Steady-State Security Region Considering Post-Contingency Cascaded DC Commutation Failure

We propose a scalable optimization framework for estimating convex inner approximations of the steady-state security sets. The framework is based on Brouwer fixed point theorem applied to a fixed-point form of the power flow equations. It establishes a certificate for the self-mapping of a polytope region constructed around a given feasible operating point. This certificate is based on the explicit bounds on the nonlinear terms that hold within the self-mapped polytope. The shape of the polytope is adapted to find the largest approximation of the steady-state security region. While the corresponding optimization problem is nonlinear and non-convex, every feasible solution found by local search defines a valid inner approximation. The number of variables scales linearly with the system size, and the general framework can naturally be applied to other nonlinear equations with affine dependence on inputs. Test cases, with the system sizes up to 1354 buses, are used to illustrate the scalability of the approach. The results show that the approximated regions are not unreasonably conservative and that they cover substantial fractions of the true steady-state security regions for most medium-sized test cases.

This chapter presents the concept and definition of the security region, and introduces major methods used in steady-state security region analysis of power system operation: the security corridor, the traditional expansion method, the enhanced expansion method, linear programming, and the fuzzy set theory. The main idea of security regions is to obtain a set of security injections explicitly so that for security assessment one need only check whether a given injection vector lies within the security region. To enhance the calculation speed of the regions, the following measures: adoption of calculation step, constraints are divided into two groups based on the threshold value shown in the equation, are adopted in the new expanding method. The chapter presents a new approach to construct the steady-state security regions of power systems, that is, the maximal security regions are directly computed using the optimization method.

Active superconducting DC fault current limiter based on flux compensation

Hybrid cascaded multi-terminal DC transmission provides a more flexible transmission method, improves the voltage stability of the inverter AC system, and reduces the probability of commutation failure. The hybrid LCC-VSC at the receiving end of the DC system is a new type of topology.In order to study the influence of coupling characteristics between VSC and LCC on commutation failure in AC power grid at receiving end,firstly,a hybrid LCC-VSC simulation model is built on PSCAD to analyze the operation characteristics of the system during the fault and verify the reactive power support ability of VSC to LCC, and the improvement effect of VSC on LCC-HVDC under different reactive power control modes is compared and analyzed. In order to study the influence of AC/DC coupling on commutation failure of LCC, commutation failure immune factor (CFII) was introduced to study the influence of AC system strength on commutation failure of LCC. Finally, considering that the influence of AC/DC coupling on commutation failure is closely related to the degree of coupling, the electrical distance between LCC and VSC is further studied. The results show that the conclusion can provide theoretical guidance for the design and analysis of related projects.

Commutation failure is one of the most frequent failures in line-commutated converter based high-voltage direct-current (LCC-HVdc) systems which may lead to the outage of HVdc links, and thus affect the performance of the power system. Most commutation failures are caused by voltage reduction due to ac system faults. In this paper, a commutation failure prevention module (CFPM) is developed, which inserts the optimal value of resistance proportional to the reduced extinction angle in series with the fault current path using a simple switching method. When a fault occurs at the inverter ac side, a self turn- off switch starts switching with a pre-specified frequency and controlled modulation index, which limits the fault current to the desired value and inhibits commutation failure. The salient feature of the proposed CFPM is that it is fully controllable. In addition, despite the available controller modification methods which are not effective for low-impedance faults, the proposed CFPM can prevent commutation failure regardless of the fault severity. The practical performance and feasibility of the developed CFPM is validated through laboratory testing, using the real-time Opal-RT hardware prototyping platform. The experimental results demonstrate that the proposed strategy can effectively eliminate the commutation failure or repetitive commutation failures under different fault types.

Commutation failure is one of the common faults in LCC-HVDC transmission systems, threatening safety and stability of power grid. To focus on the key issues of commutation failure and reduce commutation failure frequency, a review of commutation failure in LCC-HVDC transmission systems is given in this paper. Firstly, the mechanism of commutation failure is analyzed from both device and system aspects. Since commutation failures are mostly caused by AC system faults on the inverter side, commutation failures are classified into three categories: first commutation failure, continuous commutation failure during the fault period and commutation failure after the fault. Then, the influencing factors and prevention methods of commutation failure in each category are summarized. Finally, the problems to be solved in the field of commutation failure are pointed out.

The commutation failure of high-voltage direct current (HVDC) transmission system will have a huge power impact on the AC system. It is necessary to accurately identify the commutation failure of the HVDC for the security and stability control device (stability control device). At present, the stability control device which has already been applied in engineering for HVDC identifies the commutation failure of HVDC depends on the command of DC control and protection system, and there is a certain risk. The valve side current of converter transformer and the DC current of valve group that can be directly collected by the engineering are studied. Starting from the self-demand of the stability control device, new HVDC commutation failure and continuous commutation failure criteria are proposed based on the original commutation failure protection principle. Also, RTDS test verification was carried out.

The commutation failure (CF) is the most common fault in line-commutated high voltage direct current (LCC-HVDC) systems that may lead to the transient overvoltage in the sending-side system. In the worst condition, the CF may lead to large-scale wind turbine tripping. To resolve this problem, the mathematical relationship between the reactive power consumed by the rectifier and DC voltage, DC current is derived. Then, a transient overvoltage calculation method is proposed in this paper. Furthermore, the mechanism of transient overvoltage caused by the CF is analyzed; it is revealed that the reason for three times transient overvoltage is the rapid decrease of the reactive power consumed by the rectifier during the CF and the recovery period from the CF. This paper proposes a constant reactive power control (CRPC) to inhibit transient overvoltage of the sending-side AC system. The proposed CRPC can increase the reactive power consumed by the rectifier, reduce the exchange reactive power between AC and DC systems, and suppress the transient overvoltage. A simulation model in PSCAD serves to verify the proposed CRPC on the transient overvoltage suppression in the situation of different fault types, fault duration, fault severity and short circuit ratio (SCR).

Switching on no-load transformer in AC system will cause serious inrush current. When the inrush current travels to the AC bus of inverter in DC system, it will lead to the variation of fundamental voltage and distortion, which inevitably affect the commutation process of the inverter. At present, the research mainly focuses on the variation of fundamental voltage, while the commutation failure caused by voltage distortion is rare. To ensure the safety and stability of the power grid operation, this paper elaborates the mechanism of commutation failure created by voltage distortion based on harmonic voltage-time area method, and also proposes an evaluation method which can quickly identify the commutation failure caused by inrush current. Finally, the PSCAD/EMTDC simulation results are provided to illustrate the effectiveness of the proposed method.

Commutation failure (CF) is an inherent drawback of line-commutated converter high-voltage DC (LCC-HVDC) system. And CF prevention (CFPREV) control is widely applied in UHVDC lines to mitigate the subsequent CF. However, the formation of a multi-infeed UHVDC system induces power exchange and voltage interaction between AC buses during the transient process, making the behaviors and mitigation of CFs more complex. Consequently, exploring the impact of CFPREV control on adjacent stations and optimizing the CFPREV control to adapt to the interaction characteristics in a multi-infeed system is necessary. Considering the interaction characteristics, the impact of CFPREV control on the commutation process of inverters in a multi-infeed system is analyzed through formula derivation and simulation verification. It is found that although CFPREV control can effectively mitigate the CFs of the inverter near the fault, it will further increase the risk of concurrent CF (CCF) of remote inverters due to the interaction on the AC side. To solve this problem, a coordinated control scheme of CFPREV controls in a multi-infeed UHVDC system is proposed. The output of the CFPREV control of the inverter near the fault can be adjusted adaptively according to the commutation margins of remote inverters. And the probability of CCF caused by CFPREV control is consequently decreased. Case studies are conducted based on Henan provincial multi-infeed UHVDC system in China to verify the analysis result and the effectiveness of the coordinated control scheme of CFPREV controls.

Commutation failure is one of the commonest events in high voltage direct current (HVDC) systems. In the process of commutation failure, the DC voltage, current and power change drastically and impact the sending-side grid. At present, the related research lacks the quantitative analysis of transient overvoltage, which is caused by commutation failure in sending-side DC systems. Aiming at this problem, the equivalent model of the sending-side system is established. Considering characteristics of reactive power losses caused by the rectifier during commutation failure, analytical expression of transient voltage at the commutated bus is derived, then a quantitative calculation method of transient overvoltage and influencing factors are obtained. The results show that the weaker the AC system, the higher the transient voltage, and the transient over-voltage rises nonlinearly with the increase of the steady-state transmission capacity. Finally, the accuracy and effectiveness of the method are verified with the HVDC digital-physical hybrid simulation model based on Hypersim.

Commutation failure is a serious malfunction in Line-Commutated High Voltage Direct Current (HVDC) converters which is mainly caused by the inverter AC faults, and results in a temporary interruption of transmitted power and damage to the converter equipment. In this article, a Controllable Commutation Failure Inhibitor (CCFI) is developed which obviates the main drawbacks of the existing power electronic based and fault current limiting based strategies. Under normal circumstances, the developed CCFI improves the steady-state stability and the power transfer capability of the inverter AC lines, while it does not cause excessive voltage stress on the converter valves. In addition, it would reduce the risk of commutation failure occurrence, since it does not lead to any voltage drop in the commutation circuit. When a fault occurs at one of the inverter AC systems, its corresponding CCFI limits the fault current depending on the reduced extinction angle. This would not only inhibit the successive commutation failures on the HVDC converter, but also extend the lifetime of components in the inverter AC systems. The practical feasibility of the developed CCFI is assessed through laboratory testing, using a real-time Opal-RT hardware prototyping platform. The obtained results indicate that the developed CCFI can reliably inhibit the commutation failures during various types of faults.

Since a large-scale wind farm is connected, it has an impact on the hybrid High Voltage Alternating Current/Direct Current (HV AC/DC) power grid with the characteristics of 'strong DC and weak AC'. When the connected capacity of the wind farm drops instantaneously, it may cause the DC system commutation failure. In this paper, firstly, the impact of wind power grid connection on the converter bus voltage is analyzed. Then, the relationship between the commutation bus voltage variation and the critical commutation voltage reduction is established, the correlation factor of the DC commutation failure caused by the wind power grid connection is proposed, and the criterion for determining the DC commutation failure caused by the wind power grid connection is defined. As a result, the maximum variation of wind power access capacity could be obtained.

Accurate detection and effective control strategy of commutation failure (CF) of high voltage direct current (HVDC) are of great significance for keeping the safe and stable operations of the hybrid power grid. At first, a novel detection method for consecutive CF is proposed. Concretely, the 2nd and higher orders’ derivative values of direct current are summarized as the core to judge CF by analyzing the physical characteristics of the direct current waveform of the converter station in CF. Then, the Daubechies wavelet coefficient that can represent the 2nd and higher order derivative values of direct current is derived. Once the wavelet coefficients of the sampling points are detected to exceed the threshold, the occurrence of CF is confirmed. Furthermore, by instantly increasing advanced firing angle β in the inverter side, an additional emergency control strategy to prevent subsequent CF is proposed. Eventually, with simulations of the benchmark model, the effectiveness and superiorities of the proposed detection method and additional control strategy in accuracy and rapidity are verified.

HVDC commutation failure and recovery, the reactive power exchanged between HVDC and AC systems will change largely, showing the external characteristic of ‘large capacity reactive impact load’, so it is quite necessary to carry out studies on this problem. In this study, first, the HVDC sending‐side and receiving‐side reactive power equations in steady state and transient process are given. Second, the HVDC sending‐side and receiving‐side reactive power characteristics during commutation failure and recovery are analysed. Finally, the impact of the HVDC control system parameters is studied. All the contents are verified by simulation of the HVDC equivalent case system.