Control and protection strategy for MMC MTDC system under converter-side AC fault during converter blocking failure

Puyu WANG 0 Xiao-Ping ZHANG 0 Paul F. COVENTRY 0 Zhou LI 0 0 Received: 15 April 2014 / Accepted: 9 June 2014 The Author(s) 2014. This article is published with open access at Springerlink.com P. WANG, X.-P. ZHANG, University of Birmingham , Birmingham B15 2TT, UK (&) This paper investigates a control and protection strategy for a four-terminal modular multilevel converter (MMC) based high-voltage direct current (HVDC) system under a converter-side AC fault. Based on the system operating condition, a control and protection strategy against the fault with normal blocking of the converter is proposed. In practical, applications encountering such a fault, the MMC at the fault side may experience different conditions of blocking failure. The blocking failures may occur on: the whole converter; ` one converter arm; one sub-module (SM)/several SMs of one converter arm; other conditions. The phenomenon of the multi-terminal HVDC (MTDC) system following the fault is analyzed under the first three conditions with real-time simulations using the real-time digital simulator (RTDS). Based on the impact of different conditions on the MTDC system, the necessity of utilizing special control and protection is discussed. A special control and protection strategy is proposed for emergency conditions, and its effectiveness is verified by real-time simulation results. - There has been a rather long history of research on the control and protection of AC faults in electric power transmission systems [1, 2]. Regarding the high-voltage direct current (HVDC) including multi-terminal HVDC (MTDC) technologies, research effort has been made on the protection against DC faults [36]. As for AC faults, control strategies of voltage source converter (VSC) for improving the AC fault ride-through capability of VSC-HVDC systems were proposed in [7, 8]. An approach of limiting the AC unbalanced fault on the VSC MTDC grids was proposed in [9]. Control and protection strategies were investigated in [10] for a hybrid MTDC system against AC faults. In most situations, AC faults can be isolated by AC circuit breakers. However, when an AC fault occurs at the nearby AC side of the converter, which is beyond the protection area of the nearby AC circuit breakers, the fault isolation is achieved by both blocking of the converter to prevent current flowing from the DC side and tripping the AC side circuit breakers. In the above situation, previous work has assumed that the blocking of the converter is successful. However, this assumption is not necessarily valid in terms of practical operation of converters, especially in the more advanced modular multilevel converter (MMC) [11, 12] in the current system. The MMC at the fault side has different conditions of blocking failure: for the communication outage of control signals where the whole converter can be out of control and cannot be blocked, although the occurrence of such a condition is considered to be rare; ` a more common condition is that one of the six converter arms is failed to be blocked; the most common condition is that one sub-module (SM)/ several SMs on one converter arm is/are failed to be blocked; other conditions. Hence, it is worth identifying the potential risk to the MTDC system, analyzing the dynamic performance to reveal the fact that special control and Fig. 1 Configuration of four-terminal MMC HVDC system protection is necessary to be conducted when the MTDC system is significantly affected under emergency conditions, while under other non-emergency conditions, the special control and protection may not be essential. This paper investigates the control and protection strategy against a converter-side AC fault during the blocking failure of the converter of a four-terminal MMC HVDC system. The MMC technology was firstly introduced in 2001 [13] and its advantageous over the traditional VSC technology [11, 12], such as low switching losses and small harmonic proportions, have been widely recognized. The number of HVDC projects deployed the MMC technology [14, 15] has gradually been increasing. The occurrence of the aforementioned blocking failure conditions may exist and deserve to be investigated. The rest of this paper is outlined as follows. Section 2 introduces the MTDC system configuration and control strategy. Section 3 presents the control and protection strategy against a converter-side AC fault with normal blocking operation. In Section 4, the dynamic performance of the MTDC system during different conditions of MMC blocking failure following the fault is analyzed and the impact of each condition on the MTDC system is revealed by real-time simulations. An associated special control and protection strategy is proposed for certain emergency conditions where the MTDC system is significantly affected. Conclusions are drawn in Section 5. 2 MTDC system A single-line diagram of a four-terminal MMC HVDC system is shown in Fig. 1. Tn (n = 1, 2, 3, 4) denotes the HVDC terminal. On the AC side, each AC source is modeled as an ideal voltage source with a line-to-neutral voltage of 138 kV. CBn (n = 1, 2, 3, 4) represents the AC circuit breaker. On the DC side, DC ISOn denotes the DC isolation switch. The length of each DC cable is 100 km. The nominal DC voltage is 50 kV. Four converters (MMC-n) are seven-level, half-bridge MMC converters. Each MMC consists of six converter arms where each arm consists of six SMs and one arm inductor. Fig. 2 Schematic diagram of MMC system Table 1 Parameters of MTDC system Figure 2 shows a schematic diagram of the MMC system. To achieve the capacitor voltage balancing in each converter arm, the charging and discharging operations depend on the arm current direction and the capacitor voltage of each SM [12, 13] where the SM with lower capacitor voltage is charged first, while the SM with higher capacitor voltage is discharged first. Detailed parameters of the MTDC system are shown in Table 1. The control of the MMC converter station is achieved in a dq reference frame and the well-known dq decoupled control strategy is applied [1618]. The d-axis control regulates either the active power or the DC voltage, while the q-axis control regulates either the reactive power or the AC voltage magnitude. In the MTDC system, MMC-1, MMC-3 and MMC-4 use the active power control to regulate the active power at the converter AC terminals. MMC-2 applies DC voltage control to maintain the voltage of the MTDC grid. In order to reduce the power losses caused by the reactive current, the reactive power control is applied by all the four converter stations where the reactive power reference is set to 0 Mvar. 3 Control and protection strategy against converter side AC fault with normal blocking operation When an AC fault occurs at the converter AC side of one terminal of the MTDC system, say the most severe three-phase short-circuit fault, the voltage at the faulted point will drop, and the fault current will flow into the (...truncated)