Yong Cheol Kang

Fourier Transform-Based Modified Phasor Estimation Method Immune to the Effect of the DC Offsets

This paper proposes a Fourier transform-based modified phasor estimation method to eliminate the adverse influence of the exponentially decaying dc offsets when discrete Fourier transform (DFT) is used to calculate the phasor of the fundamental frequency component in a relaying signal. By subtracting the result of odd-sample-set DFT from the result of even-sample-set DFT, the information of dc offsets can be obtained. Two dc offsets in a secondary relaying signal are treated as one dc offset which is piecewise approximated in one cycle data window. The effect of the dc offsets can be eliminated by the approximated dc offset. The performance of the proposed algorithm is evaluated by using computer-simulated signals and Electromagnetic Transients Program-generated signals. The algorithm is also tested on a hardware board with TMS320C32 microprocessor. The evaluation results indicate that the proposed algorithm can estimate the accurate phasor of the fundamental frequency component regardless of not only the primary decaying dc offset but also the secondary decaying dc offset caused by CT circuit itself including its burden.

Compensation of the distortion in the secondary current caused by saturation and remanence in a CT

Current-transformer (CT) saturation may cause the maloperation of a protection relay. This is particularly onerous when the remanent flux in the core of the CT adds to the flux change caused by the fault. The CT is forced into deep saturation and the waveshape of the secondary current is severely distorted. An algorithm for compensating the distortion in the secondary current caused by saturation and remanence in a CT is described in this paper. A second-difference function detects when the CT first starts to saturate. At this instant, the negative value of the second-difference function corresponds to the magnetizing current which, in conjunction with the magnetization curve, is used to estimate the core flux. This is then used as an initial value to calculate how the flux changes during the fault. The magnetizing current is estimated by inserting the estimated core flux into the magnetization curve and added to the secondary current; the result, the compensated secondary current, is equal to the secondary referred primary current. Various test results indicate that the proposed algorithm can accurately compensate a severely distorted secondary current and is not affected by remanence. The paper concludes by describing the hardware implementation of the algorithm on a prototype compensation unit based on a digital signal processor.

Multilevel Modular DC/DC Power Converter for High-Voltage DC-Connected Offshore Wind Energy Applications

Recently, the interest in offshore wind farms has been significantly increased because of the stronger and more stable winds at sea, which will lead to a higher power production. DC/DC power conversion solutions are becoming more popular for fulfilling the growing challenges in the high-voltage (HV) dc-connected offshore wind power industry. This paper presents several multilevel modular dc/dc conversion systems based on the capacitor-clamped (CC) module concept for high-power offshore wind energy applications. Two types of the CC modules, namely, the double-switch (DS) module and the switchless (SL) module, are discussed. A soft-switching technique is adopted to achieve minimal switching losses and the maximum system efficiency. Theoretical analysis is carried out for the 2n+1-level cascaded configurations based on the CC modules. The inherent interleaving property of the proposed configurations effectively reduces the output voltage ripple without adding extra components. A cascaded hybrid topology is developed by the combination of DS and SL modules. The proposed hybrid topology achieves higher efficiency and lower component count. The cascaded hybrid approach is evaluated in terms of the power device count, reliability, and efficiency against other HV dc/dc topologies to demonstrate its advantage for HVDC-connected offshore wind farms. The experimental results of two 5-kW prototype CC converters are presented to validate the theoretical analysis and principles as well as attest the feasibility of the proposed topologies.

Releasable Kinetic Energy-Based Inertial Control of a DFIG Wind Power Plant

Wind turbine generators (WTGs) in a wind power plant (WPP) contain different levels of releasable kinetic energy (KE) because of the wake effects. This paper proposes a releasable KE-based inertial control scheme for a doubly fed induction generator (DFIG) WPP that differentiates the contributions of the WTGs depending on their stored KE. The proposed KE-based gain scheme aims to make use of the releasable KE in a WPP to raise the frequency nadir. To achieve this, two additional loops for the inertial control are implemented in each DFIG controller: the rate of change of frequency and droop loops. The proposed scheme adjusts the two loop gains in a DFIG controller depending on its rotor speed so that a DFIG operating at a higher rotor speed releases more KE. The performance of the proposed scheme was investigated under various wind conditions. The results clearly indicate that the proposed scheme successfully improves the frequency nadir more than the conventional same gain scheme by releasing more KE stored in a WPP, and it helps all WTGs to ensure stable operation during inertial control by avoiding the rotor speed reaching the minimum speed limit.

Rotor Speed-based Droop of a Wind Generator in a Wind Power Plant for the Virtual Inertial Control

The frequency of a power system should be kept within limits to produce high-quality electricity. For a power system with a high penetration of wind generators (WGs), difficulties might arise in maintaining the frequency, because modern variable speed WGs operate based on the maximum power point tracking control scheme. On the other hand, the wind speed that arrives at a downstream WG is decreased after having passed one WG due to the wake effect. The rotor speed of each WG may be different from others. This paper proposes an algorithm for assigning the droop of each WG in a wind power plant (WPP) based on the rotor speed for the virtual inertial control considering the wake effect. It assumes that each WG in the WPP has two auxiliary loops for the virtual inertial control, i.e. the frequency deviation loop and the rate of change of frequency (ROCOF) loop. To release more kinetic energy, the proposed algorithm assigns the droop of each WG, which is the gain of the frequency deviation loop, depending on the rotor speed of each WG, while the gains for the ROCOF loop of all WGs are set to be equal. The performance of the algorithm is investigated for a model system with five synchronous generators and a WPP, which consists of 15 doubly-fed induction generators, by varying the wind direction as well as the wind speed. The results clearly indicate that the algorithm successfully reduces the frequency nadir as a WG with high wind speed releases more kinetic energy for the virtual inertial control. The algorithm might help maximize the contribution of the WPP to the frequency support.

Frequency Control Support of a Doubly-Fed Induction Generator Based on the Torque Limit

This paper proposes a torque limit-based inertial control scheme of a doubly-fed induction generator (DFIG) that supports the frequency control of a power system. If a frequency deviation occurs, the proposed scheme aims to release a large amount of kinetic energy (KE) stored in the rotating masses of a DFIG to raise the frequency nadir (FN). Upon detecting the event, the scheme instantly increases its output to the torque limit and then reduces the output with the rotor speed so that it converges to the stable operating range. To restore the rotor speed while causing a small second frequency dip (SFD), after the rotor speed converges the power reference is reduced by a small amount and maintained until it meets the reference for maximum power point tracking control. The test results demonstrate that the scheme can improve the FN and maximum rate of change of frequency while causing a small SFD in any wind conditions and in a power system that has a high penetration of wind power, and thus the scheme helps maintain the required level of system reliability. The scheme releases the KE from 2.9 times to 3.7 times the Hydro-Québec requirement depending on the power reference.

Inertial Control of a DFIG-based Wind Power Plant using the Maximum Rate of Change of Frequency and the Frequency Deviation

In order to let a wind generator (WG) support the frequency control of a power system, a conventional inertial control algorithm using the rate of change of frequency (ROCOF) and frequency deviation loops was suggested. The ROCOF loop is prevailing at the initial stage of the disturbance, but the contribution becomes smaller as time goes on. Moreover, its contribution becomes negative after the frequency rebound. This paper proposes an inertial control algorithm of a wind power plant (WPP) using the maximum ROCOF and frequency deviation loops. The proposed algorithm replaces the ROCOF loop in the conventional inertial control algorithm with the maximum ROCOF loop to retain the maximum value of the ROCOF and eliminate the negative effect after the frequency rebound. The algorithm releases more kinetic energy both before and after the frequency rebound and increases the frequency nadir more than the conventional ROCOF and frequency loops. The performance of the algorithm was investigated under various wind conditions in a model system, which includes a doubly-fed induction generator-based WPP using an EMTP-RV simulator. The results indicate that the algorithm can improve the frequency drop for a disturbance by releasing more kinetic energy.

Adaptive Q–V Scheme for the Voltage Control of a DFIG-Based Wind Power Plant

Wind generators within a wind power plant (WPP) will produce different amounts of active power because of the wake effect, and therefore, they have different reactive power capabilities. This paper proposes an adaptive reactive power to the voltage (Q-V) scheme for the voltage control of a doubly fed induction generator (DFIG)-based WPP. In the proposed scheme, the WPP controller uses a voltage control mode and sends a voltage error signal to each DFIG. The DFIG controller also employs a voltage control mode utilizing the adaptive Q-V characteristics depending on the reactive power capability such that a DFIG with a larger reactive power capability will inject more reactive power to ensure fast voltage recovery. Test results indicate that the proposed scheme can recover the voltage within a short time, even for a grid fault with a small short-circuit ratio, by making use of the available reactive power of a WPP and differentiating the reactive power injection in proportion to the reactive power capability. This will, therefore, help to reduce the additional reactive power and ensure fast voltage recovery.

Hierarchical voltage control of a wind power plant using the adaptive IQ-V characteristic of a doubly-fed induction generator

Because wind generators (WGs) in a wind power plant (WPP) produce different active powers due to wake effects, the reactive power capability of each WG is different. This paper proposes a hierarchical voltage control scheme for a WPP that uses a WPP controller and WG controller. In the proposed scheme, the WPP controller determines a voltage error signal by using a PI controller and sends it to a doubly-fed induction generator (DFIG). Based on the reactive current-voltage (IQ-V) characteristic of a DFIG, the DFIG injects an appropriate reactive power corresponding to the voltage error signal. To enhance the voltage recovery capability, the gains of the IQ-V characteristic of a DFIG are modified depending on its reactive current capability so that a DFIG with greater reactive current capability may inject more reactive power. The proposed scheme enables the WPP to recover the voltage at the point of common coupling (PCC) to the nominal value within a short time after a disturbance by using the adaptive IQ-V characteristics of a DFIG. The performance of the proposed scheme was investigated for a 100 MW WPP consisting of 20 units of 5 MW DFIGs for small and larger disturbances. The results show the proposed scheme successfully recovers the PCC voltage within a short time after a disturbance.

Design and Evaluation of a Protection Relay for a Wind Generator Based on the Positive- and Negative-Sequence Fault Components

To avoid undesirable disconnection of healthy wind generators (WGs) or a wind power plant, a WG protection relay should discriminate among faults, so that it can operate instantaneously for WG, connected feeder or connection bus faults, it can operate after a delay for inter-tie or grid faults, and it can avoid operating for parallel WG or adjacent feeder faults. A WG protection relay based on the positive- and negative-sequence fault components is proposed in the paper. At stage 1, the proposed relay uses the magnitude of the positive-sequence component in the fault current to distinguish faults requiring non-operation response from those requiring instantaneous or delayed operation responses. At stage 2, the fault type is first determined using the relationships between the positive- and negative-sequence fault components. Then, the relay differentiates between instantaneous operation and delayed operation based on the magnitude of the positive-sequence fault component. Various fault scenarios involving changes in position and type of fault and faulted phases are used to verify the performance of the relay. This paper concludes by implementing the relay on a hardware platform based on a digital signal processor. Results indicate that the relay can successfully distinguish the need for instantaneous, delayed, or non-operation.