Improving the Transient Stability of the Mixed AC/DC Networks with FACTS

Received May 9, 2018 Revised Sep 6, 2018 Accepted Dec 20, 2018 It is probable that future power transmission systems will contain more HVDC-VSC links (High Voltage Direct Current-Voltage Source Converter) leading to a growing complexity in the study of its problem and so do the transient stability problems which are yet to be determined. In this context, this paper presents an efficient method to resolve this problem. Its main objective consists of improving transient stability of the AC/DC (Alternating Current/ Direct Current) power system network using FACTS (Flexible Alternating Current Transmission Systems). The overall performance of the FACTS was evaluated in an IEEE 14 bus test system by nonlinear simulations carried out using Matlab environment to check the performance of FACTS (TCSC, Thyristor Controlled Series Capacitor). The obtained results showed the effectiveness and robustness of FACTS in improving the transient stability of the systemt. Keyword:

Since its existence, the power system was produced, transported and distributed in AC [9]. But, in the last few years, the incorporation of subsets of transmission HVDC in AC transmission networks brought a significant change in the transport of the electric power [9]- [13].
In situation where the power system is subject to fault which may leads to lose its stability, clearing such fault in the adequate time is crucial to maintain the system stability. If the fault is cleared after the critical clearing time (CCT), the system stability is lost pushing the system in unstable region. The so-called critical clearing time (CCT) is therefore determined to tune our compensation apparatus.
The region of attraction in such a situation can be increased by using various controllers (FACTS), the TCSC is the best solution which provides necessary series compensation to regain synchronization and to reschedule power flows.
The rest of this paper is organised as follows, the transient stability problem and power system modeling is expressed Section 2. Methodology is described in Section 3. Experimental results are presented in Section 4; the conclusions are provided in Section 5.

POWER SYSTEM COMPONENTS MODELING
The main phase in a stability study is to create a mathematical model of the network during the transient. The elements included in the model are those affecting the acceleration (or deceleration) of the machine rotors. The complexity of the model depends upon the type of transient and system being investigated. Generally, the components of the power system that influence the electrical and mechanical torques of the machines should be included in the model, these components are elaborated in following subsections [5].

Synchronous generators
The transient stability problem in power system is defined by a differential algebraic equation, which can be resolute by time domain simulation. The swing equation for ith generators is [5], [14]- [17]:

Basic governor control system
Governor control system controls the speed and power output of the hydroelectric turbine, water level controlled power output controllers can be used for grid connected units. The block diagram of Figure 1 shows a typical governor for a hydraulic turbine Where t r is the temporary droop; r T is the reset time; s K gate servo gain; P T pilot servo time constant; g T gate power servo time constant; max g and min g are the maximum and minimum rate of change of gate position; g gate position, the parameters as shown in Table 2.

Excitation
There are a number of types of excitation system which all have the same aim. We consider here as shown in Figure 2, and the corresponding parameters are shown in Table 3.

Power system stabilizer
The power system stabilizer (PSS) is a device that measures improvements in system stability when added to a generator's automatic voltage regulator (AVR), as shown in Figure 3.

Transmission lines
The transmission lines are characterized by the equivalent  model.

Transformers
Generally, there are three types of transformers; two-winding, three-winding transformers and the phase shifting transformer. Here we consider first and second.

Loads
Constant admittance power loads are modeled as a specified quantity of real and reactive power disbursed at its bus i.
where ) (i P l and ) (i Q l are the active and reactive power at bus i;V voltage magnitude at bus i.

FACTS a. SVC
The Static VAR Compensator (SVC), is a member of the FACTS family that is connected in shunt with AC power systems, as shown in Figure 4 and the parameters as shown in Table 4. The goal functionality is to regulate the voltage at a specified bus by controlling its equivalent reactance.  b. TCSC TCSC can be seen as a controllable reactance inserted in the related transmission line. TCSC works by reducing the effective series impedance of the transmission line; hence the power transferred is increased, as shown in Figure 4.  Table 5.

HVDC-VSC
The HVDC-VSC comprises two VSCs: One operating as a rectifier and the other as an inverter. The two converters are linked either back-to-back by a DC transmission. One VSC controls DC voltage and the other acts on the active power transmission through the DC link [18].  Table 6.

METHODOLOGY
According to Section 2, a methodology was developed to find the CCT. The methodology began with the modeling of IEEE-14 bus system governed by (1)-(6) and the simulation of the block diagrams of Figures  4-6. A stability analysis was conducted for test fault situation of 3 phase fault near bus 1 (line 1-2). An appropriate fault clearing time was set for each stability analysis of the system and gradually increased till the system became unstable.
In this case, in order to maintain the transient stability, a TCSC was installed near bus 2 and in series with the transmission line between bus 2 and bus 5. The maximum compensation by TCSC was limited to 70% of the reactance of the uncompensated line where TCSC is located.

CASE STUDY AND DISCUSSIONS
The IEEE 14-bus modified test system consisted of 5 synchronous machines with IEEE type-1 exciters, 3 of which were synchronous compensators used only for reactive power support. There were 19 buses, 17 transmission lines, 8 transformers and 11 constant impedance loads. The total load demand was 259 MW and 73. 5 MVAr [19].
The IEEE 14-bus system has been modified to incorporate the HVDC-VSC link between buses 2-4 and SVC at bus 9. The results consist of 2 cases. Case 1 was to analyse the effect of a 3-phase fault at line 1-2 near bus 1 (cleared by tripping line 1-2) of the mixed AC/DC network and Case 2 was the implementation of TCSC controller and analysis on the same system as shown in Figure 7.   Figure 8 and Figure 9 shows the response of the rotor angle and electrical power for two different values of clearing time. The system however becomes unstable when the clearing time is 0.29 (s) and stable when the clearing time is 0.28 (s). However, in Case 1, it was found that system transient stability was lost following the incident disturbance at line 1-2 near bus 1. Visibly, the system cannot be operating in this mode because security of the network was violated.
In case 2, compared to the case 1, in order to maintain the transient stability, a TCSC is installed at bus 2 and in series with the transmission line between bus 2 and bus 5. The TCSC improves transient stability margins. The results in Figure 10 and Figure 11 demonstrate the aptitude of TCSC to enhance power system transient stability.
These simulation results demonstrate the effectiveness of TCSC into improving transient stability. Proper use of TCSC gives better results and ensures system transient stability following the incident.

COMPARISON TO PREVIOUS WORKS
We have benchmarked our study to previous works like [20] which the authors have used the same power system (IEEE 14 bus) but without HDVC-VSC link. We have found a clear improvement concerning the CCT; up to 0.28s instead of 0.145s in [20].
Second comparison was made that emphasize the superiority of our method on the same power system without HDVC. In this case, the active losses are taken as a parameter of comparison. We have found 7.83 MW of active power losses in our proposal instead of 15.2 MW in [21].

CONCLUSION
This paper investigates the improvement of transient stability using TCSC which is an effective FACTS device capable of controlling the electrical power and rotor angle in a transmission link by governing its series parameters.
FACTS devices, such as TCSC can be very active in case of the incident. In this paper, we have focused on the improvement of the transient stability in presence of fault via the use of TCSC. The transient stability of the mixed AC/DC network is compared with and without the use of TCSC. The simulation results prove the efficiency of the integrated TCSC on transient stability improvement.