Ramakrishna Gokaraju

An Add-On Self-Tuning Control System for a UPFC Application

This paper presents an add-on self-tuning (ST) control scheme for a Unified Power Flow Controller (UPFC) to assist its conventional PI control system in damping power oscillations. The ST control algorithm is based on a Pole Shift (PS) technique that has previously been successfully applied in Adaptive Power System Stabilizers (APSSs). For a wide range of operating conditions, the conventional PI-UPFC unless returned suffers from insufficient/nonoptimal damping performance. To overcome this problem, the authors propose supplementing the PI controllers with an ST feedback loop comprised of an identifier and a PS control algorithm. With a CRLS identifier tracking the system conditions online, this scheme eliminates the need for PI parameter retuning. Further, to correctly identify system parameters during large disturbances, a constrained recursive least squares (CRLS) identification procedure is adopted here. An improved damping performance with the proposed add-on scheme assisting the nonoptimal PI-UPFC is demonstrated for a two-area power system.

Out-of-Step Protection Using State-Plane Trajectories Analysis

Summary form only given. This paper proposes a novel out-of-step protection technique using the state-plane representation of the generator speed and power angle. The critical clearing angle is computed using the principle that the total energy of the system at the instant the fault is cleared should be equal to the maximum potential energy of the system. The critical clearing time corresponding to the value of critical clearing angle is obtained directly using the time calibration of the relative speed versus power angle solution curve. The simultaneous calculation of the critical clearing angle and the time makes the proposed approach much faster than the two-blinder scheme. The proposed state plane prediction scheme is used to detect the first swing out-of-step condition in a two-area test system using system wide information. The two generators are represented with a single machine infinite bus (SMIB) equivalent system, and the state plane algorithm is applied to the reduced equivalent. Electromagnetic transient simulations are carried out using PSCAD/EMTDC to test the proposed algorithm in the two-area test systems. The simulation studies show that the proposed method is computationally efficient, and accurate. The technique also does not require any offline studies.

Dynamic Phasor Modeling of Type 3 DFIG Wind Generators (Including SSCI Phenomenon) for Short-Circuit Calculations

Short-circuit modeling of wind generators is crucial to determine protective relay and control settings, equipment ratings, and to provide data for protection coordination. The short-circuit contribution of a Type 3 wind farm connected to a series-compensated line is affected by subsynchronous interactions, making it essential to model such behavior. Fundamental frequency models are unable to represent the majority of critical wind generator fault characteristics. The complete electromagnetic transient (EMT) models, though accurate, demand high levels of computation and modeling expertise. This paper proposes a novel modeling technique for a Type 3 wind farm based on the generalized averaging theory, where system variables are represented using time-varying Fourier coefficients known as dynamic phasors. The novelty and advantage of the proposed modeling technique is that it does not just include 60-Hz frequencies but also other dominant frequencies, such as 36 Hz, that are present due to the SSCI in the system. Methods currently used by the industry mostly rely on fundamental frequency-based analysis. Only the appropriate dynamic phasors are selected for the required fault behavior to be represented, improving computational efficiency. Once the SSCI behavior (waveforms showing resonant frequency at the point of common coupling) of a series-compensated Type 3 wind farm from real-time field data is available, the developed model could be used to simulate the scenario without necessarily having to know the exact control blocks of the wind generator controls. A 450-MW Type 3 wind farm, consisting of 150 units, was modeled using the proposed approach. The method is shown to be accurate for representing faults at the point of interconnection of the wind farm to the grid for balanced and unbalanced faults as well as for nonfundamental frequency components present in fault currents during subsynchronous interactions.

Prototype of a Negative-Sequence Turn-to-Turn Fault Detection Scheme for Transformers

Digital relays are capable of computing the negative-sequence current on both primary and secondary sides of the transformer along with the phase difference between these two negative-sequence currents. By using both phase and magnitude information, negative-sequence current could be used to detect minor turn-to-turn faults involving 3% of the transformers windings or more. Turn-to-turn faults may still occur even if no current is flowing on one side of the transformer, such as during energization. With no current flowing in the secondary windings of the transformer, negative-sequence current-based algorithms become insensitive. This paper introduces a relay prototype, using both negative-sequence current and negative-sequence voltage, which retains its sensitivity during energization. The relays performance for several commonly encountered system scenarios, such as overexcitation, current-transformer saturation, nonzero fault resistance, transformer energization, and external faults were also examined. The experimental results presented in this paper indicate that the algorithm proposed in this paper is faster and more sensitive than restrained current differential protection and is capable of detecting turn-to-turn faults occurring during transformer energization.

A New Method for Blocking Third-Zone Distance Relays During Stable Power Swings

The improper operation of the third-zone distance protection has been attributed as one of the factors for power system blackouts. The third-zone protection could incorrectly operate during power-swing scenarios. This research proposes a fast and practical methodology using local measurements for blocking the third-zone distance relays during stable power swings. The proposed scheme is referred to as the BTZ scheme. The proposed method calculates the relative speed of a fictitious equivalent machine from the local relay measurements. If the relative speed goes through a zero crossing, the swing is classified as a stable power swing; whereas if the speed does not go through a zero value, then the swing is classified as an unstable power swing. The benefit of the proposed method is that it analyzes the power swings from a system stability point of view, and does not need rigorous offline simulation studies to determine the relay settings. The performance of the proposed method in this paper is compared with an industry-standard method (conventional double blinder method). As is well known, the settings of the blinder-based methods are system specific and need several simulations to arrive at the blinder values. Transient simulation studies on a three-bus system and a modified Western Coordination Council 9-bus system are used to test the performance of the proposed approach.

Adaptive Control Using Constrained RLS and Dynamic Pole-Shift Technique for TCSCs

In this paper, an adaptive pole-shift control technique for a FACTS device, namely Thyristor Controlled Series Capacitor (TCSC), is presented. Adaptive pole-shift techniques have been successfully implemented for power system stabilizer applications in the past, but one of the difficulties in extending such a technique for transmission line control devices has been its inability to handle large disturbance conditions such as three-phase faults. In recent literature, random walk technique has been suggested during the system identification process, to overcome this problem. This paper presents a simple parameter constrained RLS identification procedure to track the large disturbance conditions. The effectiveness of the proposed methodology is demonstrated using (i) a three-area six-machine power system with a TCSC, and (ii) a IEEE 12 bus power system configuration with a TCSC.

Coordination of overexcitation limiter, field overcurrent protection and generator control

The overexcitation limit of synchronous generator plays an important role in the voltage stability of power systems. Achieving maximum use of the overexcitation capability requires adequate coordination between generator control and protection. This paper provides an example of coordination between overexcitation limiter (OEL), field overcurrent protection and automatic voltage regulator (AVR) for a synchronous generator. The coordination achieved is then verified by a model simulation using an electromagnetic transient program for practical scenarios of interest. Suggested modifications to the ST1A type exciter model are presented to represent the complete functionality of the OEL function.

Out-of-step Detection Using Energy Equilibrium Criterion in Time Domain

Abstract This article introduces a new algorithm to detect the out-of-step condition in a power system based on energy equilibrium criterion in the time domain. The proposed energy equilibrium criterion is developed using the concept of equal area criterion in the power-angle domain, and it eliminates the numerical computations required to find the critical clearing time to detect the out-of-step condition. The proposed algorithm detects the out-of-step condition based on the real-time transient energy information available from the local substations. The effectiveness of the proposed algorithm is tested on a single-machine infinite-bus system, a two-machine infinite-bus system, and a three-machine infinite-bus system. The performance of the proposed algorithm is compared with an existing concentric rectangle scheme. The simulation results show that the proposed algorithm can be applied to larger systems and is faster compared to the concentric rectangle scheme.

Out-of-step protection using the analysis of electrical power Vs speed deviation in state plane

This paper proposes a real-time out-of-step protection method using state plane analysis of generator electrical power and speed deviation. Electrical power and speed are used as input variables to the proposed scheme for transient stability assessment. The proposed technique is tested in the IEEE 12-bus test system for first swing stable, unstable, and multi-swing instability conditions to verify its performance. Electromagnetic transient modeling of the test system is done and simulations are performed using PSCAD/EMTDC™ in order to test the proposed algorithm. The simulation results indicate that the proposed method is computationally simple and accurate in predicting the transient stability of the power system.

Load carrying capability of wind integrated power systems

Wind power is a valuable renewable source of electrical energy. Due to its stochastic nature, wind power is usually considered as an energy source rather than as a power contributor to the system generating capacity. Worldwide installation of wind power is expected to continue to grow in the coming years. It is therefore necessary to recognize the operating capacity value of the added wind power. This paper presents an approach to predicting short term wind speed distributions that can be used in a probabilistic analysis of the unit commitment risk. The development of performance indices such as the Peak Load Carrying Capability (PLCC) and the PLCC benefit ratio in wind integrated power systems is demonstrated using two published test systems.