Proposing a method to protect the transmission line equipped with series capacitor

Dissertation for Master Degree in Electrical Engineering

Power Orientation

Transmission lines protected by distance relays have limitations in the presence of series compensation. When the fault loop includes a series capacitor, the impedance seen by the distance relay is reduced. The level of compensation at any moment depends on the number of capacitors connected at that time. Therefore, for the correct operation of the distance relay, it is necessary to have information about the level of compensation in the transmission line. It is possible to adapt settings in numerical relays using information collected through communication systems. PMU is an electrical device that measures the amplitude and phase of voltage and current at high speed and puts a time stamp on them with an accuracy of one microsecond. In this thesis, the phasor information obtained from the two ends of the transmission line using the application of PMU in power systems has been used to calculate the line impedance and the compensation level for different conditions, so that the settings of the protection zone 2 and 3 of the relay are adapted to the series compensated line. The proposed method for verification has been implemented in two small and large sample systems. From the obtained results, it can be seen that the proposed method estimates the compensation percentage of the transmission line with an acceptable error value. Then by correcting the settings of the distance relay according to the system conditions, the relay performs correctly against various errors.

keyword- transmission line protection equipped with series capacitor, phasor measurement unit, adaptive protection

Chapter 1: Introduction

The protection of transmission lines that are responsible for the transmission of electric power is always the attention of engineers. It is electricity and it is very important. Today, to increase stability, reduce losses and transfer maximum power in the system, various series compensators are used in transmission lines. Using series capacitors in transmission lines, especially in long transmission lines, is an effective and economical measure to increase the transmission capability of the lines and improve the transient and long-term stability of the system. In addition, the use of series compensator reduces the line voltage drop and improves the line voltage profile. Despite the above advantages in improving system performance, series capacitor and protective equipment also create serious problems for line protection.

The performance of the distance relay, which is the most common protection relay in the transmission network, is usually strongly affected by the capacitor series compensator, and this issue can cause the distance relay to malfunction. The most important problem created by series line compensation is that the impedance measured by the relay no longer represents the actual distance from the fault point to the relay location because the apparent resistance and reactance seen by the relay during the fault period are affected by the changes in the series capacitor voltage. Most of the problems faced by series compensated line relays include phenomena such as shrinking of the protection area, reversal of voltage or current, subsynchronous oscillations and transient states caused by the operation of capacitor protection equipment (MOV[1]). The rapid development of telecommunication systems in the world and the invention of the phasor measurement method with the help of phasor measurement units (PMU [2]) using the synchronization signal of the global positioning system (GPS [3]) have created a huge transformation in the visibility and control of extensive power networks. This technology is still growing at a remarkable pace. So that in the near future, the fast network coordinated control system will definitely replace the usual and less effective local control methods and the limited capabilities of the current SCADA/EMS systems to create comprehensive real-time control systems as well as slower controls such as secondary voltage control in power networks. These systems are capable of limiting the continuation of disturbances and preventing the occurrence of instabilities. A phasor measurement unit measures electrical signals in a power network (in electrical engineering, such measurements are also referred to as simultaneous phasor [4]). They are also tagged by pulses received from the Global Positioning System[5].. It is also time-stamped by the pulses received from the Global Positioning System [5] so that the phasor data of the PMUs installed at the network level can be sorted by time. Then, by performing calculations on them, improve the power system monitoring, protection and control performance. One of the most important applications of PMU in the power grid is the momentary monitoring of the amplitude and angle difference [6], which can prevent possible global blackouts [7], because network operators will be able to quickly detect a critical situation in less than a few seconds and prevent the spread of the accident. Meanwhile, PMU can be used in network stability evaluation, fault diagnosis, post-event investigation, network model determination, line emergency management, and load shedding. Today, with the development of phasor measurement units as a precise measurement tool, research on measurement applications is increasing with it. The accuracy of voltage and current measurement makes it possible to make accurate calculations of line parameters and improve decision-making in the protection system. 1-1 Research Necessity In series compensated lines with capacitors, the impedance observed by the distance relay depends on the level of compensation in the transmission lines. This compensated level depends on the number of series capacitors connected to the network. For the correct operation of the distance relay and to increase the efficiency of the protection system and its improvement, it is necessary to have real-time information at the same time, on each compensated level in the transmission lines.

In order to improve and increase the ability of the protection system of the compensated transmission lines with series capacitors, phasor measurement units (PMU[8]) that have suitable characteristics in measuring parameters with a high sampling rate can be used to calculate the level used the compensation of the transmission line and used the obtained information in setting the protection zones 2 and 3 of the distance relay. The obtained information cannot be used to improve the protection zone 1 of the distance relay because the relay operation in this protection zone is instantaneous and there is no time required for data processing.

In this thesis, using the phasor measurement unit (PMU), a method to improve the protection of compensated lines with series capacitor is presented. This method estimates the level of compensation of the line with a high sampling rate of voltage and current in a phasor at the same time and adapts the distance protection to the current conditions. 1-2 Thesis structure

The second chapter, the various methods presented to improve the protection of the compensated lines are evaluated. In this chapter, the distance protection of the transmission line equipped with series capacitor will be described first, and then the phasor measurement units will be introduced. Then the research done to improve the protection of the compensated lines is reviewed.

In the third chapter, then the proposed protection scheme is presented based on the applications of phasor measurement units in the power system. Then at the end, in order to implement the proposed method in larger systems, a method for optimal placement of phasor measurement units is described. In the fourth chapter, the simulation results in two sample systems are described.  It is assumed that the capacitor is located in the middle of the transmission line. Then, the proposed method in the first sample system has been investigated in four different modes of compensation in PSCAD/EMTDC software. The first mode is 30% compensation level, the second mode is 40% compensation level, the third mode is 70% compensation level and the fourth mode is the line without compensation. The cutoff range is estimated from the sampling performed by the PMU and checked against the actual cutoff range. Then the proposed method is evaluated in a standard IEEE 9-bus system at three levels of 30%, 40% and zero percent. In order to do this, first, the optimal location for PMUs is obtained using the integer programming algorithm. The method is implemented in DIgSILENT Power Factory software.

In the fifth chapter, conclusions and suggestions are expressed.