Literature Review on Simulink Synthetic Circuits (Essay Sample)

Literature Review on Simulink Synthetic Circuits
[Name]
[Institutional Affiliation]
Literature Review on Simulink Synthetic Circuits
It is an admitted fact that the DC circuits don’t have natural zero currents can cause some problems when the current is required to be interrupted. The breakers are the devices used to generate zero currents in the DC circuits. These devices generate the required current using two principles. One of them is the formation of the arc voltage between the electrodes of the breakers which is opposite in direction the supply voltage (Durocher, Walls, & Becker, 2011). The breaker has produced the arc voltage in a manner that it is greater the system’s voltage to produce the zero currents. The efficiency of this method relies on the effectiveness of surrounding medium to absorb all of the inductive energy in the circuit. This method often results in long arcing times which causes unnecessary erosion to electrodes of the breaker (Durocher, Walls, & Becker, 2011). The greater the inductive energy of the system, greater will be arcing time.
Another way of interrupting the current is known as current commutation. The process requires additional circuits to be connected in parallel with the main breaker. These circuits have the potential to store some amount of energy and by discharging it, a controlled current injection can be made. This produced current inject opposes the current in a circuit and thus a force current-zero is produced. It is also an admitted fact that the counter-current inject should be greater than instantaneous fault current present in the DC circuits (Durocher, Walls, & Becker, 2011). So it is necessary to evaluate the fault current of the circuits before applying this method. This method, however, has proved to be an efficient approach and causes less erosion on the device’s electrodes. A conventional commutation circuit is represented by the following diagram:
Figure 1: DC Circuit with a commutation current (Durocher, Walls, & Becker, 2011).
In the above figure the DC source ES is connected in series with a main breaker S1 which is followed by a load breaker S3 to which the load is connected in parallel. The circuit resistance and inductance includes the value of the DC source and linking lines and tracks. A commutation circuit is joined in parallel with the circuit consisting of a capacitor Cc, coil inductance LC and an auxiliary switch S2. The metal oxide varistor (MOV) is connected across S1 and has a clamp voltage to protect the devices in the circuit. The capacitor CC needs to be pre-charged and for the active commutation mode, otherwise it will be known as passive commutation mode. The circuit by nature contains the inductive load so a freewheel diode DFW is attached in parallel with the load side. The purpose of this diode is to bypass the circuit current when the current slope is negative. This installation is very useful when it is required to avoid any energy transfer between the transmission lines and the commutation capacitor Cc during an interruption (Durocher, Walls, & Becker, 2011). The source side energy transfer, however, cannot be bypassed using the freewheel diode.
In the active mode, the current oscillation provided by the pre-charged commutation circuit Cc will rise instantly and it will grow to oppose the current in main breaker S1 in case of closure for S2. A proper combination of CC and LC can result in the production of at least one zero current in the main breaker S1, the main current will commutate to the parallel path resulting in the change of polarity across the capacitor Cc (Durocher, Walls, & Becker, 2013). Oscillations will also produce a zero current across the S2 which will result in the determination of upstream line current and commutation parameters. It means the capacitor will be charged to a value governed by the initial voltage, the voltage in the system, and the voltage produced due to stored energy in the upstream line. However, the voltage the switch S2 and the rectifier circuit used in the topology are unidirectional. It means after the first zero-current is generated across the S2, the capacitor will be exposed to a high-voltage and it should be designed to withstand such voltage spikes. Finally, the load breaker S3 can be opened without any arcing. This method is also known as the one-stage interruption method (Durocher, Walls, & Becker, 2013). The disadvantages of using this method are summarized in the following points:
* An external voltage supply is required in this method with the sole aim of keep the capacitor continuously charged.
* There are high voltages produced during the commutation process and usually this method requires installation of voltage limiting devices like MOVs.
* Cc must have a large capacitance value, which in some practical scenarios is extremely difficult and expensive to achieve.
* After an interruption failure the commutation circuit might not be able to effectively perform its operations.
The generation of fault currents is also a challenge to this method. The rate of change of fault current is heavily relied on the change in the line inductance. Since the energy stored in the commutation capacitor is limited, the circuit might not be able to withstand generation of fault currents. Therefore, it is of utmost importance to measure accurate values for fault current in a DC system. This relationship is depicted in the following diagram:
Figure 2: Fault currents created in a DC system (Durocher, Walls, & Becker, 2013).
2.1 Analysis of Active Commutation Mode
A typical analysis of active commutation mode can be represented by the following figure:
Figure 3: Cycle for active commutation mode (Seeger et al., 2014).
2.1 First Interval (A)
At the time t1, the source current IS reaches a level It1. This level is presumed as the threshold for the commutation process to start. A counter current injection ic was produced to oppose the source current. Due to these facts the current through breaker iB is decreased and in that instant t2 it becomes zero. At the same time voltage of capacitor is increased to VCt2. After the current zero was produced the current is was entirely commutated from the main breaker on to a parallel path. During the time interval t1-t2 the source current is keeps on increasing. During the interval t2-t3 the source current was initially increased due to stored magnetic energy and the some of the remained energy in capacitor CC (Seeger et al., 2014). At the time t3 the source current is reached to zero and the capacitor is fully charged. The voltage across the capacitor VCE is increased to a higher value but it has opposite polarity. After this interval load breaker S3 can be opened to save the circuit from the fault currents originating from the source.
The final voltage across the capacitor is dependent on the magnetic energy stored in the system, the initial value of voltage in the system, voltage across the capacitor, and the supply voltage for circuit. When the voltage levels reached the tripping voltage of the arrester, it limited to overvoltage. The process triggers prevention of any further voltage rise as the arrester partly absorbs the inductive DC-line energy (Wm = ½ Li2) (Seeger et al., 2014). A proper choice of arrestor voltage can be important for designing such circuits. A proper choice of the absorption by the arrester can trigger a decay in the fault current and it heavily dependent on the energy levels in line inductance and the last current value in the commutator. However, the high energy inductive systems, the arrester circuit might not able to absorb such high amounts of energy. If the energy is extremely high it can result in the permanent destruction and even the damage of the load circuit and the circuit itself.
The analysis conducted below does not contain the resistance values for the sake of clarity. Moreover, the inductance on the source side is LT is considered to be greater than the inductance in the commutator coil LC. Thus, the energy required for an effective counter-current injection should be dependent on the capacitance of the commutator and the initial voltage. In an oscillatory circuit the maximum counter-current injection can be given by the following equation:
lefttop
Certainly a high-voltage might result in a high value of the required counter current. The counter current can is always plotted linearly against the other values. The voltage equation can be represented by:
The current obeys the following differential equation:
The solution of above the differential equation is:
In the above equation,
ω0=1(LT+LC)CC
Introducing a new parameter, and using the trigonometric equivalent, the current across the capacitor can be given by the following equations:
The capacitor voltage is governed by:
The values for inductance and the capacitance in the circuit can be given by:
The unit capacitance and inductance introduced by the above equations can be plotted in the figures as given below:
Figure 4: The unit base capacitor and inductance (Seeger et al., 2014).
2.2 Second Interval (B)
To make the entire analysis more realistic the resistance needs to be added in the circuit. The figure below gives the circuit diagrams when the resistance values are included. In addition to resistance values the energy absorption in the commutation capacitor is also taken into account by measuring the absorption energy across the capacitor in the load circuit. The energy absorbing circuit LA and RA only played their part in the circuit after polarity of the capacitor is changed (Venter & Da Silva, 2017). The reverse biased current D1 is required to be installed in the circuit. The signals introduced in the circuit were MS make switch and Thy-thyristor act as an auxiliary switch.
Figure 5: Cir...