Monitoramento da temperatura do óleo do transformador com operação automática do disjuntor

• O aumento da temperatura do óleo do transformador é o primeiro sinal mensurável de envelhecimento do isolamento e danos no enrolamento – deve ser monitorado continuamente, não verificado periodicamente
• Um sistema completo de monitoramento da temperatura do óleo consiste em sensores de temperatura, uma unidade de controle, um circuito de alarme, e um circuito de disparo automático do disjuntor
• A temperatura superior do óleo e a temperatura do ponto quente do enrolamento são dois pontos de medição distintos – ambos são necessários para uma proteção térmica completa
• Quando a temperatura do óleo excede o limite configurado, o sistema aciona a bobina de disparo do disjuntor e desconecta o transformador automaticamente
• A proteção em dois estágios – um alarme de alta temperatura seguido de um desarme por superaquecimento – oferece aos operadores uma janela de resposta antes da desconexão automática
• Fiber optic temperature sensors measure winding hot spot temperature directly inside the transformer tank without any metallic conductors in the oil
• CEI 60076 defines the thermal class and allowable temperature rise limits that determine where trip thresholds must be set
• Real-time online monitoring detects thermal anomalies hours or days before they escalate into a protection trip or catastrophic failure

Índice

1. Why does transformer oil temperature rise and what damage does it cause?
2. What does a transformer oil temperature monitoring system consist of?
3. Where exactly should temperature sensors be installed on a transformer?
4. How does oil temperature monitoring automatically operate a circuit breaker?
5. What is the difference between an alarm trip and an emergency trip in transformer protection?
6. How are automatic circuit breaker trip thresholds set for transformer oil temperature?
7. Why does a transformer need real-time online monitoring instead of periodic inspection?
8. Perguntas frequentes: Monitoramento da temperatura do óleo do transformador

1. Why does transformer oil temperature rise and what damage does it cause?

Every power transformer generates heat as a by-product of its normal operation. Core losses from magnetic hysteresis and eddy currents produce a steady baseline heat load regardless of the connected load. Copper losses in the primary and secondary windings add a load-dependent heat component that rises with the square of the current. Sob condições normais de operação, the transformer oil absorbs this heat and transfers it to the tank surface and cooling radiators, maintaining the winding temperature within the design envelope.

Problems begin when the heat input exceeds the cooling system’s capacity to dissipate it. Blocked radiator fins, failed cooling fans, a seized oil pump in a forced-oil-cooled unit, or a sustained overload condition all reduce the margin between heat generation and heat removal. The oil temperature climbs, and with it the temperature of every winding turn immersed in that oil.

The Arrhenius effect on insulation life

Transformer winding insulation — primarily kraft paper impregnated with oil — degrades according to an Arrhenius rate law. For every 6–8 °C rise in sustained winding temperature above the insulation’s rated thermal class, the expected service life of the insulation is approximately halved. A transformer running 20 °C above its rated top oil temperature for an extended period is consuming years of insulation life in weeks. The damage is cumulative and irreversible: once cellulose insulation has thermally degraded, no maintenance procedure restores its dielectric strength.

Failure sequence without temperature protection

In the absence of proteção térmica do transformador, the degradation sequence moves through predictable stages. Insulation brittleness increases, reducing its ability to withstand the mechanical forces of through-fault currents. Dissolved gas levels in the oil rise — detectable by Dissolved Gas Analysis (DGA) — as paper and oil begin to decompose thermally. Eventualmente, a routine fault current or switching transient that the transformer would otherwise have survived without consequence causes an inter-turn short circuit or a winding-to-tank flashover, resulting in a catastrophic failure that takes the unit out of service for months and requires complete rewinding or replacement.

2. What does a transformer oil temperature monitoring system consist of?

UM transformer oil temperature monitoring system is an integrated protection chain. Each component in the chain must function correctly for the system to deliver reliable automatic protection.

Sensores de temperatura

The sensing layer measures the actual temperature at the critical points in the transformer. Dial-type oil thermometers with micro-switch contacts are the traditional solution for top oil temperature measurement on the transformer tank. Detectores de temperatura de resistência (IDT) — typically Pt100 elements — provide accurate analogue signals compatible with electronic monitoring systems. Sensores de temperatura de fibra óptica using fluorescence decay principles are increasingly deployed for direct winding hot spot measurement inside the transformer tank, where their immunity to electromagnetic interference and the absence of metallic conductors in the oil make them the safest and most accurate available option.

Temperature controller and monitoring unit

O controlador de temperatura do transformador receives signals from all sensors, displays measured values locally, compares them against configured alarm and trip thresholds, and drives output relays when thresholds are exceeded. Modern units incorporate data logging to store temperature histories with timestamps, RS-485 or Ethernet communication ports for SCADA integration, and configurable relay outputs for cooling system control as well as alarm and trip signalling.

Alarm and trip relay circuit

The output relay contacts from the temperature controller are wired into the substation protection scheme. The alarm relay contact drives an audible or visual annunciator in the control room. The trip relay contact is wired in series with the circuit breaker trip coil — when the contact closes, it energises the trip coil and the breaker opens, disconnecting the transformer from the supply.

Controle do sistema de refrigeração

Maioria sistemas de monitoramento de temperatura de transformadores also control the cooling equipment. As oil temperature rises through defined stages, the controller switches on additional cooling fans or starts the oil circulation pump automatically, increasing cooling capacity before the alarm threshold is reached. This staged cooling response reduces the frequency of alarm events and extends transformer service life by keeping the operating temperature as low as practical.

3. Where exactly should temperature sensors be installed on a transformer?

Correct sensor placement is the foundation of effective monitoramento de temperatura do transformador. Measuring at the wrong location gives a reading that does not represent the thermal stress the insulation is actually experiencing.

Medição da temperatura superior do óleo

O temperatura superior do óleo sensor — a dial thermometer or RTD element — is installed in a purpose-built pocket on the transformer tank cover, immersed in the oil at the highest point of the tank. Because hot oil rises, the top oil temperature represents the hottest oil in the cooling circuit and gives the best available indirect indication of winding thermal stress in the absence of direct winding sensors. CEI 60076-2 specifies the measurement location and pocket dimensions for calibration purposes.

Winding hot spot temperature measurement

O temperatura do ponto quente do enrolamento is the highest temperature anywhere in the winding assembly. It occurs at a point approximately one-third to one-quarter of the winding height from the top in most transformer designs, where the combination of local heat generation and reduced oil flow velocity is most severe. Directly measuring this temperature requires a sensor installed inside the transformer tank, between the winding conductors.

Why fiber optic sensors are used for winding hot spot measurement

Sensores de temperatura de fibra óptica are the established method for direct winding hot spot measurement in oil-immersed transformers. The sensing probe — a small-diameter optical fiber with a fluorescent element at its tip — is inserted between winding conductors during transformer manufacture or rewinding. Because the probe contains no metallic conductors, it introduces no additional current path, sem risco de curto-circuito entre espiras, e nenhuma interferência eletromagnética no sinal de medição. O cabo de fibra sai do tanque através de uma glândula à prova de óleo especialmente construída e se conecta a uma unidade de interrogação montada na parte externa do tanque. A detecção por fibra óptica de fluorescência oferece precisão de medição de ±1 °C e uma vida útil da sonda superior a 30 anos sob imersão contínua em óleo.

Referência de temperatura ambiente

UM sensor de temperatura ambiente montado na sombra adjacente ao transformador fornece a leitura de referência usada para calcular o aumento de temperatura acima do ambiente - o parâmetro que a IEC 60076 usa para definir os limites térmicos em vez da temperatura absoluta, uma vez que a temperatura absoluta varia com a altitude e o clima do local.

4. How does oil temperature monitoring automatically operate a circuit breaker?

The automatic circuit breaker operation in response to transformer overtemperature is a straightforward protection logic implemented through relay contacts and the breaker’s built-in trip coil. Understanding the signal path clarifies why the system is reliable and why it responds faster than any manual intervention could.

The trip signal path

When the temperature controller determines that the measured oil or winding temperature has exceeded the configured overtemperature trip threshold, it energises an output relay. The normally-open contact of that relay closes, completing a DC circuit that flows through the circuit breaker trip coil. The trip coil generates a magnetic force that releases the breaker’s spring-loaded mechanism, and the breaker opens its main contacts within 50–100 milliseconds of the trip coil being energised. The transformer is isolated from the supply before any further thermal damage can occur.

Trip coil supervision

In well-designed protection schemes, o trip coil circuit is continuously supervised by a trip circuit supervision (TCS) relay that monitors the continuity of the trip coil and its associated wiring. If the trip coil burns out or a wire breaks, the TCS relay raises an alarm immediately — before the protection system is called upon to operate. This supervision function is essential because a failed trip circuit is a silent defect that only reveals itself at the worst possible moment.

Lockout relay integration

Para transformadores críticos, o temperature trip output is typically wired to a lockout relay (86 relay in ANSI nomenclature). The lockout relay seals in its operated state and prevents automatic re-energisation of the transformer after a temperature trip. An operator must physically reset the lockout relay at the switchboard after investigating and resolving the thermal fault — a deliberate design choice that prevents the transformer from being reconnected to a fault condition by automatic reclosure schemes.

5. What is the difference between an alarm trip and an emergency trip in transformer protection?

Two-stage thermal protection is standard practice for monitoramento de temperatura do transformador de potência. The two stages serve different purposes and drive different responses.

Estágio 1 — High temperature alarm

O alarme de alta temperatura is the first stage, set at a temperature that indicates the transformer is operating outside its normal range but has not yet reached a level that demands immediate disconnection. When this threshold is crossed, the monitoring system activates an audible or visual alarm in the control room, initiates maximum cooling (switching on all available fans and oil pumps), and logs the event with a timestamp. The transformer remains in service. Operations staff are expected to investigate the cause — a blocked radiator, a failed cooling fan, an overload condition — and take corrective action within the time available before the second-stage threshold is reached.

Estágio 2 — Overtemperature trip

O overtemperature trip is the second stage, set at a temperature above which continued operation would cause rapid and irreversible insulation damage. When this threshold is crossed, the monitoring system immediately drives the circuit breaker trip coil. No operator action is required or expected — the system disconnects the transformer automatically. The time between the Stage 1 alarm and the Stage 2 trip gives operators a defined window to attempt a load reduction or cooling restoration before the automatic disconnection occurs. In most utility and industrial protection settings, this window is between 10 e 30 minutes depending on how widely the two thresholds are separated.

Cooling failure alarm

A third alarm output — sometimes called the cooling failure alarm — is triggered when a cooling fan or pump motor fails regardless of the current oil temperature. This alarm alerts maintenance staff to restore cooling capacity before the thermal headroom is consumed, providing the earliest possible warning of a developing overtemperature condition rather than waiting for the temperature itself to rise.

6. How are automatic circuit breaker trip thresholds set for transformer oil temperature?

Threshold setting is an engineering task, not a default configuration exercise. The correct values depend on the transformer’s insulation thermal class, its cooling method, the site ambient temperature, and the load profile it serves.

CEI 60076 temperature limits

CEI 60076-2 (Temperature rise for liquid-immersed transformers) defines the maximum allowable temperature rise above a 40 °C reference ambient for each thermal class. For a standard Class A (óleo mineral, ONAN cooling) transformador, the maximum top oil temperature rise is 60 K and the maximum average winding temperature rise is 65 K, giving a maximum top oil temperature of 100 °C and a maximum average winding temperature of 105 °C at the 40 °C reference ambient. The winding hot spot is permitted to be up to 78 K above ambient — reaching 118 °C — under rated load conditions.

Practical alarm and trip settings

Na prática, o alarme de alta temperatura for top oil is commonly set at 85–90 °C, providing a margin of 10–15 °C below the IEC limit that gives the cooling system time to respond. O overtemperature trip for top oil is commonly set at 95–100 °C. Para temperatura do ponto quente do enrolamento measured directly by fiber optic sensors, the alarm is typically set at 110 °C and the trip at 120–125 °C, reflecting the higher permitted hot spot values in IEC 60076-7 (guia de carregamento).

Site-specific adjustments

Transformers installed at high altitude sites experience reduced air density that impairs convective cooling. CEI 60076-2 specifies a derating factor that reduces the allowable temperature rise for every 500 m above 1000 m altitude. Transformers in arctic climates with maximum ambient temperatures significantly below 40 °C may have their alarm thresholds adjusted upward to avoid nuisance alarms during legitimate maximum-load operation. All threshold adjustments must be documented in the protection settings record and reviewed whenever the transformer’s load profile changes significantly.

7. Why does a transformer need real-time online monitoring instead of periodic inspection?

Manual inspection of transformer oil temperature — a technician reading the dial thermometer on the tank during a site visit — is the minimum baseline practice. It is not adequate protection for any transformer whose failure would cause significant production loss, supply interruption, or safety risk.

The time gap problem

A transformer can move from normal operating temperature to a critical overtemperature condition in under an hour under the right combination of load increase and cooling failure. A weekly inspection schedule leaves a 168-hour window during which this transition can occur, progress through the insulation damage phase, and reach catastrophic failure without any external indication. Real-time online monitoring closes this window completely — the system is evaluating every temperature reading against its alarm thresholds on a continuous basis, every minute of every day.

Load-correlated trending

UM continuous transformer temperature monitoring system accumulates a temperature history correlated with the load current at every point in time. This dataset reveals patterns that no periodic inspection can identify: a transformer that consistently reaches 88 °C on weekday afternoons when load peaks, or a unit whose temperature response to a given load level has been creeping upward over six months as a cooling radiator gradually silts up. Both patterns are actionable maintenance intelligence. Neither is visible from a monthly dial reading.

Automatic response eliminates human delay

When a thermal event develops rapidly — a sudden cooling pump failure at peak load on a hot summer afternoon — the time between the overtemperature threshold being crossed and the circuit breaker opening is determined entirely by the relay operating time, measured in milliseconds. No human operator can match that response speed. O automatic circuit breaker operation driven by the online monitoring system is the only protection mechanism fast enough to intervene before serious insulation damage accumulates in a rapid overtemperature event.

Perguntas frequentes: Monitoramento da temperatura do óleo do transformador com operação automática do disjuntor

1. What is the difference between top oil temperature and winding hot spot temperature?

Temperatura máxima do óleo is the temperature of the hottest oil in the transformer tank, measured at the top of the tank where heated oil accumulates. It is an indirect indicator of winding thermal stress and is the standard measurement point on most transformer installations. Temperatura do ponto quente do enrolamento is the highest temperature at any point within the winding conductors themselves — it is always higher than the top oil temperature due to the additional heat generated in the conductors and the local reduction in oil cooling flow. CEI 60076-7 uses the hot spot temperature as the primary parameter for transformer loading calculations and insulation life assessment. Direct measurement of hot spot temperature requires a sensor — typically a sonda de fibra óptica — installed between the winding conductors inside the tank.

2. What types of transformers require oil temperature monitoring?

Any oil-immersed transformer carrying a load whose interruption would cause significant operational, financeiro, or safety consequences warrants oil temperature monitoring with automatic protection. This includes power transformers in utility substations, industrial plant transformers feeding continuous-process equipment, data centre supply transformers, hospital essential services transformers, and traction transformers in railway applications. Distribution transformers on public networks are typically protected by overcurrent and earth fault relays rather than temperature monitoring, but larger pad-mounted and network transformers in high-density urban areas increasingly incorporate temperature monitoring as part of a condition monitoring programme.

3. How does an oil temperature relay connect to a circuit breaker trip coil?

O oil temperature relay — whether a mechanical bimetallic device in a traditional dial thermometer or an electronic output relay in a digital temperature controller — provides a volt-free contact output. This contact is wired in series with the station battery DC supply and the circuit breaker trip coil. When the relay contact closes, DC current flows through the trip coil, which releases the breaker mechanism and opens the main contacts. The circuit is entirely independent of the AC supply voltage, so the protection operates correctly even during a supply voltage depression or disturbance.

4. What communication protocols do transformer monitoring systems support?

Moderno transformer temperature monitoring units typically support RS-485 with Modbus RTU as the baseline communication interface, which is natively compatible with the majority of SCADA and energy management systems. CEI 61850 is increasingly specified for new substation installations, with protocol conversion gateways mapping Modbus data to IEC 61850 GOOSE messages or MMS reports. Ethernet TCP/IP and 4G cellular interfaces are available for remote monitoring of transformers in locations without wired control room infrastructure.

5. Can transformer oil temperature monitoring integrate with SCADA or BMS?

Sim. The temperature controller outputs measured values and alarm states as Modbus registers over RS-485 or Ethernet. UM Sistema SCADA ou sistema de gerenciamento de edifícios (BMS) with a Modbus driver polls these registers and displays temperature trends, históricos de alarme, and cooling system status on the operator HMI. Integration requires only standard Modbus configuration — no bespoke software development is needed for most industrial SCADA platforms.

6. What is thermal runaway in a transformer and how does automatic protection prevent it?

Fuga térmica in a transformer occurs when the heat generated by an internal fault — typically an inter-turn short circuit or a circulating current through a damaged winding — exceeds the capacity of the cooling system to dissipate it, causing temperature to rise continuously rather than reaching a new equilibrium. As temperature rises, the fault resistance may decrease, increasing the fault current and heat generation further in a self-reinforcing cycle. Automático overtemperature circuit breaker tripping interrupts this cycle by disconnecting the transformer before the runaway condition reaches the point of insulation collapse and tank rupture.

7. Which IEC and IEEE standards apply to transformer thermal protection?

The primary standards are CEI 60076-2 (temperature rise limits for liquid-immersed transformers), CEI 60076-7 (loading guide and hot spot temperature calculations), e CEI 60255 (measuring relays and protection equipment) for the relay and trip circuit requirements. In North America, IEEE C57.91 is the equivalent loading guide and thermal model standard. NFPA 70B covers the maintenance requirements for electrical equipment including transformer thermal protection systems.

8. What is the normal operating oil temperature range for a power transformer?

For a standard mineral-oil-immersed transformer with ONAN (natural oil, natural air) cooling operating at rated load in a 40 °C ambiente, o normal top oil temperature should not exceed 95–100 °C under IEC 60076-2 limites. Na prática, a well-loaded but not overloaded transformer in a temperate climate typically operates with top oil temperatures in the 60–80 °C range during peak load periods and significantly lower during off-peak hours. Sustained operation above 85 °C under normal load conditions (not a short-term emergency overload) warrants investigation of the cooling system performance.

9. Is fiber optic temperature monitoring suitable for sealed oil-immersed transformers?

Sondas de temperatura de fibra óptica are fully compatible with sealed, hermetically-sealed, and conservator-type oil-immersed transformers. The fiber cable exits the transformer tank through an oil-tight compression gland that maintains the tank seal integrity. The probe itself — an optical fiber with a fluorescent sensing element at the tip — is chemically inert in transformer mineral oil and synthetic ester fluids, and its mechanical profile is small enough to be routed between winding conductors without disturbing the winding geometry or reducing the oil flow cross-section.

10. How do I know if my transformer needs an automatic temperature protection system?

Considerar automatic transformer temperature monitoring if any of the following conditions apply: the transformer is more than 10 years old and has not had a recent DGA oil test; the load it supplies is critical to production, segurança, or public supply continuity; previous thermographic surveys or oil tests have indicated elevated temperature or accelerated ageing; the transformer operates in a high-ambient environment or has a history of cooling system issues; or your insurance or compliance framework requires documented thermal protection. If you are unsure whether your installation warrants an automatic system, contact the engineering team at Fuzhou Innovation Electronic Scie&Companhia de tecnologia., Ltda. — established 2011, with over a decade of experience in power equipment temperature monitoring solutions. Reach us at web@fjinno.net or WhatsApp/WeChat +8613599070393.

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Isenção de responsabilidade: The information in this article is provided for general educational purposes only and does not constitute engineering advice for any specific installation. Transformer protection settings, colocação do sensor, and compliance requirements must be determined by a qualified electrical engineer in accordance with the applicable national and international standards and the transformer manufacturer’s documentation. Ciência Eletrônica de Inovação de Fuzhou&Companhia de tecnologia., Ltda. accepts no liability for decisions made solely on the basis of the general information contained in this article.

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