Transient Stability Enhancement and Critical Clearing Time Improvement for Kurdistan Region Network using Fact Configuration

The Electrical power system has become vast and more complex, so it is subjected to sudden changes in load levels. Stability is an important concept which determines the stable operation of the power system. Transient stability analysis has become one of the significant studies in the power system to ensure the system stability to withstand a considerable disturbance. The effect of temporary occurrence can lead to malfunction of electronic control equipment. The application of flexible AC transmission systems (FACTS) devices in the transmission system have introduced several changes in the power system. These changes have a significant impact on the power system protection, due to differences inline impedance, line current and voltage. On the distance relaying protection system to identify essential issues that protection engineers need to consider during the stages of design and operation of the protection system. Transient analysis can be conducted using a simulation software package. One of the commercial simulation software package used by industry worldwide is Siemens Power System Simulation for Engineering (PSS/E). The object of this work is to improve the Transient stability and to clear critical fault times of the Kurdistan Region Government (KRG) network by using optimal FACTS devices in different optimal locations under fault conditions.


INTRODUCTION
Generally, Electrical strength system is commonly nonlinear system and a compound network, consists of three essential stages which are generation, transmission and distribution substations. The exporter central part is generation stations that transmit the electricity to the load. As a reference of growing power requesting, in transmission line sometimes may be loaded more than lines capacity at construction. Generation stage, on the electric power network, is the primary point of the power system for using synchronous generators to product electrical voltages. After production for increasing voltage using the set up a transformer before transferring it to minimizing the current in the lines in order to minimize the losses in the transmission lines. After the transmission, the voltage is stepped down using step down transformers that are distributed consequently. An unexpected massive disturbance contains faults, clearing of faults, surprising load changes and persistent or uncontrollable tripping of lines and generators.
The reason for this paper is to improve transient stability and critical fault clearing time in the KRG network by using FACTs devices by using PSS/E software when it was licensed by the government. In FACTs Devices, we choose (SVC and UPFC) as the best solution in our network for now to increase power transfer capability.
(To'aima et al., 2015)he investigated that STATCOM was planned to increase the apparent power loss and explain voltage fall issue of the IEEE 5bus standard system, the result has been done by using less possible size of the reactive power injected or absorbed by the STATCOM devices, while sustaining the stability limits in order to decrease the projected installation cost of STATCOM devices. (Hassan et al., 2019) The purpose of this study is to sustain sufficient power for higher heavy loads where possible and accurate amount of load shedding while keeping the load under the accessible power threshold. The main duties are saved ultimately without any disturbance likely. The output solves the efficiency and practicality of the applied method for probable uses in power systems.
(Abd Al Hassan and Tuaimah, 2020)Investigates one of the most promising FACTS devices, UPFC is used to achieve the fundamentals (voltage regulation, reactive power and power flow controller) to make the system more efficient and reliable. The result can show the number of UPFC is increase with increase load at (15% and 20%) because one UPFC cannot reduce from overload in lines so by using tow unit of UPFC device in different position can improve maximum load ability and minimum line losses.
The motivation behind this paper is to survey and improve the voltage profile in weak buses on the KRG network by using SVC (Reactive Power Control) (Husein and AbdulFatah, 2016). This investigation is about improvements of enactment are planned applied on the interconnected 400kV, 50Hz Kurdistan and Iraq power systems using ±500kV, 300MW HVDC link.

(Roberts et al., 2015)
In this investigative show, the approximation of the (CCT) is resulting from the direct strategies of stability of the energy system. This Equation is designed to combine as many possible characteristics of transient stability analysis as possible, like different fault areas and different network circumstances after the failure. The objective of this calculation is to resolve trends instability (in terms of CCT) of energy systems under the system parameter change.
Journal of Engineering Volume 26 October 2020 Number 10 53 (Mohammed, 2019) Is considered the essential (critical) clearing time of synchronous machines in the power systems, when the non-salient pole and salient poles of synchronous machine model are using mathematical equations in power system. CCT compared two models with various cases.

Power System Stability
When there is a physical disturbance in an electrical power system returning to operating condition without losing equilibrium is called power system stability (Kundur et al., 1994). It means the power system overall connected. In the practical group of generators connected when one of them is losing the synchronism the system not lose the stability. Power system stability is classified into three types (steady state, dynamic and capable of a power system to transfer from point to another under the case of small load variation is steady state, transient) stability; dynamic stability is a type of rotor angle stability to discuss the stability when there is a significant disturbance.

TRANSIENT STABILITY
The capability of a power system to preserve of synchronism when it causes the transient fault is called transient stability; this disturbance is caused an epic trip of generators, rotor angles by nonlinear power angle linkage. The significant disruption causing loss of load and loss of generation units for returning to steady-state may be losing some generation units. (Sutter et al.,

2015)
In KRG network case the system was stable, but when there is a bus fault, or one transmission line is out of work it makes the network unstable and the CCT change according to of type of responsibilities such as clearing in the result section. For simulating power system stability and output file, we can use PSS/E software. (Kundur et al., 1994)From SMIB the mathematical equations must be as:

2.1 Swing Equation
When M = inertia constant D = damping constant Pmech = mechanical power source Pelec = electrical power product From the equivalent circuit the equations will be as: The swing equation can be written as: The estimation stability in (CCT) is a critical component to preserve the stability of the power system. The maximum duration for occurring fault in a power system is called significant clearing time (CCT) when it sets randomly, if the time for CCT is less than the time for clearing fault CFT, it causes the loss of stability. The primary purpose is to calculate protection characteristics when required in power system. (Kundur et al., 1994) 2 2 = − When the fault occurs = 0 hence the equation will be as:

fact Devices
The flexible AC transmission system is a static device construct with the growing abilities of power electronic ingredient. Equipment's with high power level can change with various voltage scales. The comprehensive at initial points for network elements preferring the reactive power the parameters of the power system. The FACT devices are mainly classified: The main application of FACT devices is used for controlling power flow, compensating reactive power by increasing the ability of transmission lines, improving and conditioning power stability and quality, controlling of voltage stability, modification of sparks. Also, FACT devices having many advantages in transmission power system are summarized as follows: 1. Increasing transmission lines loading capacity. 2. Reactive power Decrease. 3. Decrease the spark voltage. 4. Damping of power pulses. 5. Stability system, ensuring. 6. Accessibility and Security. 7. Unwavering quality and economy activity.

(SVC) Static VAR Compensator
Shunt controller's electrical equipment, Static VAR Compensator (SVC) is the oldest type generating of the FACT device family, which is designed for refining fast-acting reactive power on a high-voltage electricity transmission line. SVC is an application using for voltage regulation, dynamic stability, damping Oscillations, and reducing voltage drop. SVC is most widely installed equipment's form FACT devices in the world which can be capable of supplying reactive power in the system for improving voltage stability. It connected to the transmission system directly for controlling voltage at weak buses occasionally it connected with the control of the transmission system as shown in Fig 2. (Khoa et al., 2017) The modelling for SVC may be as (TCR-FC). In power system load is changed from time to time so that it causes confusion in the system, causing voltage instability. The essential appearances for SVC are voltage regulator and VAR switch mode. It's a stator device. Leading and lagging are terming for connecting SVC shunt devices. Worked on Voltage Profile Improvement of KR Power Network Using Reactive Power Control the result was improving stability of the system instantaneously. In our transmission line network, we suggest to use shunt fact devices SVC for improving voltage stability, voltage collapse and reactive power for increasing, transmitting power capacity, oscillation and damping by contingent on the optimal sites. SVC is separated in two quantities one is static VAR generator the second is static VAR absorber when the output may be capacitive or inductive current to keep and regulate factors of electrical power system, especially the bus voltage, in general, is thyristor Controlled Reactor (TCR) for retaining reactive power and thyristor Switched Capacitor (TSC) for reactive power supplying as shown in figure (3). The basic idea of working SVC controlling system is (a) reactive power generation for (capacitive SVC) to low voltage; (b) reactive power absorbs for (inductive SVC) for high voltage. (Virk and Garg, 2013) = (10) The reactive power injected at bus m The susceptance Equivalent equation is given by: The two thyristors is firing angle

UPFC (Unified Power Flow Controller)
The UPFC is a device of the second generation of FACT devices which can use simultaneously for controlling several line parameters for power flow, such as (line impedance, voltage and phase angle). It was produced by combining two types of fact devices: The Static Synchronous Compensator (STATCOM) and the Static Synchronous Series Compensator (SSSC) as shown in Fig.4 so that the UPFC is the best type between other FACT devices because it works as a shunt transformer by connecting to the transmission line system for improving steady-state stability, transient stability and increasing voltage stability. (Sebastian and Sajith, 2014). The fundamental principle of working UPFC is to controlling area time of dynamic compensation on AC transmission line when producing multi-functionality for improving problems in the power line system.

WITHOUT FACT DEVICES
For calculating transient stability in PSS/E first of all, we must convert the system to Dynamic system the process for converting the system to dynamic system for measuring CCT is in many steps when the system convert to dynamic we just putting the model of generators in our network Journal of Engineering Volume 26 October 2020 Number 10 59 we are using( GENROU) model and Exciter current model (SEXS) as shown in table (1) after that running the transient stability reading CCT, after that, we create three-phase fault in the system and reading CCT if the system remains stable we it means we do not have any problem, but if the order is not firm we must return back to dynamic data and changing the system till we get stability in the network this prose is shown in a flowchart in

WITH FACT DEVICES A-By connecting SVC
At normal case we have any problem in the KRG network one of them under-voltage, overload, by Connecting SVC in parallel with EGPP bus the result for CCT will be between (0.1-0.25) and the under-voltage of many busses was increased. and the transient stability for calculating CCT is increasing as Fig.8 Figure 8. CCT with SVC.

a. By connecting UPFC
Connecting UPFC in series with EGPP bus the result for CCT will be between (0.23-0.45)0.171pu as shown in Fig.9 By connecting UPFC in the KRG network many especially in Duhok and Soran area, many problems were solving such (overloading and under voltage) because the voltage in this area is very low it changed from 111Kv to 125Kv.as shown in appendix B. By this calculation, in result, we occur that connecting UPFC is much better for connecting to our KRG network. In our network having low voltage level in Duhok and Soran area if we using UPFC it will solve this problem and solving many other problems such as transient stability and dynamic stability. Also, SVC is a good compensator for improving our problem but comparing it with UPFC, UPFC is much better, but it's more costing than SVC .as shown in table 2

CONCLUSION
FACT devices are power electronics based reactive compensators that are connected in a power system and are capable of improving the power system transient performance and the quality of supply. Although individual compensations differ, all the two FACTS devices not only damp the system oscillations of the multi machine system but also reduce the oscillation settling times for generator Emf and rotor angle transient responses. This observation of KRG network is helpful to examine stability improvement in both the cases. This work can be extended to multi-machine system by using other types of FACTS controllers like SVC, UPFC etc.