Pruebas

Power systems are in general large and complex systems composed of several interconnected segments at different voltage levels (HV, MV, LV), typically generation, transmission and distribution, where a balance between the supply and the demand is continuously sought. Their evolution has been relatively slow and coherent due to the importance of such a system in the society that requires a high degree of stability and robustness to ensure a safe and reliable operation; besides investments made in this industry usually represent significant financial efforts that need to be carefully considered.

The variety of phenomena associated with large power systems concerning steady state or dynamic behavior, can have time horizons that range from minutes to milliseconds. Their operation requires ad-vanced control mechanisms to enable a reliable and economic feasible exploration of such systems. Hence, power systems are designed to operate and provide adequate and secure services to all participating enti-ties under different conditions. There are several definitions regarding the electric power system operating conditions, which mainly differ in terms of the considered functional schemes associated with the different states. In [29] the system operation is divided into states according to the following characteristics:

ˆ Normal State: Adequate generation is available to supply the demand and operation requirements are respected. The normal state can be further divided into secure and alert states depending on whether a likely event can or cannot potentially endanger the normal operation resulting in a transition from this state.

ˆ Emergency State: Adequate generation is available to supply demand but operation requirements are not completely respected.

ˆ In-Extremis: Generation is not adequate to meet the demand nor are the operation requirements respected.

ˆ Restorative State: Demand is not completely met but operation requirements are respected.

A similar approach is presented in [30] where a functional state scheme and the respective events associated with the possible transitions of each state are described. The respective state diagram is present in Fig. 2.1.

The Normal state is defined as the one where system variables are within their specified ranges and there is no overload conditions on any equipment, with the system operating in a secure state without violating any of the constraints. In the Alert state the system security is affected, with a direct impact on adequacy, given the adverse operational conditions, for instance due to weather related occurrences.

In this state the system variables are still within acceptable range and no restriction is violated, but any contingencies can have a more severe effect, which may result in more adverse operating conditions introduced for instance by an equipment overload event. The system enters the Emergency state after the occurrence of a severe disturbance has occurred when under alert, the system is still able to operate despite the incurred variations in system variables like bus voltages or equipment state of operation.

The objective is to promote the return to the Alert state by deploying emergency control actions, which may include, among others, generation control, load curtailment or fault clearing. The In Extremis state is reached if the operating conditions are aggravated due to inefficiency of emergency actions or due

Normal

Restorative Alert

In Extremis Emergency

Figure 2.1: Power System Operating States

to other externalities that severely compromised the system when operating in the Alert state. The consequences of such extreme state may include cascading outages ranging from system isolation actions up to a generalized blackout. The Restorative state is defined as being the one where control actions are taken, after recovering from the extreme operational conditions, in order to bring the system to the Normal operation. The system may go from the Restorative to the Alert mode if, in the meanwhile, operating conditions are degraded.

Preventive, corrective, emergency and restorative actions are performed pursuing the normal and secure operation state. Any event that leads to or can potentially cause a state change outside the normal operation will trigger a set of control actions. While some of these actions are initiated locally, others depend upon a communications infrastructure to implement a remote control action issued from a control center. For instance primary frequency control is a fast and local control mechanism that causes a power variation proportional to the frequency deviation, like a governor system installed in generators.

On the other hand secondary frequency control is managed by an AGC system that issues set-points to remote generating unit under its control area in order to guarantee the restoration of the frequency to its nominal value.

The reliability of power systems is a key issue, hence the need to conceive different states of operation that detail and differentiate control actions to be triggered. These states can be further detailed according to the complexity of each system and available control schemes. Nonetheless, it is impossible to ensure a fully reliable power system, as pointed out in [31], for several reasons. Among these are the large number of contingencies in modern power systems to be considered. Furthermore, the evolution of the electric grids along with the increase in complexity and the economic limitations when designing control systems to handle the wide variety of disturbances, will also have an impact on the overall system reliability.

The existing power control systems can be separated and summarized as follows [32, 33]:

1. Local Control - is a composition of systems where inputs are collected in a local area and control out-puts are triggered in the same area, thus not requiring any remote communications infrastructure, involving analog or digital input/output.

(a) Protection Systems Control - circuit breaking devices triggered by microprocessor based relays to ensure the protection of equipment and persons from faults. These devices require sensing capabilities to recognize the fault event, which can be due to high currents, frequency deviation and over or under-voltage, among other variables. These systems are required to operate within as little as a few milliseconds after the fault disturbance is detected, making it typically the fastest control mechanism;

(b) Governor Control - is a control mechanism associated with electric generators, which adjust the power output according to a set-point by sensing and varying the mechanical shaft speed.

The governor control is a form of primary frequency control that represents a very fast form of control. It can also respond to secondary frequency control set-points from AGCs;

(c) Voltage and Reactive Power Control - consist in a mechanism to control the output voltage or the input reactive energy absorption within predefined ranges. In the case of generators, this system acts over the excitation circuit allowing the control of the voltage and reactive power.

Tap changing transformers and switched capacitor banks are also used to provide voltage control. This is historically a slow control mechanism; however power electronic devices have been used to deploy fast voltage and reactive power control;

(d) Power Flow Control - it is used to balance the flow of electricity according to the capacity of the grid lines, thus preventing overload situations from occurring. In AC transmission lines the power flow control has been historically implemented through phase shifting transformers, resulting in a slow control mechanism, but recently power electronic devices have been used, like FACTS, allowing fast power flow control;

(e) Power System Stabilizer Control - is a supplementary control mechanism to generators in order to damp oscillations using local measurements.

2. Wide Area Control - is a composition of systems that require a remote communications infras-tructure to enable the collection of inputs and triggering of control outputs pertaining to different areas.

(a) Frequency Control - also known as secondary frequency control is a wide area control mech-anism that deals with frequency deviations introduced by the imbalance between generation and load. To maintain the system frequency at nominal value a coordinated control needs to be performed between control areas. The Automatic Generation Control (AGC) entity is responsible for dispatching control orders (set-points) to generators after collecting relevant state information like frequency and power flows;

(b) Voltage Control - similar to AGCs, these systems provide voltage control over wide areas to maintain voltage within predefined levels. Set-points are sent to local voltage controllers in a coordinated effort to mitigate voltage variations;

(c) Special Protection Schemes / Remedial Action Schemes - are control systems specific of wide area that rely on dedicated communications links and computer infrastructures. Metering data from distributed sensors is collected and the values are used by control and protection algorithms. These schemes require fast deployment of different decision strategies along with

very high levels of reliability. The importance of communications infrastructures and the impact they can have on such systems represents a matter of concern and, as such, to minimize their impact, redundant communication links are used as well as decentralized strategies that allow distributing the computational and communication burden [34].

The importance of communications has recently begun to rise mainly due to the implementation of wide area control and protection schemes. The rationale behind using communications in wide area power systems applications has been straightforward. The vital importance of protection systems as the last frontier in ensuring a secure operation of the electric power system has dictated the use of dedicated communication and processing infrastructures. The remainder wide area control systems, like AGCs, have comparatively less stringent requirements and can rely on shared communications networks. Depending on the objectives of different control applications and respective requirements several communications technologies can be considered, which usually include PLC, microwave and fiber optic networks [34].

The integration of additional entities and devices, such as those associated with the genesis of the SG paradigm, will demand enhanced control strategies. Moreover the increasing complexity of electric systems, due to the incorporation of non-utility related participating devices driven by economic and environmental factors, allows for a very large number of contingencies to be accounted for. These can have a significant impact of security and reliability indices of the operation of power systems. The recent evolving nature of power systems has become a greater source of unpredictability, as opposed to legacy systems where designers and operators were used to fairly stable behavior patterns [31]. The use of distributed control systems has resulted from the segmentation of operational requirements of the electric power system and the need to relieve the computational burden of centralized entities and supporting communications means. These strategies are especially important in the distribution grid, which represents the segment where more changes are expected to be introduced. As will be pointed out, the interaction of hierarchical and decentralized structures with central control entities will require a suitable communications infrastructure to ensure the necessary connectivity requirements.