CONTROLLED SWITCHING APPLICATON FOR TRANSMISSION LINES AND POWER APPARATUS

Abstract

Since the inception of power system network, utility uses transmission lines along with different power apparatus to transfer bulk amount of power from source to load end. Sometimes, it is essential to perform switching operations of transmission lines and power apparatus during normal/abnormal conditions. However, the switching operations may cause switching transients. At Extra High Voltage (EHV) and Ultra High Voltage (UHV) levels, switching transients gain more importance than lightening surge. At the same time, frequency of switching of transmission lines and power apparatus are increasing day by day due to increase in penetration of renewable energy resources in the grid, which in turn generates switching transients. The switching transients develop thermal and dielectric stresses in the power system network. They may cause problems such as degradation of quality of power supply, loss of life of power apparatus, lower efficiency, and poor reliability of the system. Two types of switching practices namely, uncontrolled switching and controlled switching are normally used by the utilities. In past, uncontrolled switching of transmission lines and power apparatus has been performed using Circuit Breakers having Pre-Insertion Resistor (PIR-CB). Though PIR-CBs offer better performance in terms of minimization of the level of switching transients compared to simple Circuit Breaker (CB) (i.e. which does not have pre-insertion resistor) they are not able to eliminate switching transients completely. In order to minimize the level of switching transients, point on wave switching technique (also known as controlled switching technique) has been reported in the literature. In this case, the level of switching transients has been limited by controlling the opening/closing instant (point on wave) of the CB. The opening/closing instants of CB are known as controlled switching targets and the device attached with the CB is known as Controlled Switching Device (CSD). Depending on the power system equipment to be switched ON/OFF, opening/closing instant of the CB is controlled by CSD. For successful implementation of the controlled switching, the targets must be repetitive in nature. This type of nature helps in predicting occurrence of point on wave target. Accordingly, the controlled switching command can be raised and mechanism of the CB can be activated. However, CB is a mechanical device with different mechanical and electrical closing instants. The electrical opening/closing target of the CB has been achieved using mechanical opening/closing target. Due to effect of Mechanical Operating Time (MOT) scatter, it is difficult to attain precise electrical opening/closing target using mechanical opening/closing target of the CB. This is one of the setback faced by the controlled switching technique during its implementation in real field. The main objective of iv this thesis is to reduce the level of switching surge during energization/re-energization of different types of transmission lines and power apparatus along with the consideration of the effect of MOT scatter of the CB. In case of uncompensated and shunt reactor compensated transmission line, energization/re-energization has been performed at zero crossing instant of gap voltage across the contacts of CB (VGAP), which is the difference between line side and source side voltages. Energization of fully discharged Un-Compensated Transmission Line (UCTL) has been carried out at zero crossing instant of supply side voltage. Due to absence of trapped charges, zero crossing instances of supply side voltage and VGAP occur at the same time. On contrary, it is required to study polarities and variations in the magnitude of the trapped charges during energization of a partially discharged transmission line (i.e. re-energization). It has been reported in the literature that the trapped charges attain unidirectional polarity during dead time of the CB in case of UCTL. Conversely, in case of Shunt Compensated Transmission Line (SCTL), the nature of trapped charges is oscillatory and achieve bidirectional polarity. Therefore, it is difficult to identify the controlled switching targets during energization of a partially discharged SCTL due to non-repetitive waveform of VGAP. In order to determine optimal controlled switching target during energization of a partially discharged SCTL, different controlled switching targets have been identified. In the presented work, supply side voltage has been considered as the reference. Seven points on wave targets from one complete cycle of supply side voltage (0 - 3600) have been tested (each at 450). Out of all seven targets, zero crossing instant of the supply voltage is considered as an optimal controlled switching target for fully and partially discharged SCTL. Whereas, half of the peak value of the supply side voltage (having same polarity of the trapped charge) is considered as an optimal controlled switching targets for a partially discharged UCTL. As the above mentioned targets are repetitive in nature, they can be easily implemented in real field. Here, the transmission line has been modeled by the frequency dependent phase model, available in Power System Computer Aided Design (PSCAD)/Electro Magnetic Transients including DC (EMTDC) software package, as it is commonly used for transient overvoltage study. In order to evaluate performance of the proposed controlled switching target, a simulation study has been carried out. At this point, different power system parameters such as fault type, fault inception angel, fault distance, compensation level, load angel, and fault duration are varied to generate large numbers of cases. Further, the proposed technique has been tested considering effect of MOT scatter of the CB and Short Circuit Current Level (SCCL) of the source. Considering all above cases, the suggested technique is able to limit the level of switching surge up to 1.9 p.u. v and hence, can be easily adopted by utilities. At last, comparative evaluation of the proposed scheme with the existing scheme clearly indicates its superiority in terms of reduction in the level of the switching surge for fully and partially discharged SCTL. Similarly, controlled switching targets for energization of fully and partly discharged SCTL have been obtained using analysis of line side voltages. In order to determine optimal controlled switching target, Discrete Fourier Transform (DFT) of the line side voltages has been performed. It has been observed that the derived optimal controlled switching target is repetitive in nature. The simulation model has been developed in PSCAD/EMTDC software package environment considering an existing 400 kV Indian power transmission network. In order to test wide range applicability of the proposed technique, a simulation study of energization of a partially discharged SCTL has been performed. Variation in fault type, load angel, fault duration, compensation level, and distance of fault have generated large numbers of simulation cases. The proposed technique has also been evaluated considering impact of Switching Arrestor (SA), effect of MOT scatter of the CB, and different SCCL of the sources. It has been observed that the proposed technique is capable to maintain the level of switching surge below 1.5 p.u. At the end, comparison of the proposed scheme with the existing scheme proves the supremacy of the proposed scheme. The presented work has been further extended to identify controlled switching target for Single Phase Auto Reclosing (SPAR) for SCTL, which has been determined using line side voltages. In order to assess the performance of the proposed technique, a simulation study has been carried out. Large numbers of simulation cases have been generated by varying power system parameters such as fault duration, load angel, instant of fault inception, fault distance, and compensation level. The modeling of an existing 400 kV power transmission network has been carried out in PSCAD/EMTDC software package. Further, the performance of the proposed technique has been verified considering effect of MOT scatter of the CB and different SCCL of the grid sources. Unlike the existing technique, the proposed technique is able to restrict the level of switching surge up to 1.5 p.u. Shunt reactors are widely used in power system network at 400 kV and above level. They are switched ON and OFF multiple times in a day for reactive power management. Uncontrolled energization of the shunt reactor draws high charging current from the supply, which creates high thermal stress in the winding of the shunt reactor. As this charging current contains asymmetric dc component it takes a very long time to decay. At the same time, presence of asymmetric dc component in the charging current may saturate the instrument transformers due to which the protective device may maloperate. Based on analytical analysis, optimal controlled vi switching targets have been determined during energization of the shunt reactor. The simulation model has been developed in PSCAD/EMTDC software package. The proposed technique has been tested considering different connection configurations of the shunt reactor. Moreover, the effect of MOT scatter of the CB is also analyzed. The proposed technique has also been implemented in real field and the results are discussed thoroughly. It has been observed that the proposed technique is able to limit the level of charging current up to 1.1 p.u., which authenticates its wide range of applicability. Occasional switching of power transformers is converted in to frequent switching due to penetration of more and more renewable energy sources because of deregulation of power system network. Uncontrolled energization of power transformer draws very high level of inrush current from the source. This inrush current contains asymmetric dc component and different harmonics component, which degrades power quality and may develop severe thermal stresses in the winding of the transformer. At the same time, it also creates Transient Voltage Dip (TVD) for the equipment connected with the same line. The level of inrush current depends on winding resistance, residual magnetic field, and instant of switching. In order to limit the level of inrush current during controlled energization of a power transformer, it is essential to minimize the effect of MOT scatter of the CB. In the presented work, a detailed study of the parameters, which are responsible for the MOT scatter of the CB, has been carried out. Further, the behaviour of the dynamic fluxes is also investigated, as it is entirely different for electrically and magnetically coupled power transformers. The performance of the proposed technique has been evaluated by modeling two different types of power transformers (electrically and magnetically coupled) in PSCAD/EMTDC environment. In order to validate the simulation model, field testing for implementation of the proposed technique for electrically (1500 MVA, 765 kV) and magnetically (500 MVA, 400 kV) coupled power transformers have been performed. It has been observed that the proposed technique effectively limits the level of inrush within 1.0 p.u. At the end, comparison of the proposed scheme with the existing scheme proves the supremacy of the proposed technique in terms of reduction of the level of inrush current. The presented work is likely to contribute significantly to the area of controlled switching applications for transmission lines and different power apparatus. The different techniques developed will be particularly useful for shunt compensated transmission line, shunt reactor, and power transformer. Some suggestions, based on observations and simulations in this area, are proposed at the end of the thesis for the benefit of potential researchers.