance Alarm: Triggered by abnormal divergence among the three phase currents.

Phase-to-phase comparative analysis ($\Delta T$) proves most effective. Since symmetrical phases carry comparable load current, a distinct thermal divergence on a single phase definitively isolates contact degradation rather than ambient solar loading.

Measurement Accuracy Compared to Infrared Cameras

Direct contact sensor readings were benchmarked against professional handheld thermal cameras during field verification. Discrepancies ranged between 0.4 °C and 3.2 °C, well within the combined uncertainty of contact thermistors and optical infrared devices. Optical infrared cameras feature a base accuracy of ±2 °C or ±2% and are susceptible to surface emissivity variations and measurement angles.

IEEE Std 2969-2025 (Annex A.2) underscores a pivotal industry reality: "The accuracy of the measurement is potentially less important than detecting temperature changes over time." A clear upward temperature trend on one phase provides decisive maintenance intelligence that periodic spot-checks cannot match.

Real Equipment Anomalies Detected

Incident 1 — 110 kV Bushing Terminal on Transformer T2, Hoi Hop Substation

Temperatures on Phases A and C of Transformer T2's 110 kV bushing terminals climbed substantially above Phase B during evening peak loading, tripping the inter-phase delta-T threshold. Follow-up infrared imaging confirmed a copper-aluminum transition hotspot inside the terminal clamp. Because the anomaly developed during the evening load peak, daytime manual surveys had consistently missed it.

Incident 2 — Phase C Outgoing MCCB, Ao Hai Substation

The monitoring system flagged an accelerated thermal rise on Phase C of branch MCCB 2 in the 0.4 kV board. Field maintenance investigated on November 21, 2023 and verified high contact resistance. During a scheduled maintenance outage on November 27, 2023, the circuit was isolated, contact faces cleaned, and bolts torqued to specification. Upon re-energization, Phase C temperature returned to equilibrium — resolving the defect in less than one week with zero unscheduled customer outages.

Operational Lessons and Next-Generation Engineering Solutions

The 2023 pilot yielded key engineering insights that have been integrated into the current VTI-TS commercial product series:

Benchmarking Against IEEE 2969, CIGRE TB 999, and NFPA 70B

The pilot substations encompass three voltage classes (110 kV, 22 kV, and 0.4 kV). The table below compares the applicable international standards:

Critical Notice for Facilities Operating Under NFPA 70B: Under Sections 9.2.1.1–9.2.1.3, an unresolved high-priority thermal alarm shifts equipment from Condition 1 (60-month maintenance interval per Table 9.3.2) down to Condition 3 (reduced to 12 months). Nuisance alarms cause unnecessary maintenance expense. VTI's phase-comparison algorithm effectively screens out false alarms triggered by ambient weather or balanced load surges.

VTI-TS Continuous Thermal Monitoring Product Line

The current production VTI thermal monitoring system is built around the VTI-TS wireless sensor series (formerly TBR22) and the GW22 / GW22-D dual-band gateway hub.