Young-Hyun Moon

Extended integral control for load frequency control with the consideration of generation-rate constraints

This paper presents an extended integral control to load frequency control (LFC) scheme with the presence of generation rate constraints (GRC) in order to get rid of overshoot of the conventional proportional-integral (PI) control. The conventional LFC scheme does not yield adequate control performance with the consideration of the singularities of speed-governor such as rate limits on valve position and GRC. In order to overcome this drawback, an extended integral control is developed for the PI control of the speed governor in the presence of GRC. The key idea of the extended integral control is using a decaying factor to reduce the effects of the error in the past. The decaying factor greatly affects the control performance, and should be carefully selected. This study determines the decaying factor in proportion to the degree of deviation in several levels. The computer simulation has been conducted for the single machine system with various load changes. The simulation results show that the proposed controller based on extended integral control yields much improved control performance, compared to the conventional PI controller.

Power system load frequency control using noise-tolerable PID feedback

This paper presents a new PID (proportional, integral and differential) control scheme based on the feedback of averaged derivatives to realize a noise-tolerable differential control with its application to the load frequency control in the power system. It is well known that the LFC (load frequency control) is exposed to the quite noisy environment. The noisy environment has made it difficult to adopt the differential feedback loop since the derivatives of the signals deteriorated by high frequency noise causes the system instability. This paper proposed a new PID control scheme adopting averaged derivative of the signal as the differential feedback signal in order to remove the effects of high frequency noise. This study deals with an application of the average differential feedback to the LFC problem in the power systems. The proposed control scheme has been tested for the load frequency control of power systems.

Transmission Loss Allocation Algorithm Using Path-Integral Based on Transaction Strategy

This paper presents a new algorithm of accurate bus-wise transmission loss allocation based on path-integrals. With rigorous theoretical analysis, a new path-integral method is developed by integrating the partial differential of the system loss along a path reflecting the transaction strategy. This path-integral enables us to remarkably enhance the accuracy in loss allocation with full consideration of nonlinearity. The accuracy has been further improved by using the ac power flow. Determining the integral path is discussed to reflect various situations of the power market. Given an integral path, the proposed algorithm provides a unique and accurate solution to the loss allocation.

Improvement of system damping by using the differential feedback in the load frequency control

The power system stability mostly depends on the surplus kinetic energy which is stored in the generator during the fault period. The steam valve control is the most direct method to control the surplus kinetic energy. This paper proposes a new LFC scheme with a modified PID controller which improves the system damping. The proposed frequency controller is designed to guarantee the stability of frequency control for any change in the feedback gain of frequency deviation. The proposed LFC scheme adopts the nonwindup cutoff model for the steam valve position. It has been shown that the steam valve control is the most direct method to control the surplus kinetic energy stored in the generator during the fault period and control performance significantly depends on steam valve modeling. The proposed LFC scheme has been applied for two sample systems, 4-machine 10-bus and New England 39-bus systems.

Uniqueness of static voltage stability analysis in power systems

Diverse theories have been established in voltage stability analysis since various aspects have been observed during voltage collapse phenomena. Through rigorous mathematical investigation, this paper shows that all the major methods used in static voltage stability analysis, i.e. Jacobian method, voltage sensitivity method, real and reactive power loss sensitivity method and energy function method-provide an identical result to show the uniqueness of static voltage stability analysis in theory. The tests for sample systems have shown that an identical result can be obtained from the various analysis methods.

Modified PID load-frequency control with the consideration of valve position limits

This paper proposes a new load frequency control (LFC) scheme with a modified PID controller which guarantees the stability of the LFC loop for the wide control ranges of PID feedback gains. By observing the fact that the ratios between PID gains have more information than the individual gains, we proposed a modified PID controller by implementing feedback circuits by differentiating and integrating the output of the flyball speed governor. The proposed control scheme guarantees wide-range stability boundary of the PID gain controls, which can be easily checked by observing the root locus diagram. This paper shows that the valve position limits can be modeled in several ways, and that the control performance significantly depends on steam valve modeling in the case of large disturbances. The proposed LFC scheme has been tested for a sample 4-machine 10-bus system.

Fuzzy logic based extended integral control for load frequency control

This study presents an application of fuzzy logic based method to determine the optimal parameters for the extended integral control scheme. The performance of the extended integral control is greatly dependent on the decaying factor. For an optimal or near optimal performance, it is necessary that the decaying factor as well as the feedback gains should be changed very quickly in response to changes in the system dynamics. However, because of its time-varing characteristic, the optimal decaying factor is difficult to be selected analytically. By adopting fuzzy set theory, the decaying factor can be determined quickly to respond to the variation of the feedback signals. This study builds a fuzzy rule base with use of the change of frequency and its rate as inputs. A set of decision rules is established to relate input signals to the decaying factor. The computer simulation has been conducted for the single machine system. The simulation results show that the proposed fuzzy logic based controller yields more improved control performance than the conventional PI controller.

The development of equivalent system technique for deriving an energy function reflecting transfer conductances

This paper shows that a well-defined energy function can be developed to reflect the transfer conductances for multi-machine power systems under an assumption that all transmission lines have uniform R/X ratios. The energy function is derived by introducing a pure reactive equivalent system for the given system. In this study, a static energy function reflecting transfer conductances is also derived as well as the transient energy function. The proposed static energy function is applied to voltage stability analysis and tested for various sample systems. The test results show that the accuracy of voltage stability analysis can be considerably improved by reflecting transfer conductances into the energy function.

Slack-bus independent penalty factor for spot pricing under deregulation

Recently, deregulation has been a hot issue in electric power industries with the introduction of the bidding system to electric power markets. One of the main topics associated with the deregulation is spot pricing, where the exact analysis of transmission loss and the method of calculating reliable penalty factor play an important role. Penalty factors calculated with conventional methods is highly dependent on the location of the slack bus, which brings about unfair competition to IPPs (independent power producers). This paper presents a new method of calculating the exact and reliable penalty factor independent of the change of the slack bus. The proposed method is tested on IEEE 14 bus system, New England 39 bus system and OH-145 bus system. The test results show that the proposed method can give more reliable information of the penalty factors to the spot pricing system.

Brief Estimating the domain of attraction for power systems via a group of damping-reflected energy functions

The well-known Lyapunov direct method provides precise and rigorous theoretical backgrounds for stability analysis of nonlinear systems. However, incorporating damping effects into accurate estimates of the domain of attraction has proved difficult, which is pointed as a minor but crucial drawback. This paper presents a new method to derive a group of Lyapunov functions to reflect the damping effects by considering the first integral of motion obtained from the system governing equations when transfer conductances are neglected. A systematical approach of converting some part of damping loss into some appropriate system energy terms is developed. Examples show that the proposed method remarkably improves the estimation of the region of attraction compared with the conventional approach.