With the development of the IT industry, a lot of loads that are sensitive to power quality problems are being widely utilized . Furthermore, because of the tendency to widely utilize distributed generators that are sensitive to natural phenomena such as strong winds, lightning, and heavy snow, more reliable and stable power supply technologies are needed , .
Power grid breakdowns occur frequently due to unwanted contact between power lines and the surrounding environment. If a quick break of fault current is not performed, serious damages may occur due to a rapid increase in the fault current and its resulting electrical fires . Even durring the short-duration faults caused by strong wind or trees, the power grid should be able to supply utility power to loads through quick fault recovery from the initial fault breaking operation. After an initial fault breaking operation, a long-duration sustained interruption without fault recovery may result in a lot of serious economic losses .
For this reason, IEEE std C37.09 prescribes a circuit breaker’s reclosing and rebreaking operations should be able to be performed repeatedly .
In order to ensure a good power quality for the AC power grid, the technology of a circuit breaker must identify a voltage sag/swell, overcurrent, or short circuit fault, and then immediately interrupt it. Furthermore, the circuit breaker should be able to perform a rapid reclosing operation even under a short circuit of the load side, and interrupt the fault.
Power devices for breaking grid faults include the mechanical breaker and the SSCB (Solid-State Circuit Breaker). The mechanical breaker is capable of interrupting faults in several tens [ms] due to its physical structure, which results in a slow interruption capability. Thus, it is difficult to prevent damage to sensitive loads .
In contrast, the SSCB can interrupt faults within 4 [ms] in a much lower current level than the allowable peak fault current level so that it can reduce the damage to grid devices . There are several types of semiconductor switching elements used to implement the SSCB such as IGBTs, GCTs, SCRs, and GTOs. Although the SCR requires a commutation circuit for turn-off, it is highly suitable for use in AC SSCB because it is economical and its on-state loss is relatively small .
Fig. 1 shows an existing AC SSCB circuit using a SCR that can break the fault current of a single-phase AC circuit . In the circuit of Fig. 1, AC power is supplied through the main SCRs, Tmain1 and Tmain2. When a fault occurs, the fault current can be broken immediately as the auxiliary SCR, Taux1 or Taux2 is turned on depending on the direction of the current. However, as shown in Fig. 1, in order to turn off the main SCR, the commutation capacitor must be charged in advance.
Fig. 1.Previous basic forced commutation circuit.
Fig. 2 shows a circuit for charging the commutation capacitor of Fig. 1 . It is possible to charge the commutation capacitor to a required voltage by using the line voltage and varistor.
Fig. 2.Previous varistor charging circuit.
Fig. 3 shows a previous three-phase SSCB using the circuits of Fig. 1 and Fig. 2 . The thick line of Fig. 3 shows the loop where the capacitors C21 and C22 on the b-phase are charged by the line voltage Vab. In the circuit of Fig. 3, when a short circuit fault occurs, the previous three-phase SSCB may break rapidly because it has the same breaking principle as shown in Fig. 1. However, the circuit of Fig. 3 has a significant disadvantage since it is unable to perform the reclosing and rebreaking operations. If explained in further detail, if a short circuit faults lasts on the load side, as shown in Fig. 4, the capacitors C21 and C22 may not be charged even if the SCRs, T11 and T22 are turned on, which makes it impossible to reclose and rebreak the previous AC SSCB of Fig. 3.
Fig. 3.Previous three-phase SSCB and charging loop of C21, C22.
Fig. 4.Charging loop when short fault occurs in Fig. 3.
To overcome such shortcomings, this paper proposes a new AC SSCB that has quick reclosing and rebreaking capabilities. Thus, it can perform the operating duties of reclosing and rebreaking regardless of the circuit state on the load side. In addition, this paper suggests design guidelines in order to apply this AC SSCB to systems of different capacities and applications regardless of the ground system. The proposed AC SSCB is designed and implemented in a rated power of 5 [kW] and a line voltage of 220 [V]. Thus, the operating characteristics are confirmed by simulation and experimental results.
II. PROPOSED AC SOLID-STATE CIRCUIT BREAKER
Fig. 5 shows the proposed AC SSCB circuit. Even under a short circuit fault on the load side, the proposed SSCB can charge the commutation capacitor. In other words, since the recharging operation of the commutation capacitor is possible regardless of the fault state on the load side, the proposed AC SSCB can perform the operation duties of reclosing and rebreaking as prescribed in the circuit breaker standard .
Fig. 5.Proposed AC Solid-State Circuit Breaker(AC SSCB).
The operation of the proposed SSCB may be divided into four modes. The four operation modes are composed of the charging mode, to charge the commutation capacitor; the normal mode, to supply energy to the load; the breaking mode, to break the fault current; and the recharging mode, for reclosing.
In order to confirm the operating characteristics of the proposed SSCB, this paper verifies the breaking and reclosing operations of the SSCB by simulating a three-phase short circuit fault, and as the worst-case, one with the largest fault current value.
Fig. 6 shows the circuit operation according to the modes of the proposed AC SSCB, and Fig. 7 shows the operation waveform corresponding to each mode. The operating characteristics of each mode are as follows.
Fig. 6.Operating modes of the proposed AC SSCB.
Fig. 7.Operating waveforms of the proposed AC SSCB.
A. Charging Mode (t0 ≤ t < t5)
In the charging mode, all of the commutation capacitors of the AC SSCB must be charged before a faults take place so that the SSCB can interrupt the fault current by using the pre-charged commutation capacitors. Thus, during the charging mode (t0-t5) of the SSCB, the commutation capacitor is charged to the voltage level required for breaking by using the line voltage and varistor. The commutation capacitors are divided into two groups; that is, C11, C21, and C31 are charged naturally by the line voltages, and C12, C22, and C32 are charged through the SCRs.
Fig. 8 shows the loop where the capacitors C11 and C12 of the a-phase are charged by the line voltage Vca. In the circuit (a) of Fig. 8, the capacitor C11 has a charging loop that does not contain the SCR. Therefore, the natural charging (t0-t0’) is performed when the line voltage Vca is higher than the breakdown voltage Vv of the varistor.
Fig. 8.Charging loop in charging mode
However, the capacitor C12 cannot be charged naturally because the charging loop contains the SCR, T12 as shown in Fig. 8(b). The capacitor C12 can be charged by turning on the SCR, T12 at t3 where the line voltage Vab is higher than the breakdown voltage of the varistor.
By using the charging principle shown in Fig. 8, it is possible to charge all of the capacitors in each phase. However, individually turning on the SCRs, T12, T22, and T32 according to the magnitude and phase of the line voltage may require complex control. Thus, as in the charging mode of Fig. 7, it is possible to charge the capacitors C12, C22, and C32 together by turning on the SCRs, T12, T22, and T32 at the same time during t1-t5 (1-3 cycles of the power grid). This enables all of the capacitors to be charged with a simple SCR control.
Of course there may be a charging loop that contains the load, as shown in Fig. 9. The additional charging loop of Fig. 9 does not matter because the magnitude of the charging current is small due to the load impedance. Thus, turning on the SCRs, T12, T22, and T32 at the same time can charge all of the capacitors with a simple SCR control.
Fig. 9.Charging loop of capacitor C32 (t2~t3).
B. Normal Mode (t5 ≤ t < t6)
The normal mode is the steady-state operating mode of the AC SSCB. Since all of the SCRs, Tall (T11, T12, T21, T22, T31, and T32) are turned on, energy is supplied to the load as shown in circuit B of Fig. 6. In this mode, such faults as sags/swells of the voltage and over-current are monitored by detecting the line voltages and line currents.
C. Normal Mode (short circuit fault : t6 ≤ t < t7)
Mode C is the section where the fault current increases when a three-phase short circuit fault occurs at t6. Although a short circuit fault has already occurred, the SSCB operates as in the normal mode because the magnitude of the fault current is still smaller than the preset reference current to determine a short circuit fault.
As the a-phase current ia gradually increases to be equal to the reference value, the AC SSCB starts to break the short circuit fault at t7. Thus, the breaking mode begins.
D. Breaking Mode (t7 ≤ t < t12)
The breaking mode is the section where the fault current is interrupted. If the auxiliary SCRs, S11, S22, and S32 are turned on, as shown in circuit D of Fig. 6, according to the directions of each phase current, the related main SCRs, T11, T22, and T32 are turned off by the pre-charged voltage of the capacitors C11, C22, and C32. As the current through R1, L1, and C flows in each phase, the fault current is interrupted. The capacitors C11, C22, or C32 discharged for fault current interruption are reversely charged as the waveform VC of Fig. 7.
E. Breaking Mode (t12 ≤ t < t13)
This mode is the section where the fault current is completely broken. Thus, no current flows in the SSCB.
F. Recharging Mode (t13 ≤ t < t18)
This recharging mode is the section where the commutation capacitors discharged in the previous breaking mode are recharged again. Figures 10 and 11 show the recharging loops of the capacitors C11, C22, and C32 at the recharging mode (t13-t18).
Fig. 10.Charging loops of capacitors in recharging mode.
Fig. 11.Additionally possible charging loop of capacitor C32 (t14~t15).
This mode is divided into two time durations. During the first time duration without thyristor triggering signals, the capacitor C11 is naturally charged by the line voltage Vca. During the second time duration with thyristor triggering signals, the capacitors C22 and C32 are recharged through the turned-on SCR, T22 and T32. As shown in Figures 10 and 11, if the fault keeps going, the recharging modes are carried out without passing through the load side.
However, if the fault is removed, the recharging loop may also exist through the load side, as shown in Fig. 9, during the second time duration of simultaneously turning on the SCRs, T12, T22, and T32 together.
In other words, the proposed SSCB can recharge the commutation capacitors even under a short circuit on the load side. If the recharging mode is already completed, the SSCB may perform a reclosing operation by turning on the main SCRs because even in the case of a short circuit fault on the load side, the proposed SSCB can rebreak the fault current.
III. ANALYSIS AND DESIGN OF THE PROPOSED AC SSCB
The design specification and system parameters of the proposed SSCB are shown in Table I. Rv, C, SCR, and the varistor of the SSCB should be selected considering all of the currents through each device and all of the voltages across each device during the charging mode, breaking mode, and recharging mode.
TABLE IDESIGN SPECIFICATION AND PARAMETERS
A. Charging Mode (t0 ≤ t < t5)
The charging mode is divided into two cases. The first case is where a single capacitor is charged, as shown in Fig. 12(a), and the other case is where two capacitors are charged by turning on the SCR, S22, as shown in Fig. 12(b). The charging current begins to flow when the line voltage Vab is higher than the breakdown voltage Vv of the varistor. The input voltage Vab, the charging currents ich1 and ich2 of the charging circuit, and the charged voltage Vch of the commutation capacitor are expressed as Equations (1), (2), (3), and (4), respectively.
Fig. 12.Equivalent circuit of charging mode.
In the charging mode, it should be noted that the commutation capacitor might be charged to a voltage higher than the desired voltage in the case of an underdamped circuit condition. If so, the voltage rating of the commutation capacitor may increase and the peak value of the fault current may increase, resulting in larger SCR capacity. To limit the charged voltage of the commutation capacitor during the charging mode, the resistor Rv should be selected to satisfy the condition of Equation (5). The charging current ich and charged voltage Vch of the charging mode are lower than those of the other modes. Thus, the capacitor C should be selected earlier than RV by considering the breaking mode and recharging mode instead of the charging mode.
B. Charging Mode (short circuit fault: t7 ≤ t < t13)
Fig. 13 shows the operation of the SSCB in the breaking mode (t7-t13) in greater detail.
Fig. 13.Detailed sub operating modes of breaking mode.
When a three-phase short circuit fault occurs, the fault current has the largest peak value, which is the worst-case condition. Fig. 14 shows the current waveform through each device on the a-phase for the breaking mode (t7-t12) in a three-phase short circuit fault.
Fig. 14.a-Phase current waveforms in breaking mode.
The components of the AC SSCB should be selected considering the worst-case current flowing through the power devices during the breaking mode operation. To achieve this, it is necessary to know the peak current of the a-phase current ia of Figs. 13 and 14. In Fig. 14, the a-phase current ia flows at t8-t9 or t9-t10 through the varistor of the breakdown voltage Vv and the capacitor of the capacitance C. Thus, the peak value of the a-phase current ia is determined by L1, Vv, and C, as shown in Fig. 15 and Fig. 16.
Fig. 15.Peak value of current ia when C and L1 change.
Fig. 16.Peak value of current ia when C and VV change.
As the breakdown voltage Vv of the varistor becomes higher, the charged voltage of the capacitor becomes lower by Equation (4). Therefore, as shown in Fig. 16, if the capacitance C and charged voltage Vch become smaller, the peak value of the a-phase current ia becomes smaller.
However, as the capacitance C of the commutation capacitor and the charged voltage Vch become smaller, the time duration where the commutation capacitor can apply a negative voltage to the turned-off main thyristor becomes shorter. If the time duration is shorter than the turn-off time tq of the main SCRs, the main thyristors might fail to be turned off. Thus, the breakdown voltage VV of the varistor and the capacitance C of the commutation capacitor should be selected considering the tq of the main SCRs.
The capacitor voltage Vc is reversely charged by the current iS11, as shown in the breaking mode of Fig. 7. At the time instant t10 of Fig. 14 the current iS11 becomes zero, and the capacitor voltage reaches the maximum value.
As the resistance Rv increases, the current iRv decreases and the current iS11 increases. Thus, the capacitor voltage Vc becomes higher. As a result, the maximum value of the capacitor voltage is determined in accordance with the resistance RV and the breakdown voltage VV of the varistor, as shown in Fig. 17.
Fig. 17.Maximum voltage of C when VV and RV change.
C. Recharging Mode (t13 ≤ t < t18)
Fig. 18 shows the equivalent circuit of Fig. 10-(a) where the capacitor C11 is recharged in the recharging mode. The equivalent circuit of Fig. 11 including the short circuit on the load side is the same as that in Fig. 18. In the recharging mode, the commutation capacitor is charged in the reverse direction unlike the charging mode. At t13 of Fig. 7, the line voltage Vca is less than the breakdown voltage VV of the varistor. However, if Equation (6) is met, the recharging operation of the capacitor begins.
Fig. 18.Equivalent circuit of recharging mode.
The input voltage Vca of the recharging circuit and the charging current ich are expressed as Equations (7) and (8), respectively. The final voltage recharged in the capacitor is the same as the result in Equation (4) of the charging mode.
Since the breakdown voltage VV of the varistor and the capacitor C are properly selected in the breaking mode, the recharging time of the capacitor may be controlled by using the resistor Rv. As the resistance of Rv becomes larger, the recharging time of the capacitor increases and the maximum voltage of the capacitor becomes higher as shown in Fig. 17. Therefore, the resistance of Rv should be selected at the lowest resistance within the operation range to satisfy Equation (5).
D. Recharging Mode in a Neutral Grounding 3-phase Power Grid
So far the proposed AC SSCB has been described and designed under the assumption of a non-grounded 3-phase power grid. More importantly, it can also be applied to a neutral grounding 3-phase power grid. For that purpose the proposed AC SSCB does not need to be structurally changed, and the operation characteristics of charging and breaking modes are the same. Fig. 19 shows the recharging loop under the worst condition of a three-phase line-to-ground fault. When the three-phase line-to-ground fault occurs at a power grid, the proposed AC SSCB can quickly break the fault current by using the proposed triggering method shown in Figs. 20 and 22.
Fig. 19.Recharging loop of AC SSCB in the 3-phase line to ground fault.
Fig. 20.Triggering method A of Recharging Mode in neutral grounding 3-phase power grid.
Fig. 21.Recharging loop of C22 in the 3-phase ground fault by the triggering method A.
Fig. 22.Triggering method B of Recharging Mode in neutral grounding 3-phase power grid.
(1) Triggering Method A of the Recharging Mode
In the case of a neutral grounding 3-phase power grid, when operating the recharging mode of the SSCB according to the triggering method of Fig. 20, both stable and reliable recharging and rebreaking can be obtained.
Fig. 21 shows the recharging loop of the capacitor C22 after the first fault breaking under the line-to-ground fault condition. In the initial stage of the recharging mode, the SSCB has no turn-on signal for 2-3 cycles of the SCR, T12, T22, and T32. The capacitor C22 is naturally recharged to the voltage Va-Vv by the recharging loop of Fig. 21. By this principle, the capacitors C12 and C32 are also naturally recharged at sections t13-t15. At this time C11, C21, and C31 are also naturally recharged.
The recharging voltages of the capacitors C12, C22, and C32 are determined as the minimum voltage necessary to break the fault current, which enables the reclosing and rebreaking of the SSCB even under the line-to-ground fault condition.
At sections t15-t18 of Fig. 20, the proposed AC SSCB turns on the SCR, T12, T22, and T32. If the three-phase line-to-ground fault is not cleared yet, as shown Fig. 19, the proposed SSCB can carry out the rebreaking operation by using the already recharged capacitor at the t13-t15 section. In other words, turning on the SCR, T12, T22, and T32 at sections t15-t18 of the recharging mode is a kind of pre-test procedure for the reclosing operation. If the fault is no longer detected, the original reclosing operation can start.
(2) Triggering Method B of the Recharging Mode
As shown in Fig. 22, this method initially detects whether the line-to-ground fault lasts or not. The load situation can be detected by turning on the SCRs, T11, T21, and T31 as shown in Fig. 23. These turn-on operations can be initiated since the capacitors C11, C21, and C31 are already naturally recharged regardless of the line-to-ground fault. At this stage, the turn-on operation of the SCRs, T11, T21, and T31 is a kind of reclosing operation in the case of a fault situation. After that, if the load side is in the normal state but not in the fault state, the proposed SSCB recharges the capacitors C12, C22, and C32 by turning on the SCRs, T12, T22, and T32, which enables the original reclosing and rebreaking operations.
Fig. 23.Recharging loop of C22 in the 3-phase ground fault by the triggering method B.
IV. SIMULATION AND EXPERIMENTAL RESULTS
Table II shows the specifications of the prototype designed to implement the proposed AC SSCB shown in Fig. 5. In particular, the operating characteristics of the proposed SSCB are verified under the three-phase short circuit fault condition. The experimental results of each mode are as follows.
TABLE IIEXPERIMENTALMODEL PARAMETERS
A. Charging Mode (t0 ≤ t < t5)
Fig. 24 shows the simulation waveforms of the three-phase currents ia, ib, and ic and capacitor voltages in the charging mode (t0-t5). Fig. 25 shows the measured waveforms corresponding to Fig. 24. This confirms that the charging current at the time interval (t2-t3) of Fig. 9 flows when turning on the SCRs, T12, T22, and T32 at t2. It also shows that the commutation capacitor voltages are established well enough to interrupt the fault current.
Fig. 24.Current ia, ib, ic and voltage VC11, VC12, VC21, VC22, VC31, VC32 simulation waveforms in charging mode.
Fig. 25.Measured current ia, ib, ic and voltage VC11, VC12, VC22, VC32 waveforms in charging mode.
B. Breaking Mode (short circuit fault: t7 ≤ t < t13)
Fig. 26 shows the simulation waveforms of ia, iRv, iS11, and iT11 in the breaking mode and their enlarged waveforms. Fig. 27 shows the measured waveforms corresponding to Fig. 26. It shows that the fault current is interrupted rapidly within 400 usec.
Fig. 26.Current ia, iRv, iT11 and iS11 simulation waveforms in breaking mode.
Fig. 27.Measured current ia, iRv, iT11 and iS11 waveforms in breaking mode.
C. Recharging Mode (t13 ≤ t < t18)
Fig. 28 shows the simulation waveforms of the phase current ia, ib, and ic in the recharging mode (t13-t18) and the capacitor voltages VC11, VC22, and VC32. Fig. 29 shows the measured waveforms corresponding to Fig. 28. It shows that the commutation capacitors are recharged well by recharging the loop of Fig. 10 even in the worst state where the three-phase short circuit lasts.
Fig. 28.Current ia, ib, ic and voltage VC11, VC22, VC32 simulation waveforms in recharging mode
Fig. 29.Measured current ia, ib, ic and voltage VC11, VC22, VC32 waveforms in recharging mode.
Fig. 30 shows the enlarged simulation waveforms of the three-phase currents ia, ib, and ic in the recharging mode, while Fig. 31 shows the measured waveforms corresponding to Fig. 30. Fig. 32 shows the experimental waveforms of the recharging currents that include the recharging loop of Fig. 11 when turning on SCR T12, SCR T22, and SCR T32 at t14.
Fig. 30.Phase current ia, ib, ic simulation waveforms in recharging mode.
Fig. 31.Measured phase current ia, ib, ic waveforms in recharging mode
Fig. 32.Measured phase current ia, ib, ic waveforms in recharging mode that include 3-phase short circuit
This shows that a large fault current does not flow even if a charging current that occurs at t14 passes through the short circuit. Figures 30, 31, and 32 show that the commutation capacitors are stably recharged even in the worst case where the short circuit condition lasts.
This paper proposes a new AC SSCB that can perform the operating duty of reclosing and rebreaking without a lot of additional devices. Even in the worst case where a short circuit fault lasts, the proposed AC SSCB can charge the commutation capacitor. As a result, it can also perform the operating duty of reclosing and rebreaking regardless of the fault state on the load side. In addition, it is more economical since it reduces the number of varistors and uses low-priced resistors.
Even in a neutral grounding 3-phase power grid, since the proposed AC SSCB can detect line-to-ground faults with a simple control, it can be applied to both neutral floating and neutral-grounding three-phase power systems.
The operating characteristics of the proposed SSCB were verified by simulation and experimental results on three-phase short circuit faults. In addition, this paper suggests design guidelines so that it can be applied to a lot of applications. It is anticipated that the proposed AC SSCB will be widely utilized to realize the power schemes of high power quality systems.