Adoption of response surface methodology for optimization of benzotriazole additive in ester fluids as transformer insulant

This paper reports the critical investigation on the streaming current of ester fluids towards power transformer. The spinning disc methodology is adopted for measuring the streaming current at the liquid and pressboard interface. Based on the measurement of the streaming current, the optimization of the benzotriazole (BTA) additive is evaluated, considering its impact with and without temperature. According to the experimental findings, a negative streaming current is observed when BTA concentrations reaches 130 ppm in the ester fluids, and the effect of temperature is found to enhance the magnitude of streaming current. Only a minimal change is noticed in the viscosity of ester fluids with the addition of BTA but to the contradictory, an increased dissipation factor is observed. The diffusion of BTA additive in ester fluids towards reduction in streaming phenomenon can impact the other dielectric properties inside the power transformers. Thus, it is required to identify the optimal concentration of BTA in ester fluids before implementing to real-time applications. In the present research work, the response surface methodology (RSM) is being utilized as an optimization technique with central composite design (CCD) as a tool to identify the permissible level of BTA under transformer standstill and normal operating temperature conditions. During transformer standstill conditions, the optimal concentration level of BTA provided by RSM-CCD towards lower streaming current is 106.863 ppm with its permissible disc velocity of 300 rpm. A quadratic model is being followed for standstill conditions with its R2 value equivalent to 0.9889. Considering this BTA concentration as a base input to RSM-CCD during transformer operating conditions, the optimal concentration of BTA is found to be 104.265 ppm with temperature limit of 65 °C resulting in magnitude of streaming current, viscosity, and dissipation factor to 1145.33 pA, 18.43 mPa.s and 0.0159, respectively.


List of symbols
The streaming electrification of power transformers traces back to early nineteenth century with the first hazardous incident reported in Japan [1].Since then, numerous researchers worldwide have been conducting experiments on the streaming electrification phenomenon with different laboratory models [2,3].Initially, the pilot scale model simulating the actual power transformer was investigated [4] which then advanced towards cylindrical system [5] due to the process involved in the pre-conditioning and lower cost involved in the technique.Recently, the spinning disc technology [6, 123 7] has gained more importance for assessing the streaming current due to its simplicity in the measurement.The mineral oil which is widely used in power transformers for the last 50 years have been tested by different models towards streaming phenomenon with pressboard material as its interface [2].Although the aromatic additives present in mineral oil inhibits the charge separation process [8], its lower biodegradability and flash point poses a disadvantage.Thus, alternate ester fluids are currently in research to overcome the limitations of mineral oil towards power transformer applications.Before introducing any new insulating fluid, the transformer design specifications should be altered [9] for a proper functioning of fluid as an insulant.Ester fluids possess a higher fire class behaviour than mineral oil [10], and hence its viable for transformers to be located near the load centres.The higher affinity of ester fluids towards moisture content removes the water diffused towards the cellulosic pressboard insulation and increases its lifetime inside the transformers compared to mineral oil [11].The antioxidants present naturally in the ester fluids observes a higher breakdown strength under different field conditions [12,13] and exhibits a very minimal change in its dielectric properties after subjected to thermal ageing [14].Although the ester fluids have a better dielectric strength than mineral oil, the higher viscous nature can impact the heat transfer mechanism between the winding conductors and the charge formation at the interfaces [15].Salama et al. [16] simulated a thermal model for transformers analysing the hotspot and top-oil temperature levels of ester fluids with mineral oil.The ester fluids provided a superior thermal behaviour under ageing conditions matching the same load with its operational power factor lower than the mineral oil.The interfacial region between the pressboard insulation spacers and insulating fluids is the major site towards the development of streaming electrification [17].Kolcunová et al. [18] concluded that mineral oil provided a lower streaming current than natural ester fluid but to the contradictory, the temperature influence on streaming current was inferred to be higher in the former compared to latter.Among the different ester fluids tested towards streaming phenomenon, the synthetic ester fluid was found to have a streaming current magnitude three times higher than natural ester fluid [19].
There are different additives added towards the insulating fluids for improving the dielectric properties [20,21].Zdanowski et al. [22] have used C 60 as an inhibitor towards streaming electrification of insulating fluid and concluded that concentrations between 100 and 200 mg/L are suitable for suppressing the electrostatic charging tendency (ECT).Around 10 ppm of benzotriazole (BTA) additive was found to reduce the streaming current in mineral oil [2], and in addition, BTA can also prevent the erosion of copper winding and pressboard insulation [23].The addition of such additives for streaming current mitigation can impact the other dielectric properties of ester fluids such as partial discharge, breakdown voltage, viscosity, and dissipation factor [24].There are currently no specific regulations formulated for the maximum additive concentrations permitted in ester fluids towards power transformer applications.Thus, the optimization of such additives should be investigated on ester fluids to reduce its charge formation along with its tendency towards other dielectric properties, thereby improving the service lifetime of power transformers.The machine learning techniques towards the optimization problem has now gained more importance in the various engineering domains with the development of new algorithms which can be used for current research problem [25].There are different optimization techniques adopted for specific research problems such as genetic algorithm (GA), Taguchi method, response surface methodology (RSM), and artificial neural network (ANN) [26][27][28].The constrained optimization is most widely used with its limits bounded on its input parameters.The step size used in the optimization algorithms affects the speed and accuracy of results, with lower step size avoiding the zigzag movement of search space and improving the probability of identifying the global optimum [29].Among the abovementioned techniques towards optimization problems, the RSM technique with central composite design (CCD) is used in the present research work.The RSM-CCD formulates the design of experiments (DOE) based on the limits provided on the input variables [30].This technique is more effective in identifying the interaction between different independent input variable towards output responses with the analysis of variance (ANOVA) suggesting the prediction of suitable model [31].Raj et al. [33] studied the transesterification reaction of Pongamia Pinnata oil considering the parameters of time, temperature, and catalyst where its optimization using RSM provided a higher breakdown strength and flash point with less viscosity suitable for transformer applications.Duraisamy et al. [34] studied the reclamation of transformer oil using different adsorbents and identified optimal amount of bentonite and kappa cotton using RSM improving its dielectric properties.In the current work, as a first step the streaming current of ester fluids are experimented with spinning disc system, and its effect with the additive concentrations is understood with respect to angular velocity and temperature.Based on the experimental data findings, the RSM-CCD is performed for the transformer standstill conditions which is then considered as a base for input parameters during operating conditions.The optimization of BTA additive for streaming current with temperature is being performed through RSM-CCD along with its viscosity and dissipation factor.

Preparation of ester fluids with additives
The ester fluids from MIDEL 1215 were used in the present work with the BTA additive procured from Sigma-Aldrich (purity of 98.5%).The BTA additive was heated at 150 °C for 8 h to remove the moisture content.The ester fluids were then added with different concentrations of BTA which was mixed using a magnetic stirring for 30 min.For better stable dispersion of BTA in ester fluids, it was further allowed for sonication at 40 °C for 3 h.The overall mixture of ester fluid containing BTA were allowed to remain in a glass vessel for 24 h to confirm its stability before performing the experiments.

Streaming electrification
The streaming electrification analysis has been experimented using the spinning disc system (Fig. 1a) which is widely used for laboratory conditions, and its schematic sketch on the connections is shown in Fig. 1b.This technique involves a metallic disc coated with pressboard specimen on its both sides diffused inside an aluminium vessel which is tested for its streaming current towards the centrifugal motion of insulating fluids.The dimensions of the disc and aluminium vessel are considered as per the CIGRE specification.The rotational flow of the disc is controlled externally with its speed ranging up to 600 rpm and a digital thermometer is used for continuous monitoring of temperature.Since the current measured is in the range of pA, the external noises can interrupt the actual streaming current, and thus, a Faraday cage is provided as a shielding towards overall experimental unit.In addition, the aluminium vessel housing the insulating fluid is placed on Teflon material to prevent the charges from entraining towards the external metallic interfaces.

Physical and dielectric analysis
The rheometer using the cone-plate geometry was used to measure the viscosity (Fig. 2a) with its continuous monitoring on the temperature measured using Peltier temperature control device (P-PTD200/AIR Plate).Before starting the experiment, the samples were maintained under high vacuum for 10 min for its equilibration.The set zero gap was performed before any measurement, while the gap spacing between the cone and plate was maintained at 0.080 mm during its measurement period.The dissipation factor of ester fluids was measured at different temperatures as per IEC 60247 standard using OMICRON-DIRANA unit (Fig. 2b).The heating system was provided to the test cell with its temperature variation from 30 to 90 °C, and temperature equilibrium was maintained throughout the measurement period.3 Results and discussions

Streaming current
Figure 3(a) shows the current of ester fluids measured with respect to time.During the initial time period (up to 50 s), the current increases marginally due to the sudden change in the rotation velocity.So, it was continuously recorded until the steady-state condition (300 s) has been attained, and this final steady-state value is being considered as the streaming current.The current increased with disc velocity due to the frictional stress between pressboard and ester fluid along with the change in Debye length [6].Without the addition of BTA (0 ppm), the ester fluid observed a streaming current magnitude of 520 pA at 600 rpm (Fig. 3b).The different BTA concentrations were tested on ester fluids at an interval of 25 ppm.At 25 ppm, a reduction of 42.31% was observed in the streaming current.The dissociation of BTA results in the triazole ring along with hydrogen ion.The nitrogen ions present in the external structure of benzene interacts with electrical double layer between pressboard and ester fluids, reducing its ECT.The unsaturated double bonds present in the external triazole ring of the BTA tries to prevent the positive ions from diffusing into the insulating fluid, and thus, 25 ppm of BTA observed a lower current compared to its influence without BTA at 500 rpm.The reduction in the streaming current was noticed on the subsequent concentrations of BTA (50, 75, 100 ppm).Upon further increase in the BTA, the streaming current reversed its polarity.This gives us an indication on the neutralization of charges in the double layer by the negative ions from the triazole ring, and thus, the optimal additive concentrations range between 100 and 130 ppm.
The temperature influence of BTA additives on ester fluids towards streaming current was investigated maintaining a constant flow velocity of 600 rpm (Fig. 4).The contour plot involves multiple colour where the yellow colour represents higher streaming current, red colour indicates the mid-region of streaming current, and dark brown indicates the lower streaming current.The disc velocity of 600 rpm is being considered further towards optimization problems with temperature since its flow characteristics are similar to 1.2 m/s which is typical inflow velocity allowed in power transformers during operation [6].The normal operating temperature of liquid immersed power transformers are in between 55 and 65°C.In the present work, the streaming current is recorded from the ambient temperature till 100 °C.An increase in the streaming current is associated with rise in temperature, and this could be related to the change in its viscous nature that induces more charges at the interfaces.A linear increase in the streaming current is noticed with temperature for different concentrations of additives towards BTA.The change in the streaming current with the addition of temperature observed a minimal change upon the influence of BTA.Without BTA, the streaming current reached a magnitude of 1853 pA at 100 °C, whereas it was around 1683 pA with BTA of 130 ppm.For lower BTA concentrations (25-100 ppm), the streaming current of ester fluids increased compared to its influence without the addition of BTA.The dissociation of hydrogen ions along with the triazole ring is responsible for higher current at lower additive concentrations.With increase in temperature, the conductivity of fluid increases which also requires a higher negative ion to suppress the charge formed at the interfaces.Above 100 ppm, the BTA additive diffuses more negative ions compared to the formation of positive ions formed in the ester fluid with temperature and thus causing a reduction in its streaming current.Further, there was no charge relaxation observed under the influence of temperature like the mineral oil [34] giving an indication that ester fluids do not involve any relaxation with temperature after its tribocharging mechanism.

Viscosity and dissipation factor
The streaming current exhibited inside the power transformers is dependent on the rheology and dissipation factor of the insulating fluid.The flow patterns (laminar, turbulent) governs frictional stress between the interfaces, whereas a higher dissipation factor induces more charges on pressboard surface leaving equal ions to diffuse into the insulating fluid.Both parameters govern the streaming phenomenon at the insulating liquid and pressboard interfaces.The flow behaviour of ester fluids with respect to temperature is assessed in terms of viscosity as shown in Table 2.The addition of BTA to ester fluids observed an increase in the viscosity to around 10% at ambient temperature, whereas it got reduced substantially at the operating temperature ranges of power transformers (70 °C).The dissipation factor of ester fluids with BTA was very minimal at 30 °C, whereas at higher temperatures (> 70 °C), an increased dissipation factor is noticed.The less interaction between the molecules of ester fluids at high temperatures causes an increased dissipation factor.Further, dissociation of BTA used for suppressing the static charge formation can result in an increased conductivity at higher temperature which could also be the reason for higher dissipation factor.

Transformer standstill condition
The streaming electrification can result in charge formation even when the transformers are unenergized or in standstill condition.Initially, the insulating fluid is injected into the transformers to allow the pressboard/paper insulation to be diffused for more than 24 h.During such conditions, the forced insulating liquid can create certain remnant charges on both pressboard and paper insulation which should be minimized by optimizing the inlet flow velocity.Hence, RSM is being used to formulate the design experiments using CCD to minimize the current with corresponding BTA additive and disc velocity.Based on the experimental results, the BTA additive was considered in the range of 100-130 ppm, and dynamic zone of angular velocity (300-600 rpm) towards streaming current was provided as input towards the RSM model (Table 3).The objective function for minimizing the streaming current during transformer standstill condition is given by, where I st is the streaming current (pA) at transformer standstill condition, BT A is additive concentration (ppm), and v is the disc velocity.The total number of experimental runs in RSM-CCD are generated based on the interaction of axial points, factorial points, and centre points of each factor.The input values in Table 3 with three decimal points is because From Eq. (3), it is observed that viscosity and the squared terms of both input factors has a positive effect on the response streaming current (I st ) at standstill, whereas the linear term BTA and the interaction terms of both input factors have a negative effect on I st .
Figure 5 shows the input variables (BTA, disc velocity) on the predicted streaming current using RSM.The increase in the disc velocity increases the standstill streaming current and increase in BTA reduces the current magnitude but above a certain concentration level, it starts increasing in the negative polarity.Thus, the optimal criteria to minimize the streaming  4. The fitted model followed the quadratic equation with actual R 2 value of 0.9889 and predicted R 2 value of 0.9208.The overall model is found to be significant with p-value < 0.0001.

Transformer operating conditions
The optimal BTA concentration level obtained for mitigating the streaming current during standstill condition has been considered as a standard towards its normal operating temperature conditions.Nevertheless, the disc velocity of 300 rpm might be suitable under transformer standstill conditions but during operating conditions, the flow velocity of 600 rpm would be required as per the real-time standards [6] for reducing the heat formed in the winding conductors.The addition of BTA reduces the streaming current but it can impact the other dielectric properties of ester fluids.Thus, the optimization of BTA with the impact of temperature is simulated using RSM model towards the output responses of streaming current, viscosity and dissipation factor.The upper and lower bounds of BTA are selected as 100 ppm and 106.863 ppm, whereas the temperature was considered between 65 and 90 °C with a constant disc velocity of 600 rpm.The objective function during transformer operating conditions for the streaming current, viscosity and dissipation factor is given by, subject to constraints 100ppm < BTA < 106.863ppm and 65 where I T is the streaming current (pA) at transformer operating conditions, T is the temperature, η is the dynamic viscosity and tanδ is the dissipation factor, respectively.The experimental runs in RSM-CCD for transformer operating conditions (Table 4) were generated similar to transformer standstill condition mentioned in Table 3.These constraints were used for formulating the design experiments with the influence of temperature as shown in Table 5.From Eq. ( 6), it is observed that temperature, interaction terms of both BTA and temperature, and squared terms of BTA show a positive effect on the response streaming current (I T ) at operating conditions, whereas the BTA and squared terms of temperature indicate a negative effect on I T .Equation (7) for viscosity (η) observes a positive effect with BTA and negative effect with temperature.Equation (8) for dissipation factor (tanδ) indicates a positive interaction with temperature, combined effect of both input factors, and squared terms of BTA, whereas negative interaction is seen with BTA and squared terms of temperature.The 3D plot indicating the effect of BTA and temperature towards streaming current, viscosity, and dissipation factor is shown in Fig. 6.The streaming current and dissipation factor during transformer operating condition increases individually with respect to BTA concentration and temperature, whereas the viscosity reduces with temperature and increases slightly with BTA concentration.Considering the minimization of all the above responses (I T , η, tanδ), the optimal BTA additive and operating temperature are observed to be 104.265ppm and 65 °C with the streaming current, viscosity, and dissipation factor of 1145.33 pA, 18.4298 mPa.s, and 0.01592, respectively.Table 6 represents the ANOVA for the streaming current, viscosity, and dissipation factor during transformer operating conditions.The fitted model followed a quadratic equation for I T and tanδ with its actual R 2 value of 0.9968 and 0.9891, whereas the predicted R 2 value were 0.9770 and   0.9226, respectively.To the contradictory, a linear model was followed for η with actual R 2 value of 0.9829 and predicted R 2 value of 0.9633.All the responses (I T , η, and tanδ) are found to be statistically significant with p-value < 0.0001.Considering the above observations, these operating conditions could be a useful standard for the insulation engineers before introducing the ester fluids for power transformer applications.

Conclusions
The following points have been postulated as a major conclusion from the present study: • The streaming current of ester fluids tested with different concentrations of BTA additive resulted in optimal level between 100 and 130 ppm.• An increased streaming current is noticed with increase in temperature, and the maximum magnitude of current observed was around 2000 pA at 100 °C.• The addition of BTA increased the viscosity of ester fluids only at ambient temperature (30 °C), whereas it had no effect at higher temperatures.The dissipation factor of ester fluids increased to a higher magnitude with the addition of BTA at higher temperatures greater than 70 °C.The present study investigated the optimization of BTA considering only the streaming current, temperature, viscosity, and dissipation factor.In addition to these parameters, the other criteria such as breakdown voltage, fire point, permittivity, and nanoparticles are also be considered towards the performance of ester fluids which will be considered as a future scope of present research work.
I st Streaming current at transformer standstill conditions BTA Concentration of benzotriazole additive in ppm v Disc velocity in rpm I T Streaming current at transformer operating conditions 1 Introduction

Fig. 1 aFig. 2
Fig. 1 a Experimental setup used for streaming electrification, b Sketch on the spinning disc connections

Fig. 3 Fig. 4
Fig. 3 Streaming current of ester fluid containing BTA with a time, b disc velocity

Fig. 6
Fig. 6 Three-dimensional surface plot with the effect of BTA and temperature on the a streaming current, b viscosity, c tan δ

Table 1
Properties of natural ester fluids

Table 3
Design of experiments using CCD with input variables (BTA, disc velocity) and output response of streaming current

Table 4
ANOVA table for streaming current with respect to BTA and disc velocity

Table 6
• The RSM-CCD formulated towards the optimization of BTA under transformer standstill condition is possible with concentration level of 106.863 with flow velocity of 300 rpm.During transformer operating conditions, the BTA concentration of 104.265 ppm with temperature of 65 °C concludes the lower magnitudes of streaming current, viscosity, and dissipation factor to be around 1145.33 pA, 18.4298 mPa.s, and 0.01592, respectively.