The types of load in modern power systems range "from simple resistive load to more complicated loads with electronic controllers" [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Power system complexity and their characteristic nonlinearity increase with increase in controllers and loads. This in turn makes power systems to exhibit increasing instability problem. Power instability problems can cause partial or total blackout. This problems can be categorized into "voltage, phase angle and frequency related problems" [1][2][3][4][5][6][7].
In order to address power systems disturbances many devices have been invented and a number of solutions have been proffered to enhance the effectiveness of these devices. These include fast exciter or Automatic Voltage Regulators (AVR), Power System Stabilizer (PSS) which helps to produce the fine adjustment needed to damp out electromechanical or low frequency oscillations in power systems [1,[7][8][9][10]. Existing study [1] examined the "Impact of Active Distribution Network Cell (ADNC) on Power System Oscillation." The objective of the work is to examined the impact of Passive Cell (PC) on Active Distribution Network Cell (ADNC) and to analyze the small and large signal stability changes and to analyze the power flow signal [1,7].
System Oscillation requires modelling of power system components. The system covers the well knows Two Areas-System benchmark. The benchmark is a model was created by Canadian Association to presents the various types of oscillations that may happen in large/small power systems. It is also known as Kunder's system which "is specifically designed to study low frequency electromechanical oscillations in large interconnected power systems" [1].
The 2-Area System was modelled using Power System Analysis Toolbox (PSAT) embedded in Matlab Environment. Figure 1 shows the design overview. The analysis of the system model is summarized below: i.)
The System consists of two areas: Area I region is indicated by the buses on the left half side of Figure 1 and Area II region is indicated by the buses on the right half.
ii.)
The two areas are connected together by a transmission line of power, voltage rating and frequency of 100MVA, 20kV and 60Hz respectively.
iii.)
The system generally consists of 4 PSSs, 4 AVRs, 5 Synchronous Generators, 3 PV Generators, 3 ZIP Loads, 3 PQ Constants, 11 Buses, 5 Transformers, 5 transmission lines, 1 Slack Bus, and 1 Induction Machine.
iv.) Each of the Area consists of 2 Synchronous Generators, each of rating 900MVA of Power and 20kV of Voltage.
v.) Each generator is equipped with AVR and PSS.
The loads are applied at Bus 6 and Bus 5.
The Slack Bus, PSS, AVR, Synchronous Generator, Transformers, and Transmission Lines are all linked together by Buses of rating 20kV.
There are three cases considered in the system design, namely: Base Case, in which the system is considered neutrally as depicted in Figure 1; Passive Cell (PC) Case and Active Distributive Network Cell (ADNC) which are varied at Bus 5 and Bus 6 as shown in Figure 2 (a) and 2 (b) respectively.
The system with passive network cell consists of Induction Machine, constant PQ load and ZIP load.
The base case load benchmark is P= 2734 MW, with 100 MW transferred from Area I to Area II over the tie-line. Results in Figures 3 and 4 show the power flow and the eigenvalues for the base case system.
In order to study and analyze the impact of Passive Cell in the Power System Oscillation, Passive Cells which consists of Loads (ZIP and PQ) and Induction Machine are applied to the system (Figure 5).
The power flow of system with PC at bus 6, Area II is as shown in Figure 6. Figure 7 is the eigenvalues curve for the system with PC.
Figure 7: Eigenvalues report at Area II for system with PC Figure 8 shows the power flow analysis of the system with PC implementation at Area I while Figure 9 is the eigenvalue for the same system.
Table 1 shows the report of the Total Generation, Total Load and Total Losses of both Real and Reactive Power of the model for each of the cases i.e. base case, PC case and ADNC case.
From the table, it can be seen that ADNC has a great impact on power system oscillation than any of the other in term of the total generation, total losses and total load of the model.
In term of the design model, when solving the power flow from the existing design, there are errors generated, shown as follows:
Definition of system connections ...
Error: Block <Line3> cannot be connected to block <Line4>.
Error: Block <Line4> cannot be connected to block <Line3>.
Error: Block <Line5> cannot be connected to block <Line6>.
Error: Block <Line6> cannot be connected to block <Line5>.
Failed conversion from Simulink model: Simulink model is not well-formed (check links).
Attempted to access idx(2); index out of bounds because numel(idx)=1.
These errors make the existing model heavily error-prone. The errors are corrected using the new model by removing the transmission lines (compare Figure 15 with Figure 16). Since the existing model is invalidated, some of the responses of the analysis analyzed are not gotten when using the new model. New dimensions were then taken to analyze the impact of both passive cell and active distribution network cell on power system oscillation.
In term of the continuous and time domain analysis, only the base case has continuous and time domain flow. The passive cell and ADNC (both Area I and Area II) do not have continuous state variable.
As shown by the simulation of the base case, where there is no additional elements connected to the two area system, it has been found that all the PSS types exhibit damping effect under small disturbances.
Then, in the subsequent simulation model, adding a regular load and induction motor as a passive cell, in different locations, and their impact on the system loading have been analyzed. The results revealed that the system becomes instable when the passive cell is connected at the midpoint of tie line. Thus, small signal analysis using PSAT where conducted to find the reason of the instability.
Small signal analysis revealed that one of the induction motor eigenvalues has a positive real part when the passive cell is installed at the mid-point which is the cause for system instability.
Then, the impact of the active distribution network cell (ADNC) on the power system oscillation was been investigated. Increasing in the loading level is actually reflected as an additional stress on the tie-line that may cause system instability. Results show that ADNC has a great impact on power system oscillation than any of the other models in term of the total generation, total losses and total load of the model. The power flow signals also show that there is a great impact by passive cell and ADNC on power system oscillation.