Compared to the conventional power systems, large wind power plants (WPPs) present a more challenging system, where the interactions between the passive elements and the wideband control systems of power converters may result in harmonic instability and new resonance frequencies. Most of researches about harmonic stability focus on small-scale systems, and it has not paid much attention yet to identify the mentioned resonance frequencies. This paper models and analyzes the harmonic instability and resonance frequencies in large permanent magnet synchronous generator (PMSG) based WPPs with full-scale converters, where linearized models of inner control loops of the power converters are considered. A large PMSG-based WPP introduces as a multi-input mulit-output (MIMO) control system, therefore, the stability of the whole power system is analyzed based on the real parts of the poles of the introduced MIMO system and the resonance frequencies are identified based on the element amplitudes of the MIMO matrix. An active damping controller is used to set the poles of the WPP in a desired location in order to mitigate the harmonic instability problems. Multiple case studies are provided to depict that wind turbine connections or disconnections in a WPP, as well as grid impedance variations can affect the harmonic stability and resonance frequencies. The effectiveness of the presented theoretical analysis is validated by time-domain simulations of a 400-MW WPP in PSCAD/EMTDC software environment.

This paper proposes an online steady-state voltage stability assessment scheme to evaluate the proximity to voltage collapse at each bus of a load area. Using a non-iterative holomorphic embedding method (HEM) with a proposed physical germ solution, an accurate loading limit at each load bus can be calculated based on online state estimation on the entire load area and a measurement-based equivalent for the external system. The HEM employs a power series to calculate an accurate Power-Voltage (P-V) curve at each load bus and accordingly evaluates the voltage stability margin considering load variations in the next period. An adaptive two-stage Pade approximants (PA) method is proposed to improve the convergence of the power series for accurate determination of the nose point on the P-V curve with moderate computational burden. The proposed method is illustrated in detail on a 4-bus test system and then demonstrated on a load area of the Northeast Power Coordinating Council (NPCC) 48-generator, 140-bus power system.

Compared with the conventional power systems, large-scale power electronics based power systems present a more complex situation, where harmonic instability may be induced by the mutual interactions between the inner control loops of the converters. This paper presents an approach to locate which power converters and buses are more sensitive and have significant contribution to the harmonic instability. In the approach, a power electronics based system is introduced as a Multi-Input Multi-Output (MIMO) dynamic system by means of a dynamic admittance matrix. Bus Participation Factors (PFs) are calculated by the oscillatory mode sensitivity analysis versus the elements of the MIMO transfer function matrix. The PF analysis detects which power electronic converters or buses have a higher participation in harmonic instability excitation than others or at which buses such instability problems have a higher impact. In order to confirm the effectiveness of the presented approach, time-domain simulation results are provided for a 400-MW wind farm in PSCAD software environment.

In this article, distinctive fault characteristics of converter-interfaced renewable energy sources (CIRESs) will result in the misoperation of traditional distance relays. To handle this issue, a new fault coordination control method is proposed to ensure the correct operation of distance relays. Combined with sequence boundary conditions, the relationship between the apparent reactance and the CIRES positive-sequence current angle is revealed. Based on this, a reasonable current angle is generated by adjusting current references of the controller to make the apparent reactance equal to the actual line replica reactance, so distance relays can detect the fault distance correctly. The proposed control method shows good performance for high resistance faults and operation in weak grids. Meanwhile, the proposed method is economical because it does not require revising the controller structure, and only local measured data are used. Furthermore, fault ride through requirements such as current limitation and reactive current injection are still being realized. Simulation analysis is performed in PSCAD and real-time digital simulator to validate the recommended scheme.

The traditional methodology for defining the ampacity of overhead lines is based on conservative criteria regarding the operating conditions of the line, leading to the so-called static line rating. Although this procedure has been considered satisfactory for decades, it is nowadays sensible to account for more realistic line operating conditions when calculating its dynamic ampacity. Dynamic line rating is a technology used to improve the ampacity of overhead transmission lines based on the assumption that ampacity is not a static value but a function of weather and line's operating conditions. This paper presents an algorithm to estimate and predict the temperature in overhead line conductors using an Extended Kalman Filter. The proposed algorithm assumes both actual weather and current intensity flowing along the conductor as control variables. The temperature of the conductor, mechanical tension and sag of the catenary are used as measurements because the common practice is to measure these values with dynamic line rating hardware. The algorithm has been validated by both simulations and measurements. The results of this study conclude that it is possible to implement the algorithm into Dynamic Line Rating systems, leading to a more accurate estimation and prediction of temperature.

Unique fault behaviors of renewable energy sources (RESs) may lead to the misoperation of traditional pilot protection. To cope with this issue, this article proposes a new pilot protection method using the improved Euclidean distance. For normal operation or external faults, the currents on both ends are completely opposite, so the Euclidean distance of current absolute values on both ends is equal to 0. However, it will be much larger than 0 for internal faults because transient currents on both ends will have a big difference at this time. Therefore, internal and external faults can be detected reliably. In order to facilitate the setting calculation, the Euclidean distance is normalized, and a stability factor is introduced to avoid invalid calculation results. The proposed method can be applied to different RES types and different fault ride through strategies. Meanwhile, it can withstand larger fault resistance and noise interference. Compared with other methods using the RES fault currents, this approach can operate correctly without any additional criteria when the circuit breaker recloses on a permanent fault or RESs output a low power. PSCAD simulation and real-time digital simulator experiment verify this method.