Large AC Systems Research Articles

Real-Time FPGA-RTDS Co-Simulator for Power Systems

This paper proposes a co-simulation platform using field-programmable gate array (FPGA) and real-time digital simulator (RTDS) for the simulation of large power systems. It combines the advantages of high computational power from FPGA and better modelling flexibility from RTDS together. The FPGA therefore acts as an efficient and economical extension to RTDS especially when simulating large ac systems. One of the significant advantages of the proposed co-simulator is that it avoids the potential interface error existing in the conventional approach of interfacing transient stability program with electromagnetic transient programs. Two key aspects of the proposed co-simulator are discussed: 1) the interface design between FPGA and RTDS and 2) the hardware implementation and expandability of the platform. Two case studies are presented to verify the simulation accuracy and capability of the proposed co-simulator. The first case simulates a two-area four-machine power system with one area simulated in FPGA and the other area in RTDS. Comparisons are made with the case where the complete system is simulated in RTDS. The second case simulates a system of 141 buses in FPGA to demonstrate the simulator’s capability in simulating large power systems.

Enhanced Virtual Synchronous Generator Control with Fuzzy Logic Controller for Parallel Inverters in Microgrids

Objective: To reduce the power oscillations and also to provide accurate active power and reactive power sharing. Methods: In this paper enhanced Virtual Synchronous Generator (VSG) control strategy along with Fuzzy Logic Controller (FLC) is employed for parallel inverters in microgrids. It deals with design of the system and simulation results shows the improvement obtained through it. Findings: By using enhanced VSG control, the oscillation damping and proper transient active power sharing are achieved by adjusting the virtual stator reactance and also accurate reactive power sharing is achieved based on inversed voltage droop control. Furthermore, FLC is used as suitable controllable mechanism (which was PI in basic VSG) for controlling the parallel inverters in microgrids with which power oscillations are further damped and accurate active and reactive power sharing are achieved. The simulation results prove that the proposed method is better than the basic VSG for parallel inverters in microgrids. Application: This scheme is used for control of inverters in distributed source environments. This is essential for the operation of large AC systems, where distances between inverters make communication impractical. Use of multilevel inverters or an adaptive neuro-fuzzy inference system (ANFIS) can be considered as improvement for this paper. Keywords: Distributed Power Generation, Droop Control, Fuzzy logic controller, Microgrids; Reactive Power Control, Virtual Synchronous Generator Control1.

HVDC Grid Segmentation Analysis for Blackouts Reduction

Segmentation of large AC systems through DC links introduces a new concept that utilizes the advantages of direct current transmission to improve network reliability and increase power transfer capacity. Technical literature argues that the segmentation of the AC network and the introduction of DC links at these systems connection points bring benefits to system operation, once contingencies generated on one side of the DC connection point would not be reflected on the other side of DC connection, thereby reducing the likelihood of cascading shutdowns and blackouts due to load restraint on transmission lines and transformers. Amidst this scenario, this paper presents a study of the main topics regarding the use of this new network segmentation philosophy, bringing a practical point of view for the use of this concept at the electrical power system planning. The effect of DC segmentation before a contingency that would initiate major outages in an adapted electrical system model IEEE-14 bus is studied and simulations have been performed with test HVAC systems and segmented by HVDC link. The results have been compared, principally in relation to the voltage bus, reactive power generation, system losses and power flow at the lines, and demonstrated that this new concept improved the grid reliability.

Stability Enhancement of a DC-Segmented AC Power System

This paper proposes and investigates a new line-commutated current-sourced converter (LCC)-high voltage dc current (HVDC) global supplementary control (GSC) strategy for stabilizing and enhancing the dynamic performance of a large ac system that is segmented by LCC-HVDC links. The GSC is designed based on a linear quadratic Gaussian (LQG) method and enables coordinated supplementary control action of multiple HVDC links that participate in segmentation. The GSC can stabilize the ac system while either minimizing the propagation of oscillatory dynamics from one segment to other segments (GSC1) or enabling their controlled transfer from a disturbed segment to other segments (GSC2). The study results show that in a fully-dc-segmented system: 1) GSC1 and GSC2 are able to stabilize the system; 2) under GSC1, each segment can experience major disturbances without causing adjacent segments to experience the disturbances with the same degree of severity; and 3) GSC2 enables controlled transfer of the oscillations among the segments and, depending on the system configuration, can further reduce the magnitude and duration of oscillatory dynamics. The studies are conducted on a three-segment ac system including three interconnecting LCC-HVDC links.

Steady State Overvoltages of TCSC Terminals and Their Impact on Degree of Compensation and Transmitted Power of Radical Power System

This paper studies, in a tutorial form, the impact of location of the TCSC on the degree of compensation and the transmitted power of a radial power system. The voltages of the two terminals of the TCSC have been examined at different locations with increase in the degree of compensation and transmitted power. The objective is to limit these voltages to 1.05 pu and obtaining maximum degree of compensation. Two cases were studied: (1) The case when the receiving end is a large AC system where the receiving end voltage is always maintained at 1.0 pu which is designated as case # 1 and (2) The case when the receiving end is an inductive load (R-L) load and it is designated as case # 2. For case #1, it has been found that sitting the TCSC at the sending or receiving end causes a large overvoltage. In contrast, sitting it at the midpoint could not cause overvoltage up to 0.5 degree of compensation. If it is higher than this value, overvoltage has been found. Thus placing the TCSC at the midpoint provides a compensation range up to 0.5. If the TCSC are disparted in two or three smaller TCSCs, which are equal in size and equally spaced along the transmission line, it has been found that higher degree of compensation can be obtained. This scheme has been found to be best compensation scheme to obtain a wide compensation range. In case 2, the sending end positioning has been found to cause overvoltage while the receiving end positioning provides a wide compensation range up to 1 without overvoltage at any point. Similar to case # 1, sitting the TCSC at the midpoint causes overvoltage at a degree of compensation greater than 0.5. For distributed TCSCs, a degree of compensation greater 0.5 can be obtained and the maximum degree of compensation depends upon the number of TCSCs. For case # 2, the best position has been found to be at the load end. In addition to the two study cases, a relation between the number of the distributed TCSCs and the maximum degree of compensation has been developed.

Point-to-point HVdc systems using 36-pulse operation

HVdc systems are adopted in the case of long distance power transmission, interconnection of two large ac systems or power transmission by cables to overloaded districts or isolated islands. Moreover, a division of a huge ac system by dc systems can reduce its short-circuit capacity. HVdc systems, however, need both ac and dc filters, which attempt to prevent the HVdc converters from injecting all its harmonic output into both the ac and dc systems. Unfortunately, filters have a complex design and performance, and constitute a considerable part of the volume and cost of present dc terminal stations. Thus, any possible new solution is welcome. In this paper a 36-pulse configuration for point-to-point HVdc systems is proposed, whereby both ac and dc harmonics are diminished throughout the system, thus opening the possibility of reducing both the ac and dc filtering, together with the associated drawbacks. Prospective advantages regarding reactive power requirements and reliability for the proposed configuration are also evaluated.

Effect of HVDC line faults on transient torsional torques of turbine-generator shafts

This paper investigates the effects of HVDC line faults, line de-energization, and line re-energization on the transient torsional stresses of steam turbine-generator (T-G) units. The studies are conducted on a bipole HVDC system which connects a T-G set to a large AC system. The shaft transient stresses of the T-G set as a result of HVAC line fault, fault clearing, and automatic reclosure are also determined when the HVDC transmission system is replaced by an equivalent double-line HVAC system. The EMTDC program is used for the simulation studies. >

Control of parallel connected inverters in standalone AC supply systems

A scheme for controlling parallel-connected inverters in a standalone AC supply system is presented. This scheme is suitable for control of inverters in distributed source environments such as in isolated AC systems, large and distributed uninterruptible power supply (UPS) systems, photovoltaic systems connected to AC grids, and low-voltage DC power transmission meshes. A key feature of the control scheme is that it uses feedback of only those variables that can be measured locally at the inverter and does not need communication of control signals between the inverters. This is essential for the operation of large AC systems, where distances between inverters make communication impractical. It is also important in high-reliability UPS systems where system operation can be maintained in the face of a communication breakdown. Real and reactive power sharing between inverters can be achieved by controlling two independent quantities: the power angle and the fundamental inverter voltage magnitude. Simulation results are presented. >

Enhancement of AC/DC system performance by modulation of a proposed multiterminal DC system in the southwestern US

Modulation of power transmitted over a proposed multiterminal DC system to provide effective stability enhancement of a large AC system in the southwestern US is demonstrated. This system, connecting the Phoenix, Mead, and Los Angeles areas, would be in parallel with an extra-high-voltage (EHV) AC transmission network that could be operating at heavy loading conditions. Studies have indicated that the utilization of large power and reactive power modulation on the DC system can provide transient stability enhancements and improved AC system damping. The application of modulation permits the release of converter station VArs to support the AC system voltage and thus ease demands on the capability of voltage support devices.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Load Flow and Stability Analysis of Multimachine Power Systems with Controlled Multiterminal HVDC Links

The paper presents the implementation of the fast decoupled load flow technique for the inclusion of a multiterminal HVDC link in a large AC system. It is shown in the paper that reliability, computational speed and storage advantages, offered by the basic fast decoupled algorithms are preserved as far as the AC network is concerned. The method described in the paper is versatile to include any particular control mode of the HVDC link by changing some of the elements associated with the DC Jacobian matrix. Further tha outages of ac and dc lines are simulated by modifying the power injected to the affected buses using the fast decoupled line flow algorithms.A test example is worked out to highlight the formulas presented in the paper. The dynamic stability mcdeloof the multiterminal HVDC system is then attempted by using the linearised version of the DC Jacobian matrix and replacing the AC network by equivalent generations and loads.