Yük-frekans kontrolunun incelenmesi

Abstract

Elektrik şebekeleri her geçen gün büyümektedir, öyle ki bir ülke sadece kendi içinde enterkonekte sistemler oluşturmakla yetinmemekte, komşu sistemlere bağlanarak en uygun çalışma maliyetleriyle güç alışverişi yapmak istemektedir. Ülkemizin son yıllarda karşı karşıya kaldığı enerji darboğazından çıkabilmesi için hızlı bir şekilde elektrik enerjisi yatırımları yapılması yanında komşu ülkelerin elektrik enerjisi sistemlerine bağlanıp ekonomik bir şekilde ihtiyacını karşılaması da mümkündür. Fakat enterkonekte sistemleri birbirine bağlamak kolay değildir, öncelikle her enterkonekte sistemin kendi içinde kararlı çalışması gerekir. Her iki sistemin frekanslarını, üretimlerini ve birbirlerine verdikleri aktif güç değerlerini kontrol etmeleri gerekmektedir. Tüm bu kontrol işlemleri, yük - frekans kontrolü ile gerçekleştirilir. Frekansın kontrolü, aktif güç kontrolü ile ilgili olduğundan herhangi birindeki değişimden diğeri de etkilenecektir. Herhangi bir yük değişimi, üretim birimi kaybı gibi durumlar sonucunda her enterkonekte sistemin frekansını, üretimini ve diğer sistemlerle gerçekleştirdiği güç alışverişini yeniden düzenlemesi gerekir. Enterkonekte sistemlerde kararlılığı bozucu bu olayların etkisi, tek basma çalışan sistemlerden daha azdır. Bu da enterkonekte sistemlerin tercih edilme nedenlerinden biridir. Tezde, yük - frekans kontrolü analizine uygun olacak şekilde generator, devir sayısı regülatörü ve türbin modelleri oluşturulmuştur. Yük - frekans kontrol kademeleri olan ilk hız kontrolü ve destekleyici hız kontrolü sırasıyla tanıtılmıştır. Enterkonekte sistemler arasında üretimin ekonomik şekilde paylaşımını sağlayan kontrol işlemine de değinilmiştir. En son kısımda da, hız kontrolörlerinin tek tek olması durumu ve ikisinin bir arada bulunması durumunda herhangi bir yük değişimi olursa frekansın nasıl değişeceği bilgisayar simülasyonu ile gösterilmiştir.

The flows of active power and reactive power in a transmission network are fairly independent of each other and are influenced by different control actions. Hence, they may be studied separately for a large class of problems. Active power control is closely related to frequency control and reactive power control is closely related to voltage control. As constancy of frequency and voltage are important factors in determining the quality of power supply, the control of active and reactive power is vital to the satisfactory performance of power systems. For satisfactory operation of a power system, the frequency should remain nearly constant. Relatively close control of frequency ensures constancy of speed of induction and synchronous motors. Constancy of speed of motor drives is particularly important for satisfactory performance of generating units as they are highly dependent on the performance of all the auxiliary drives associated with the fuel, the feed - water and the combustion air supply systems. In a network, considerable drop in frequency could result in high magnetizing currents in induction motors and transformers. The extensive use of electric clocks and the use of frequency for other timing purposes require accurate maintenance of synchronous time that is proportional to integral of frequency. As a consequence, it is necessary to regulate not only the frequency itself but also its integral. A system operating at constant frequency must be in a state of power balance. That is to say, the total system - generated real power must equal the real power loads, losses and tie flows out of system. When this balance is upset, the system frequency will begin to change. Too much generation, and the system frequency will increase; too little and the frequency will drop. As frequency is a common factor throughout the system, a change in active power demand at one point is reflected throughout the system by a change in frequency. Because there are many generators supplying power into the system, some means must be provided to allocate change demand to the generators. The frequency of a generator's generated voltage is determined by the speed of its rotor. Thus, controlling the frequency is equivalent to controlling generator - turbine speed. Since the system frequency is essentially the same everywhere, the generators must be running at the same speed or synchronized. Fortunately, synchronized generators operating in parallel are inherently stable. If this were not the case, mulugenerator systems would be impossible. By stable, the machines can normally recover from small random perturbing forces and still remain synchronized. Consider an isolated mulugenerator power system operating at constant frequency in a state of perfect power balance. By isolated, there are no ties to external systems. Now, consider that a consumer suddenly switches in a load at an arbitrary point in the system. This modifies the network topology instantly, changing the vu voltage magnitude and phase at the terminals of each generator. The real power requirements of the system load plus losses are instantly met by the generators. However, the generator powers are produced through an electromagnetic torque on the generator - turbine rotor in opposition to rotation. Prior to the load increase, this torque was counterbalanced by an equal and opposite torque provided by the drive turbine. The sudden increase in load causes the generator torque to increase suddenly. The turbine - generator torques are now out of balance, with the net balance opposing rotation. The rotors thus begin to decelerate. Several different effects come into play that arrest this accumulating reduction in speed. First, steam and hydraulic turbines inherently develop more torque at slower speeds and would even if unregulated eventually reduce the torque unbalance to zero. Furthermore, turbines have speed control systems such that if a low- speed condition is detected, control valves open to increase the flow and thus the torque. Also, it is an almost universal charasteristic of motor mechanical loads that they increase with motor speed. Since motor speed typically varies directly with system frequency, if the frequency drops, total system load will typically also drop, balancing out the original load increase. In addition to these effects, there is a system frequency control system designed to hold the integral of frequency error (i.e., deviation from a reference level) to within certain tolerances. Thus, the average frequency is adjusted precisely to the reference value. This control action is again implemented by adjusting the turbine control valves. Since the power systems are very large, it is good idea to divide into groups of generators. The system is divided into coherent groups of generators interconnected by tie lines. Each coherent group is called an area according to load - frequency control law. Each area must meet its own load changes. In an interconnected system with two or more independently controlled areas, in addition to control of frequency, the generation within each area has to be controlled so as to maintain scheduled power interchange. The control of generation and frequency is commonly referred to as load - frequency control ( LFC ). The frequency control is accomplished by two different control actions. The primary speed control and supplementary or secondary speed control actions. The primary speed control makes the initial coarse readjustment of frequency. By its actions the various generators in the control area track the shifting load and share it in proportion to their size. The speed of response is limited only by the natural time lags of the turbine and the system itself. Depending upon turbine type the primary loop responds in 2 to 20 seconds, typically. The supplementary speed control takes over the fine adjustment of the frequency by resetting, through integral action, the frequency error to zero. The relationship between speed and load can be adjusted by changing an input ' load reference setpoint '. In practice, the adjustment of load reference setpoint is accomplished by operating the 'speed - changer motor'. The output of each unit at any given system frequency can be varied only by changing its load reference, which in effect moves the speed - droop characteristic up and down. This control is vui considerably slower and goes into action only when the primary speed control has done its job. Response time may be of the order of one minute. To adjust the frequency is used to speed - governing system. An isochronous governor adjusts the turbine valve / gate to bring the frequency back to the nominal or scheduled value. An isochronous governor works satisfactorily when a generator is supplying an isolated load or when only one generator in a multigenerator system is required to respond to changes in load. For power load sharing between generators connected to the system, speed regulation or droop characteristic must be provided. The speed - droop or regulation characteristic may be obtained by adding a steady - state feedback loop around the integrator. For load - frequency studies, the generator simply is represented by APm - APe = Ms Aco where M is expressed as inertia constant. APm= The change in mechanical power. APe = The change in electrical power. Aco = The deviation in speed. In general, power system loads are a composite of a variety of electrical devises. For resistive loads, such as lighting and heating loads, the electrical power is independent of frequency. In the case of motor loads, such as fans and pumps, the electrical power changes with frequency due to changes in motor speed. The overall frequency - dependent characteristic of a composite load may be expressed as APe = APL + DAcor where APl = non -frequency - sensitive load change DAcûr = frequency - sensitive load change D = load -damping constant. The damping constant is expressed as a percent change in load for one percent change in frequency. Typical values of D are 1 to 2 percent. + »? Load reference set point MR Governor Prime mover Ö i 1+«ÎG AP., Rotating mass & load / Ali, A?._ 1 Ms+D Au Figure 1. Block diagram of governor, prime mover, rotating mass. IX The turbine model is simply shown in figure. The transfer functions of the steam turbine and hydraulic turbine are different. The block diagram includes representation of the speed governor, turbine, rotating mass and load, appropriate for load - frequency analysis. As system load is continually changing, it is necessary to change the output of generators automatically. The primary objectives of automatic generation control (AGC ) are to regulate frequency to the specified nominal value and to maintain the interchange power between control areas at the scheduled values by adjusting the output of selected generators. A secondary objective is to distribute the required change in generation among units to minimize operating costs. The area control error (ACE) represents the required change in area generation. ACE is a control signal which made up of tie line flow deviation added to frequency deviation weighted by a bias factor (B). ACE = AP + B Af As noted earlier, an important secondary function of automatic generation control is to allocate generation so that each power source is loaded most economically. This function is referred to as economic dispatch control (EDC). The theory of economic dispatch is based on the principle of equal incremental costs. For control of tie line power and frequency, it is necessary to send signals to generating plants to control generation. It is possible to use these signals to control generation to satisfy economic dispatch criteria. Thus, the requirement for EDC can be handled as part of the AGC function. Under normal conditions, with each area able to carry out its control obligations, steady - state corrective action of AGC is confined to the area where the deficit or excess of generation occurs. Interarea power transfers are maintained at scheduled levels and system frequency is held constant. Under abnormal conditions, one or more areas may be unable to correct for the generation - load mismatch doe to insufficient generation reserve on AGC. In such an event, other areas assist by permitting the interarea power transfers to deviate from scheduled values and by allowing system frequency to depart from its per- disturbance value. Each area participates in frequency regulation in proportion to its available regulating capacity relative to that of the overall system. In modern AGC schemes, the control actions are usually determined for each control area at a central location called the dispatch centre. Information pertaining to tie line flows, system frequency and unit MW loadings is telemetered to the central location where the control actions are determined by a digital computer. The control signals are transmitted via the same telemetering channels to the generating units on AGC as shown in figure 2. The normal practice is to transmit raise or lower pulses of varying lengths to the units. The control equipment at the plants then changes the reference setpoints of the units up or down in proportion to the pulse length. Figure 2 illustrates the implementation of AGC for one control area ( normally the service area of an individual utility ). Each control area of an interconnected system is controlled in a similar manner, but independently of the other control areas. That is, the control of generation in the interconnected system is ' area - wise decentralized.' Early AGC systems, developed in the 1950s, were based on analog control equipment. These were gradually superseded by digital systems beginning in the late 1960s. Now, all - digital systems are the universal choice for AGC applications. Raise/lower pulses Telemetered tie flows Regulating and economic allocation algorithm Speed changer position Scheduled net interchanee Generatinc units on AGC Figure 2. AGC control logic for each area. From a practical point of view, the problems of frequency control of interconnected areas, or power pools, are more important than those of isolated areas. Practically, all power systems today are tied together with neighboring areas and the problem of load - frequency control becomes a joint undertaking. Closely associated is the problem of controlling the power flows on the interties. Many advantages can be derived from interconnected areas and they can be summarized in two words, mutual assistance. In the third chapter, results of computer simulation are given. Block diagrams which include the governor,the turbine, the rotating mass and load are shown and the simulation results of these diagrams were obtained by simulation program.The simulation results which represent the change in frequency caused by the change in load are obtained by Matlab( version 4.0) simulation program. XI BOLUM 1 GİRİŞ Bir enerji iletim şebekesinde aktif ve reaktif gücün akışı birbirinden bağımsızdır ve farklı kontrol olayları ile kontrol edilirler. Bu nedenle aktif ve reaktif güç kontrolundaki problemlerin çoğu için ayrı ayrı çalışmalar yapılır. Aktif güç kontrolü, frekans kontrolü ile yakından ilgiliyken reaktif güç kontrolü, gerilim kontrolü ile ilgilidir. Bu tezde frekans kontrolü dolayısıyla üretim birimlerinin aktif güç kontrolü ve enterkonekte sistemleri birbirine bağlayan bağlantı hatları üzerinden akan aktif gücün kontrolü incelenmiştir. Bir sistemin frekansı, aktif güç dengesine bağlıdır. Frekans sistemin her yerinde bilinen bir faktör olduğundan bir noktadaki aktif güç isteğindeki değişim, frekanta bir değişim ile sistemin her yerine yansıtılabilir. Frekansın kontrolü, iki farklı kontrol olayı ile gerçekleştirilir [1-3]. İlk hız kontrolü dediğimiz kontrol ile frekans ayarlaması oldukça kaba bir şekilde yapılır. Bu kontrol ile frekansın sabitliği sağlanır fakat frekans hatası sıfir yapılamaz. İlk hız kontrolün etkisi ile kontrol alanındaki generatörler, değişen yükü büyüklükleriyle orantılı bir şekilde paylaşırlar. İkinci kontrol olayı ise destekleyici hız kontrolüdür. Bu hız kontrolü frekans hatasını sıfırlayan integral kontrolör içerdiğinden frekans ayarını en iyi şekilde yapar. Destekleyici hız kontrolü oldukça yavaştır ve ilk hız kontrolü görevini yaptıktan sonra etki etmeye başlar. Aynı zamanda ilk hız kontrolü ile yük değişiminin yeri önemli olmaksızın aktif güç değişimi yapılırken, destekleyici hız kontrol ile sadece yük değişiminin olduğu üretim birimlerinde aktif güç değişimi yapma imkanı vardır. Destekleyici hız kontrol olayı sonucunda, istenen değerlerde frekans ve komşu alanlarla net aktif güç alışverişi sağlanmış olur. Ayrıca her üretim birimini en ekonomik şekilde yükleyecek üretimi, alanlar arasında paylaştırmak gerekir. Bu paylaşım işlemi ekonomik paylaşım kontrolü ile gerçekleştirilir.