1. Introduction
In the electric power industry, alternating-current (AC) systems have been overwhelmingly dominant over the direct-current (DC) option for a long time. However, this scenario is changing in recent years with DC systems playing an ever increasing role in the overall power systems due to several potential benefits: long distance water crossing, lower losses, controllability, limit short circuit currents, lesser corona loss, and the fact they requires less insulation [
1]. Voltage source converter (VSC)-based high voltage direct current (HVDC) transmission is increasing research interest in the study of the smart grid. The VSC-based HVDC concept was introduced by manufacturers in the late 1990s and this new technology was also named HVDC Light by ABB [
2,
3] and HVDC Plus [
4] by Siemens. Compared to the classic line commutated converter HVDC (LCC-HVDC), the former has several potential benefits such as self-regulating control of active and reactive power, increased power quality, comfortable integration of large-scale wind farm, and easy operation with weak AC grids, no reactive power demand, and less operational cost [
5,
6,
7,
8,
9].
Moreover, VSC-HVDC offers a solution for many problems faced nowadays by power networks such as network congestion, grid reinforcement, multi-terminal DC (MTDC) operation and asynchronous operations of two different grids [
10]. Different types of VSC topologies are proposed in the literature [
11,
12] including two level, three level and multilevel converters for HVDC transmission. Multilevel means more than two voltage levels can be achieved in one phase leg, which reduces the switching times of valve and makes the voltage wave form closer to a sinusoidal curve. Line to neutral voltage waveforms of both two-level and three-level converters with PWM are discussed and compared in [
12].
However, despite the numerous advantages, VSC-HVDC systems face difficulties in dealing with different grid faults [
13]. Fault ride-through (FRT) capability enhancement is one of the main requirements for wind farm-integrated VSC-HVDC systems [
14,
15]. During system faults, bulk power interruptions must be avoided by keeping the HVDC system energized, otherwise, the system may face serious instability due to this bulk power interruption. Modular multilevel converter-based VSC-HVDC system topologies can provide enhanced fault ride-through capability [
16]. In [
17,
18] FRT capability as well as transient stability improvement have been reported by applying various VSC control techniques. A power synchronization control technique has been proposed in [
19] with coordination between wind turbines and HVDC controllers in order to achieve FRT and frequency response capabilities, relying solely on offshore frequency modulation. Power oscillations minimization and DC link ripples reduction have been presented [
20] with a negative sequence current controller. However, the power from the sending end wind power plant is reduced to zero which results in serious stresses on the mechanical system.
The security and stability of electric power systems are becoming ever more significant for the future due to their complex nature. A prospective solution to the stability and security issues mentioned above is to employ fault current limiters. Different categories of fault current limiter such as resistive, inductive, superconducting, flux-lock, DC reactor, and resonance FCL [
21,
22,
23,
24,
25] have been presented for limiting fault currents as well as improving the stability of power systems. Resistive-type and inductive-type FCLs provide approximately zero impedance under normal condition, while they provide high-impedance resistors or inductors under a fault condition. Recently, a number of practical superconducting fault current limiter (SFCL) devices have been successfully developed and demonstrated by in-grid tests. The current motivation on the applied superconductivity technology to build SFCL devices has been moving from the 10-kV distribution level [
26,
27] to the 100-kV transmission level [
27,
28].
So far, fault current limiters have been comprehensively studied and applied in AC power grids to limit fault currents and to improve the dynamic performance of the system. However, the application of FCLs in VSC-HVDC systems has not been fully investigated [
29]. In [
30], the authors introduced a flux-coupling-type superconducting FCL to reduce the possibility of commutation failure in classic LCC-HVDC systems. DC fault current amplitude, commutation voltage, commutation failure duration, and successive commutation reduction have been observed with this FCL. However, SFCL parameter optimization, economic performance analysis, design of electrical insulation are not yet investigated. Superconducting fault current limiters have been studied in VSC-HVDC systems [
31,
32,
33,
34,
35,
36,
37]. A novel SFCL with both a superconducting coil and a resistance is presented for limiting rapidly growing current in VSC-HVDC systems during severe disturbances [
32]. A DC protection scheme of multiterminal VSC-HVDC with resistive type SFCL was presented [
34]. Resistive type SFCLs reduce the current rating of DC circuit breakers. Maximum fault current, dissipated energy stress on HVDC circuit breaker and interruption time have been reduced with SFCLs by absorbing the fault energy during system disturbances [
37]. A resistive-type SFCL with a chopper controlled resistor was presented [
38] for transient stability augmentation of VSC-HVDC systems. Reduction in DC link voltage fluctuation and current is observed with the offered resistive SFCL scheme.
Among the different types of FCLs the non-superconducting bride-type fault current limiter (BFCL) is a new technology offering the capability of limiting fault currents as well as improving the dynamic performance of the power grid [
39,
40]. Diodes and IGBT switches are required for BFCLs which can be implemented easily [
41]. Moreover, inductor and resistor required for the current limiting part of the BFCL are non-superconducting in nature. This reduces the implementation cost greatly compared to superconducting fault current limiters [
42]. However, to the best of the authors’ knowledge this innovative technology in enhancing FRT capability as well as the stability of VSC-HVDC systems has not been examined so far. Since the necessity of auxiliary devices in improving dynamic performance of VSC-HVDC systems cannot be ignored and cost of applications needs to be minimized, BFCL represents a potential solution in this regard. In conclusion, there is a lack of works exploring the potential of BFCL to enhance transient stability as well as the FRT capability of VSC-HVDC system.
This research proposes a BFCL-based approach for fault current reduction and fault ride-through capability enhancement as well as stability improvement of VSC-HVDC systems in different configurations of two-grids connected mode and wind farm integrated mode. To the best of our knowledge, the potential of BFCL has not been examined so far in VSC-HVDC to enhance system dynamic performance. Some literatures [
43,
44] have presented series dynamic braking resistor (SDBR) as a potential solution to limit fault current. In this work, the proposed BFCL for VSC-HVDC systems is compared with SDBR in order to show the effectiveness of BFCL to reduce fault currents and improve system stability. The main contributions of this research are as follows:
- ▪
The proposed BFCL solution is able to limit fault currents of VSC-HVDC system for both symmetrical and unsymmetrical faults.
- ▪
DC link voltage fluctuation is greatly suppressed with the proposed BFCL.
- ▪
Active power oscillation is significantly reduced with BFCL.
- ▪
Wind generator speed oscillation is notably suppressed with BFCL in wind-integrated VSC-HVDC systems.
- ▪
BFCL shows better performance over SDBR in all cases considered.
2. Bridge Type Fault Current Limiter
In this work, BFCL is proposed as a potential solution to the fault problems in VSC-HVDC systems. Its structure, operation and control technique are described in the following subsections.
2.1. BFCL Structrue, Operation and Design Consideration
BFCL is composed of a bridge with a parallel shunt branch [
39,
45] as shown in
Figure 1a.
The bridge part consists a diode rectifier, very small DC resistor and inductor with antiparallel diode, and an IGBT switch. The shunt branch consists of a series resistance and reactance. The main function of BFCL is to insert resistance and reactance by connecting its shunt branch with the line.
During normal operating conditions of the system, the IGBT switch is turned on by its controller, so for a positive half cycle, current conducts thorough the path D1-LDC-RDC-IGBT-D4. During a negative half cycle, current passes thorough the path D2-LDC-RDC-IGBT-D3. Therefore, for both the cycles, the current has a unified direction through LDC and RDC. Consequently, LDC is charged to the peak value of the line current and acts like a short circuit. Since the values of RDC and LDC are very small, insignificant voltage drops across them. Eventually, bridge is short circuited during normal system operation and it has no effect on this operating condition. On the other hand, during system disturbances, the IGBT switch is turned off by BFCL controller. As a result, the shunt branch is connected to the line by considering the open circuit of the bridge part. Insertion of this resistance and reactance during system disturbances limits fault currents and improves the dynamic stability of the VSC-HVDC system.
In this work, BFCL parameters are designed based on the pre-fault power flow through each line [
46]. In order to have least effect of the fault on the system, BFCL should consume equal or higher amount of active power as the pre-fault value. Pre-fault power consumption by the BFCL is given as below:
where
PG,
VPCC,
Rsh and
Xsh are power delivered by the grid or wind farm, voltage at point of common coupling (PCC), shunt resistance, and shunt reactance, respectively. The above two equations give the following expression:
The necessary condition for the
Rsh to be a real value is as follows:
The same approach is used to find the value of Rsh. For the BFCL to be practical, small values of LDC and RDC are chosen so that the voltage drop across them is negligible and the DC current flowing through them is smooth.
2.2. BFCL Control Technique
Disturbances in AC/DC systems can be detected either by voltage dips or by over-current at the point of common coupling (PCC) [
47]. Voltage dip at the PCC has been used in this work to sense faults and generate IGBT gate control pulses as shown in
Figure 1b. A comparator compares the PCC voltage (
VPCC) with a predefined threshold voltage (
VT). Under normal operating conditions when
VPCC is higher than the
VT, the comparator output goes on low, so the step voltage generation part of the BFCL controller generates a high voltage signal to make the IGBT switch on. During grid abnormalities
VPCC decreases and becomes lower than
VT, and as a result the comparator output becomes high. In this condition, a low voltage signal is generated by step voltage generation to turn off the IGBT. Consequently, the shunt branch of BFCL comes into operation to limit the fault current and enhance the system stability. Afterwards, when
VPCC exceeds
VT due to fault clearance or PCC voltage support by any compensation technique, IGBT receives a high voltage signal and consequently turns on. In this way, BFCL has no effect on normal operating condition and inserts resistance and reactance in order to limit fault current and augment fault ride-through capability of VSC-HVDC system. It is worth mentioning that during fault initiation the
LDC line current tends to rise drastically; however,
LDC limits this current. Therefore, the IGBT switch is protected from high
di/dt.