Due to the low impedance characteristic of the high voltage direct current (HVDC) grid, the fault current rises extremely fast after a DC-side fault occurs, and this phenomenon seriously endangers the safety of the HVDC grid. In order to suppress the rising speed of the fault current and reduce the current interruption requirements of the main breaker (MB), a fault current limiting hybrid DC circuit breaker (FCL-HCB) has been proposed in this paper, and it has the capability of bidirectional fault current limiting and fault current interruption. After the occurrence of the overcurrent in the HVDC grid, the current limiting circuit (CLC) of FCL-HCB is put into operation immediately, and whether the protected line is cut off or resumed to normal operation is decided according to the fault detection result. Compared with the traditional hybrid DC circuit breaker (HCB), the required number of semiconductor switches and the peak value of fault current after fault occurs are greatly reduced by adopting the proposed device. Extensive simulations also verify the effectiveness of the proposed FCL-HCB.
Compared with the traditional line commutated converter (LCC)-based high-voltage DC (HVDC) transmission systems, HVDC grids based on modular multilevel converter (MMC) have more advantages, such as independent control of active and reactive power, easiness to form a multi-terminal network, power supply for passive networks, etc. [
One of the important factors restricting the development of the HVDC grid is the fault isolation technology [
Currently, the proposed DCCBs can be divided into three main types, namely mechanical DC circuit breakers (MCBs), solid-state DC circuit breakers (SSCBs) and hybrid DC circuit breakers (HCBs) [
Due to the low-impedance characteristic of the HVDC grid, the fault current develops extremely fast, and within a few milliseconds it can reach several times or even tens of times the rated current. To suppress the rising speed of the fault current, many fault current limiters have been proposed [
Most of the fault current limiters proposed at present are independent devices, so installing the fault current limiter requires additional investment costs. Fault current limiting devices have fault current limiting function, HCB on the cut off the fault current has a superior performance. HCB which has the capability of fault current limiting can be implemented in fault current limiting. FCL-HCB can further inhibit the rise of the fault current, and reduce the pressure of the HCB to cut off the fault current, Therefore, a current limiting hybrid DC circuit breaker topology is proposed [
The rest of the paper is organized as follows. The traditional HCB and the proposed FCL-HCB are introduced in
The HCB has been first proposed by ABB [
The traditional HCB consists of a residual current breaker (RCB), a load current branch and an MB (
When the HCB receives the trip command from the protection, the MB is turned on and the LCS is turned off. At this time, the fault current starts to be transferred to the MB. When the fault current of the load current branch drops to zero, the UFD starts the opening action. The opening time of UFD in zero current state is about 2 ms [
The topology of the FCL-HCB proposed in this paper is shown in
For the safety of the electronic devices, the HVDC grid usually has very high requirements for fault isolation speed. Taking the Zhangbei four-terminal HVDC grid in China as an example, the protection is required to send trip signal within 3 ms, and the HCB is required to interrupt fault current within 3 ms [
In order to solve the above problems, the FCL-HCB proposed in this paper puts the CLC into operation immediately when the overcurrent of the protected line is detected, instead of waiting for the trip command of the protection system. Because the detection time of the overcurrent can be extremely short (less than 0.5 ms), the CLC in the FCL-HCB can greatly reduce the rising speed of the fault current. If the cause of the overcurrent is a fault in the protected line, the FCL-HCB can continue to operate to interrupt the fault current. Otherwise, if the cause of the overcurrent is a non-fault factor or the fault is an external fault, the CLC can exit and the normal operation can be resumed. It is worth noting that during the process of fault current interruption of the FCL-HCB, because the current limiting inductor in the CLC is bypassed by the energy dissipation resistor Re, the energy consumed by the arresters in the MB and the total energy consumption time are both greatly reduced. Thereby, the pressure of devices in the HVDC grid to withstand large currents is greatly reduced.
The detailed operation principle of the proposed FCL-HCB is as follows:
1. Stage I:
The two-terminal test system is utilized to analyze the operation principle of the FCL-HCB (
2. Stage II:
When the overcurrent is detected at time
3. Stage III:
At the end of the opening action of the UFD at time
4. Stage IV:
After the block of the thyristor T1, the capacitor
According to
The initial value conditions of the above differential equations can be described by the following equations:
The expressions of the current
where
5. Stage V:
After time delay, the trip signal is received by the FCL-HCB from the protection system at time
6. Stage VI:
After the fault current interruption at time
If the fault detection result obtained at time
Firstly, the UFD is commanded to close. After a small period, the close action of the UFD is completed. Then, the LCS is turned on. After that, because the equivalent resistance of the load current path is much smaller than the total equivalent resistance of the current path D1-
According to the above analysis, the parameters of the main components in the proposed FCL-HCB are designed, including the selection of inductive components and capacitance components. At present, current limiting reactants are widely used in fault current limiting of MMC-HVDC transmission lines. Considering thet current limiting reactants used in current projects are generally 150 mH, and combined with the current limiting demand of 500 kV MMC-HVDC, the inductance value of current limiting reactance is selected as 200 mH in this paper. Considering the maximum forward voltage the capacitor must withstand is:
The capacitance value is selected according to the maximum forward voltage to be sustained by the capacitor voltage. In this paper, the capacitance of 10 uF is selected.
According to
Based on the
To prove the effectiveness of the proposed FCL-HCB, a unipolar test system us built with the PSCAD/EMTDC software (
Parameter | Value |
---|---|
Nominal voltage |
500 kV |
DC inductance |
50 mH |
Nominal power of S1, S2 | 500, 500 MW |
Number of SMs per arm | 100 |
SM capacitance | 10 mF |
Arm inductance | 29 mH |
Nominal voltage of MOV | 500 kV |
Protection threshold of MOV | 800 kV |
Capacitance |
10 μF |
Current limiting inductance |
200 mH |
Energy absorption resistance |
20 Ω |
Inductance of the DC line | 0.85 mH/km |
Resistance of the DC line | 9.32 mΩ/km |
Length of the DC line | 200 km |
The short-circuit fault
During the current limiting process of the FCL-HCB, the current has transferred from the thyristor branch T1 to the thyristor branch T2, and then it transfers from the thyristor branch T2 to the current limiting inductor
It is assumed that an overcurrent interference occurs at 4.0 s, and the overcurrent is detected after 0.5 ms. Then, the FCL-HCB changes from the normal operation state to the fault current limiting state. At 4.006 s, the FCL-HCB starts to exit the fault current limiting state, and the operation process is shown in
Compared the FCL-HCB proposed in this paper with the traditional HCB, and the traditional HCB is used to interrupt the fault current. The fault condition is the same as that in
The simulation results show that the peak value of the fault current flowing through the DC line is about 8.7 kA, the fault current absorption time is about 9.3 ms, and the energy consumption of the arrester in the MB is about 31.4 MJ (
Because the traditional HCB and the FCL-HCB both have the RCB and the load current, their costs will no longer be considered in the economic comparison. It is assumed that the IGBT FZ3600R17HE4PHPSA1 (1.7 kV, $1524.85), the diode VS-SD1100C20C (2 kV, $80.18) and the thyristor VS-ST1230C16K1 (1.6 kV, $416.33) are used [
For the traditional HCB and the FCL-HCB, their MB both needs to withstand the protection voltage of the arresters which is 800 kV. In addition, for the diode branches D1–D4 of FCL-HCB, they also need to withstand the protection voltage of the arresters. For the thyristor branches T1, T2 and the diode branch
DCCB type | IGBT | Thyristor | Diode | Total cost |
---|---|---|---|---|
Traditional HCB | 1884 | 0 | 0 | $2872817.4 |
FCL-HCB | 942 | 1750 | 3900 | $2477688.2 |
The cost of semiconductor switches of the FCL-HCB is less than that of the traditional HCB (
This paper proposes an FCL-HCB which has the ability of bidirectional current limiting and fault current interruption. Once an overcurrent current is detected, the CLC of FCL-HCB can be quickly put into operation, thereby the rising speed of the current is suppressed. Then, once the internal fault is identified, the FCL-HCB can quickly interrupt the fault current. Extensive simulations have proved the effectiveness of the FCL-HCB.
Compared with the traditional HCB, the FCL-HCB reduces the peak value of the fault current by 35.63%, the energy consumption of the arresters in the MB is reduced by 47.13%, and the energy dissipation time is shortened by 18.28%. In addition, the economic analysis results show FCL-HCB is more economical.