Facts Controllers

ABSTRACT

With the ongoing expansion and growth of the electric utility industry, electricity has become a highly engineered product; it is increasingly being considered and handled as a commodity. In the evolving utility environment, financial and market forces are, and will continue to, demand a more optimal and profitable operation of the power system with respect to generation, transmission and distribution. Power electronics based equipment, Flexible AC Transmission Systems (FACTS), provide proven technical solutions to address these new operating challenges being presented today. The potential benefits of the FACTS equipment are now widely recognized by the power systems engineering and T&D communities.

This paper focuses on the summary of issues and benefits of applying FACTS controllers to AC power systems. The overall process for the system studies and the analysis associated with the FACTS installation projects and the need for FACTS controller models is also discussed. Finally an introduction to the basic circuits of several FACTS controllers is provided with a focus on their systems performance characteristics.

INTRODUCTION

FACTS technologies allow for improved transmission system operation with minimal infrastructure investment, environmental impact, and implementation time compared to the construction of new transmission lines. FACTS technologies provide advanced solutions as cost-effective alternatives to new transmission line construction. With respect to FACTS equipment, voltage sourced converter (VSC) technology, which utilizes self-commutated thyristors/transistors such as GTOs, GCTs, IGCTs, and IGBTs, has been successfully applied in a number of installations world-wide for Static Synchronous Compensators (STATCOM), Unified Power Flow Controllers (UPFC), Convertible Series Compensators(CSC), back-to-back dc ties (VSC-BTB) and VSC transmission. These aforementioned transmission system installations are in addition to the earlier generation of power electronics systems that utilize line-commutated thyristor technology for Static Var Compensators (SVC) and Thyristor Controlled Series Compensators (TCSC).

Although power electronic based equipment is prevalent in each of the three areas, Generation, Transmission, Distribution the focus of this paper is on transmission The limitations of the transmission system can take many forms and may involve power transfer between areas (referred to here as transmission bottlenecks) or within a single area or region (referred to here as a regional constraint) and may include one or more of the following characteristics:

Steady-State Power Transfer Limit

пЂ Voltage Stability Limit

пЂ пЂ Transient Stability Limit

пЂ Power System Oscillation Damping Limit

пЂ Short-Circuit Current Limit

пЂ Others

Controllability of Power Systems

To illustrate that the power system only has certain variables that can be impacted by control, consider the basic and well-known power-angle curve, shown in fig1.Although this is a steady-state curve and the implementation of FACTS is primarily for dynamic issues, this illustration demonstrates that there are primarily three main variables that can be directly controlled in the power system to impact its performance. These are:

fig1: Power angle curve

One could also make the point that direct control of power is a fourth variable of controllability in power systems. With the establishment of “what” variables can be controlled in a power system, the next question is “how” these variables can be controlled. The answer is presented in two parts: namely conventional equipment and FACTS controllers.

FACTS controllers Vs Conventional equipment

The key to solving transmission system problems in the most cost- effective and coordinated manner is by thorough systems analysis which includes comparing the system benefits available by conventional equipment and from FACTS controllers. There is an important distinction to make when considering the differences in these two solution options.

Fig 2: Illustration of speed of power system control

Figure 2 is an illustration of a few cycles of voltage at power system frequency. This figure shows that the speed of mechanical switches (primarily circuit breakers) for conventional equipment solutions can be as fast as a couple of cycles of 60 (or 50) Hz. This speed of switching may be fast enough to solve many power system constraints by itself. There is a vast improvement in switching time from mechanical to power electronic based solutions. The main benefit that FACTS controller solutions provide is the “cycling/repeatability” and “smooth control” that accompanies the power electronic based switching.

PHASES OF POWER SYSTEM STUDIES FOR FACTS INSTALLATION PROJECTS

The overall process for system studies associated with FACTS installation projects includes various phases of power system studies and what items must be focused on for each phase including the modeling requirements. The process will start with initial feasibility studies to determine system constraints and reinforcement needs, typically undertaken by the utility/transmission owners, all the way through to the system studies and modeling issues associated with the every-day operation of an installed FACTS controller in a specific power system. The following subsections provide the basic objectives and selected details for each study phase.

Fig 3:Phases of power system studies for FACTS installation projects

Phase 1: Initial Feasibility Studies to Determine System Constraints and Reinforcement Needs

The key objectives for Phase 1 type studies are to identify characteristics of the Power System and identify System Performance Problems. The performance problems include transient instability, oscillatory instability, dynamic