Transformer monitoring.

As part of an initiative to safeguard reliable power output and reduce risk, a biomass power plant in the north of the Netherlands has installed a continuous transformer monitor. The Vaisala MHT410 continuously measures three key parameters in transformer oil – moisture, hydrogen and temperature. Owned and operated by the energy company Eneco, the Bio Golden Raand plant produces steam for local industry as well as power, with a capacity of approximately 135 MW thermal and 49.9 MW electrical.

Background.
Transformer materials can deteriorate over time, resulting in the potential for costly faults, repairs and downtime. However, the development of transformer faults results in the accumulation of dissolved gases in the transformer oil, so this oil is routinely tested as part of a preventative maintenance program. Eneco’s transformer gas monitor was installed to provide continuous data and reduce the requirement for periodic oil sampling and laboratory analysis.

Eneco Transformer

“We discussed our requirement for the early detection of potential issues with our local service provider, Flux Transformer Services,” explains Laurens Freriksen, a project manager and maintenance specialist for Eneco. “They suggested that we should consider using one of Vaisala’s online monitors to support the planning and optimization of preventive maintenance; to extend the lifespan of our transformer, and reduce the risk of unexpected and costly outages.”

Vaisala’s Andreas Hilgers then visited the site to demonstrate two options; the MHT410 which monitors a single gas (hydrogen), and the OPT100 which is a multi-gas DGA (dissolved gas analysis) monitor. Subsequently, the MHT410 instrument was installed by Flux TS, and Laurens says: “We have benefited from online measurements for around one year now, and it has been very reassuring to see low hydrogen levels in the transformer oil, irrespective of the transformer load.”

Bio Golden Raand Power Station.
Biomass is used as a feedstock at the Bio Golden Raand plant (NL) to generate energy from wood waste. This form of power generation helps to reduce the consumption of fossil fuels. The plant uses non-hazardous Grade B waste wood. This type of waste may contain Grade A wood (mainly from packaging waste, scrap pallets, packing cases and cable drums, and process off-cuts) together with other waste wood sourced from construction and demolition activities, transfer stations, civic amenity sites and the manufacture of furniture from solid wood.

Every year, the plant processes approximately 300,000 tons of waste wood that arrives in Delfzijl (NL) by ship and truck from the Netherlands and surrounding countries. The wood is passed to a boiler fitted with a circulating fluidized bed furnace operating at 900°C. Flue gases from the furnace pass through three heat exchangers to a water steam circuit, where steam at 90 bar with a temperature of 520°C is produced. This steam can be delivered directly to local industry, or it can be used to drive a steam turbine, which in turn drives a generator that produces electricity.

Generator step-up transformers provide the critical link between a power station and the transmission network. These transformers step up the voltage from generator level to the transmission voltage level, which steps down the current and thereby reduces the loss of energy as heat and ensures efficient power transmission over long distances. Typically, generator transformers operate continuously 24/7 so they need to be extremely reliable.

Transformer oil.
Generator transformers are generally oil-filled to provide insulation and cooling. The transformer at Bio Golden Raand, for example, contains around 20 tonnes of oil. A special mineral oil is used for its chemical properties and dielectric strength, and this is routinely tested as part of an effective preventative maintenance program for the transformer.

Oil degradation occurs when its molecules break down under the influence of thermal and electrical stresses due to transformer faults such as discharges or hot spots, for example. This degradation raises the levels of hydrogen, carbon oxides and hydrocarbon gases in the oil. Hydrogen concentration increases with all fault types, but the ratio of hydrocarbons depends on the fault type.

Testing and monitoring transformer oil.
Traditionally, transformer oil samples are collected once or twice per year and sent for laboratory analysis to determine the level of gases. This spot sampling method provides an indication of dissolved gases and oil quality at one moment in time. The main advantages of continuous monitors therefore, are that they are able to reveal trends so that users can correlate gas levels with transformer load, for example. Importantly, by measuring continuously, DGA monitors can provide early warnings of faults.

The levels and trends of dissolved gases can be used for fault identification, and this is the subject of a Cigré Technical Brochure (Ref.783) on DGA Monitoring Systems. This document describes the different types of DGA monitors, and includes an impressive performance evaluation of monitors including Vaisala’s OPT100.

In addition to hydrogen, the MHT410 also measures temperature, which is a key indicator of faults. Moisture in oil is also measured by the device because moisture decreases the dielectric strength, accelerates cellulose (insulation) decomposition, and increases the risk of bubble formation at high temperatures.

DGA monitoring at the plant.
Explaining the reasoning behind the installation of the Vaisala MHT410, Laurens Freriksen says: “A power plant transformer is one of the most valuable assets in a power network - ours is around 10 years old and operates continuously. However, there is no redundancy so it is important for us to be able to monitor its condition and performance closely.

“We chose the MHT410 because it offered an opportunity for the early identification of potential problems, which is an important risk reduction measure. Early fault identification enables timely corrective measures such as transformer service, oil change, repairs and possibly transformer load management through customer engagement. If the transformer indicates a fault through increased hydrogen levels, it may also be necessary to deploy a multigas monitor such as the OPT100, in order to obtain a full fault diagnosis.”

Laurens Freriksen

Data from the MHT410 continuously feeds into the Eneco digital control system, which Laurens is able to access from his laptop. This means, for example, that he is able to track transformer load on the same screen as the MHT410 measurements. “The digitization of assets is an important issue for our industry, and this is a good example,” Laurens adds. “Remote access to live data informs decision making, lowers risk and was an enormous benefit during the Covid lockdown, because it meant that we could check the transformer at any time, from anywhere.”

The MHT410 has been set to raise a high level alarm at 100ppm hydrogen and a very high level alarm at 150ppm. In addition, an alarm will be issued if there is a sudden increase in the readings for hydrogen, moisture or temperature. However, Laurens is pleased to confirm that during the first year of operation there have been no alarms, with hydrogen readings typically ranging between 2 and 15ppm, with no significant effects during periods of high load.

Robust power transformer monitor.
Designed for quick and easy installation with almost no maintenance requirement, the MHT410 has a low cost of ownership. “This is important,” explains Andreas Hilgers. “However, the cost of DGA monitors is negligible in comparison with the value of the assets that they help protect, or in comparison with the cost of outages.

MHT410 Installation

“One of the main advantages of digitization is a reduction in the need for unnecessary site visits, so it makes sense to install a Vaisala monitor that does not incur an extra service requirement." Frans van Hofwegen from Flux TS agrees. He says: “This is a good example of cooperation between Vaisala, Flux and the end-user; combining knowledge and experience to achieve the desired goals. As the first MHT410 that we have installed in the Netherlands, the device at Bio Golden Raand represents a great start to our relationship.”

The MHT410 takes measurements with an in-situ probe, so there is no requirement for pumps or membranes. With no consumables or moving parts that could fail, the instrument is encased in an IP66-rated metal housing equipped with a weather shield. Every unit is individually tested for a pressure of at least 10 bar and also withstands vacuum conditions. Special attention has been given to EMC tolerance; for example, all electrical connections are isolated. In addition, the MHT410 has been designed to tolerate short-term power outages.

In brief.
For Eneco, the installation of the Vaisala monitor is essentially a risk reduction measure, but as Laurens explains: “It is very reassuring to have constant visibility of the transformer’s condition. However, the key advantage is that it buys us time – time to plan an effective strategy if oil conditions deteriorate - to optimize the performance of the transformer and to extend its working life.

“We are proud to be leading the way in the Netherlands with this form of digitization, and since the MHT410 was installed, we have received a great deal of interest from our colleagues in other sectors, such as wind power.”

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