Reactive Power Resources Research Articles

Optimal planning of static and dynamic reactive power resources

This study proposes an optimisation-based method of planning static and dynamic VAR sources for the enhancement of electric power transmission systems under a set of contingencies. The overall objective function is to minimise the total installation cost of static and dynamic VAR sources, while satisfying the requirements of long-term voltage stability margin and transient voltage dip under a contingency list. The backward/forward search algorithm with linear complexity is used to select candidate locations for reactive power compensation. Optimal locations and amounts of static and dynamic VAR sources are obtained by solving a sequence of mixed integer programming problems. The New England 39 bus system is adopted to illustrate the effectiveness of the proposed method. The systematic applicability of the proposed method to a large scale practical system is also illustrated using a 16173-bus U.S. Eastern Interconnection system.

A Coordinated Volt/Var Control Strategy for Wind Farm Group

A coordinated Volt / Var control strategy for wind farm group is proposed. The area voltage control center monitors the pilot bus in the central substation where many wind farms are connected to, and coordinates all kinds of reactive power resources in different wind farms to regulate the voltage profile of all wind farms. The genetic algorithm is used to solve such a multi-objective optimization model. The simulation results of a typical wind power system show that the coordinated control strategy has better performance than the current individual one in respects of security and economy of the grid.

Adaptive and Automatic Closed-Loop Coordinated Voltage Control Based Model for Optimal Voltage Control: a Case Study

Effective and efficient management of voltage control devices and reactive power resources is one of the most challenging tasks in power system operation. The need for proper voltage control is mainly to fulfill the following requirements: system security, economic operation and voltage quality. In Malaysian Transmission System (MTS), most of the voltage control devices and reactive power resources are manually control. Since the guidelines for controlling the primary and secondary voltage control are broad in nature, control actions could vary from one operator to another depending on experience, system information and knowledge. Tenaga Nasional Berhad (TNB) as system operator for MTS System believes that improvement in the approach and methods for voltage controls are desirable. This paper presents the newly developed hierarchical voltage control system called Adaptive and Automatic Closed-Loop Coordinated Voltage Control (A2CLCVC) based system model as a possible solution for better voltage control within Malaysia Transmission System. The A2CLCVC system model is basically a hierarchical voltage control approach that operates at three different voltage control levels which consists of Tertiary, Secondary and Primary voltage control modules. The A2CLCVC based system model will be used to coordinate and optimize the voltage control operation in order to improve security, quality and economic of Malaysian Transmission System. To verify the A2CLCVC model system performance, two new simulation methodologies based on PSS/E simulation software has been developed: 24-hours Load Flow Python-programming (24LFP-PSS/EM) and 24-hours dynamic simulation based method (24DS-PSS/EM). The two simulation methodologies results has shown that the proposed A2CLCVC system can provide great benefits to TNB especially for better voltage control and power system operational. The comparative studies with the commercial automatic voltage control system have shown that the performance of A2CLCVC system is much similar.

A new hybrid technique for power systems multi-facts optimization design

SUMMARY The purpose of this paper is to develop a new tool to the dispatching and optimization of reactive power resources using two kinds of the flexible alternating current transmission system devices, namely the static volt ampere reactive compensator and the Thyristor controlled series compensator. The fast voltage stability index and line stability index (Lmn) are used to identify the critical buses and lines, which will receive the flexible alternating current transmission system devices. The methodology carried out in this work is deployed in two phases. In the first one, we consider that the power system network is secure and operates within its stability limits and the balance between consumption and generation is satisfied. Thus, the solution of optimization program of the fuel cost minimization problem using the interior point method gives a feasible solution where the reactive power compensation is not considered. In the second phase, the power system network is unsecure and operates close to its secure limits. Although the optimization program of the fuel cost minimization problem gives a non-feasible solution, the reactive power compensation is, in this case, introduced to solve the problem. The power transmission line factor minimization problem is applied after each phase to improve the solution of the problem by minimizing the power flow in the transmissions lines. The reactive power planning and the power transmission line factor minimization problems are solved with a new hybrid technique combining particle swarm optimization with a nonlinear interior point method. Furthermore, the fuel cost minimization problem is solved with the interior point method. The methodology has been tested with the IEEE 57-bus system, and the simulation results show the effectiveness of the proposed approach for improving the reactive power planning problem. Copyright © 2014 John Wiley & Sons, Ltd.

Optimization of Voltage and Reactive Power in Regional Power Network Based on MCR

Along with the increasing use of cables in power grid and the increasing ration of distributed power sources’ synchronization, such as small hydropower’s synchronization, increasing the reactive power transmission on the line, make it difficult to achieve the balance of reactive hierarchical partition. Take a certain region’s power grid for actual examples, after the installation of magnetic control reactor (MCR), using immune genetic algorithm (IGA) to coordinate the capacity of magnetic control reactor and the existing reactive power resources, the results show that the magnetic control reactor does much good to absorb the system’s excessive reactive power and limit the voltage’s increasing.

Smart DER control for minimizing power losses in distribution feeders

Distributed energy resources, such as distributed generators and storage systems, are generally considered as active power sources. Indeed, they can also be used as reactive power resources, which contribute to reduce the power losses in a distribution feeder. Adopting a decentralized approach, in this paper an on-line Optimization Strategy is introduced to define the optimal set point of a classical reactive power control system for distributed energy resources so as to minimize the power losses along the feeder. Using only local measurements, firstly the actual operating conditions of the distribution system are estimated and, then, a constraint minimization problem is solved so as to calculate the optimal set point. A suitable change of coordinates yields to evaluate the optimal solution in an analytic closed form. Numerical simulation results are also presented to give evidence of the reduction of power losses and of the performance of the proposed smart control scheme.

Capability Chart for Distributed Reactive Power Resources

The ubiquity of synchronous generation should not be assumed in highly renewable power systems. Various problems may consequently arise, not least of which are the difficulties entailed in maintaining regional reactive power balance. Offering a potential solution to such problems, modern renewable generator technologies offer controllable reactive power resources. As many of these generators will be embedded in distribution networks, their incorporation into transmission system operational and planning activities appears challenging. An extension of the capability chart concept offers insight here: for a given active power exchange between the transmission system and a distribution network section, the range of controllable reactive power typically available is of interest. This aggregate capability depends on the innate machine capabilities of the distributed generators and on the prevailing conditions within the distribution network. Novel optimisation techniques are useful in addressing the latter point, offering a means to identify the combination of power flow profiles within the distribution system most restrictive to reactive power provision. The capability chart thus derived gives the dependable range of reactive power available, under the assumption that each generator is operated to locally maximize its own reactive power contribution. Such a description can be applied in transmission system planning or to quantify the effects of modifications to the distribution system.

Optimal Control and Operation of Grid-Connected Photovoltaic Production Units for Voltage Support in Medium-Voltage Networks

The strong increase in the number of installed photovoltaic (PV) production plants has slowly changed the assets and the operating conditions of the electricity power system. In this context, the supply and control of the reactive power from the renewable generation plants are becoming an important issue to be exploited that can facilitate the integration of PV and wind power in electricity grids. The aim of this paper is to propose an optimized control strategy to manage the reactive power resource generated by PV plants in order to improve the quality of the low/medium voltage distribution network and meet the latest technical requirements listed by distribution system operators in their grid codes. A methodology to define the optimal reactive power references for each PV unit connected to a generic distribution network is presented for a specified goal (e.g., the achievement of suitable voltage levels at the grid nodes). Such reference signals are then provided to the PV plants regulation systems, which are designed in order to guarantee the decoupling between the active and reactive power channels, due to the use of the Feedback Linearization control technique. The problem is formulated in a generalized way, so that it can be extended to any possible grid configuration.

Optimal Allocation of Different Types of DG Sources Considering the Impact on Power Losses and Voltage in Distribution Networks

As the distribution energies are becoming the future trend to solve the tense fossil fuel supplying and environmental issues, further research on the management of DGs connected to system is necessary. Management of reactive power resources is vital for stable and secure operation of power systems in power losses and voltage quality. Base on this, an optimal power allocation strategy of different types of DG units which result in the minimum line losses and relatively good voltage profile is proposed in this paper.

Decentralized Control of Distributed Generation for Voltage Profile Optimization in Smart Feeders

Distributed generation, which is installed to exploit renewable energy sources, can also be used as a reactive power resource and contribute to tackle the voltage regulation problem in smart distribution grids. Adopting a decentralized approach with off-line coordination, the paper proposes an optimal set-point design for the voltage/reactive power control scheme of distributed generation. The objective is to improve the voltage profile along the feeders of a distribution system. Using only local measurements, the actual operating conditions of the feeder are firstly estimated; then, the set-point is evaluated by solving an optimal voltage profile problem. The off-line coordination avoids any modification to the architecture of existing control systems and requires communication only in the case of significant changes of the distribution system topology. The results of numerical simulations are presented for MV feeders with wind and photovoltaic generations. A comparison with standard control schemes is reported, as well as considerations about the off-line coordination among the distributed generations and the tap-changer of the HV/MV transformer.

Optimal power flow solution of wind integrated power system using modified bacteria foraging algorithm

Owing to the intermittent nature of wind flow, optimal power flow in a power system with considerable wind energy penetration is a challenging issue. In this paper an OPF solution is proposed for IEEE 30 bus power system modified by replacing three conventional generators with equivalent wind energy conversion systems (WECS).The uncertain nature of wind power has the risk of over or under estimating the capacity of available wind power. This uncertainty has been suitably modeled and included in the OPF framework. Moreover, to justify the limitation of reactive power generation capability of doubly fed induction generator based WECS during under estimation, additional cost component corresponding to external reactive power (Q) generating sources has been added in the objective function. The scheduling problem of WECS integrated power system is solved by formulating it as an optimization problem. Genetic algorithm and a modified bacteria foraging algorithm are employed separately to determine the optimal schedule. The optimal solution obtained with a modified version of BFA has been found to give better results compared to GA. The results depict the impact of wind and thermal scheduling on total system cost and reiterate the need of additional support of reactive power resources to maintain stable voltage profiles of the wind–thermal system.

Capability curve based enhanced reactive power control strategy for stability enhancement and network voltage management

Reactive power has become a vital resource in modern electricity networks due to increased penetration of distributed generation. This paper examines the extended reactive power capability of DFIGs to improve network stability and capability to manage network voltage profile during transient faults and dynamic operating conditions. A coordinated reactive power controller is designed by considering the reactive power capabilities of the rotor-side converter (RSC) and the grid-side converter (GSC) of the DFIG in order to maximise the reactive power support from DFIGs. The study has illustrated that, a significant reactive power contribution can be obtained from partially loaded DFIG wind farms for stability enhancement by using the proposed capability curve based reactive power controller; hence DFIG wind farms can function as vital dynamic reactive power resources for power utilities without commissioning additional dynamic reactive power devices. Several network adaptive droop control schemes are also proposed for network voltage management and their performance has been investigated during variable wind conditions. Furthermore, the influence of reactive power capability on network adaptive droop control strategies has been investigated and it has also been shown that enhanced reactive power capability of DFIGs can substantially improve the voltage control performance.

On the inter-area optimal voltage and reactive power control

Scheduling voltage and reactive power is one of the major problems and concerns of the Transmission System Operators (TSOs) especially after deregulation of power market. Due to locally provision of the reactive power resources, each TSO has developed its own specific method. Hence, the voltage and reactive power coordination in the interconnected system has got less attention. In this respect, current practices of different TSOs in Europe and North America, as examples of interconnected power systems, are investigated in this paper. Afterwards, it focuses on the inter-area optimization of the voltage and reactive power. Instead of performing the studies around one particular optimization, different aspects of the mathematical formulation of the optimization problem in the interconnected power system, including objective function, constraints and appropriate modeling of neighboring areas, are revisited more in depth. Various possible implementations of coordinated approaches, including centralized and decentralized structures, as well as non-coordinated approach in collaborative and non-collaborative environments are studied. In this respect, new contributions are proposed in this paper by using the distributed slack bus model and the limitation of the voltage and reactive power in the interconnections links. The comparative analysis between the available and proposed methods are discussed in terms of sub-optimality and time to convergence. The discussions are based on New England 39 bus system and the presented results in the literatures.

Multiple Dispatching Reactive Power and Voltage Coordinated Control Method

This paper studies the reactive power and voltage coordinated control scheme. According to the characteristics of Hunan power grid, the coordinated schemes about Hunan power grid with Central China Power Grid, as well as Changsha power grid are proposed. At the same time, this paper builds a two-way interactive and multiple dispatching reactive power and voltage coordinated control mode, and can be successfully applied in Hunan power grid. The operation results show that this control scheme fulfills the ability of large power grids in optimal allocating of resources, effectively integrates the reactive power resources of the entire grid, achieves the purpose of reducing power grid loss, improving voltage quality, reducing the operating numbers of the reactive power equipment.

Preventive reactive power management for improving voltage stability margin

Voltage stability imposes important limitations on the power systems operation. Adequate voltage stability margin needs to be obtained through the appropriate scheduling of the reactive power resources. The main countermeasures against voltage instability could be distinctly classified into preventive and corrective control actions. This paper proposes a preventive countermeasure to improve the voltage stability margin through the management of the reactive power and its reserve. The voltage and reactive power management is studied from the generator's point of view to maximize effective generator reactive power reserve (EGRPR). Detailed model of the generators including the armature and field current limits, as well as the switch mode between the voltage control and the reactive power limitations are considered to maximize the reactive power capability of the generators in emergency states. One-stage and two-stage optimization approaches are utilized to find the optimum solution. The proposed optimization procedure is applied on a 6-bus system and the New England 39-bus system to illustrate the effectiveness of the method.

Voltage/VAR Control in Distribution Networks via Reactive Power Injection Through Distributed Generators

This paper demonstrates how reactive power injection from distributed generators can be used to mitigate the voltage/VAR control problem of a distribution network. Firstly, power flow equations are formulated with arbitrarily located distributed generators in the network. Since reactive power injection is limited by economic viability and power electronics interface, we formulate voltage/VAR control as a constrained optimization problem. The formulation aims to minimize the combined reactive power injection by distributed generators, with constraints on: 1) power flow equations; 2) voltage regulation; 3) phase imbalance correction; and 4) maximum and minimum reactive power injection. The formulation is a nonconvex problem thereby making the search for an optimal solution extremely complex. So, a suboptimal approach is proposed based on methods of sequential convex programming (SCP). Comparing our suboptimal approach with the optimal solution obtained from branch and bound method, we show the trade-off in quality of our solution with runtime. We also validate our approach on the IEEE 123 node test feeder and illustrate the efficacy of using distributed generators as distributed reactive power resource.

Optimal reactive power dispatch for improving voltage stability margin using a local voltage stability index

Management of reactive power resources is vital for stable and secure operation of power systems in the view point of voltage stability. This paper deals with the management of on-load tap changers (OLTCs) and dynamic VAR sources (including synchronous generators, synchronous condensers, and shunt reactive power compensators) to improve voltage stability margin (VSM) of power systems. This problem is usually called optimal reactive power dispatch (ORD) in the literature. The main contribution of the paper is to introduce a new objective function for the ORD problem. The proposed objective function is derived based on a local voltage stability index, called DSY, and has a strong correlation with VSM. This strong correlation makes the objective function effective for improving VSM, which is the main purpose of ORD. The proposed objective function is tested on the New England 39-bus test system and its performance is compared with some of the most common objective functions used in ORD. The obtained results show that solving ORD problem using the proposed objective function yields considerable increase in VSM.

An Authenticated Control Framework for Distributed Voltage Support on the Smart Grid

Existing and forthcoming devices at the residential level have the ability to provide reactive power support. Inverters which connect distributed generation such as solar panels and pluggable hybrid electric vehicles (PHEVs) to the grid are an example. Such devices are not currently utilized by the power system. We investigate the integration of these end-user reactive-power-capable devices to provide voltage support to the grid via a secure communications infrastructure. We determine effective locations in the transmission system and show how reactive power resources connected at those buses can be controlled. Buses belong to reactive support groups which parallel the regions of the secure communications architecture that is presented. Ultimately, our goal is to present how the smart grid can enable the utilization of available end-user devices as a resource to mitigate power system problems such as voltage collapse.

A Study on Local Coordinative Reactive Power Control between STATCOM and Other Reactive Power Controllers for Voltage Stability Improvement at Substation

Installation of FACTS(Flexible AC Transmission System) device can maximize utilization of existing power facilities and reliability of power system. STATCOM has excellent characteristics in operating cost, maintaining facilities, loss and so on. However, STATCOM has a disadvantage of facility cost over capacity. So it is effective to coordinate STATCOM and existing external bank(capacitor and reactor) and OLTC(On Load Tap Changer). This paper mainly proposes coordinative control method between STATCOM installed within substation and other reactive power resources including Shunt Reactors and Shunt Capacitors and OLTC for voltage stability improvement. The proposed coordinative control method is developed for the STATCOM of Mi-gum substation and the simulation results of EMTP/RV model show the effectiveness of the proposed method.

Electric Power System Under-Voltage Load Shedding Protection Can Become a Trap

Problem statement: Under-Voltage Load Shedding (UVLS) protection of Electric Power Systems (EPS) is frequently used against Voltage Collapse (VC), however when there is automatic bus voltage regulation with excessive capacitive compensation, the UVLS scheme may not trip. In this case, load shedding must be based in a Voltage Collapse Proximity Indicator (VCPI). Many UVLS procedures may not be appropriate today. Approach: In order to elucidate the problem stated, several studies were carried out using MatLab/SimPowerSystems. In the first case, it was simulated a reduced electric system consisting of an infinite-bus feeding a load through a large impedance line. Two other cases were simulated now including a fixed capacitive impedance (representing a saturated SVC or similar) with 25 and 60 MVAr, both with a generator regulating the load bus voltage. Graphic curves representing the load bus voltage versus time were obtained with the application of a ramp power load. Results: In all cases the curves showed if there was sufficient time to command the UVLS scheme. The usual UVLS criteria failed for the third case. As the capacitive reactive power of the saturated compensation devices was increased, their equivalent capacitance, corresponding to the sum of maximum MVAr capacities, grows. The load demand increase, after MVAr saturation, can cause a voltage decrement which is too fast for UVLS adequate operation. Conclusion/Recommendations: Based in past experiences, any operator could be confident on existing UVLS protection of some area, but a VC can occur with the current situation without UVLS trip, as stated. It was suggested to check the current UVLS operation conditions, especially in areas where there was a growth of both load demand and reactive power resources. When UVLS method is found ineffective, then a suggestion is to replace it by a technique based upon some VCPI.