Monday, September 26, 2011

Evolution of Power Systems

Protective Relaying (Part5)    
       Power systems have evolved from isolated generators feeding their own loads to huge interconnected power systems spanning an entire country. The evolution has progressed from low-voltage systems to high-voltage systems and low-power handling capacities to high-power handling capacities. The requirements imposed on the protective system are closely linked to the nature of the power system.
1.1 Isolated Power System
       The protection of an isolated power system is simpler because firstly, there is no concentration of generating capacity and secondly, a single synchronous alternator does not suffer from the stability problem as faced by a multi-machine system. Further, when there is a fault and the protective relays remove the generator from the system, the system may suffer from a blackout unless there is a standby source of power. As shown in Section (1.2. in second post) , the steady-state fault current in a single machine power system may even be less than the full-load current. Such a fault will, however, cause other effects like speeding up of the generator because of the disturbed balance between the input mechanical power and the output electrical power, and therefore should be quickly attended to. Although, there are no longer any isolated power systems supplying residential or industrial loads, we do encounter such situations in case of emergency diesel generators powering the uninterrupted power supplies as well as critical auxiliaries in a thermal or nuclear power station.
1.2 lnterconnected Power System
       An interconnected power system has evolved because it is more reliable than an isolated power system In case of disruption in one part of the system, power can be fed from alternate paths, thus, maintaining continuity of service. An interconnected power system also makes it possible to implement an economic load dispatch.
       The generators in an interconnected system could be of varied types such as turboalternators (in coal fired, gas fired or nuclear power plants), generators in hydroelectric power plants, wind-powered generators, fuel cells or even sol%-powered photovoltaic cells.
       Figure 1. shows a simple interconnected power system. Most of the generators operate at the voltage level of around 20 kV For bulk transmission of power, voltage levels of the order of 400 kV or higher are used. At the receiving end, the voltage is stepped down to the distribution level, which is further stepped down before it reaches the consumers.
       It can be seen that the EHV lines are the tie lines which interconnect two or more generators whereas the low voltage lines are radial in nature which terminate in loads at the remote ends. 
There is interconnection at various EHV voltage levels.
1.3 Negative Synergy of an Interconnected System 
       There are other undesirable effects of interconnection. It is very difficult to maintain stability in a massively interconnected system. Disturbances quickly propagate throughout the system endangering the integrity of the whole system. Possibility of cascade tripping due to loss of stability is always looming large. In addition to the angle stability problem, an interconnected system also suffers from the voltage stability problem. Further, undesirable effects, such as harmonic distortion, propagate throughout the system rather than remain localized. Also, there is the possibility of cyber-attacks and acts of malicious hacking, which have a greater footprint in the case of an interconnected.
Figure 1. Single-line diagram of a simple interconnected power system
       power system. This can be called the synergy, of the negative type, which inevitably accompanies the interconnection. However, these are the perils of the so-called modern way of life that we have adopted and have to be taken as an opportunity to devise newer and novel methods of protection.
1.4 Various States of Operation of a Power System
       A power system is a dynamic entity. Its state is likely to drift from one state to the other as shown in Figure 2.When the power system is operating in steady state, it is said to be in the normal operating state. In this state, there is enough generation capacity available to meet the load, therefore, the frequency is stable around the nominal frequency of 50 Hz or 60 Hz. This state is also characterized by reactive power balance between generation and load. This is reflected as a flat voltage profile with normal voltage throughout the system. The above is almost a theoretical proposition and a real power system rarely finds itself in this state.
Fig.2  Various states of the power system
       Another possible state of the power system is the alert state. In this state, all the parameters are within the limits but a major disturbing event is imminent, for example, a mighty storm, accompanied by lightening, which threatens to put some major EHV tie line out of service. From this state the system may hurtle into the in-extremis state, as a result of major tripping, after passing through an emergency state. In the emergency state there could be overloading in certain tie lines, the system frequency may take a significant plunge or may surge and the voltage profile may be far from flat. The emergency situation may lead to total ac failure as a result of cascade tripping. This is indeed the nightmare and the system controllers at the load dispatch centres try their best to avoid it.
       What has all these states of the power system to do with relaying? A little thought will show that on the one hand relaying is profoundly affected by the state of the system while on the other, the system's fate is decided by the settings of the relays. Ideally, relay settings must be live to the system state and must change so as to operate in the best interest of system stability and security. There are, however, certain other ground realities which prevent us from implementing a totally adaptive online relay setting philosophy. The relay engineers want their protective system to be as simple as possible. There is overwhelming evidence that the simpler the systems are, the more reliable they are. Thus the relay engineers follow the KISS philosophy; Keep It Simple, Stupid!
1.5 From Natural Monopoly to the Deregulated Power System
        The electrical power system was always considered to be a natural monopoly Recently, however, the world over, there is a paradigm shift. The industry is being deregulated. In this scenario, big electric utility companies are no longer monopolizing the generation of electric power. The field is open for smaller players. The customers have a choice to buy electricity from the cheapest bidder (albeit through the distribution company). This has opened many technical and administrative issues which, up till now, were non-existent. The relaying engineer cannot remain unaffected by this change which is sweeping through the power industry. We shall, however, not discuss this aspect further in this introductory text.

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