Using a Remote System to Study the Thermal Properties of a Vegetable Oil Filled Power Transformer: How Does Operation Differ from Mineral Oil D. Martin, W. Guo, N. Lelekakis and N. Heyward
Abstract--There has been much discussion on the properties of natural esters and how they can be best applied as a transformer dielectric. The thermal properties of natural esters are different from those of mineral oil. A difficulty with investigating the thermal properties is that it is uncommon to find the same transformer design filled with different fluids. Two 90MVA power transformers were manufactured to the same design and installed in the south east of England, UK, where they were connected to the internet via a private connection. The transformers have been filled with different dielectrics, one being mineral oil and the other a natural ester. Remote monitoring has allowed us to perform a field study on the thermal properties of a natural ester and their effect on the operation of the transformer. The investigation shows that despite the viscosity affecting the cooling capability a power transformer can be designed so that there is little effect on its temperature. This study also demonstrates the advantage of remote monitoring, where a team in Australia can oversee a transformer on the other side of the world.
N
I. INTRODUCTION
ATURAL ester based transformer dielectrics have been used in transformers since the late 1990s [1]. The transformer class that such fluids have been used in has been steadily increasing as the industry has become more accustomed and competent in. The dielectric strength of these fluids has been much discussed in the past, with general agreement that the breakdown voltage is similar [2-5]. A transformer oil is not only an insulator but is also a coolant. Therefore, in order to successfully use these fluids in large transformers, care must be taken to ensure that the thermal properties of these dielectrics are not different to the degree where they affect the operation of the transformer. A comparison of the thermal properties of two types of fluid is given in Table I. The properties of a mineral oil are compared to two types of natural ester. The flash and fire points of these esters are around 200°C higher than that of the mineral oil. This is This work was supported by UK Power Networks, UK. D. Martin, W. Guo & N. Lelekakis are with the Department of Electrical & Computer Systems Engineering, Monash University, Clayton, Victoria, Australia (e-mails:
[email protected],
[email protected] and
[email protected]). N. Heyward is with UK Power Networks, Crawley, UK (e-mail:
[email protected]).
advantageous as these oils are less likely to fuel a fire. Esters are slightly denser than mineral oil, which affects transformer weight. The viscosity of an ester is higher than that of mineral oil, and so an ester will theoretically be a less efficient coolant than mineral oil. Therefore, the question is whether the higher viscosity causes a notable increase in transformer temperature. TABLE I COMPARISON OF FLUID PROPERTIES Mineral Natural ester A Oil Density 0.88 0.92 (g/ml) @ 20°C @ 20°C Expansion coefficient 7.5 x 10-4 7.5 x 10-4 (°C) Viscosity (cSt) 32-33 8-10 @ 40°C 8 2 @ 100°C Specific heat (J/kg K) 1860 1880 @ 20-25°C 0.126 0.167 Thermal conductivity @ 20°C @ 25°C (W/mK) @ 20-25°C Pour point -57 -18 to -21 (°C) Flash point 145 330 (°C) Fire point 160 360 (°C)
Natural ester B 0.91 @ ambient 6.88 x 10-4 42 9 1970 0.17 @ 25°C -15 330 360
In this study we set out to determine whether the temperature profiles of two identical transformers, one filled with a natural ester and the other with mineral oil, were different. Some thermal properties are affected by temperature. An insulating oil must remain liquid under ambient temperatures to be an effective coolant. The pour point of these natural esters is lower than the average minimum temperature for most UK winters [6]. However, there is the occasional extreme whether event such as -27.2°C recorded in Scotland in 1995 [6]. The lowest temperature recorded in Australia is -23°C, measured in NSW in 1994 [7]. Temperatures in North America can fall further so Rapp investigated the behaviour of ester oils near their pour point [8]. Rapp froze then thawed an ester finding that its dielectric strength did not change. A comparison of the viscosity of mineral oil and the studied natural ester, as a function of temperature, is given in Fig. 1.
978-1-4577-0875-6/11/$26.00@2011 IEEE
The viscosity was calculated, for this range of temperature, from two measurements using standard ASTM D341 [9]. The A & B coefficients required for use with ASTM D341 were calculated as 7.102 and 2.772. As can be seen the viscosity of the natural ester is around two and a half times higher than that of the mineral oil. The thermal capability of a dielectric is also related to the design of the transformer, such as the winding and oil duct arrangement, the ease which oil flows through the ducts and height of the heat exchanger [10], [11]. Aubin investigated the effect of oil viscosity at low temperatures, noted that heat transfer was also dependent on the width of the cooling duct [12]. Since the transformers are of the same type it is assumed that any differences are due to the fluid and not the design. For a transformer in ONAN mode (Oil flow natural and air flow natural) the cooling capability of an oil is related to its flow speed [13], (1): ∆
II. STUDIED TRANSFORMER We investigated two 132kV 90MVA transformers operating near London, UK, built in 2006 (Fig. 2). The oil is not pumped around either transformer. The radiator banks are cooled using fans. Since the transformers, including peripheral equipment, are the same then any differences in operation are most likely to be caused by the different fluid properties. The transformer is one tonne heavier when filled with the natural ester, due to its higher density (Table II). The mass of 22500 litres of fluid is 20,700kg when filled with natural ester A and 19,800kg when filled with the mineral oil. A manufacturer may need to be aware of such a difference when transporting a unit to site.
(1)
Where p is pressure, v is kinematic viscosity, l and d are channel dimensions, γ is the specific weight, w is the oil speed in channel and g is the gravity constant. Merging the factors which are constant at a particular temperature to a constant, f, (1) can be written as (2). (2) The equation shows that the oil speed falls as the viscosity increases. The Reynolds number (Re) is a measure of the heat exchange between the transformer and the oil (3). The higher the Reynolds number the better the oil cooling capability. (3) By combining (2) and (3) to produce (4), we show that the Reynolds number falls as a function of the square of the viscosity. (4) Given that the viscosity of the studied natural ester is between two and three times that of the mineral oil, its Reynolds number be lower. This reduced cooling capability can be offset by changing the dimension (d) of the oil ducts, which in turn increases the Reynolds number.
The transformers are equipped with an MS2000 monitoring system which allowed us to download and analyse data. At regular intervals the monitoring system recorded load current, internal temperatures, ambient temperature, oil water content and dissolved gas analysis data. Other condition monitoring data was available offline, such as oil quality measurements. TABLE II TRANSFORMER RATINGS Transformer GT1 GT2 Oil type Mineral oil Natural ester Cooling ONAN / ONAF KNAN / KNAF Oil Volume in Transformer (litres) 22500 Oil Volume in Cooler Bank, including pipework (litres) Complete Transformer Mass with Oil, but without Cooling Plant (tonnes)
4500 93
94
III. ANALYSIS OF HEAT RUN DATA
120 Natural ester
100 Kinematic viscosity (cSt)
Fig. 2. Transformer used in this investigation
Mineral oil
80 60 40 20 0 0
20
40 60 80 100 Temperature (°C) Fig. 1. Comparison of oil viscosity as a function of temperature
During a heat run test the transformer is run at rated load, allowing the thermal performance of the transformer to be evaluated. The FR3 heat run test continued for longer than the test performed on the mineral oil transformer. Both transformers meet the specification of
