13-08-2012, 11:19 AM
POWER SYSTEM CONTROL WITH DISTRIBUTED FLEXIBLE AC TRANSMISSION SYSTEM DEVICES
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ABSTRACT
Distributed flexible AC transmission system (D-FACTS) devices offer many potential benefits to power systems. This work examines the impact of installing D-FACTS devices by studying the sensitivities of power system quantities such as voltage magnitude, voltage angle, bus power injections, line power flows, and real power losses with respect to line impedance. These sensitivities enable us to identify and develop appropriate applications for the use of D-FACTS devices for the enhanced operation and control of the grid. Specific applications of D-FACTS devices for line flow control, real power loss minimization, and voltage control are investigated.
INTRODUCTION
Power flow in alternating current (AC) systems is unlike other flow problems such as in transportation or telecommunications. In a transportation system, trucks can be routed along a desired path from a source to a destination. Similarly, in a communications system, packets can be routed such that they travel along a shortest path between a sender and a receiver. However, electricity must follow the laws of physics, so power flow is not routable and cannot be directly controlled. Power flow control is also different from other types of flow problems since electricity must also be produced exactly when it is needed. Generation must constantly track the load as the customers’ demands change. In other systems for distributing goods and services, products are stored in a warehouse until they need to be sent to the end user. If the desired supply is unavailable, the end user can wait and it will arrive later. In power systems, customers are in control of how much power they use and always expect that amount of power to be available.
POWER FLOW CONTROL CONCEPTS
To control power flow, it is necessary to be able to maintain or change line impedances, bus voltage magnitudes, or phase angle differences. In this work, a power flow control device refers to any device that changes or maintains one or more of these parameters. Power flow control devices can be coordinated to affect system states in a way which attains some objective. There are many power flow control devices, including the well-studied FACTS devices.
Power flow control devices often work by changing an effective admittance or impedance. Effective impedance can either be changed through the use of physical capacitors and inductors or through the use of a voltage source to perform active impedance injection.
Passive Impedance Injection
Passive impedance injection refers to inserting fixed impedance devices such as series capacitors. The control of the fixed impedance devices may be accomplished using power electronics switching circuits. Passive impedance FACTS devices include fixed series capacitors as well as thyristor-controlled compensators such as static volt-ampere reactive (VAr) compensators (SVCs) and thyristor-controlled series capacitors (TCSCs).
Distributed Flexible AC Transmission System (D-FACTS) Controllers
Widespread use of conventional FACTS controllers has not extensively occurred due in part to size, expense, and installation effort. Technology improvements since the inception of FACTS allow the genre of power flow control concepts to be revisited. As shown in the rest of this thesis, the use of distributed flexible AC transmission system (D-FACTS) devices may facilitate the realization of a comprehensively controllable power system. Large-scale power flow control may finally be achievable.
In this work, we analyze in particular the use of distributed static series compensators (DSSCs). DSSCs are series power flow control D-FACTS devices which change the effective impedance of transmission lines through the use of active impedance injection with a synchronous voltage source (SVS) [10]. DSSCs are comprised of a low-rated single phase inverter and a single turn transformer and provide control similar to the SSSC, but are smaller and will be less expensive. Hereafter, DSSCs will only be referred to as D-FACTS devices.
ANALYSIS OF LINE IMPEDANCE SENSITIVITIES
Sensitivities are linearized relationships between variables and are often used in power systems analysis [12
]. Linearized relationships can reveal the impact of a small change in a particular variable on the rest of the system. Linear approximations in nonlinear systems are useful because they can provide insight into how variables depend on other variables when such relationships may otherwise be difficult to characterize. Since D-FACTS devices change effective line impedance, line impedance sensitivities are useful to determine potential benefits of D-FACTS devices.