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Term Paper on Power Flow

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it is a power flow model of statcom
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  POWER FLOW MODEL OF STATCOM Deepak Porwal Reg no-11108002 ,Section-E3E14,RollNo-A-30 (Lovely Professional University Punjab) (Department of Electrical & Electronics) ABSTRACT: The performance of power systems decreases with the size, the loading and the complexity of the networks. This is related to problems with load flow, power oscillations and voltage quality. Where contractual power flows do no more follow the initial design criteria of the existing network configuration. Additional problems can arise in case of large system interconnections, especially when the connecting AC links are weak. FACTS devices, however, provide the necessary features to avoid technical problems in the power systems and they increase the transmission efficiency. This paper presents a study on the design of a shunt connected FACTS device (STATCOM) and investigates the application of this device to control voltage dynamics and to damp out the oscillation in electric power system. STATCOM is one of the key shunt controllers in flexible alternating current transmission system (FACTS) to control the transmission line voltage and can be used to enhance the load ability of transmission line and extend the voltage stability margin.   Keywords: FACTS, STATCOM, transient stability INTRODUCTION- The power system is an interconnection of generating units to load centres through high voltage electric transmission lines and in general is mechanically controlled. It can be divided into three subsystems: generation, transmission and distribution subsystems. In order to provide cheaper electricity the deregulation of power system, which will produce separate generation, transmission and distribution companies, is already  being performed. At the same time electric power demand continues to grow and also building of the new generating units and transmission circuits is  becoming more difficult because of economic and environmental reasons. Therefore, power utilities are forced to rely on utilization of existing generating units and to load existing transmission lines close to their thermal limits. However stability has to be maintained at all times. Hence, in order to operate power system effectively, without reduction in the system security and quality of supply, even in the case of contingency conditions such as loss of transmission lines and/or generating units, which occur frequently, and will most  probably occur at a higher frequency under deregulation, a new controlstrategies need to be implemented. The future growth of power systems will rely more on increasing capability of already existing transmission systems, rather than on  building new transmission lines and power stations, for economic and environmental reasons. Ideally, these new controllers should be able to control voltage levels and flow of active and reactive  power on transmission lines to allow for their secure loading, to full thermal capability in some cases, with no reduction of system stability and security.The location of STATCOM for power flow control in transmission system has been  presented. The FACTS devices are introduced in the power system transmission for the reduction of the transmission line losses and also to increase the transfer capability. STATCOM is VSC based controller to regulate the voltage by varying the reactive power in a long transmission line. The effectiveness of SVC and STATCOM of same rating for the enhancement of power flow has been demonstrated. The modelling of converter-based controllers when two or more VSCs are coupled to a dc link has been presented .The optimal location of shunt FACTS devices in transmission line for highest possible benefit under normal condition and has been investigated .A shunt connected controllable source of reactive power, and two series connected voltage-sourced converters - one on each side of the shunt device was presented. An overview of how series connected and combined series/shunt connected FACTS controllers are studied in an AC system has been highlighted. The optimum required rating of series and shunt flexible ac transmission systems controllers for EHVAC long transmission lines by computing `optimum compensation requirement' (OCR) for different loading conditions has been demonstrated  . A series passive compensation and shunt active compensation provided by a static synchronous compensator (STATCOM) connected at the electrical centre of the transmission line to minimize the effects of SSR has been presented. A novel approach for damping inter-area oscillations in a large power network using multiple STATCOM II. STATCOM A STATCOM is a controlled reactive-power source. It provides the desired reactive-power generation and absorption entirely by means of electronic processing of the voltage and current waveforms in a voltage source converter (VSC). A single-line STATCOM power circuit is shown in Fig. 1.(a), where a VSC is connected to a utility  bus through magnetic coupling. In Fig. 1.(b), a STATCOM is seen as an adjustable voltage source  behind a reactance meaning that capacitor banks and shunt reactors are not needed for reactive- power generation and absorption, thereby giving a STATCOM a compact design, or small footprint, as well as low noise and low magnetic impact.The exchange of reactive power between the converter and the ac system can be controlled by varying the amplitude of the 3-phase output voltage  Es , of the converter, as illustrated in Fig. 1.(c). That is, if the amplitude of the output voltage is increased above that of the utility bus voltage, Et, then a current flows through the reactance from the converter to the ac system and the converter generates capacitive-reactive power for the ac system. If the amplitude of the output voltage is decreased  below the utility bus voltage, then the current flows from the ac system to the converter and the converter absorbs inductive-reactive . Fig. 1. The STATCOM principle diagram: (a) a  power circuit; (b) an equivalent circuit; and (c) a  power exchange power from the ac system. Principle of STATCOM- The STATCOM is made up of a shunt transformer, a voltage source converter (VSC), a DC capacitor, a magnetic circuit, and a controller. If there is no energy storage device coupled to the DC link and the losses are neglected, neither shunt converter is capable of absorbing or generating real power so that only operating in the reactive domain impossible. The reactive power exchange of STATCOM with the AC system is controlled by regulating the output voltage amplitude of VSC. If the amplitude is increased above that of the AC system, the current flows through the shunt transformer from the STATCOM to the AC system, and the device generates reactive power (capacitive). If the amplitude is decreased to a level  below that of the ac system, then the current flows from the AC system to STATCOM. The capacitor is used to maintain DC voltage to the VSC, which itself keeps the capacitor charged to the required levels. Thus, by controlling the VSC output voltage lead or lag with respect to the AC system voltage, the capacitor DC voltage can be decreased or increased, respectively, to control the reactive  power output of the device. When the VSC voltage leads the bus voltage, the capacitor supplies active  power to the system, reducing its voltage; on the other hand, when the VSC voltage lags the bus voltage, the capacitor is charged by Consuming active power from the system. The structure of a STATCOM with phase and PWM based .   THE V-I CHARACTERISTIC A typical V  -  I characteristic of a STATCOM is depicted in Fig.1. As can be seen, the STATCOM can supply both the capacitive and the inductive compensation and is able to independently control its output current over the rated maximum capacitive or inductive range irrespective of the amount of ac-system voltage That is, the STATCOM can provide full capacitive-reactive  power at any system voltage even as low as 0.15pu. The characteristic of a STATCOM reveals strength of this technology: that it is capable of yielding the full output of capacitive generation almost independently of the system voltage (constant current output at lower voltages). This capability is  particularly useful for situations in which the STATCOM is needed to support the system voltage during and after faults where voltage collapse would otherwise be a limiting Factor    Fig. 2. The V  -  I characteristic of the STATCOM MODEL OF STATCOM IN POWER SYSTEMS - Without losing generality, a three-bus STATCOM system shown in Fig. is employed to derive the model. It resembles the case where power is transmitted through an electrical transmission line connecting various generators and loads at its sending and receiving end. i t should be noted that except the STATCOM parameters, all the transmission network parameters are not known in  practice. Fig. 3. Three bus system model with STATCOM. ADVANTAGES OF STATCOM- (a) Faster response (b) Requires less space as bulky passive components (such as reactors) are eliminated (c) Inherently modular and re locatable (d) It can be interfaced with real power sources such as battery, fuel cellar SMES (superconducting magnetic energy storage) (e) A STATCOM has superior performance during low voltage condition as the reactive current can be maintained constant (In a SVC, the capacitive reactive current drops linearly with the voltage at the limit (of capacitive susceptance). It is even  possible to increase the reactive current in a STATCOM under transient conditions if the devices are rated for the transient overload. In a SVC, the maximumreactive current is determined  by the rating of the passive components  –   reactors and capacitors. A STATCOM is comparable to a Synchronous Condenser (or Compensator) which can variable reactive power and regulate the voltage of the bus where it is connected. NEED FOR REACTIVE POWER COMPENSATION  –    The main reason for reactive power compensation in a system is: 1) the voltage regulation; 2) increased system stability; 3) better utilization of machines connected to the system; 4) reducing losses associated with the system; and 5) to prevent voltage collapse as well as voltage sag. The impedance of transmission lines and the need for lagging VAR by most machines in a generating system results in the consumption of reactive  power, thus affecting the stability limits of the system as well as transmission lines. Unnecessary voltage drops lead to increased losses which needs to be supplied by the source and in turn leading to outages in the line due to increased stress on the system to carry this imaginary power. Thus we can infer that the compensation of reactive power not only mitigates all these effects but also helps in  better transient response to faults and disturbances. In recent times there has been an increased focus on the techniques used for the compensation and with better devices included in the technology, the compensation is made more effective. It is very much required that the lines be relieved of the obligation to carry the reactive power, which is  better provided near the generators or the loads. Shunt compensation can be installed near the load, in a distribution substation or transmission substation. APPLICATIONS OF STATCOM- C ONTROL S TRATEGY AND C ONTROL S YSTEMS FOR STATCOM- STATCOM by injecting current in parallel with transmission line could control bus voltage and active power. For this purpose, the STSTCOM sampling from DC-link capacitor voltage as well as the UPFC connected bus voltage and then by converting these values to dq0 parameters by Parks transformation and calculating voltages in per unit as follows: V  bus V  d    V  q V  bus and V  dc are compared by distinct values according to the block diagram depicted in Fig. 1, and generates error signals as follows: Generated error signals carried to controllers and output of controllers converted to abc  parameters again and enter to the PWM pulse generating unit. By adjusting control parameters of controllers and switching, finally error signals carried to zero. And accordingly V  bus and V  dc stabled in their reference Values.  SWITCHING METHOD Switching technique is shown in figure 4. According to this figure, SPWM compares reference signal and carrier signal which is a triangular signal to generate switching pulses. In this technique, if the reference signal is more than carrier one then switch will be on, else off. Pulse widths could be changed to control the output voltage. Although pulse widths could be different to each other, we can select the pulse width in such way that desired harmonics come out and the most common technique is sinusoidal pulse width modulation (SPWM). As thereference signal in this way is a sinusoidal form, this PWM technique is called SPWM. In SPWM, DF is unity and power factor will be corrected. Fig. 4. SPWM technique for switching Modulation amplitude index, m a and Modulation Frequency modulation, m  f are expressed using following equations Ma=Vm.sin(e)/Vm,tri = Vm,sin(e) Mf=Fsine/Ftri Vout=MaVdcsin (2*3.14*Fsinet-Q) MULTILAYER PERCEPTION (MLP)-  Neural Networks (NNs) have succeeded to solve several power system problems, such as: planning; control; analysis; protection; design; load forecasting; and fault diagnosis. The last three ones are the most popular . The simple neuron model is made from studies of the human  brain neurons. A neuron in the brain receives its chemical input from other neurons through its dendrites. If the input exceeds a certain threshold, the neuron fires its own impulse on to the neurons it is connected to by its axon .  A. Concept of  perceptron Type of NN has been created based on computations unit,that is named perceptron. A  perceptron receives vector ofinputs with real values and calculates liner composition of these inputs. If result is more than threshold value, perceptron output is 1 and otherwise is -1. This concept has  been shown in Fig.5. The perceptron can be trained by adjusting the weights of the inputs with Supervised Learning. In this learning technique, the patterns to be recognized are known in advance, and a training set of input values are already classified with the desired output. Before commencing, the weights are initialized with random values. Each training set is then presented for the perceptron in turn. For every input set the output from the perceptron is compared to the desired output. If the output is correct, no weights are altered. However, if the output is wrong, we have to distinguish which of the patterns we would like the result to be, and adjust the weights on the currently active inputs towards the desired result . For training of a  perceptron used relationship Structure of MLP : Building on the algorithm of the simple Perceptron, the MLP model not only gives a perceptron structure for representing more than two classes, it also defines a learning rule for this kind of network. The MLP is divided into three layers: the input layer, the hidden layer and the output layer, where each layer in this order gives the input to the next. The extra layers gives the structure needed to recognise non-linearly separable classes. The structure has been illustrated in Fig.6.

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