Capacity Margin Research Articles

Transmission capacity margin in market clearing

To ensure the secure operation of a power market, transmission networks must maintain an appropriate margin to account for various uncertainties. The uncertainties have increased significantly by the increasing penetration of renewable energy generation. A steady-state security distance (SSD) is an effective security margin that describes the distance of an operation point (OP) from steady-state security region boundaries. It captures both the available capacity and topological location of each transmission line that are important considerations for the ability of each line to withstand uncertainties. The proposed SSD-based transmission capacity margin is integrated into the market clearing model. The proposed SSD-constrained market clearing (SSDC-MC) model ensures that the SSD between the scheduled OP and security boundaries does not fall below a specified threshold. The SSDC-MC is formulated as a bi-level optimization problem. A novel algorithm is developed by dynamically identifying the binding constraints in the lower-level model. The SSDs are expressed as piecewise analytic functions of the scheduled OP. As a result, the bi-level problem is transformed into a single-level one. Two test cases are used to compare the SSDC-MC method with a different transmission margin determination method, which imposes constraints on loading of all transmission lines.

High efficient valley-filling strategy for centralized coordinated charging of large-scale electric vehicles

With the rapid development of electric vehicles (EVs), the impacts on power grids arising from EV charging have drawn increasing attention worldwide. Uncoordinated charging of large-scale EVs will inevitably elevate load peaks at rush hours, therefore, poses serious challenges to the stability and security of power grid. Coordinated charging is expected to alleviate these negative impacts by utilizing the surplus power in the lower-demand hours with the help of the so-called valley filling algorithm. Nevertheless, when the amount of EVs involved reaches 1 million or above, the complexity of the scheduling method becomes a critical issue. In this paper, a very high efficient valley-filling strategy is proposed. Two key indexes, viz., capacity margin index and charging priority index are defined, the former one is used to select the target time slots on which the power grid has abundant surplus power for EV charging, and the latter one is used to determine the charging priority of EVs on each time slot. The simulation results demonstrate that the coordinated charging scheme with the proposed valley-filling strategy significantly outperforms the uncoordinated charging in the aspect of suppressing the elevated peak loads of power grid. Moreover, the complexity analysis shows that the proposed algorithm is much high-efficient than its existing counterparts.

Coordinated Virtual Inertia Control Strategy for D-PMSG Considering Frequency Regulation Ability

In the process of virtual inertia control (VIC), the frequency regulation capability of the directly-driven wind turbine with permanent-magnet synchronous generator (D-PMSG) on wind farm is related to its rotor kinetic energy and capacity margin. This paper proposes the method for assessing the D-PMSG frequency regulation capability and defining its coefficient according to the operating state of wind power generators. In addition, the calculating method of parameters in VIC is also discussed according to the principles of primary frequency regulation and inertia response of synchronous generators. Then, by introducing the capability coefficient into the proportion-differential virtual inertia control (PD-VIC) for power coordination, a coordinated virtual inertia control (C-VIC) strategy is developed, with the consideration of the difference in frequency regulation capability between wind power generators. The proposed control method can not only give full play to the frequency regulation capability of wind power generators, decrease the movements of the pitch angle control system but also bring some self-coordination capability to different wind power generators thus to avoid a secondary drop in system frequency. The simulations and experiments prove the proposed method to be effective and practicable.

Margin Credit and Stock Return Predictability

Margin capacity, defined as the aggregate excess debt capacity of investors buying securities on margin, strongly predicts (a) lower S&P 500 returns, (b) lower growth in aggregate earnings, dividends, employment, and overall economic activity, (c) higher macro, financial, and policy uncertainty, (d) lower interest rates, (e) tighter lending standards by banks, and (f) lower intermediary equity capital. High margin capacity is a precursor, not a response, to borrowing and intermediary constraints and higher volatility. It typically arises when levered investors with profitable past positions limit their leverage. We interpret that it reflects informed investors' conservatism ahead of bad times.

Dynamic Voltage and Current Assignment Strategies of Nine-Switch-Converter-Based DFIG Wind Power System for Low-Voltage Ride-Through (LVRT) Under Symmetrical Grid Voltage Dip

This paper proposes the dynamic voltage and current assignment (DVCA) strategies for doubly fed induction generator (DFIG) wind power system, which is fed by nine-switch converter (NSC). With the proposed DVCA strategies, the voltage and current capacities of switches in NSC are dynamically assigned to the rotor- and grid-side branches, depending on different grid conditions. With dynamic voltage assignment, the rotor-side gets more voltage to suppress the overcurrent when the symmetrical grid voltage dips, and with dynamic current assignment, the grid-side gets more current capacity for reactive current compensation. Based on the proposed DVCA strategies and configuration, NSC-based DFIG system can be globally optimized, so that the rating capacities designed in the normal condition can also be fully used during low-voltage ride-through (LVRT) period, and no extra capacity margins or auxiliary circuits are needed. The proposed DVCA strategies and global optimization are designed, and also compared with those typical B2B-based DFIG systems. The experimental results from a 1.5-kW NSC-based DFIG system verify the validity of the analysis.

Economic hazards of a forced energy transition: inferences from the UK’s renewable energy and climate strategy

The UK government has recently announced a reorientation of its energy and climate policy, scaling back subsidies to renewables, suggesting that uncontrollable generators, such as wind may be required to meet their own system costs, and emphasizing the need for research and development towards an as yet undiscovered, fundamentally economic, low carbon transition. The government also aims to open the way for nuclear power, and the maximization of oil and gas recovery both from the North Sea, and, on-shore, from hydraulic fracking. The present authors argue that although this policy is self-characterised as a re-liberalisation of the markets, the revision is only in part political, and is better understood as a force majeure response to cost and technical problems with the previous renewables-centred policy. Specifically, subsidies have led to an overheated renewables sector with high costs that will exceed Treasury limits and place heavy burdens on consumers. Subsidies to renewables have also weakened investment signals to conventional generation, leading to low capacity margins that necessitate a costly Capacity Mechanism, in effect a subsidy, to guarantee security of supply. Taken together, these costs are significant, and are a matter for particular concern, since there are already signs of a trend towards a de-electrification of the UK economy, a trend which is undesirable for many reasons, including climate policy.

Multi‐objective control of multi‐functional grid‐connected inverter for renewable energy integration and power quality service

Multi-functional grid-connected inverters (MFGCIs) not only interface renewable energy sources into the utility, but also provide ancillary power quality enhancement service. Therefore, extra installment of power quality conditioners can be partially avoided in a micro-grid including MFGCIs. Because the capacity of an MFGCI employed for power quality compensation is limited, how to balance the multiple functions and optimally utilise the limited capacity becomes a challenging for MFGCI application, and this is studied in details in this paper. First, to set up a benchmark for balancing the multiple functions of the MFGCI, a comprehensive power quality evaluation (CPQE) index is presented based on the catastrophe decision theory to quantify the power quality of a micro-grid. Then, for the strategic utilisation of the limited capacity, a multi-objective optimal compensation model is proposed in which the objectives are to optimise the CPQE index and minimise the occupied capacity of an MFGCI for power quality compensation. Finally, the solutions of the model are derived on the basis of Pareto approach. As a result, the MFGCI can flexibly customise the power quality of the micro-grid according to its available capacity margin and the users’ requirement. Finally, the experimental results performed on a 10 kVA MFGCI prototype have confirmed the validity of the proposed model.

Developing an optimal electricity generation mix for the UK 2050 future

The UK electricity sector is undergoing a transition driven by domestic and regional climate change and environmental policies. Aging electricity generating infrastructure is set to affect capacity margins after 2015. These developments, coupled with the increased proportion of inflexible and variable generation technologies will impact on the security of electricity supply. Investment in low-carbon technologies is central to the UK meeting its energy policy objectives. The complexity of these challenges over the future development of the UK electricity generation sector has motivated this study which aims to develop a policy-informed optimal electricity generation scenario to assess the sector's transition to 2050. The study analyses the level of deployment of electricity generating technologies in line with the 80% by 2050 emission target. This is achieved by using an excel-based “Energy Optimisation Calculator” which captures the interaction of various inputs to produce a least-cost generation mix. The key results focus on the least-cost electricity generation portfolio, emission intensity, and total investment required to assemble a sustainable electricity generation mix. A carbon neutral electricity sector is feasible if low-carbon technologies are deployed on a large scale. This requires a robust policy framework that supports the development and deployment of mature and emerging technologies.

Economies of scale and scope in expansion of the U.S. natural gas pipeline network

I analyze cost, capacity, mileage, and technical data for 254 U.S. natural gas pipeline projects over the period 1997–2012. Although project costs exhibit economies of scale over the capacity margin and economies of scope over the spatial margin, network expansion costs may not exhibit cost economies overall. That is, proportional increases in both transmission capacity and length (in miles) may result in a proportional (or even greater-than-proportional) increase in expansion costs. Moreover, large projects (high-capacity pipelines spanning long distances) likely require installation of compression horsepower, which has direct cost effects. My results suggest such projects exhibit significant diseconomies in cost structure. As a result, pipeline tariffs based on cost-of-service pricing likely present a disincentive for prospective pipeline customers to commit to long-term contracts—which are necessary for the pipeline to acquire regulatory permission to build—particularly for large, long-distance expansion projects. The implication is that cost-of-service pricing may inhibit network expansion, exacerbating congestion issues.

Considerations for a Framework for Specification and Measurement of Reserve Capacity in Software‐Intensive Systems

AbstractFor some classes of software‐intensive systems, especially long life‐cycle embedded systems with evolving requirements, the customer needs some margin of reserve data processing capacity in the delivered system. This is typically needed to assure system responsiveness and stability under transient loading conditions and to provide for future software maintenance, capability growth and changes in usage without the need for processing hardware upgrades. The specification, management and verification of reserve data processing resources so as to achieve this are challenging and there is a lack of universally agreed or standardised approaches. It is proposed to create a framework for characterising run‐time resource utilisation in a data processing system, such that effective reserve capacity can be described and measured in ways that are well defined but not design‐specific. This paper outlines the need for and desirable attributes of such a framework.

Hybrid simulation of bridge pier uplifting

Substructured Pseudodynamic testing is used to study the transverse seismic response of a bridge with box-girder deck monolithically connected to circular piers, but free to move laterally at the abutments. The tests are of two types, each one with two variants: with the pier footing rocking on elastic soil and uplifting from it, or fixed to rigid ground. The first type concerns a two-span deck connected monolithically to a central pier and supported at each end on the abutment through a pair of elastomeric bearings, which are stiff and strong in compression but very soft and weak in tension. The second one concerns the interior piers of multi-span bridges having all spans equal and all piers similar. The pier was physically tested at 1:2 scale; everything else was numerically simulated online with Opensees. The deck, the soil and the bearings were modeled as elastic, but not allowing tension to develop at the soil-footing interface, and the bearings two orders of magnitude more flexible in tension than in compression. In the first type of tests, at a peak ground acceleration (PGA) of 0.15 g, the footing uplifted, the bearings developed tension, but the pier stayed elastic. In the second type of tests, with a PGA of 0.4 g, uplifting of the footing did not prevent plastic hinging at the pier base, but reduced significantly the pier damage. A cyclic quasi-static test in the end shows the deformation capacity margins of the pier.

Collapse response assessment of low-rise buildings with irregularities in plan

The present paper aims at evaluating damage and collapse behavior of low-rise buildings with unidirectional mass irregularities in plan (torsional buildings). In previous earthquake events, such buildings have been exposed to extensive damages and even total collapse in some cases. To investigate the performance and collapse behavior of such buildings from probabilistic points of view, three-dimensional three and six-story reinforced concrete models with unidirectional mass eccentricities ranging from 0% to 30% and designed with modern seismic design code provisions specific to intermediate ductility class were subjected to nonlinear static as well as extensive nonlinear incremental dynamic analysis (IDA) under a set of far-field real ground motions containing 21 two-component records. Performance of each model was then examined by means of calculating conventional seismic design parameters including the response reduction (R), structural overstrength (<TEX>${\Omega}$</TEX>) and structural ductility (<TEX>${\mu}$</TEX>) factors, calculation of probability distribution of maximum inter-story drift responses in two orthogonal directions and calculation collapse margin ratio (CMR) as an indicator of performance. Results demonstrate that substantial differences exist between the behavior of regular and irregular buildings in terms of lateral load capacity and collapse margin ratio. Also, results indicate that current seismic design parameters could be non-conservative for buildings with high levels of plan eccentricity and such structures do not meet the target "life safety" performance level based on safety margin against collapse. The adverse effects of plan irregularity on collapse safety of structures are more pronounced as the number of stories increases.

A Convex Model of Risk-Based Unit Commitment for Day-Ahead Market Clearing Considering Wind Power Uncertainty

The integration of wind power requires the power system to be sufficiently flexible to accommodate its forecast errors. In the market clearing process, the scheduling of flexibility relies on the manner in which the wind power uncertainty is addressed in the unit commitment (UC) model. This paper presents a novel risk-based day-ahead unit commitment (RUC) model that considers the risks of the loss of load, wind curtailment and branch overflow caused by wind power uncertainty. These risks are formulated in detail using the probabilistic distributions of wind power probabilistic forecast and are considered in both the objective functions and the constraints. The RUC model is shown to be convex and is transformed into a mixed integer linear programming (MILP) problem using relaxation and piecewise linearization. The proposed RUC model is tested using a three-bus system and an IEEE RTS79 system with wind power integration. The results show that the model can dynamically schedule the spinning reserves and hold the transmission capacity margins according to the uncertainty of the wind power. A comparison between the results of the RUC, a deterministic UC and two scenario-based UC models shows that the risk modeling facilitates a strategic market clearing procedure with a reasonable computational expense.

Available Transmission Capacity Analysis for the Receiving-End Grid of UHV-DC Transmission System

UHV-DC transmission system presents certain adverse reactive characteristics to its receiving-end grid. Based on the approach of P-V curve and V-Q curve, this paper presents a method of voltage stability analysis for the receiving-end grid of UHV-DC transmission system, which gives its available transmission capacity (ATC), voltage limit and reactive power margin, with the consideration of the normal, N-1 and N-2 operation situations and the situation of the commutation failure. Different with traditional ATC analysis, the characteristics of UHV-DC transmission system is included here. With the real UHV-DC project of ±800kV HaZheng in China, the effectiveness and efficiency of the proposed mothed are verified.

Experimental study on the flexural behavior of stone beams strengthened with a combination of angle steels and PET belts

This paper focuses on a novel strengthening method for stone beams. An experimental study on the flexural behavior of six full scale stone beams, including three beams unstrengthened and three beams strengthened with a combination of angle steels and PET (polyethylene terephthalate plastic) belts, was performed. The failure modes, flexural capacity, deformability, and influence of different types and dimensions of stones, along with the different specifications of angle steels on the flexural behavior, were analyzed. The experimental results indicated that: unstrengthened greenish white marble beams exhibited a typical brittle failure mode, and unstrengthened white marble beams with better deformation properties exhibited a non-fully brittle failure mode; each of the strengthened stone beams exhibited a non-fully brittle failure mode. Furthermore, the flexural capacity and deformation capacity of the strengthened stone beams were improved to some extent. After the ultimate load, strengthened stone beams still had a certain flexural capacity, namely the flexural capacity margin, because of the action of angle steels. A theoretical analysis of the flexural capacity of stone beams was conducted, and a simplified calculation formula of the ultimate flexural capacity and flexural capacity margin was proposed.

On the energy cost of robustness and resiliency in IP networks

Despite the growing concern for the energy consumption of the Internet, green strategies for network and traffic management cannot undermine Quality of Service (QoS) and network survivability. In particular, two very important issues that may be affected by green networking techniques are resilience to node and link failures, and robustness to traffic variations.In this paper, we study how achieving different levels of resiliency and robustness impacts the network energy-aware efficiency. We propose novel optimization models to minimize the energy consumption of IP networks that explicitly guarantee network survivability to failures and robustness to traffic variations. Energy consumption is reduced by putting in sleep mode idle line cards and nodes according to traffic variations in different periods of the day. To guarantee network survivability we consider two different schemes, dedicated and shared protection, which assign a backup path to each traffic demand and some spare capacity on the links along the path. Robustness to traffic variations is provided by tuning the capacity margin on active links in order to accommodate load variations of different magnitude. Furthermore, we impose some inter-period constraints to guarantee network stability and preserve device lifetime. Both exact and heuristic methods are proposed.Experimentations carried out on realistic networks operated with flow-based routing protocols (like MPLS) allow us to quantitatively analyze the trade-off between energy cost and level of protection and robustness. Results show that significant savings, up to 30%, may be achieved even when both survivability and robustness are fully guaranteed, both with exact and heuristic approaches.

Planning scheduling strategies to suppress the magnitudes of fault currents

With traditional circuit breakers, the interrupter capacity margin is insufficient. To increase interrupting capacity, the circuit breaker can be upgraded or a device can be installed to suppress the fault current, thereby correcting the small fault current margin. In this paper, the power dispatch method is used to suppress the fault current. Simulations are performed and summarized to show the best scheduling policy for increasing the breaker capacity margin to allow for safe operation of the system. Scheduling policies take advantage of existing equipment, so there is no need for any additional purchases for the system. The proposed method will be able to significantly reduce the fault current directly and effectively compared to traditional practices.

Effects of renewables penetration on the security of Portuguese electricity supply

The increase of renewables in power sector, together with the increase of their electricity share in final energy consumption, is changing our perception about energy security with diverse and contradictory statements. The Portuguese security of electricity supply is analyzed in this study by comparing selected indicators for 2years before and after the high increase of onshore wind since 2005. Our goal is to find how the security of electricity supply was impacted by the penetration of renewables, taking a supply chain approach. Our analysis highlights that the penetration of renewables has decreased the energy dependence of the power sector by more than 20% between 2004 and 2011, while risks related to the concentration of natural gas suppliers and to the still-high share of fossil fuels suffering from price volatility are discussed. We observed a significant improvement in power interconnections with Spain, as well as an increase of the de-rated generation capacity margin, allowing proper management of renewable power intermittency if necessary, thereby improving power security. Although the share of intermittent renewables almost quadrupled in total installed capacity between those years, the indicators reveal an improvement in the quality of transport and distribution when delivering electricity to end-users. Although electricity prices increased, mainly due to taxes, the lack of energy efficiency is an aspect deserving improvement to alleviate the pressure on electricity security, mainly at high peak demands.

Applicable Indices to Connecting Industrial Park Load and Renewable Sources to a Local Power System

This paper presents the applicable methods to connect industrial park load and renewable energy to a small-scaled power system. The connecting procedure consists of three items namely Optimal Order and Domain-Link Method and Weakness Degree in Energy Supply (WDES) as well. The optimal order makes dispersed hydropower and thermal power first developed to improve P-V nodes and capacity of small power system. The domain-link method can selects more stable buses that have multi-branch of power supply in order to connect industrial park and renewable energy. WDES shows weakness information of each bus to avoid connecting industrial park load and renewable energy to unstable buses in practical operation. To testify the operational characteristics, the system power flow is used to indicate the margin of buses voltage and carrying capacity of power lines after introducing industrial park and renewable energy. To elaborate on the application process in detail, this paper takes the several diagrams and tables as the theoretical basis and makes the all connection process more systematic.

The need to maintain quality and build capacity for emergency medicine training

The need to maintain quality and build capacity for emergency medicine training