Satish Natti, Ph.D. Senior Consultant satish.natti@siemens.com	Paloma de Arizon, Ph.D. Staff Consultant paloma.dearizon@siemens.com	Nelson J. Bacalao, Ph.D. Senior Manager Consulting nelson.bacalao@siemens.com

Synopsis
The main objective of switching studies is to make sure that the transmission system can be energized in a secure manner that guarantees that the system equipment can withstand the overvoltages due to the corresponding maneuvers. These studies generally include steady-state evaluation of every energization step, electromagnetic transient studies of switching of selected topologies (most critical cases) and verification of the absence of sustained overvoltages due, for example, to self-excitation of generators during load rejection events. Siemens PTI has recently conducted such studies on the Uzbekistan-Afghanistan 220 kV interconnection, for Advanced Engineering Associates International (AEAI). This interconnection was energized weeks later and is now supplying power to several communities in northern Afghanistan. This article summarizes the results of the study completed to verify the proposed energization procedure.

The North East Power System (NEPS) is a 220 kV grid system, built in Afghanistan to import power from neighboring Uzbekistan. An overview of the NEPS is shown in Figure 1. Power will be transported to the main load center in Kabul and to the various communities along the way. The interconnection is radial from the Surkhan 220 kV bus and supplies load that is isolated from the rest of the Afghanistan power system.

Steady-State Evaluation
The energization procedure provided by AEAI was reviewed and modified in order to preserve the system integrity while serving the maximum load in the Kabul region. The recommended energization sequence has 18 steps, and is described in Table 1. The amount of total load connected was less than that originally proposed to maintain acceptable voltage levels. Several sensitivity cases were also performed in order to maximize the power transfer to Kabul.

Figure 1 - North East Power System (NEPS)

Table 1 - Recommended Energization Sequence

Load Rejection Study
The main objective of the load rejection study is to evaluate the possible sustained overvoltages in the system due to the switching events. Surkhan generator is modeled as a conventional generator with governor and excitation, as no additional data is provided by the client. Table 2 shows the load rejection scenarios and the associated system condition. The studies are performed using the PSS®E transient stability program. The system is stable for all switching events considered in the study. However, for the loss of the Chimtalah transformer and reactor, the Pul-e-Khumri and Chimtalah 220 kV buses experience high voltages. The loss of reactor may activate the overvoltage protection at Chimtalah substation.

Table 2 - Load Rejection Study Results

Electromagnetic Transient Evaluation
The objective of this study is to verify the adequacy of the line insulation during the energization process. The switching of 220 kV lines, interconnecting the Surkhan substation to the Chimtalah substation, is simulated, as energization of 220 kV lines represents the worst scenario from a transient overvoltage point of view. The study is performed using MicroTran (EMTP program). Statistical (uncontrolled) switching is simulated for the considered switching sequences, and voltages at the 220 kV buses are observed. The voltage at the Chimtalah 220 kV bus for the loss of reactor is shown in Figure 2. For the installations of voltage levels 245 kV and lower, the Basic Insulation Level (BIL) is the most demanding condition. The results show that the maximum instantaneous overvoltage is 2.9 pu (502 kV crest) and the BIL for the 220 kV substations is 1050 kV, with a minimum of 950 kV for some power transformers. Therefore, as per the results of this study, the equipment insulation should withstand the switching overvoltages. The sustained overvoltage levels after the line energization maneuver is simulated are under 1.1 pu, and are consistent with the steady-state simulations described above.

Figure 2 - Voltage at Chimtalah Substation for the Loss of Reactor

Conclusion
An energization/switching study involves steady-state evaluation of each switching step, transient evaluation of switching of critical lines, and verification of the absence of sustained overvoltages due to load rejection. The results of a study of this type, performed on the Uzbekistan-Afghanistan 220 kV interconnection, have been presented in this article. The proposed energization sequence was modified based on this study. The load rejection study results show that the system is stable for all switching actions considered in the study, except for a sudden loss of reactor at Chimtalah, which may trigger the overvoltage protection. The transient overvoltage evaluation of line switching indicates that the 220 kV lines have sufficient insulation to withstand the overvoltages during the switching process. This line was successfully energized at the end of April and is supplying power to Afghanistan.

Download document as .pdf