This application relates to methods and apparatus for control apparatus of a voltage source converter for direct current transmission of electrical power over a transmission line. The control apparatus comprises regulation control for generating a regulation signal for the converter to regulate an electrical parameter of the voltage source converter, such as voltage or power, so as to regulate power transmission over the transmission line. The regulation signal is based on the difference between a measured value of said parameter and a reference value of said parameter, as may be supplied from a reference control block and may be derived from a demand signal indicating the voltage or power demand. The apparatus also includes damping control configured to generate a damping signal for modulating said regulation signal to reduce the effect of resonance within the bandwidth of control of the converter. The damping signal is generated as a function of a measured voltage or current of the transmission line and an impedance value for the transmission line. The damping signal may be high-pass filtered so as to have no substantial impact on steady state operation.

In the field of high voltage direct current (HVDC) power transmission networks, there is a need for improvements to allow a single power converter to control individual AC network voltages carried by multiple AC transmission conduits to multiple AC network elements, such as respective wind parks.

Disclosed are a voltage source converter based high voltage direct current (VSC-HVDC) high-frequency resonance suppression method, system, and device. The method includes: when an effective value of an actually input alternating current (AC) voltage is reduced from a normal value to meet a preset condition, making the virtual electrical quantity completely equal to the actual electrical quantity, and performing full real-time tracking for the actual electrical quantity to improve dynamic characteristics of a power system at the moment of a fault; and after performing the full tracking for a period of time, if the effective value of the actual AC voltage is less than a preset threshold, performing adaptive tracking until the actual electrical quantity recovers to a stable value. The present disclosure can reduce a risk of high-frequency resonance of a VSC-HVDC, avoid deteriorating dynamic characteristics of the VSC-HVDC, and improve safety of fault ride-through of the VSC-HVDC.

An electrical assembly includes a power converter having first and second DC terminals which are connectable to a DC electrical network. The power converter also includes converter limbs connected between the first and second DC terminals. Each converter limb includes an AC terminal that is connectable to a respective AC phase of a multi-phase AC electrical network. Each converter limb also includes limb portions, each connected between a corresponding AC terminal and a respective one of the first and second DC terminals. Each limb portion includes switching element(s). The electrical assembly includes a single grounding circuit having a reactor configured to provide a current path for alternating current with a high impedance to ground and a current path for direct current with a low impedance to ground. The grounding circuit is arranged so that only one of the AC phases is connected to ground via the grounding circuit.

Methods for analyzing a power system is presented. In some embodiments, a method may analyze system data, reduce the analyzed system data based on a set reduction level, model a device and a high voltage direct-current (HVDC) system interlinked to the device, and reflect the modeled result to the reduced system data.

There is provided an electrical assembly for use in an electrical system. The electrical assembly comprises a DC path. The DC path includes: a DC power transmission medium; and a current commutation device, the current commutation device including a switching element and an energy absorbing element, the switching element arranged to permit a current flowing, in use, through the DC path to flow through the switching element and at the same time bypass the energy absorbing element, wherein the electrical assembly further includes a control unit programmed to selectively control the switching of the switching element to commutate the current directly from the switching element to the energy absorbing element in order to increase the resultant voltage drop caused by the flow of direct current through the DC path in which the current commutation device is connected and thereby oppose the flow of the current through the DC path.

The present disclosure relates to a method and system for evaluating a grid condition of an AC power grid. A time series of voltage and current signals of the AC power grid is captured and a local frequency of the AC power grid is identified. The time series of the voltage and current signals is transformed at the identified local frequency of the AC power grid into transformed signals. Equivalent circuit parameters are estimated from the transformed signals based on a description of an equivalent circuit of the AC power grid. A value of a grid condition indicator is determined based on the calculated equivalent circuit parameters and the identified local frequency of the AC power grid and processed in a grid monitoring system for evaluating the grid condition of the AC power grid.

An energy supporting device for a high voltage direct current, HVDC, transmission system is provided. The HVDC transmission system includes a first HVDC converter and a second HVDC converter connected to each other via an HVDC link. The energy supporting device comprises a plurality of cells configured to be connected in series to the HVDC link, a resistor electrically connected between the plurality of cells and an electrical reference potential, and a bypass switch configured to allow the resistor to be bypassed. A cell includes a full-bridge arrangement of power switches and an energy storage device electrically connected to the full-bridge arrangement. The energy supporting device is configured to be operated in: a charging mode, in which the bypass switch is in a closed state for allowing the resistor to be bypassed, and in which the energy storage device of at least one cell of the plurality of cells is receiving electrical energy from the HVDC link, and in a dissipation mode, in which the bypass switch is in an open state for allowing the resistor to dissipate electrical energy from the HVDC link, and in which the energy storage devices of the plurality of cells are bypassed, and in an energy release mode, in which the energy storage device of at least one cell of the plurality of cells is discharging electrical energy to the HVDC link.

A power transmission network including: a variable power source; an AC transmission link for AC power transmission from the variable power source to at least one source side converter; at least one source side converter including: an AC connecting point operably connected to the AC transmission link; and a DC connecting point for connection to a DC transmission link; and a control system configured to operate the source side converter or at least one of the source side converters in a frequency damping mode to control an AC voltage at its AC connecting point and thereby damp at least one frequency component at its AC connecting point and/or in the AC transmission link.

A relay protection method against LC parallel circuit detuning faults comprises the steps of: a relay protection device samples a current of a parallel LC, that is, a reactor and a capacitor, and samples a total current flowing through the whole LC; convert the current of the reactor into a current of an equivalent capacitor; calculate amplitudes of the current of the equivalent capacitor and a current of a realistic capacitor and calculate an amplitude of the total current flowing through the LC; calculate a current amplitude ratio of the equivalent capacitor to the realistic capacitor; and when the amplitude of the total current flowing through the LC is large enough, send an alarm signal or a trip after a setting time delay if the current ratio exceeds a preset upper and lower limit range. Also provided is a corresponding relay protection device.