A phase-locked-loop design for the smooth operation of a hybrid microgrid

A microgrid contains both distributed generators (DGs) and loads and can be viewed by a controllable load by utilities. The DGs can be either inertial synchronous generators or non-inertial converter interfaced. Moreover, some of them can come online or go offline in plug and play fashion. The combination of these various types of operation makes the microgrid control a challenging task, especially when the microgrid operates in an autonomous mode. In this paper, a new phase locked loop (PLL) algorithm is proposed for smooth synchronization of plug and play DGs. A frequency droop for power sharing is used and a pseudo inertia has been introduced to non-inertial DGs in order to match their response with inertial DGs. The proposed strategy is validated through PSCAD simulation studies.

Primary control level of parallel distributed energy resources converters in system of multiple interconnected autonomous microgrids within self-healing networks

To minimise the number of load sheddings in a microgrid (MG) during autonomous operation, islanded neighbour MGs can be interconnected if they are on a self-healing network and an extra generation capacity is available in the distributed energy resources (DER) of one of the MGs. In this way, the total load in the system of interconnected MGs can be shared by all the DERs within those MGs. However, for this purpose, carefully designed self-healing and supply restoration control algorithm, protection systems and communication infrastructure are required at the network and MG levels. In this study, first, a hierarchical control structure is discussed for interconnecting the neighbour autonomous MGs where the introduced primary control level is the main focus of this study. Through the developed primary control level, this study demonstrates how the parallel DERs in the system of multiple interconnected autonomous MGs can properly share the load of the system. This controller is designed such that the converter-interfaced DERs operate in a voltage-controlled mode following a decentralised power sharing algorithm based on droop control. DER converters are controlled based on a per-phase technique instead of a conventional direct-quadratic transformation technique. In addition, linear quadratic regulator-based state feedback controllers, which are more stable than conventional proportional integrator controllers, are utilised to prevent instability and weak dynamic performances of the DERs when autonomous MGs are interconnected. The efficacy of the primary control level of the DERs in the system of multiple interconnected autonomous MGs is validated through the PSCAD/EMTDC simulations considering detailed dynamic models of DERs and converters.

Interconnected autonomous microgrids in smart grids with self-healing

In order to minimize the number of load shedding in a Microgrid during autonomous operation, islanded neighbour microgrids can be interconnected if they are on a self-healing network and an extra generation capacity is available in Distributed Energy Resources (DER) in one of the microgrids. In this way, the total load in the system of interconnected microgrids can be shared by all the DERs within these microgrids. However, for this purpose, carefully designed self-healing and supply restoration control algorithm, protection systems and communication infrastructure are required at the network and microgrid levels. In this chapter, first a hierarchical control structure is discussed for interconnecting the neighbour autonomous microgrids where the introduced primary control level is the main focus. Through the developed primary control level, it demonstrates how the parallel DERs in the system of multiple interconnected autonomous microgrids can properly share the load in the system. This controller is designed such that the converter-interfaced DERs operate in a voltage-controlled mode following a decentralized power sharing algorithm based on droop control. The switching in the converters is controlled using a linear quadratic regulator based state feedback which is more stable than conventional proportional integrator controllers and this prevents instability among parallel DERs when two microgrids are interconnected. The efficacy of the primary control level of DERs in the system of multiple interconnected autonomous microgrids is validated through simulations considering detailed dynamic models of DERs and converters.

Diode-clamped three-level inverter-based battery/supercapacitor direct integration scheme for renewable energy systems

This paper describes a diode-clamped three-level inverter-based battery/supercapacitor direct integration scheme for renewable energy systems. The study is carried out for three different cases. In the first case, one of the two dc-link capacitors of the inverter is replaced by a battery bank and the other by a supercapacitor bank. In the second case, dc-link capacitors are replaced by two battery banks. In the third case, ordinary dc-link capacitors are replaced by two supercapacitor banks. The first system is supposed to mitigate both long-term and short-term power fluctuations while the last two systems are intended for smoothening long-term and short-term power fluctuations, respectively. These topologies eliminate the need for interfacing dc-dc converters and thus considerably improve the overall system efficiency. The major issue in aforementioned systems is the unavoidable imbalance in dc-link voltages. An analysis on the effects of unbalance and a space vector modulation method, which can produce undistorted current even in the presence of such unbalances, are presented in this paper. Furthermore, small vector selection-based power sharing and state of charge balancing techniques are proposed. Experimental results, obtained from a laboratory prototype, are presented to verify the efficacy of the proposed modulation and control techniques.

Battery clamped three-level inverter for renewable energy systems

This paper presents a novel battery direct integration scheme for renewable energy systems. The idea is to replace ordinary capacitors of a three-level flying-capacitor inverter by three battery banks to alleviate power fluctuations in renewable generation. This approach eliminates the need for interfacing dc-dc converters and thus considerably improves the overall efficiency. However, the major problem with this approach is the uneven distribution of space vectors which is due to unavoidable unbalance in clamping voltages. A detailed analysis on the effects of this issue and a novel carrier based pulse width modulation method, which can generate undistorted currents even in the presence of unevenly distributed space vectors, are presented in this paper. A charge/discharge controller is also proposed for power sharing and state of charge balancing of battery banks. Simulation results are presented to verify the efficacy of the proposed system, modulation method and power sharing controller.

A direct integration scheme for battery-supercapacitor hybrid energy storage systems with the use of grid side inverter

Battery-supercapacitor hybrid energy storage systems are becoming popular in the renewable energy sector due to their improved power and energy performances. These hybrid systems require separate dc-dc converters, or at least one dc-dc converter for the supercapacitor bank, to connect them to the dc-link of the grid interfacing inverter. These additional dc-dc converters increase power losses, complexity and cost. Therefore, possibility of their direct connection is investigated in this paper. The inverter system used in this study is formed by cascading two 3-level inverters, named as the “main inverter” and the “auxiliary inverter”, through a coupling transformer. In the test system the main inverter is connected with the rectified output of a wind generator while the auxiliary inverter is directly attached to a battery and a supercapacitor bank. The major issues with this approach are the dynamic changes in dc-link voltages and inevitable imbalances in the auxiliary inverter voltages, which results in unevenly distributed space vectors. A modified SVM technique is proposed to solve this issue. A PWM based time sharing method is proposed for power sharing between the battery and the supercapacitor. Simulation results are presented to verify the efficacy of the proposed modulation and control techniques.

A modular matrix converter for transformer-less PMSG wind generation systems

An offshore wind turbine usually has the grid step-up transformer integrated in the nacelle. This increases mechanical loading of the tower. In that context, a transformer-less, high voltage, highly-reliable and compact converter system for nacelle installation would be an attractive solution for large offshore wind turbines. This paper, therefore, presents a transformer-less grid integration topology for PMSG based large wind turbine generator systems using modular matrix converters. Each matrix converter module is fed from three generator coils of the PMSG which are phase shifted by 120°. Outputs of matrix converter modules are connected in series to increase the output voltage and thus eliminate the need of a coupling step-up transformer. Moreover, dc-link capacitors found in conventional back-to-back converter topologies are eliminated in the proposed system. Proper multilevel output voltage generation and power sharing between converter modules are achieved through an advanced switching strategy. Simulation results are presented to validate the proposed modular matrix converter system, modulation method and control techniques.

Connecting two wind turbine generators to the grid using only one three level NPC inverter

This paper explores the possibility of connecting two Wind Turbine Generators (WTG) to the grid using a single three level inverter. In the proposed system the rectified output of one WTG is connected across the upper dc-link capacitor of a standard diode clamped three level inverter. Similarly the rectified output of the other WTG is connected across the lower capacitor. This particular combination has several advantages such as, direct connection to the grid, reduced parts count, improved reliability and high power capacity. However, the major problem in the proposed system is the imminent imbalance of dc-link voltages. Under such conditions conventional modulation methods fail to produce desired voltage and current waveforms. A detailed analysis on this issue and a novel space vector modulation method, as the solution, are proposed in this paper. To track the Maximum power point of each WTG a power sharing algorithm is proposed. Simulation results are presented to attest the efficacy of the proposed system.

A supervisory load-leveling approach to improve the voltage profile in distribution network

This paper suggests a supervisory control for storage units to provide load leveling in distribution networks. This approach coordinates storage units to charge during high generation and discharge during peak load times, while utilized to improve the network voltage profile indirectly. The aim of this control strategy is to establish power sharing on a pro rata basis for storage units. As a case study, a practical distribution network with 30 buses is simulated and the results are provided.

Christian-Muslim Dialogue : Perspectives of Four Lebanese Thinkers

This study describes and analyses two Lebanese Muslims and two Lebanese Christians ideas about Christian-Muslim dialogue, its nature, aims, and methods and its different dimensions, which include doctrinal, ethical, and social dimensions. On the basis of the analysis, the four thinkers contributions for promoting constructive dialogue are evaluated. The persons studied are two religious authorities, the Shiite Great Ayatollah Muhammad Husayn Fadlallah (b. 1935) and the Eastern Orthodox Metropolitan of Mount Lebanon, Georges Khodr (b. 1923), and two academic scholars, Doctor Mahmoud Ayoub (b. 1935) and Doctor, Father Mouchir Aoun (b. 1964), from the Shiite and Greek Catholic communities, respectively. The method of the study is systematic analysis. The sources consist of the four thinkers writings on Christian-Muslim relations, the most of which have been published in Lebanon in the 1990s and 2000s in the Arabic language. In their general guidelines for Christian-Muslim dialogue, the four authors do not offer any novel or unusual insights. However, their dialogue visions are multi-faceted, motivating interreligious encounter both on religious and practical grounds and clarifying the theological grounds and socio-political conditions of this endeavour. The major challenge appears to be the tension between loyalty to one s own convictions and taking into account the particular self-understanding of the other. While this tension may be ultimately unsolvable, it is obvious that linking dialogue tightly to missionary motivations or certain theological agenda imposed on the others is not conducive for better mutual understanding. As for how diverse theologies of religions affect interreligious dialogue, narrow exclusivism hardly promotes mutual knowledge and appreciation, but also inclusive and pluralistic positions have their particular dilemmas. In the end, dialogue is possible from diverse positions on theology of religions. All the authors discuss the theological themes of divine revelation, concept of God, and human condition and ultimate destiny. The two religions particular views on these issues cannot be reconciled, but the authors offer diverse means to facilitate mutual understanding on them, such as increasing mutual knowledge, questioning certain traditional condemnations, showing theological parallels between the two religions, and transcending doctrinal disagreements by stressing common religious experience or ethical concerns. Among the theological themes, especially the concept of God seems to offer possibilities for better understanding than has traditionally been the case. Significantly, all the four authors maintain that Christians and Muslims share the faith in the one God, irrespective of their disagreements about the nature of his oneness. Basic ethical principles are not discussed as widely by the four authors as might be expected, which may reflect the shared cultural background and common ethical values of the Lebanese Muslims and Christians. On this level, Christians alienation from the Islamic law appears as the most significant challenge to mutual understanding, while neighbourly love and the golden rule of ethics offer a fruitful basis for further dialogue. As for the issue of political power-sharing in Lebanon, it is clear that the proposal of an Islamic state is problematic in a country with a sizable Christian minority and a heterogeneous Muslim population. Some form of democracy seems more viable for a multireligious country, but the question remains how to retain religion as a vital force in society, which is felt to be important by all the four Lebanese authors.

企业组织领导参与决策行为研究

In the processes of Chinese economic and political reforms, making decision democratically and scientifically is one of the most important problems for the managers. Through the investigations of 319 managers from 13 companies in 3 cities of China, the participation or the behavior of influence and power sharing in managerial decision-making was systematically analyzed. The research was concerned with three aspects: (1) the descriptive study of managerial decision-making; (2) the relationship between managerial decision-making and a set of specified contingent situational factors; (3) the relationship between managerial decision-making and various outcome variables, such as job satisfaction and organizational effectiveness. The principal results of the research showed. 1. The managers used different methods of decision-making in different situations. The wildly used method by the managers was "prior consultation with subordinate". 2. Compared with the developed countries, Chinese managers tended to use more centralized methods of decision-making. In the comparisons among the different districts of China, the managers in the districts of the higher level economic and cultural development tended to use more participative methods in the processes of decision-making. 3. The behavior of managerial decision-making was influenced by the various contingent factors, such as the uncertainties of the environments, the job constraints, and the variables related to persons, etc. 4. The behavior of the managerial decision-making correlated significantly with the job satisfaction and organizational effectiveness. The more influence and power managers owned in the processes of decision-making, the more positive they evaluated their job satisfaction and organizational effectiveness.

Consociationalism and its critics: Evidence from the historic Northern Ireland Assembly Election of 2007:

Critics of consociational power-sharing institutional arrangements in deeply divided societies argue that such arrangements solidify the underlying conflict cleavage and render it all-important for party competition and voter behaviour. I find evidence to the contrary in the case of voter behaviour at the historic 2007 Assembly election in Northern Ireland. At least in the unionist bloc, I find the effective disappearance of the ethno-national conflict cleavage as a determinant of voter choice. This suggests that consociational arrangements have led to both inclusion and moderation, rather than polarisation and ‘ethnic outbidding’

Northern Ireland: The Politics of Constraint

The article surveys the interrupted experience of devolution in Northern Ireland since 1999 and draws a number of comparisons between the first devolved Assembly and Executive and their successors elected in 2007. It underlines the significance of the changed political, electoral and paramilitary context in the period leading up to the 2007 Assembly election which, together with a number of procedural changes effected by the 2006 St Andrews Agreement, enabled the restoration of power-sharing devolution to occur. Against the background of its legislative and policy record and the wider altered state of Northern Ireland, it concludes that the contrived consociational model of governance can work, up to a point, but perhaps as much because of the politics of constraint than consociationalism's much vaunted promise to reflect and engender the politics of accommodation.

Hanging Flower Baskets, Blowing in the Wind? Third Sector Groups, Cross-Border Partnerships and the EU Peace Programmes in Ireland

The EU’s Peace programmes in Ireland have promoted the cross-border activity of Third sector groups. Potentially, such activity gives substantive meaning to regional cross-border governance and helps to ameliorate ethno-national conflict by providing positive sum outcomes for ‘post-conflict’ communities. The paper mobilizes focused research conducted by the authors to explore this potential. It finds that while regional cross-border governance has indeed developed under the Peace programmes, the sustainability of the social partnerships underpinning this governance is uncertain and its significance for conflict resolution is qualified by difficulties in forming a stable power-sharing arrangement at the political elite level.

Committing to Effective Climate Governance in Northern Ireland: A Defining Test of Devolution

This is the second of a two-part analysis exploring the interaction between UK devolution and governance of the national low carbon transition. It argues that devolution shaped the national climate governance regime created by the Climate Change Act 2008, but will itself be tested and even altered as the traction of the low carbon imperative intensifies. This dynamic is explored in the specific context of the UK’s most devolved region. The first article argued that devolution facilitated and arguably forced Northern Ireland’s devolved administration to give a highly qualified and potentially illusory consent to the regional application of the UK Act. The second article argues that making a more effective commitment to climate governance will be a defining test of its devolution arrangements but will require constitutional arrangements designed for conflict resolution to mature. Failure to do so will have important implications for the UK’s putative ‘national’ low carbon transition and the longer-term viability of devolution in the region.