Concerning the defects of double-thyristor method in clearing fault on the DC side of MMC-HVDC system, a new MMC sub-module topology is designed, and a protection scheme for DC-side fault based on the new topology is proposed. First, the characteristic of short circuit current when fault occurs on the DC side of MMC-HVDC system after converter blocking is analyzed, as well as the attenuation pattern and influencing factors. And then, concerning the defects of double-thyristor method, an RC absorber is added to the sub-module to avoid the mis-triggering of thyristor in normal operation state by utilizing the characteristic that the capacitor voltage cannot jump. At the same time, a group of inverse-series IGBT and diode is added to the sub-module, thus the unidirectional conduction characteristic of diode can be used to block the flow of short circuit current in the diode. On this basis, a new MMC sub-module topology is designed, and a DC-side protection scheme based on fault self-clearing is proposed, which combined with reclosing measures can quickly restore the system to normal operation. RTDS simulation results demonstrate that, the protection method based on the new sub-module can rapidly reduce the fault current on DC side, effectively protect the sub-module devices, and quickly restore the system to normal operation in the case of instantaneous fault.

Utilities across the world are seeing increased penetration of inverter-based renewable generation (RC) in their systems. These RGs have ride through curves programmed in them which define operating conditions that need to be satisfied during electrical disturbances. If they are violated, the RGs are tripped offline. In systems with large amounts of RGs tightly coupled electrically, there can be disturbances that cause sudden loss of a large number of RGs, which will then considerably exacerbate the stability of the system. Such a phenomenon is not captured by the existing direct methods for TSA. In this paper, by treating RG-rich systems as non-linear switched systems as opposed to the traditional approach of treating this tripping phenomenon as an instability, an approach utilizing multiple low voltage ride through constrained stability regions (CSRs) is proposed for capturing unstable fault clearing times. The CSRs are estimated through Lyapunov functions found using sum of squares programming. The effectiveness of the proposed technique is demonstrated using a three-machine system.

Concerning the variation of power system operating condition caused by time-varying factors such as wind generator output power fluctuation and load change, an adaptive control strategy for time-varying power system based on continuous Markov model is proposed in this paper. First, the Lyapunov functional containing the continuous Markov time-varying power system model is constructed, and the robust stochastic stability theorem with H infinity norm bound gamma is derived using the Dynkin lemma. On this basis, according to the Schur complement theorem, the robust stochastic stability LMI (linear matrix inequality) which satisfies the disturbance attenuation degree gamma and the minimum variance constraint is derived considering the system operating conditions steady-state variance constraint. And then, through transforming the LMI to the minimization problem of linear objective function, the matching controller of each operating condition is solved. Finally, the stochastic gradient method is used to identify the system operating condition and determine the weighting coefficient of each controller, thus the adaptive control strategy is established. Time-domain simulation tests show that, the proposed control strategy could effectively suppress cascading disturbances as well as avoid the problem of mismatch between current operating condition and the fixed fault set. Besides, the impact on power system caused by controller switching is reduced.

In view of the low reliability of traditional protection in the distribution network caused by the massive integration of DGs (distributed generation), an adaptive directional current protection scheme based on fault steady-state component is proposed in this paper. First, by analyzing the fault transient characteristics and fault equalization methods of different types of DGs, the components of system short circuit current are calculated, and the relationship between three phase components of the short circuit current is revealed. On this basis, according to the fault boundary conditions, the fault steady-state components are obtained. And then, combined with the measured voltage and measured current at the relaying point, the equivalent voltage and equivalent impedance at the backside of protection are calculated. Thus the adaptive directional current protection criteria for different fault types are formed. Simulation results demonstrate that, the proposed scheme is not affected by the type and power output of DG and the system operation mode, and could effectively prevent the protection from refusing to operate due to voltage drop.

The variation characteristics of electrical variables and the variation pattern of oscillation centre in multi-source oscillation scenes are revealed, and a desynchronising centre positioning method is proposed. First, based on the multi-source oscillation model, the expressions of voltage and current in multi-source oscillation scenes are derived. And then, according to the definition of oscillation centre, the oscillation centre position function is constructed, as a quantitative description of the position of oscillation centre. On this basis, the impacts of power angle variation trend (oscillation mode), system operation mode variation and unequal emf amplitudes on the oscillation centre are analysed. Simulation results demonstrate that, the oscillation mode is the main factor that affects the drifting pattern of oscillation centre, system operation mode is the main factor that determines the drifting boundary of oscillation centre, and unequal emf amplitudes cause the oscillation centre to deviate towards the side with lower amplitude. Finally, according to the relationship between system emfs when the desynchronising centre appears, the desynchronising centre position function is derived, so that the position of desynchronising centre can be identified. Simulation results on Real Time Digital Simulator of multi-machine system verify the correctness of the analysis results.

In view of frequent multi-machine power system oscillations in power grid which may cause traditional oscillation blocking and re-opening schemes to be completely invalid, a distance protection oscillation blocking and re-opening scheme in multi-machine power system oscillation scenes is proposed in this paper. First, the ride-through trajectory of measured impedance in multi-machine power system oscillation cases and its effect on the operation characteristic of distance protection is analyzed, and the oscillation blocking scheme is formed by combining the operation status information of protection components on 2 line ends. And then, in the oscillation blocking duration, by calculating the estimated error of the dynamic phasor of measured current at the relaying point, the protection re-opening scheme for symmetrical fault during oscillation is established. Finally, simulation tests on PSCAD in 3-machine system and IEEE 39-bus system verify that the proposed scheme could reliably block the protection when oscillation occurs in multi-machine power system, and re-open the protection accurately and reliably when symmetrical fault occurs during oscillation.

Traditional Lyapunov-based transient stability assessment approaches focus on identifying the stability region (SR) of the equilibrium point under study. When trying to estimate this region using Lyapunov functions, the shape of the final estimate is often limited by the degree of the function chosen - a limitation that results in conservativeness in the estimate of the SR. More conservative the estimate is in a particular region in state space, smaller is the estimate of the critical clearing time (CCT) for disturbances that drive the system towards that region. In order to reduce this conservativeness, we propose a methodology that uses the disturbance trajectory data to skew the shape of the final Lyapunov-based SR estimate. We exploit the advances made in the theory of sum of squares decomposition to algorithmically estimate this region. The effectiveness of this technique is demonstrated on a power systems classical model.

Wide area measurement system relies on phasor measurement unit (PMU) data to monitor, protect, and control high-voltage transmission networks. However, errors in instrument transformers (ITs) located at the inputs of a PMU can significantly degrade its output quality. This study proposes two methodologies for voltage and current transformers calibration using PMU data. The first method calibrates ITs using one good quality voltage measurement located at a tie-line. This method tolerates errors in both the ITs (which are to be estimated) as well as the PMUs. The second method attains the same objective as the first one, with the additional constraint that some portion of the data is unusable. Thus, the second method can be used even when the incoming data is intermittent.

A line overload emergency control strategy based on the source-load synergy coefficient is proposed in this paper. First, the definition of the source-load synergy coefficient is introduced. When line overload is detected, the source-load branch synergy coefficient and source-load distribution synergy coefficient are calculated according to the real-time operation mode of the system. Second, the generator tripping and load shedding control node set is determined according to the source-load branch synergy coefficient. And then, according to the line overload condition, the control quantity of each control node is determined using the Double Fitness Particle Swarm Optimization (DFPSO), with minimum system economic loss as the objective function. Thus load shedding for the overloaded line could be realized. On this basis, in order to guarantee continuous and reliable power supply, on the condition that no new line overload is caused, some of the untripped generators are selected according to the source-load distribution synergy coefficient to increase power output. Thus power supply could be restored to some of the shedded loads, and the economic loss caused by emergency control could be minimized. Simulation tests on the IEEE 10-machine 39-bus system verify the effectiveness and feasibility of the proposed strategy.

An oscillation blocking and re-opening scheme for distance protection in multisource oscillation scenes is proposed in this paper. First, the expression of measured impedance in multisource oscillation scenes is derived and the variation characteristic of the trajectory of measured impedance is analyzed. Then, according to the correlation between the measured impedance and the position of desynchronizing center, the desynchronizing center position factor (DCPF) is defined, and protection blocking and re-opening criterion based on DCPF is constructed. Real Time Digital Simulator test results demonstrate that the proposed scheme can effectively block the protection during oscillation even when fault occurs at the same time as the desynchronizing center appears, and re-open the protection quickly when a fault occurs.

Many cascading methodologies tend to be steady-state load flow analyses to avoid the computational burden of examining cascades from a dynamic standpoint. Load flow, however, has the drawback of losing information about generator transient angular stability. On the other hand, time-domain simulations can accurately identify angular stability issues at the cost of extra computational burden. This work aims to incorporate time-domain angular stability in load-flow through machine learning. A Random Forest (RF) machine learning classifier is trained on generator transient angular instability data obtained from time-domain simulations with machine features derived from steady-state load-flow analysis. RFs require very little computation once trained and can have high predictive accuracy, resulting in a classifier that can be used in steady-state cascading methodologies to predict transient stability.

In view of the sensitivity and selectivity problems in DFIG (doubly-fed induction generator) wind farm collector line protection, a new adaptive distance protection scheme is proposed. First, according to the geometric distribution characteristics of the collector line voltage and current, the equation of voltage drop from the relaying point to the fault point is established and the adaptive setting coefficient is calculated. On this basis, a new adaptive distance protection criterion is formed according to the phase relationship between the fault current and the measured current. Wind farm simulation tests on RTDS verify that, the proposed method is well adaptable to different operation modes, strongly immune to the fault resistance and unaffected by the weak feed characteristics of the wind generator collector system.

Concerning multi-source oscillation scenes, an oscillation centre identification method based on the variation trend of bus frequency is proposed. First, according to the amplitude and phase relationship between voltage and current at any point in the system, the functional expression of the frequency of voltage and current is derived. Then, the variation trend and distribution of the frequency of voltage and current in multi-source oscillation scenes are analysed. On this basis, the relationship between system frequency distribution and the oscillation centre and desynchronising centre is revealed, and an oscillation centre identification scheme for multi-source oscillation scenes is put forward. Simulation results based on DIgSILENT/PowerFactory demonstrate that the proposed scheme can effectively identify the location of oscillation centre in multi-source oscillation system.

There has been significant increase in penetration of renewable generation (RG) sources all over the world. Localized concentration of many such generators could initiate a cascade tripping sequence that might threaten the stability of the entire system. Understanding the impact of cascade tripping process would help the system planner identify trip sequences that must be blocked in order to increase stability. In this work, we attempt to understand the consequences of cascade tripping mechanism through a Lyapunov approach. A conservative definition for the stability region (SR) along with its estimation for a given cascading sequence using sum of squares (SOS) programming is proposed. Finally, a simple probabilistic definition of the SR is used to visualize the risk of instability and understand the impact of blocking trip sequences. A 3-machine system with significant RG penetration is used to demonstrate the idea.

Several different events connected with the early application of phasor measurements prompted consideration of the propagation of transient events in power systems. The first is typical of what is shown in Fig. 11.1.

A method to analyze the angle stability of power system with multiple operating conditions considering cascading failure is proposed in this paper. First, considering stochastic events such as uncertain operation of protection/breaker and system hardware failure, the system operating condition set is determined based on the flow transfer theory and the matrix of transfer probability between different operating conditions is calculated. On this basis, the discrete Markov power system model with multiple operating conditions considering cascading failure is established. And then, construct the Lyapunov functional containing the Markov model, and derive the robust stochastic stability criterion which satisfies the disturbance attenuation degree γ by iteration and the angle stability of power system could be identified according to the criterion using linear matrix inequality tools. Finally, stability index μ is defined to establish the relationship between transfer probability and system stability. Time-domain simulation tests on the IEEE 16-machine 68-bus system verify that, the proposed method could identify the stability of power system with multiple operating conditions effectively, with no need to obtain the operating trajectory of the system. Compared with traditional time-domain simulation method, the computation amount is reduced. Besides, it is simple and easy to implement.

In view of the incorrect operation of local protection caused by multiple distributed generations (DGs) and branches in distribution system, a regional protection scheme for distribution network based on logical information is proposed here. Moreover, a regional centralised protection system in parallel with and independent from the existing protection system is constructed. First, the problems in local current protection of lines with multiple DGs and branches are analysed. On this basis, protection regionalisation is conducted according to the configuration of circuit breakers. Also then, by combining the start-up information of existing over-current protection set-I, set-II, and set-III, the operation criterion of regional protection is formed. Simulation tests on the IEEE 33-node distribution system verify that, the proposed scheme could accurately locate in-zone fault and block out-of-zone fault, accelerate protection operation, and shorten the outage time. Besides, the proposed criterion is fault tolerant due to redundancy in the multi-source information used. With small amount of information transmission, the requirement on information synchronisation is low.

The increasing penetration of inverter based renewable generation (RG) in the form of solar photo-voltaic (PV) or wind has introduced numerous operational challenges and uncertainties. According to the standards [1], [2], these generators are made to trip offline if their operating requirements are not met. In an RG-rich system, this might alter the system dynamics and/or cause shifting of the equilibrium points to the extent that a cascaded tripping scenario is manifested. The present work attempts at avoiding such scenarios by estimating the constrained stability region (CSR) inside which the system must operate using maximal level set of a Lyapunov function estimated through sum of squares (SOS) technique. A time-independent conservative approximation of the LVRT constraint is initially derived for a classical model of the power system. The proposed approach is eventually validated by evaluating the stability of a 3 machine test system with trip-able RG.