DETECTION OF INTERNAL WINDING FAULTS IN POWER TRANSFORMER : MODELING AND ANALYSIS

The power transformer is an important and costly element in electric power systems. The continuity of transformer operation is vital in maintaining stability and in improving the reliability of power systems. Any fault in a power transformer affects the operation of the connected power system. In order to reduce the losses caused by the severe currents and avoid the instability in the power system, the correct and prompt diagnosis of fault in damaged power transformers is of great importance. A new and efficient approach for diagnosing the occurrence of incipient turn to turn short-circuits in the

windings of power transformers is discussed here. The transformer is modeled in steady state and no-load operation. So the phase difference between the input voltage and input current in the faulty primary winding is changed in a manner that makes it a good measure to find the fault.

CHAPTER 1

INTRODUCTION

Electrical power transformer plays a vital role in power transmission. It is an important and expensive element in the electric power system. It is a device that consists of a coil wrapped around an electromagnet that transfers electricity from one circuit to another without changing the frequency of the electrical energy. The availability and longevity of power transformers have a major impact on grid reliability and profitability. Any fault in the power transformer affects the operation of the connected power system.

Therefore correct diagnosis of a fault in a damaged power transformer is essential in order to reduce the losses caused by severe currents. 70 %-80 % of the number of transformer failures lead eventually to internal winding faults. Winding faults are a result of the degradation of the transformer winding due to aging, high voltages, etc., which tend to cause a breakdown in the dielectric strength of the insulation. This breakdown either causes adjacent windings to short (turn to turn) or a winding to be shorted to ground. The induced EMF in shorted turns is the source of current in shorted turns. The induced current will flow in a direction so that its magnetic field opposes the main flux in the core. Then the input current is increased to keep the main flux constant. The impedance of shorted turns is very low; therefore, a very high current will flow in shorted turns. in power transformers, for incipient faults or when the number of shorted turns is less than 10 % of total winding turns, there is no significant change in the terminal current to provide protection. Therefore detection and diagnosis of fewer winding inter-turn faults is a difficult task. These faults can lead to a catastrophic failure and

hence cause outages if they are not detected in the early stages. Conventional protection for internal winding faults in power transformers is the percentage differential relay. However, this protection does not completely protect the transformer for fewer numbers of shorted turns. Dissolved Gas Analysis (DGA) is currently used for detection of incipient faults. DGA can be used to verify the effects of faults even in the early stages of fault development. The results obtained from the DGA, which are applicable to an in-service transformer, do not clearly determine the cause and origin of faults. Also, it is not applicable for the dry-type, naturally cooled (NC), and air-forced (AF) cooling based transformers. FRA (Frequency Response Analysis) is also a well-known and popular method in fault diagnosis in power transformers. This method is used for detecting winding deformation and inter-turn short circuit. Conventional FRA has been relying on a graphical analysis for diagnosis of transformers, which requires trained experts to interpret test results. Hence fault detection by this method especially in the first stages is difficult and controversial.

The method discussed here is based on measurement and variations of the phase difference between the no-load input voltages and currents in primary windings of power transformers. The transformer is modeled in a steady state. In modeling under no-load operation, shorted turns in the faulty phase behave similarly to the secondary of an auto-transformer. A custom-built, 3-kVA, 50-Hz, 380/220-V, 3 phase transformer is designed and constructed in our laboratory to perform a set of experiments. The experimental results show that the proposed method is very efficient and encouraging for the detection of internal faults in the windings of distribution and small power transformers even in the early stage.

CHAPTER 2

DETERIORATING INSULATION MODEL

Electrical properties of dielectric material in power transformers may be altered significantly due to operating conditions and diverse factors,. Deterioration and aging of the insulations in a transformer are usually due to the strong electric field that the transformer is encountered with. Incipient internal winding fault in transformers is initiated by deteriorating the insulation between the turns in the windings. Thermal, electrical and mechanical stresses, moisture, and so on are the other factors which generally affect the aging and deterioration of insulation. Generally, the electrical behavior of dielectric material is modeled as a parallel equivalent circuit as is shown in Fig 2.1. This model will be used to simulate the incipient internal faults in the transformer winding. The resistance Rp represents the power loss in the dielectric. The capacitance Cp in this model is given by, where C0 is the equivalent capacitance in the vacuum of the insulation and ǫr is the relative permittivity or dielectric constant.

The insulation thickness between two layers or two turns in a winding is very thin. So the capacitance in the equivalent circuit has a very small value (about nF). The capacitive reactance hence will be very large around the power frequency and can be ignored. Therefore, a proper model that simulates the dielectric behavior is an external resistance between the turns. The resistance decreases with more deterioration of the insulation properties. In other words, losses for perfect insulation is almost zero,

but with degradation and aging of insulation, these losses are rising which leads to a reduction in insulation resistance and when the insulation was broken down completely, this resistance will be almost zero.

CHAPTER 3

ANALYSIS AND THEORETICAL BACKGROUND

3.1 EQUIVALENT CIRCUIT OF THE TRANSFORMER IN PRESENCE OF SHORT CIRCUIT

If there is an internal fault in one of the primary winding of a power transformer under no load operation, the faulty winding can be considered as the primary of an autotransformer. Short-circuited turns are considered as the secondary winding and the fault impedance acts as an autotransformer load. Magnetic flux induces an EMF in the faulty turns. As the short circuit occurs across a few turns of the winding, current flows in the faulty turns. This current generates a magnetic flux that opposes to the initial magnetic flux of the core. Now the input current in the faulty winding increases so that it compensates the effect of current in the shorted turns. The transformer in presence of a fault in the primary winding is shown in figure 3.1. In this case, the equivalent circuit of the transformer can be considered as shown in figure 3.2.

3.2 THEORETICAL BACKGROUND

Figure 3.3 shows the equivalent circuit of one phase of the transformer under no-load

the condition where there are no shorted turns in its windings.

the input impedance of a healthy transformer neglecting the primary winding impedance

is given by

At no-load condition, the transformer acts as a single winding with high self-inductance so that for most of distribution and power transformers the no-load power factor averages about 0.15 lagging. This means that the angle of input impedance at no-load condition is around 80 degrees and the value of core resistance (Rc) is approximately 6-7 times the value of magnetizing reactance (Xm) (so Xn = 6-7 times Rn). Therefore, the transformer input current lags approximately the input voltage by an angle of 80 degrees. For some extra high voltage transformers, this is not true because the no-load power factor may be around 0.85. Therefore, the analysis and the proposed method of fault diagnosis in this paper are valid for those transformers with lower values of no-load power factors.

3.2.2 Faulty condition

If there is a complete internal short circuit across a few turns in the primary winding, it is difficult to detect and diagnose the fault. This method is used in such cases. the analysis is valid for the case where the magnetic fluxes of the two healthy phases are not altered due to the fault current in shorted turns. At a high number of shorted turns, the magnetic flux distribution is fundamentally altered in a way that the magnetic fluxes of the other two phases are changed and so their no-load currents. 

According to the equivalent circuit in figure 3.3, the input impedance can be evaluated. The actual value of resistance and leakage inductance of shorted turns are very low especially for a low number of shorted turns i.e. one, two or five turns. However, the ratio of the number of turns in the primary winding to the number of turns that is short-circuited is actually high. Therefore the referred values of these quantities to the primary side are so that their combination with Zn=Rn+jXn determine the major part of the input impedance of the faulty phase winding. On the other hand, the impedance of primary winding (Z1 = R1 + jX1) is negligible to both Zn and Zf = Rf + jXf and their series combination. The input impedance is given by,

It is seen that under the faulty condition, the above angle is dependent on Rf and Xf.

If this dependence obeys a systematic rule, it is possible to find a method to detect the

inter-turn fault in a transformer.

The angle of input impedance varies considerably as the number of shorted turns

varies. Suppose that < Zinf is smaller than < Zinh. To satisfy this condition, the

following inequality has to be checked:

If the following inequality is satisfied, then the angle of input impedance in the faulty condition is decreased compared to the normal condition. Any increase in the number of shorted turns leads to more decrease in the angle of input impedance.

CHAPTER 4

EXPERIMENTAL RESULTS

A three-phase, YY, core type, 3-kVA, the 380-V/220-V transformer was designed and constructed for the experiments. The primary winding has 270 turns and the secondary has 150 turns. Short circuit and open circuit tests are performed to obtain the equivalent circuit parameters of the transformer. The results are shown in Table 4.1.Table 4.1: Calculated Phase Difference Between the Fundamental Components of the input Voltage and the Input Current in R- Phase Under Different Conditions (Y-Y

Connection)

To establish the short circuit between the primary turns, additional wires are soldered to the selected turns, 250th, 255th, 257th and 258th turn in the primary winding. Wires are extracted out to create the short circuit faults in the primary winding. It should be noticed that the resistance of these wires, acts as the fault resistance Rfault.

Angles of input impedance are calculated in the normal and in the faulty conditions, separately using the equivalent circuit obtained for the laboratory transformer. In the experiments, the short circuit is established through the external wires. The resistance of external wires has also been considered in the calculations. Table 4.1 shows the resistance of external wires and the phase difference between the fundamental components of R-phase voltage and R-phase current under normal and faulty conditions.

The results in Table 4.1 show that as the number of shorted turns increases, the angle of input impedance or the phase difference between the fundamental components of phase voltage and phase current in faulty phase decreases considerably.

The experimental investigation was carried out to verify the presented model and theoretical bases. Steady-state input voltages and currents were analyzed. High voltage windings were connected to the 380-V line to line voltage and experiments are performed under no load condition. Waveforms of the input voltage and the input current in Rphase (faulty phase) are shown in Fig. 4.1, 4.2, 4.3, 4.4. Per-unit values of voltages and currents are plotted. It is seen that as the fault occurs and expands in

the primary winding, the phase difference of the input voltage and the input current decreases significantly in the faulty winding. Experimental results show that the phase difference between the input voltage and the input current in the other two healthy windings has no significant variation while the number of shorted turns is low. Table 4.2 gives the phase difference between the fundamental components of Rphase voltage and R-phase current under normal and faulty conditions. These results have a very close agreement with the calculated results shown in Table 4.1.

Small differences between the calculated and experimental results are due to some unbalances in applied line voltages.

Alternatively, instead of the measurement of phase difference between the fundamental components, we can measure the phase difference between the overall input voltage and the overall input current in the faulty phase. Our investigation shows that the error due to this approximation is negligible. However, it is much easier to measure the phase difference using the overall signals and not their fundamentals. Summary of the results using the overall signals is given in Table 4.3. To confirm and to show the effectiveness of the analytical approach and this method for fault detection for various winding connections, the primary side of the laboratory transformer is changed to delta connection. The faulty winding is connected between phase R and phase S of the three-phase power supply. We name the faulty winding as RS winding. Table VIII gives the phase difference between the overall input phase voltage and the overall input phase current in the R and S phases, respectively. It is seen that as the fault occurs and expands in RS winding, the phase difference between the input phase voltage and the input phase current in phase R and phase S decrease significantly. Experimental results show that the phase difference between the input phase voltage and the input phase current in phase T has no significant variation. The faulty winding has no direct connection to Phase T. So the experimental results show that the method is applicable to DY connection too.

CHAPTER 5

CONCLUSION

Under steady state, no-load operation power factor of a power transformer is approximately very low. Hence its behavior is similar to a winding with high inductance. If there is any turn to turn short circuit in one of the primary windings, this winding and short-circuited turns can be considered as the primary and secondary windings of an autotransformer, respectively. Magnetic flux induces an EMF in the faulty turns. As the short circuit occurs across these turns, current flows in the faulty turns. This current generates a magnetic flux that opposes to the initial magnetic flux of the core. Now the input current to the faulty winding increases to compensate for the effect of current in the short-circuited turns. For internal faults with a low number of turns, the equivalent impedance of faulty turns is mostly resistive. So the current through the faulty winding is approximately in phase with its applied voltage. This method is applicable to various winding connections Y or D. But it is suitable only for the no-load operation of transformers.

REFERENCES

[1] Nahid Asadi, Homayoun Meshgin Kelk, “Modeling, Analysis, and Detection of Internal Winding Faults in Power Transformers ”, IEEE Transactions on PowerDelivery, September 2015.

[2] Karen L. Butler and Adedayo Kuforiji, and D. P. Kothari, “Experimental results from short-circuiting faults on a distribution transformer ”, 2008 IEEE Transmission and Distribution Conf., vol.1, Oct. 2001, pp. 299-306

[3] J.C. Meza and A. H. Samra, “Zero-sequence harmonics current minimization using zero-blocking reactor and zig-zag transformer”, IEEE DRPT, pp. 1758-1764,

2008.

[4] C. E. Lin, J. M. Ling, and C. L. Huang, “An expert system for transformer fault diagnosis using dissolved gas analysis”, IEEE Trans. Power Deliveryvol. 8, no. 1, pp. 231-238, January 1993.

[5] E.P. Dick and C.C. Erven “Transformer diagnostic testing by frequency response analysis”, IEEE Trans. on Power Apparatus and Systems, vol. PAS-97,

no. 6, pp. 2144-2153, Nov. 1978.

[6] Jong-Wook Kim, Byung-Koo Park, Seung Cheol Jeong “Fault diagnosis of a power

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Power Delivery ., vol. 20, no. 1, pp. 169 178, January 2005

INFLUENCE OF COOLING FLUID PARAMETER ON THE FLUID FLOW AND END PART TEMPERATURE IN END REGION OF A LARGE TURBOGENERATOR

In order to study the influence of the cooling fluid parameter on the fluid flow and end part temperature in the end region of the large turbogenerator, 330-MW water- hydrogen hydrogen-cooled turbogenerator is analyzed. The fluid velocity and pressure values from the flow network calculations are applied to the end region as boundary conditions and the losses obtained from 3-D transient electromagnetic field calculations are applied to the end parts as heat sources in the fluid and thermal coupling analysis.

After solving the fluid and thermal equations of fluid-solid conjugated heat transfer, the fluid velocity and end part temperature in the turbogenerator end region are gained under the different cooling fluid parameters. The influence of different fluid velocities and fluid temperatures in the water pipe inlet, and in the fan inlet on the fluid flow and end part temperature in the turbogenerator end region is researched. The calculation results of copper shield temperature are compared with the measured values. The calculation results coincident well with the measured values. These provide the important

reference for better cooling turbogenerator end region.

CHAPTER 1

INTRODUCTION

Turbogenerator is a kind of equipment which converts mechanical energy into electrical energy. When the turbogenerator runs, copper losses in the stator-end copper coil and current losses, in the end, parts are produced in the turbogenerator end region. These losses will be converted into the heat during the operation of the turbogenerator. On the one hand, these losses result in the temperature rise of the end parts, an, on the other hand, the heat of the end parts could be taken away by the cooling

fluid. The poor choice of cooling fluid parameters may lead to insufficient heat removal capability in the end region components. This will result in a very high temperature in the end region and shorten the service life of the turbogenerator. Therefore, it has an important engineering significance about studying the influence of cooling fluid parameter on the fluid ow and end part temperature in the end region of the large turbogenerator. Based on the results of the flow network and three-dimensional nonlinear transient electromagnetic field, the fluid and thermal equations of fluid-solid conjugated heat transfer, in the end, the region is calculated under the different cooling fluid parameters. The distribution laws of fluid velocity and end part temperature in the turbogenerator end region are researched under the different fluid velocities and fluid temperatures in the water pipe inlet and in the fan inlet.

In order to avoid confusion, fluid velocity and fluid temperature in the water pipe inlet are defined as water velocity and water temperature in the water pipe inlet. Fluid velocity and fluid temperature in the fan inlet are defined as gas velocity and gas temperature in the fan inlet in this paper. These provide an important reference for selecting a reasonable cooling fluid parameter in the larger capacity turbogenerator.

CHAPTER 2

Establishment of Flow Network and Calculation Results Based on the actual structure of a 330 MW water-hydrogen- hydrogen-cooled turbogenerator, flow network in the half of a turbogenerator is established. Fig.2.1 shows ventilation cooling system of the turbogenerator half-axial segment. Fig.2.2 shows a flow network in one half of the turbogenerator. After solving flow network equations within the half of turbogenerator under rated-load conditions, the distributions of gas rate and pressure in the turbogenerator are gained. The gas velocity in the fan inlet is 32.43 m/s. Fig.2.3 shows the pressure values in all the outlets of the end region.

                    Figure 2.1: Ventilation cooling system of a turbogenerator half axial segment

The gas velocity and pressure values from the flow network calculations are applied

to the end region of a 330 MW water- hydrogen-hydrogen cooled turbogenerator as boundary conditions. It provides an important theoretical basis for studying the influence of cooling fluid parameter on fluid velocity and end part temperature, in the end, the region of the turbogenerator.

                            Figure 2.2: Flow network in one half of the turbogenerator

                           Figure 2.3: Pressure values in all the outlets of the end region

CHAPTER 3

Fluid-Thermal Coupling Analysis Model for Turbogenerator End Region

3.1 Model for Coupling Analysis Based on the actual shape and size of the 330 MW turbogenerator end region, 3-D fluid and thermal coupling analysis model of the turbogenerator end region is es-

established. Fig.3.1 gives the turbogenerator end parts and end region inlets. Fig.3.2 shows the end region outlets.

      Figure 3.1: Turbogenerator end parts and region inlets

The gas velocity in the fan inlet of the turbogenerator is 32.43 m/s under rated-load

conditions, which is obtained from the calculated results of the flow network. The gas

temperature in the fan inlet is 41 Celsius. zone outlet between the end core and wind

board, air-gap outlet, No. 2 and No. 4 cold-gas zone outlets in the turbogenerator end

                                                     Figure 3.2: End region outlets

region are given as pressure-outlet and the pressure values are obtained from the calculated results of the flow network under rated-load conditions. The water temperature in the water pipe inlet is 40 Celsius and water velocity in the water pipe inlet is 1 m/s in the turbogenerator end region under rated-load conditions. These are applied as boundary conditions of the 3-D fluid and thermal coupling analysis model of the turbogenerator end region.

3.2 Determination of Eddy Current Losses of End Parts in the Turbogenerator End Region

Based on the classical electromagnetic theory, mathematical and geometric models of the 3-D nonlinear transient electromagnetic field are established in the turbogenerator end region. Fig.3.3 shows the 3-D electromagnetic solving region for the turbogenerator end region. The equations for the 3-D nonlinear transient field have been calculated. According to Equation, the eddy current loss in the end parts can be determined once the eddy current density has been obtained:

where Pe is the eddy current loss (in W), Tc is the period of time, k is the total number of elements in the various end parts, i is the element volume, Ji is the element eddy current density, and r is the end-part conductivity.

Fig.3.4 shows the current losses of end parts in the turbogenerator end region. It

can be seen from Fig.3.4 that the largest eddy current loss which locates copper shield

                     Figure 3.3: 3-D electromagnetic solving region of the turbogenerator end region

inner circle zone is 12872 W. However, the smallest eddy current loss which locates press finger is 946 W.

                         Figure 3.4: Current losses of end parts in the turbogenerator end region

CHAPTER 4

The Influence of Water Parameter in the Water Pipe Inlet on End Part Temperature in the Turbogenerator End Region

4.1 Influence of Water Velocity

After solving the fluid and thermal equations of fluid-solid conjugated heat transfer, the influence of water velocity in the water pipe inlet on the temperature of end parts is studied. Fig.4.1 shows the highest temperatures of end parts under the different water velocities in the water pipe inlet. Fig.4.1 shows the highest temperatures of end parts decrease as water velocity in the water pipe inlet increases. When water velocity in the water pipe inlet is between 0.5 m/s and 3 m/s, the highest temperatures of the copper shield, stator-end copper coil, press finger, and stator-end winding insulation decrease obviously as water velocity in the water pipe inlet increases. However, the highest temperatures of these end parts decrease a little after water velocity in the water pipe inlet is larger than 3 m/s (between 3 and 6 m/s). Since the stator-end copper coil is cooled by the water in the water pipe inlet, the highest temperature of the stator-end copper coil decreases obviously as water velocity in the water pipe inlet increases. The highest temperature of stator-end copper coil drops to 47.4 Celsius when water velocity in the water pipe inlet is 3 m/s. The highest temperature (when water velocity in the water pipe inlet is 3 m/s) of stator-end copper coil is 16.2 Celsius lower than that of the stator-end copper coil when water velocity in the water pipe inlet is 0.5 m/s. The highest temperature of the press plate decreases a little as water velocity in the water pipe inlet increases.

Fig.4.2 shows the highest temperature of copper shield drops to 58.2 Celsius when water velocity in the water pipe inlet is 6 m/s. The highest temperature of the copper shield is 2 Celsius lower than that of the copper shield when water velocity in the water pipe inlet is 1 m/s. Fig.4.3 shows the highest temperature and average temperature of the stator-end copper coil are 47 and 43.2 Celsius when water velocity in the water pipe inlet is 6 m/s. The highest temperature and the average temperature of the stator-end copper coil (when water velocity in the water pipe inlet is 6 m/s)

Figure 4.1: Highest temperatures of end parts under the different water velocities in

the water pipe inlet.

are 7 and 8.9 Celsius lower than those of stator-end copper coil when water velocity in the water pipe inlet is 1 m/s, respectively. As the cooling water flows in the water pipe, the water temperature gradually increases and the cooling effect of water becomes worse. It results in a higher temperature in the involute segment of stator-end copper coil.

4.2 Influence of Water Temperature

 In order to study the influence of water temperature in the water pipe inlet on end part temperature, fluid and thermal equations of fluid-solid conjugated heat transfer are calculated when water temperatures in the water pipe inlet are 35, 37.5, 40 Celsius (original scheme), 42.5, and 45 Celsius, respectively. As water temperature in the water pipe inlet increases, it has little effect on the highest temperatures and average temperatures of the copper shield, press finger, and press plate. However, the increased temperatures of the stator-end copper coil and stator-end winding insulation are almost proportional to increased water temperature in the water pipe inlet. Fig.4.4 gives the highest temperatures of stator-end copper coil and stator- end winding insulation under the different water temperatures in the water pipe inlet.

Figure 4.4: Highest temperatures of the stator-end copper coil and stator-end winding

insulation under the different water temperatures in the water pipe inlet coil

CHAPTER 5

The Influence of Gas Parameters in the Fan Inlet on End Part Temperature in Turbogenerator End Region

5.1 Influence of Gas Velocity

Based on the theory of fluid mechanics and heat transfer, the influence of gas velocity in the fan inlet on the gas velocity and end part temperature in the end region is researched: Case One, gas velocity in the fan inlet decreases by 15%; Case Two, gas velocity in the fan inlet decreases by 10%; Case Three, gas velocity in the fan inlet decreases by 5%; Case Four, gas velocity in the fan inlet increases by 5%; Case Five, gas velocity in the fan inlet increases by 10%; Case Six, gas velocity in the fan inlet increases by 15%. After solving flow network equations, the fan inlet velocities and pressure values of the end region outlet in the 3-D fluid and thermal coupling analysis model of the turbogenerator end region are obtained under the different cases, as shown in fig.5.1 and Fig.5.2. It can be seen from Fig.5.2 that all the pressure values of the end region outlet increase gradually as gas velocity in the fan inlet increases. As gas velocity in the fan inlet increases, the pressure values of No. 2 cold-gas zone outlet, No. 4 cold-gas zone outlet, and air-gap outlet increase obviously, while the pressure value of zone outlet between the end core and windboard increases a little. These provide the boundary condition for 3-D fluid and thermal coupling analysis model of the turbogenerator end the region to study the influence of gas velocity in the fan inlet on the gas velocity and end part temperature in the turbogenerator end region. 

To facilitate analysis, the ventilation ducts between the copper shield inner circle zone, copper shield transitional circle zone, copper shield external circle zone and press plate are marked by Part AB, Part BC, and Part CD, respectively. The arrow direction of the dotted line represents the direction of gas flow, as shown in Fig.5.3. 

Fig.5.4 shows average gas velocity in the ventilation duct between the copper shield

and press plate rest increases gradually and then decreases gradually along the direction of gas flow from Part AB to Part CD. As gas velocity in the fan inlet increases, average gas velocity in the ventilation duct between the copper shield and press plate increases gradually. When gas velocity in the fan inlet increases by 15%, the average gas velocities in the Part AB, Part BC, and Part CD between the copper shield and press plate reach the highest values and they are 39.75, 67.81, and 49.1 m/s, respectively. When gas velocity in the fan inlet decreases by 15%, the average gas velocities in the Part AB, Part BC and Part CD between the copper shield and press plate reach the lowest values and they are 28.98, 46.02, and 32.39 m/s, respectively.

5.2 Influence of Gas Temperature

In order to study the influence of gas temperature in the fan inlet on end part temperature, the highest temperatures of the end parts under the different gas temperatures in the fan inlet are given, as shown in Fig.5.5. The figure shows the highest temperatures of end parts increase almost linearly as the gas temperature in the fan inlet increases. As the gas temperature in the fan inlet increases, the highest temperatures of the copper shield, press plate, press finger, and stator-end winding insulation increase

obviously, especially to the highest temperature of the copper shield. The reason is that copper shield completely contacts with the cooling gas. The temperature of the cooling gas has a great effect on the temperature of the copper shield. Since the stator-end copper coil is wrapped in the stator-end winding insulation and doesn't contact with the cooling gas, the gas temperature in the fan inlet has a small effect on the stator-end copper coil. The highest temperature of the stator-end copper coil increases a little like the gas temperature in the fan inlet increases. The highest temperatures of the copper shield, press plate, press finger, stator-end winding insulation, and stator-end copper coil increase by 10, 7.5, 8.5, 5.7, and 1.7 Celsius, respectively, when the gas temperature in the fan inlet increases from 36 to 46 Celsius.

CHAPTER 6

Calculated Results and Measured Values in the Turbogenerator End Region

After solving the fluid and thermal equations of fluid-solid conjugated heat transfer, the position of the highest temperature in the turbogenerator end region is determined. The highest temperature in the turbogenerator end region appears in the copper shield inner circle zone. The highest temperature and average temperature of the copper shield inner circle zone are 60.2 and 57.3 Celsius under rated-load conditions. In order to verify the accuracy of the calculation results, the temperature of the copper shield is measured online at the power plant. To facilitate analysis, the copper shield inner circle zone is divided into three equal volumes along the generator axial direction, which is represented by M1, M2, and M3, as shown in Fig.6.1. Three temperature sensors are embedded evenly at a 120 angle along the circumference in the same radius of the copper shield inner circle zone. They are embedded in Region M1 close to Region M2. At the same load and power factor (rated power is 330 MW, rated voltage is 20 kV, rated current is 11200 A, and power factor (lagging) is 0.85), measured values and calculated results are obtained. Fig.5.2 shows measured values and calculated results in the Region M1 of the copper shield inner circle zone. The calculated results and three measured values are compared. The calculated results are closer to the measured values. In addition, it not only calculates this one geometry but also calculates lots of different geometries of practical engineering cases using the method. For example, fluid velocities and temperature distributions of parts are obtained in the wind-driven generator, permanent magnet generator, large hydro-generator, and induction motor using the method of this paper. The calculated temperature results and measured values are compared in the different geometries of these electrical machines. The errors meet the engineering requirement in these different engineering cases. It shows the calculation method is credible.

The reasons for the error between the calculated results and the measured values are that different gap distances between the water conduits of the stator-end windings and between the stator-end windings, together with the rough surface of the copper shield and the installation error, result in an asymmetric velocity distribution in the ventilation duct between the copper shield and the press plate.

CHAPTER 7

CONCLUSIONS

When the water velocity in the water pipe inlet is smaller than 3 m/s, the temperatures of the copper shield, stator-end copper coil, press finger, and stator-end winding insulation decrease obviously as water velocity in the water pipe inlet increases. However, the temperatures of these end parts decrease a little after water velocity in the water pipe inlet is larger than 3 m/s. The highest temperature (when water velocity in the water pipe inlet is 3 m/s) of stator- end copper coil is 16.22 Celsius lower than that of the stator-end copper coil when water velocity in the water pipe inlet is 0.5 m/s. When water velocity in the water pipe inlet is 3 m/s, it can reduce effectively the temperature of end parts in the turbogenerator end region. As water temperature in the water pipe inlet increases, it has little effect on the temperatures of the copper shield, press finger, and press plate. The increased temperatures of the stator-end copper coil and stator-end winding insulation are almost proportional to increased water temperature in the water pipe inlet. The water temperature in the water pipe inlet has a great effect on the temperature of stator- end copper coil. However, it has a small effect on the temperature of stator-end winding insulation. As the gas temperature in the fan inlet increases, the highest temperatures and average temperatures of end parts increase almost linearly. The temperature of end parts can be reduced obviously when the water velocity in the water pipe inlet is 3 m/s and gas velocity in the fan inlet is 37.29 m/s. This could enhance the temperature distribution in the turbogenerator end region and could provide an important reference for the cooling system design in the turbogenerator.

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[6] S. Nategh, O. Wallmark, M. Leksell, and S. Zhao,, Thermal analysis of a PMaSRM using partial FEA and lumped parameter modeling, IEEE Trans. Energy Convers., vol. 27, no. 2, pp. 477488, Jun. 2012.

[7] H. Jichao, L. Weili, W. Likun, Z. Xingfu, Z. Xiaochen, and L. Yong,, Calculation and analysis of the surface heat-transfer coefficient and temperature fields on the three-dimensional complex end windings of the large turbogenerator,, IEEE Trans. Ind. Electron., vol. 61, no. 10, pp. 52225231, Oct. 2014.

വിമാനങ്ങളില്‍ 50Hzന് പകരം 400Hz ഉപയോഗിക്കുന്നത് എന്തു കൊണ്ട്?

1994ല്‍ ഈസ്റ്റ്‌വെസ്റ്റ് എയര്‍ലൈന്‍സില്‍ ജോലി ചെയ്യുന്ന കാലത്താണ് വിമാനത്തിലെ വൈദ്യുതി ഉപകരണങ്ങള്‍ വളരെ ചെറുതും ഭാരം കുറഞ്ഞവയുമാണെന്ന് മനസ്സിലായത്. അന്ന് മുംബൈ സാന്റാക്രൂസ് എയര്‍ക്രാഫ്റ്റ് മെയിന്റനന്‍സ് സ്റ്റോറിന്റെ തലവനായിരുന്ന മഹാരാഷ്ട്ര സ്വദേശി കട്ടാരെ സാറാണ് ഇതിന്റെ പിന്നിലെ ഗുട്ടന്‍സ് പറഞ്ഞു തന്നത്.

വിമാനങ്ങളിലും ബഹിരാകാശ വാഹനങ്ങളിലും ഉപയോഗിക്കുന്ന യന്ത്ര സാമഗ്രികളുടെ ഭാരം കുറച്ചു നിര്‍ത്തുക വളരെ പ്രധാനമാണ്. വിമാനങ്ങളില്‍ ഉപകരണങ്ങളുടെ ഭാരം ഒരു കിലോ കുറച്ചാല്‍ 5 കിലോ ഭാരമെങ്കിലും ആകെ ലാഭിക്കാന്‍ പറ്റും. ഉപകരണങ്ങളെ താങ്ങി നിര്‍ത്താനുള്ള സംവിധാനങ്ങളുടെ ഭാരവും കൂടുതല്‍ ഇന്ധനത്തിന്റെ ആവശ്യകതയും കാരണമാണ് ഈ വര്‍ദ്ധന ഉണ്ടാകുന്നത്.  പലപ്പോഴും ഭാരം കുറക്കാന്‍ വിട്ടു വീഴ്ചകള്‍ ചെയ്യുമ്പോള്‍ മറ്റെന്തെങ്കിലും ദോഷങ്ങളും ഉണ്ടാകും. വൈദ്യുതി ഉപകരണങ്ങളുടെ രൂപകല്‍പ്പനയിലും ഇത്തരം വിട്ടുവീഴ്ചകള്‍ ചെയ്യുന്നതിനാലാണ് വളരെ വലിപ്പം കുറഞ്ഞ ഉപകരണങ്ങള്‍ നിര്‍മ്മിക്കാന്‍ സാധിക്കുന്നത്.

ആദ്യ കാലങ്ങളില്‍ വിമാനങ്ങളിലെ ഉപകരണങ്ങള്‍ പ്രവര്‍ത്തിച്ചത് ഡി.സി വൈദ്യുതി ഉപയോഗപ്പെടുത്തി ആയിരുന്നു. എന്നാല്‍ ആധുനിക വിമാനങ്ങളില്‍ റഡാറുകള്‍, ആയുധ സംവിധാനങ്ങള്‍, കോക്ക്പിറ്റ് ഡിസ്പ്ലേ സംവിധാനങ്ങള്‍, വാര്‍ത്താ വിനിമയ സംവിധാനങ്ങള്‍ എന്നിവയ്ക്ക് ധാരാളം വൈദ്യുതി ആവശ്യമായതോടെ എ.സി വൈദ്യുതി സംവിധാനങ്ങള്‍ വ്യാപകമായി.

ഫ്രീക്വന്‍സി വര്‍ദ്ധിപ്പിച്ചാല്‍ വൈദ്യുതി ഉപകരണങ്ങളുടെ ഭാരവും വലുപ്പവും കുറക്കാന്‍ പറ്റുന്നതിനാല്‍ വിമാനങ്ങളില്‍ 400Hz ആണ് ഫ്രീക്വന്‍സി. ഇന്‍ഡക്ഷന്‍ മോട്ടോറുകള്‍ കറങ്ങുന്നത് ഫ്രീക്വന്‍സിക്ക് ആനുപാതികമായാണ്. അതു കൊണ്ട് ഫ്രീക്വന്‍സി കൂടുമ്പോള്‍ ഒരേ വ്യാപ്തത്തിലും ഭാരത്തിലും നിന്ന് കൂടുതല്‍ പവര്‍ ഉത്പാദിപ്പിക്കാന്‍ സാധിക്കും. ട്രാന്‍സ്ഫോര്‍മറുകളില്‍ കുറഞ്ഞ മാഗനറ്റിക് കോര്‍ മതിയാകും എന്നതിനാല്‍ ഭാരവും വലുപ്പവും കുത്തനെ കുറയും. കപ്പാസിറ്ററുകളുടെ വലുപ്പം ഏഴില്‍ ഒന്നായി കുറക്കാന്‍ പറ്റും.

ദീര്‍ഘദൂര ട്രാന്‍സ്മിഷന്‍ ലൈനിലെ ഇന്‍ഡക്ടന്‍സ് കാരണം ഫ്രീക്വന്‍സി കൂടുമ്പോള്‍ സീരീസ് ഇംപിഡന്‍സ് കൂടുകയും വോള്‍ട്ടേജ് ന്ഷ്ടം വര്‍ദ്ധിക്കുകയും ചെയ്യുന്നതിനാല്‍ സാധാരണ വൈദ്യുതി നെറ്റ്‌വര്‍ക്കുകളില്‍ ഉയര്‍ന്ന ഫ്രീക്വന്‍സി ഉപയോഗിക്കുന്നത് ലാഭകരമല്ല. അതു കൊണ്ട് 400Hz സിസ്റ്റം നെറ്റ്‌വര്‍ക്കിന്റെ നീളം വളരെ കുറവായ ഡാറ്റാ സെന്ററുകളിലും അന്തര്‍വാഹിനികളിലും വിമാനങ്ങളിലും പട്ടാള ഉപകരണങ്ങളിലും മാത്രം ഒതുങ്ങി. ലോകത്തിലെ വിവിധ രാജ്യങ്ങളിലെ വൈദ്യുതി നെറ്റ്‌വര്‍ക്കുകളില്‍ 60/50 Hz ആണ് ഫ്രീക്വന്‍സി എങ്കിലും എല്ലാ രാജ്യങ്ങളിലും വിമാനങ്ങളില്‍ 400Hz തന്നെയാണ് ഉപയോഗിക്കുന്നത്.

WHAT is CASCADING BLACKOUT and REASONS of BLACKOUT

  CASCADING BLACKOUT

1. A power outage (also called a power cut, a power blackout, power failure or a blackout) is a short-term or a long-term loss of the electric power to a particular area.
2. There are many causes of power failures in an electricity network. Examples of these causes include faults at power stations, damage to electric transmission lines,   substations or other parts of the distribution system, a  short circuit, or the overloading of electricity mains.
3. Power failures are particularly critical at sites where the environment and public safety are at risk. Institutions such as hospitals, sewage treatment plants, mines, shelters and the like will usually have backup power sources such as standby generators, which will automatically start up when electrical power is lost. Other critical systems, such as telecommunication, are also required to have emergency power. The battery room of a telephone exchange usually has arrays of lead-acid batteries for backup and also a socket for connecting a generator during extended periods of outage.
4. Power outages are categorized into three different phenomena, relating to the duration and effect of the outage:
5. A permanent fault is a massive loss of power typically caused by a fault on a power line. Power is automatically restored once the fault is cleared.
6. A  brownout is a drop in voltage in an electrical power supply. The term brownout comes from the dimming experienced by lighting when the voltage sags. Brownouts can cause poor performance of equipment or even incorrect operation.
7. A blackout is the total loss of power to an area and is the most severe form of the power outage that can occur. Blackouts which result from or result in power stations tripping are particularly difficult to recover from quickly. Outages may last from a few minutes to a few weeks depending on the nature of the blackout and the configuration of the electrical network.

Figure 3.2 shows the photographs of blackout when cascading failure is affected.

 Figure 3.2 The photographs of blackout when cascading failure is affected.

3.3  REASONS OF BLACKOUT

Cascade tripping occurs due to unbalance in the grid, mainly due to under frequency situation. Under frequency in power system occurs in the following circumstances.
1. During a steady condition when an online generating capability is inadequate to meet the load requires a steady decline of frequency occurs.
2. When a sudden loss of large generating unit occurs or there is a sudden tripping major of a transmission line carrying the bulk load in a system, a sudden decline in frequency is observed.
For all practical purposes, electricity flowing through the grid cannot be stored. Once it is generated electricity must flow somewhere. If there is not enough demand it will cause voltage spikes, if it is too little, it will cause voltage dips.
For high voltage power to remain stable synchronism must be maintained when the synchronism is disturbed by inevitable local events such as sudden loss of major transmission line or generator power can begin to flow in an uncontrolled manner causing automatic safety devices to trip and isolate parts of the system to prevent damage to equipment.
Blackout result when generation is separated from the load. The grid typically will withstand any single event (single generator failure or single transmission- line failure) under worst case conditions. This called “N-1” contingency planning. But the system can collapse if several failures take place in rapid succession when the grid is already stressed. Such events include.
- Multiple lightning strikes
- Falling trees
- Equipment failure
- Human error
- Wires sagging into the underbrush
- Overloads, voltage sags, frequency deviations.
- Sabotage.
- Fire.

  Unseal operation of grid occurs when:
- Under frequency condition in power grid causes unusual operation of machines in the grid. This kind of situation causes automatic safety devices to trip to prevent damage to equipment.
- When demand is more than the power generated, the machine will overload. So speed will decrease. So frequency goes down. Sometimes it may happen that demand is very much less than the generation, it will also cause unbalance condition. If there is not enough demand it will voltage spikes, if it is too little, it will cause voltage dips.

  In thermal plants under frequency causes the following effect in the generator:
- Higher flux density resulting in machine saturation and higher field requirement.
- Excessive core losses.
- Heating of core and other parts.
- Reduced speed reduced cooling effect.
- The reduced reactance of the machine resulting in higher fault currents.


In hydropower plants under frequency causes the following effect in the generator.
- Increases flux level and magnetic saturation. - More iron losses.
- Overheating and overloading.
- Decreased speed and poor ventilation and hence overheating.

What is SF6-Sulfur Hexafluoride

What is SF6-Sulfur hexafluoride 

Sulfur hexafluoride is an inert, nontoxic, colorless, odorless, tasteless, and non-flammable gas consisting of a sulfur atom surrounded by and tightly bonded to six fluorine atoms. It is about five times as dense as air. SF6 is used in GIS at pressures from 400 to 600 kPa absolute. The pressure is chosen so that the SF6 will not condense into a liquid at the lowest temperatures the equipment experiences.

SF6 has two to three times the insulating ability of air at the same pressure. SF6 is about 100 times better than air for interrupting arcs. It is the universally used interrupting medium for high voltage circuit breakers, replacing the older mediums of oil and air. SF6 decomposes in the high temperature of an electric arc, but the decomposed gas recombines back into SF6 so well that it is not necessary to replenish the SF6 in GIS.

There are some reactive decomposition byproducts formed because of the trace presence of moisture, air, and other contaminants. The quantities formed are very small. Molecular sieve absorbs ants inside the GIS enclosure eliminate these reactive byproducts. SF6 is supplied in 50-kg gas cylinders in a liquid state at a pressure of about 6000 kPa for convenient storage and transport. Gas handling systems with filters, compressors, and vacuum pumps are commercially available. Best practices and the personal safety aspects of SF6 gas handling are covered in international standards (IEC, 1995).