water content monitoring in operating transformers under changing ... results of
laboratory coulometric tests have been found and observed [4],[5],[15].
IEEE International Conference on Dielectric Liquids 2011, Trondheim
Experimental evaluation of water content determination in transformer oil by moisture sensor T. Gradnik, M. Kon an-Gradnik, N. Petric, N. Muc Elektroinstitut Milan Vidmar Ljubljana, Slovenia
[email protected] Abstract —Paper presents experimental research on temperature dependency of water solubility in mineral transformer oils. Moisture sensor measurements and absolute water content determination by Karl Fisher titration method were performed under controlled laboratory conditions to investigate solubility models of different types and conditions of mineral transformer oils. Results of experiments demonstrate that preset moisture solubility model of the moisture sensor affects accuracy of water content determination. Test setup and optimised procedure for verification and calibration of the moisture sensor with oil-specific solubility parameters is described, tested and evaluated. This allows higher accuracy of on-line water content monitoring in operating transformers under changing temperature conditions.
Keywords- power transformer; oil; solubility model; moisture; relative water saturation; sensor; water content; on-line monitoring; calibration; temperature; I.
INTRODUCTION
Power transformer represents a vital element of power transmission system with a typical life-expectancy over 40 years. Supervising transformer condition not only plays an important role in securing longevity of transformer, but also assures reliability of its operation. Majority of large power transformers are filled with mineral oils because of their excellent combination of dielectric, cooling and oxidation stability properties. Presence of moisture in mineral oil reduces dielectric performance of the insulation system and accelerates ageing of the paper insulation, therefore it is vital to assure low moisture content in new and maintain it low in operating transformers. To control and maintain oil quality, periodic oil tests are performed according to international standards [1],[2], with maintenance frequency depending on the condition and position of the transformer as well as maintenance strategy of the transformer operator. In the standards, maximum limits of water content are specified as mg of water per kg of oil, (i.e. ppmw value), for new and aged transformers depending on their voltage levels. For decades, these limits have been widely accepted, assuring longevity and reliability of transformer operation. In the last decade an increased trend in use of transformer on-line moisture monitoring devices can be observed. Moisture sensor consists of two electrodes, connected by a hygroscopic thin-film polymer and a temperature probe, preferably positioned in the oil flow of the transformer cooling system piping. Water molecules penetrate from oil into the polymer and change its capacitance proportionally to the relative saturation of the oil. Water content in oil is calculated by multiplying the measured relative saturation of the oil with
absolute saturation content of the oil. The latter is a material specific property, which is also temperature dependant. Temperature dependency of the oil solubility is described by a solubility curve. Some moisture monitoring applications rely on the assumption that unified solubility curve can be used with variety of new oils. Moisture sensor measurements are further used to calculate water content in paper insulation by use of moisture equilibrium curves. An overview of moisture equilibrium curves and their use is given in [3], [14]. More observations of unexpected deviations between water content in oil results obtained by on-line moisture sensor and results of laboratory coulometric tests have been found and observed [4],[5],[15]. A possible reason for the deviations was that the solubility model used in moisture sensor relied on the preset oil solubility model, thus neglecting specific solubility characteristics of different oils. Researchers at SINTEF reported a deviation between experimentally obtained solubility model and a model found in the IEC 60422 standard [6]. To estimate the error derived from using unified solubility model with different oils and analyse the improvements in accuracy using the oil-specific solubility model a laboratory investigation was performed as follows. II. WATER SOLUBILITY IN TRANSFORMER OIL MODEL AND DEFINITIONS
Mineral transformer oils have low affinity for water; one drop of water at room temperature is enough to saturate one litre of new oil. Water is attracted to polar oil components such as aromatic hydrocarbons and polar impurities. Lower weight molecular acids, produced during thermal ageing of mainly paper insulation [7], also increase moisture solubility of water in oil. Water in transformer exists in four typical forms: dissolved, when water concentration in oil is below its saturation level, chemically bound water (presence of polar oil components) free water (water concentration exceeds the oil saturation) water adsorbed in cellulose insulation Relative saturation of the oil Rs, expressed in per-cent form and its equivalent, water activity (aw), expressed in per unit form, represent the ratio of dissolved water content in oil (Woil, in ppmw) and water saturation content (Wsat, in ppmw). Water saturation is a function of oil temperature, chemical oil properties and ambient pressure. The temperature dependency of the water saturation content can be approximated in form of simplified Arrhenius equation: Wsat ã 10
A–
B T
,
(1)
1/4
where T is the absolute temperature in Kelvin, A and B are oil specific solubility coefficients. Different formulations of the water saturation content in exponential form (Eq. 2) exist, [8-12] with corresponding oil solubility coefficients (W0 and B’, Eq 3) can be found in literature, [8-12], outputting equal solubility curves for equal types of oil. Wsat
W0 e
–
B T
III. EXPERIMENT
(2)
where A ln øW0 ÷ , B=B log ø e ÷
(3)
In Tab. 1 and Fig. 1, oil solubility parameters from different sources [8-12] are recalculated into form of Eq. 1. for the purpose of comparison to experimentally obtained results, discussed in chapter III. Table 1. Comparison of literature and estimated oil solubility parameters Cigre Oomen Griffin GE Hydran Vaisala EIMV [9] [10] [11] M2 [12] MMT330 [8] (Tab. 2) A B
7,23 1640
7,42 1670
7,09 1567
7,71 1783
7,37 1662
5,5 - 6,8 1320-1520
Figure 1. Comparison of literature and experimental oil solubility curves
To calculate absolute water content from relative saturation measurements, Eq. 4 is applied: Woil
aw Wsat ø T ÷ ,
(4)
Oil solubility coefficients can be experimentally determined from two relative saturation measurements in a sealed oil sample with constant water content at two different temperatures by applying the Eq. 5 and 6. A simple test setup description and below equations are found in [13]. B A ã log øWsat øT1 ÷ ÷ õ , T1
Bã–
ø
(5)
÷
log Wsat øT2 ÷ – log øWsat ø T1 ÷ ÷ , 1 1 – T2 T1
corresponding temperature, calculated from Eq. 4. In this method of calculation of solubility parameters it is assumed that the isotherm of water activity is linear to water concentration (linearity of the moisture sensor indication) and that the solubility curve has the form of Eq. 1 (Arrhenius form).
(6)
where Wsat(Tx) is the water saturation content at
During transformer operation, partitioning of water content between oil and cellulose insulation in transformer is depending on temperature changes that affect water solubility properties. Time constant of the moisture migration is a variable, depending on the oil-paper insulation temperature. To investigate solely the response of the moisture sensor, the water content during laboratory experiment needs to stay constant, which means that the laboratory test cell needs to be free of any hydrophilic materials that could adsorb and absorb water to oil as oil temperature changes during test. Laboratory test setup in Fig. 2 was used to obtain constant moisture content in the test cell. A 100 ml Erlenmeyer glass flask with hydrophobic polytetrafluoroethylene bushings was used and a gas-sealed oil dilatation system was setup to prevent ambient moisture ingress during KF oil sampling and coolingdown period. Magnatic stirrer and heater with external temperature regulation probe were used to obtain a homogenised and constant temperature of the test cell. A calibrated Vaisala HMT338 moisture and temperature sensor was inserted into the oil flask and connected to a personal computer through a digital rs-232 output, enabling continuous logging of temperature, relative saturation and water content. According to the Calibration report of the test instrument its accuracy was within the manufacturer specifications (temperature accuracy error
