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CHAPTER 1 INTRODUCTION 1.1 INTRODUCTION As commercial and industrial customers become more and more reliant on highquality and high-reliability electric power, utilities have considered approaches that would provide different options or levels of premium power for those customers who require something more than what the bulk power system can provide. Insufficient power quality can be caused by (1) failures and switching operations in the network, which mainly result in voltage dips, interruptions, and transients and (2) network disturbances from loads that mainly result in flicker (fast voltage variations), harmonics, and phase imbalance. Momentary voltage sags and interruptions are by far the most common disturbances that adversely impact electric customer process operations in large distribution systems. In fact, an event lasting less than one-sixtieth of a second (onecycle) can cause a multimillion-dollar process disruption for a single industrial customer. Several compensation devices are available to mitigate the impacts of momentary voltage sags and interruptions. When PQ problems are arising from nonlinear customer loads, such as arc furnaces, welding operations, voltage flicker and harmonic problems can affect the entire distribution feeder. Several devices have been designed to minimize or reduce the impact of these variations. The primary concept is to provide dynamic capacitance and reactance to stabilize the power system. This is typically accomplished by using static switching devices to control the capacitance and reactance, or by using an injection transformer to supply the reactive power to the system. Custom power is formally defined as the employment of power electronic or static controllers in distribution systems rated up to 38 kV for the purpose of supplying a level of reliability or power quality that is needed by electric power customers who are sensitive to power variations. Custom power devices or controllers, include static switches, inverters, converters, injection transformers, master-control modules and energy-storage modules that have the ability to perform current-interruption and voltageregulation functions within a distribution system. Each custom power device can be

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considered to be a type of power-conditioning device. In general, power-conditioning technology includes all devices used to correct end-user problems in response to voltage sags, voltage interruptions, voltage flicker, harmonic distortion and voltage-regulation problems. The solution to the above power quality problem is to use Flexible AC Transmission Systems (FACTS)[2] and Custom Power products like DSTATCOM (Distribution Static synchronous Compensator), DVR (Dynamic Voltage Restorer), UPQC (Unified Power Quality Conditioner) etc. These devices deal with the issues related to power quality using similar control strategies and concepts. Basically, they are different only in the location in a power system where they are deployed and the objectives for which they are employed. Most of the electricity produced today is generated in large generating stations, which is then transmitted at high voltage to the load centers and transmitted to consumers at reduced voltage through local distribution systems. In contrast with large generating stations, distributed generation (DG) produce power on a customer's site or at a local distribution network. Distribution generation has started gaining importance all over the world and can become the answer for increasing power failure some times during fault occurrences. Power failure leads power interruption leading to insecure and unreliable power system. Conventionally, power plants have been large, centralized units. A new trend is developing toward distributed energy generation, which means that energy conversion units are situated close to energy consumers, and smaller ones substitute large units. In the ultimate case, distributed energy generation means that single buildings can be completely self-supporting in terms of electricity, heat, and cooling energy. The relation between distributed generation and power quality is an ambiguous one. On the one hand, many authors stress the healing effects of distributed generation for power quality problems. For example, in areas where voltage support is difficult, distributed generation can contribute because connecting distributed generation generally leads to a rise in voltage in the network. At the same time if any faults are occurring in the system the DG must be capable of providing the power supply without any problems to the customers. Hence in order to ensure the reliability of power to the customers it is

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necessary to install some compensating devices such that these devices provides the required reactive power to the generators during fault instants such that the reactive power drawn from the supply will be nil and the other customers will not be affected. 1.2 PROBLEM DEFINITION The main objective of the thesis is to show that using DISTRIBUTION STATCOM (DSTATCOM) it is possible to reduce the voltage fluctuations like sag and swell conditions in distribution systems. The DSTATCOM which can be used at the PCC for improving power quality is modeled and simulated using proposed control strategy and the performance is compared by applying it to a radial distribution system with and without DSTATCOM. DSTATCOM is applied to a simple Distributed Generation system consisting of AC generators like Induction and Synchronous Generators and the system is analyzed by applying faults at various points. Finally the best generator to be installed with DSTATCOM is chosen. 1.3 OUTLINE OF THE THESIS The complete thesis is divided into 8 chapters. In this, chapter 1 discusses about the power quality and the available solutions followed by literature survey for DSTATCOM and for Distributed Generation. in chapter 2. Chapter 3 of the thesis summarizes the solutions to different power quality problems in the form of custom power devices and also in this chapter the power quality problems in DG are discussed. Chapter 4 describes about DSTATCOM and its operating principle. In chapter 5, the modeling part of DSTATCOM along with the voltage regulation technique is shown. In Chapter 6, the modeled DSTATCOM is applied to a distribution system with the two loads switched at different times for sag condition and source voltage is increased to a particular time for swell condition. And the waveforms show that the DSTATCOM improves the terminal voltage to 1pu in sag and swell conditions. Also the Chapter 6 shows the simulation results simulated with the DG and DSTATCOM. The Simulation is completed using MATLAB / SIMULINK version 7.0.1.

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CHAPTER 2 LITERATURE SURVEY The last decade has seen a marked increase on the deployment of end user equipment that is highly sensitive to poor quality control electricity supply. Several large industrial users are reported to have experienced large financial losses as a result of even minor lapses in the quality of electricity supply [1]- [3]. A great many efforts have been made to remedy the situation, where the solutions based on the use of latest power electronic technology figure prominently. Indeed custom power technology, the low voltage counterpart of the more widely known flexible as transmission system (FACTS) technology, aimed at high voltage power transmission applications, has emerged as a credible solution to solve many of the problems relating to continuity of supply at the end user level. The various power quality Problems at the Distribution level are voltage sag and swells, fluctuations, harmonics, flickering etc [7]. Recently, various power electronic technology devices have been proposed especially to be applied to medium voltage networks, generally named custom power. Custom power concept introduced by N.G.Hingorani [1] has been proposed to ensure high quality of power supply in distribution networks using power electronics devices. Additionally, various custom power devices are based on the voltage source converter technology introduced by N.G.Hingorani and L.Gyugyi [2]. At present, wide range of very flexible controllers, which capitalize on newly available power electronics components, are emerging for custom power applications. Among these the Distribution static Compensator (DSTATCOM) and dynamic voltage restorer (DVR), both of them based on the VSC principle given by L.Gyugyi [2], and the SSTS are the controllers which have received the most attention. The modeling and analysis of these custom power devices has applied for the study of power quality by Olimpo Anaya-Lara and E Acha [4] presenting comprehensive results to assess the performance of each device as a potential custom power application. The different control techniques of DSTATCOM are discussed in papers [8]-[12]. Sung- Min Woo, Dae- wook kang, Woo-Chol Lee, Dong-Seok Hyun, have demonstrated a new control technique for

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reducing effect of Voltage Sag and Swell with DSTATCOM [8]. In this Thesis the DSTATCOM is simulated with Voltage Regulation Technique [9]. The interest in distributed generation has considerably increased due to market deregulation, technological advances, governmental incentives, and environment impact concerns [18]. At present, most distributed generation [19] Installations employ induction and synchronous machines, which can be used in thermal, hydro, and wind generation plants [18]. Although such technologies are well known, there is no consensus on what is the best choice under a wide technical perspective. In the paper by Prof.Mrs. P.R.Khatri, Prof.Mrs. V.S.Jape, Prof.Mrs. N.M.Lokhande, Prof.Mrs. B.S.Motling, [18], they discussed the main problems associated with DG and also how to interface the DG to the utility systems. M. I. Marei, E. F. El-Saadany and M. M. A. Salama, in their work dealt with the Flexible Distributed Generation proposed a novel control scheme for the nonlinear link connecting DG to the distribution network using a current controlled Voltage Source Inverter (VSI).

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CHAPTER 3 POWER QUALITY ISSUES AND SOLUTIONS IN DISTRIBUTION SYSTEM 3.1 INTRODUCTION FACTS use the latest power electronic devices and methods to control electronically the high-voltage side of the network. Custom Power focuses on lowvoltage distribution, and it is a technology born in response to reports of poor power quality and reliability of supply affecting factories, offices and homes [1]-[3]. With Custom Power solutions in place, the end-user will see tighter voltage regulation, nearzero power interruptions, low harmonic voltages, and acceptance of rapidly fluctuating and other non-linear loads in the vicinity. A Custom Power specification may include provision for  No power interruption.  Tight voltage regulation including short duration sags or swells  Low harmonic voltages  Acceptance of fluctuating and non linear loads without effect on terminal voltage The family of emerging power electronic devices being offered to achieve these custom power objectives includes:  Distribution Static Compensator (DSTATCOM) to protect the distribution system from the effects of a polluting e.g. fluctuating, voltage sags and swells and non-linear loads.  Dynamic voltage restorer (DVR) to protect a critical load from disturbances e.g. sags swells, transients or harmonics, originating on the interconnected distribution system.  Unified Power Quality Conditioner (UPQC) is the combination of series and shunt APF, which compensates supply voltage and load current imperfections in the distribution system.

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 Solid State Breaker (SSB) to provide power quality improvement through instantaneous current interruption there by protecting the sensitive loads from disturbances that conventional electromechanical breaker cannot eliminate.  Solid- State Transfer switch (SSTS) to instantaneously transfer sensitive loads from a disturbance on the normal feed to the undisturbed alternate feed. 3.2 Available Custom Power Devices This section presents an overview of the VSC-based custom power controllers mentioned above. 3.2.1

Dynamic Voltage Restorer (DVR): The DVR is a powerful controller that is

commonly used for voltage sags mitigation at the point of connection. The DVR employs the same blocks as the D-STATCOM, but in this application the coupling transformer is connected in series with the ac system [5]-[6], as illustrated in Fig 3.1. The VSC generates a three-phase ac output voltage, which is controllable in phase and magnitude. These voltages are injected into the ac distribution system in order to maintain the load voltage at the desired voltage reference.

Fig 3.1 Schematic representation of the DVR The DVR is a solid state dc to ac switching power converter that injects a set of three single phase ac output voltages in series with the distribution feeder and in synchronism with the voltages of the distribution system. By injecting voltages of controllable amplitude, phase angle and frequency (harmonic) into the distribution feeder in instantaneous real time via a series injection transformer, the DVR can restore the

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quality of voltage at its load side terminals when the quality of the source side terminal voltage is significantly out of specification for sensitive load equipment. The reactive power exchanged between the DVR and distribution system is internally generated by the DVR without any ac passive reactive components, i.e. reactors and capacitors. For large variations in the source voltage, the DVR supplies partial power to the load from a rechargeable energy source attached to the DVR dc terminal. The DVR, with its three single phase independent control and inverter design is able to restore line voltage to critical loads during sags caused by unsymmetrical L-G, LL, L-L-G, as well as symmetrical three phase faults on adjacent feeders or disturbances that may originate many miles away on the higher voltage interconnected transmission system.Connection to the distribution network is via three single-phase series transformers there by allowing the DVR to be applied to all classes of distribution voltages. At the point of connection the DVR will, within the limits of its inverter, provide a highly regulated clean output voltage. 3.2.2

Unified Power Quality Conditioner (UPQC): The Universal Power Quality

Conditioner (UPQC) is a more complete solution for the power quality problem. The basic structure of this equipment is shown in shown in Fig 3.2. In this figure, the UPQC is an association of a series and shunt active filter based on two converters with common dc link [5], [6]. The series converter has the function to compensate for the harmonic components

Fig 3.2 Basic Block Diagram of UPQC 8

(Including unbalances) present in the source voltages in such a way that the voltage on the load is sinusoidal and balanced. The shunt active filter has the function of eliminating the harmonic components of nonlinear loads in such a way that the source current is sinusoidal and balanced. This equipment is a good solution for the case when the voltage source presents distortion and a harmonic sensitive load is close to a nonlinear load as shown in Fig3.2. .3.2.3 Solid State Transfer Switch (SSTS): The SSTS consists of two three-phase static switches, each constituted in turn by two anti-parallel thyristors per phase. Normally, the static switch on the primary source is fired regularly, while the other one is off. In the event of a voltage disturbance, the SSTS [6] is used to transfer the load from the preferred source to an alternative healthy source. This results in a very effective way of mitigating the effects of both interruptions and voltage dips by limiting their duration as seen by the load.

Fig 3.3 Basic Block diagram of SSTS The SSTS can be used very effectively to protect sensitive loads against voltage sags, swells and other electrical disturbances. The SSTS ensures continuous high-quality power supply to sensitive loads by transferring, within a time scale of milliseconds, the load from a faulted bus to a healthy one. The basic configuration of this device consists of

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two three-phase solid-state switches, one for the main feeder and one for the backup feeder. These switches have an arrangement of back-to-back connected thyristors, as illustrated in the schematic diagram of Fig 3.3. Each time a fault condition is detected in the main feeder, the control system swaps the firing signals to the thyristors in both switches, i.e., Switch 1 in the main feeder is deactivated and Switch 2 in the backup feeder is activated. The control system measures the peak value of the voltage waveform at every half cycle and checks whether or not it is within a prespecified range. If it is outside limits, an abnormal condition is detected and the firing signals to the thyristors are changed to transfer the load to the healthy feeder. 3.2.4 Solid State Breaker (SSB): It offers a solution to several problems originated in distribution systems since it can act as [6]: a) Transfer switch by transferring sensitive loads from the normal supply that experiences a disturbance to an alternate supply unaffected by the disturbance b) A substation bus-tie switch that is normally open; a fault on one feeder leads to Opening its circuit breaker, the bus-tie switch will close to serve the loads from other feeder as soon as the faulty feeder is separated from loads c) A current limiter that conducts inrush and fault currents for several cycles

Fig 3.4 Block Diagram Of SSB

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Fig 3.4. Shows a schematic diagram of a SSB acting as a fault current limiter (FCL), constructed from a number of antiparallel GTO modules; in normal operation GTO elements are closed, under abnormal condition the breaker detects the rise of both the steady state current and the rate of current change, di/dt, and opens rapidly. The SSB in its present form is not likely to replace the conventional circuit breaker. However it has a number of applications where, used in place of a circuit breaker could provide uninterrupted power by providing rapid transfer to a secondary feeder or limit reactive inrush currents by pulse width modulating the current. 3.3 DISTRIBUTED GENERATION Most of the electricity produced today is generated in large generating stations, which is then transmitted at high voltage to the load centers and transmitted to consumers at reduced voltage through local distribution systems. In contrast with large generating stations, distributed generation (DG) produce power on a customer's site or at a local distribution network. The International Energy Agency (IEA) [20] defines distributed generation as the following: "Distributed generation is a generating plant serving a customer on-site or providing support to a distribution network, connected to the grid at distribution level voltages. The technologies include engines, small (and micro) turbines, fuel cells, and photovoltaic systems [18]. They generate electricity through various smallscale power generation technologies. Distributed energy resources (DE) refers to a variety of small, modular power-generating technologies that can be combined with energy management and storage systems and used to improve the operation of the electricity delivery system, whether or not those technologies are connected to an electricity grid. 3.3.1 Advantages of Distributed Generation Distributed generation has some economic advantages over power from the grid, particularly for on-site power production [18]. 1) On-site production avoids transmission and distribution costs, which otherwise amount to about 30% of the cost of delivered electricity.

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2) Onsite power production by fossil fuels generates waste heat that can be used by the customer. Distributed generation may also be better positioned to use inexpensive fuels such as landfill gas. 3) End-user perspective: End users who place a high value on electric power can generally benefit greatly by having back up generation to provide improved reliability. There are also substantial benefits in high efficiency applications, such as combined heat and power, where the total energy bill is reduced. End users may also be able to receive compensation for making their generation capacity available to the power systems in area where there are potential power shortages. 4) Distribution utility perspective: The distribution utility is interested in selling power to end users through its existing network of lines and substation. It can be used for transmission and distribution capacity relief. Thus it can also serve as a hedge against uncertain load growth and high price hikes on the power market, if permitted by regulatory agencies. 5) Commercial power producer perspective: Those looking at DG from this perspective are mainly interested in selling power in the power market. Commercial aggregators will bid the capacities of the units generated by them. The DG then can be directly interconnected into the grid or simply serve the load off- grid. However the perspectives on interconnected DG of typical utility distribution are very conservative in their approach to planning and operation. 3.3.2 Power Quality problems with DG The Main Power Quality Issues affected by Distributed Generation are [18]. 1. Sustained Interruption: This is the traditional reliability area. Many generators are designed to provide backup power to the load in case of power interruption. However, Distributed Generation has the potential to increase the number of interruptions in some cases. 2. Voltage Regulation: This is often the most limiting factor for how much Distributed Generation (DG) can be accommodated on a distribution feeder without making changes.

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3. Harmonics: There are harmonics concerns with both rotating machines and inverters, although concern with inverters is less with modern technologies. 4. Voltage Sag: The most common power quality problem is the voltage sag. 3.3.3 Interfacing to the Utility System While the energy conversion technology may play some role in the power quality, most power quality issues relates to the type of electrical system interface [18]. However some notable exceptions are: 1. The power variation from renewable sources such as wind and solar can cause voltage fluctuations. 2. Some fuel cells and micro turbines do not follow step changes in load and must be supplemented with battery or flywheel storage to achieve improved reliability. 3. Misfiring of the engine sets can lead to persistent and irritating type of flicker which is more prominent when magnified by the response of power system. The main types of electrical system interfaces however are 1) Synchronous machine. 2) Asynchronous or Induction machine. 3) Electronic power inverters. The most common type of distributed generation employs ac rotating machines i.e. Induction generator and Synchronous generator. Though the synchronous machines are most commonly used technology and are well understood. The machine can follow any load within its designed capability. It is possible for such machine, which is large enough relative to the capacity of the system at the PCC to regulate the utility system voltage, which can be a power quality advantage in certain weak systems. Generators should be sized or designed considerably larger than the load to achieve satisfactory power quality in isolated operation. Though it is very simple to interface induction machine to the utility system as no special synchronizing equipment is necessary. The chief issue however is that a simple induction generator requires reactive power to excite the machine from the power where it is connected. Another problem that is prominent in such machines is that the capacitor bank yields resonance that coincides with the harmonics produced. Most of the DG

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technologies nowadays have to use electronic power inverter to interface with the electrical power system. However to achieve better control and to avoid harmonics problems the inverter technology has changed to switched, pulse width modulated technologies.

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CHAPTER 4 DISTRIBUTION STATCOM 4.1 INTRODUCTION This chapter presents the operating principles of DSTATCOM. The DSTATCOM is basically one of the custom power devices. It is nothing but a STATCOM but used at the Distribution level. The key component of the DSTATCOM is a power VSC that is based on high power electronics technologies. The Distribution STATCOM is a versatile device for providing reactive compensation in ac networks. The control of reactive power is achieved via the regulation of a controlled voltage source behind the leakage impedance of a transformer, in much the same way as a conventional synchronous compensator. However, unlike the conventional synchronous compensator, which is essentially a synchronous generator where the field current is used to adjust the regulated voltage, the DSTATCOM uses an electronic voltage sourced converter (VSC), to achieve the same regulation task. The fast control of the VSC permits the STATCOM to have a rapid rate of response. The DSTATCOM is the solid – state based power converter version of the SVC. Operating as a shunt – connected SVC, its capacitive or inductive output currents can be controlled independently from its connected AC bus voltage. Because of the fastswitching characteristic of power converters, the DSTATCOM provides much faster response as compare to SVC. DSTATCOM is a shunt connected, reactive compensation equipment, which is capable of generating and or absorbing reactive power whose output can be varied so as to maintain control of specific parameters of the electric power system. DSTATCOM provides operating characteristics similar to a rotating synchronous compensator without mechanical inertia, due to the DSTATCOM employ solid state power switching devices it provides rapid controllability of the three phase voltages, both in magnitude and phase angle. In addition, in the event of a rapid change in system voltage, the capacitor voltage does not change instantaneously; therefore the DSTATCOM reacts for the desired responses. For example, if the system voltage drops for any reason, there is a tendency for the DSTATCOM inject capacitive power to support the dipped voltages.

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4.2 Operating Principle of the DSTATCOM Basically, the DSTATCOM system is comprised of three main parts: a VSC, a set of coupling reactors and a controller. The basic principle of a DSTATCOM installed in a power system is the generation of a controllable ac voltage source by a voltage source inverter (VSI) connected to a dc capacitor (energy storage device). The ac voltage source, in general, appears behind a transformer leakage reactance. The active and reactive power transfer between the power system and the DSTATCOM is caused by the voltage difference across this reactance. The DSTATCOM is connected to the power networks at a PCC, where the voltage-quality problem is a concern. All required voltages and currents are measured and are fed into the controller to be compared with the commands. The controller then performs feedback control and outputs a set of switching signals to drive the main semiconductor switches (IGBT’s, which are used at the distribution level) of the power converter accordingly. The basic diagram of the DSTATCOM is illustrated in Fig 4.1.

Fig 4.1 Block Diagram of the voltage source converter based DSTATCOM The ac voltage control is achieved by firing angle control. Ideally the output voltage of the VSI is in phase with the bus (where the DSTATCOM is connected) voltage. In steady state, the dc side capacitance is maintained at a fixed voltage and there is no real power exchange, except for losses. The DSTATCOM differs from other reactive

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power generating devices (such as shunt Capacitors, Static Var Compensators etc.) in the sense that the ability for energy storage is not a rigid necessity but is only required for system unbalance or harmonic absorption. There are two control objectives implemented in the DSTATCOM. One is the ac voltage regulation of the power system at the bus where the DSTATCOM is connected and the other is dc voltage control across the capacitor inside the DSTATCOM. It is widely known that shunt reactive power injection can be used to control the bus voltage. In conventional control scheme, there are two voltage regulators designed for these purposes: ac voltage regulator for bus voltage control and dc voltage regulator for capacitor voltage control. In the simplest strategy, both the regulators are proportional integral (PI) type controllers. Thus, the shunt current is split into d-axis and q-axis components. The reference values for these currents are obtained by separate PI regulators from dc voltage and ac-bus voltage errors, respectively. Then, subsequently, these reference currents are regulated by another set of PI regulators whose outputs are the d-axis and q-axis control voltages for the DSTATCOM. 4.3 Principle of Voltage Regulation 1) Voltage Regulation without Compensator: Consider a simple circuit as shown in Fig 4.2. It consists of a source Voltage E, V is the voltage at a PCC and a load drawing the current Il. Without a voltage compensator [8], the PCC voltage drop caused by the load current Il, shown in fig as ∆V,

∆V = E − V = Z s * I l ,

S = VI * , so S * = V * I From above equation,

Il =

Pl − j * Ql V

so that,

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Pl − jQl ) V ( R P − X s Ql ) ( X P + Rs Ql ) = s l +j s l V V = ∆Vr + ∆Vx

∆V = ( Rs + jX s )(

The voltage change has a component ∆Vr in phase with V and component ∆Vx, which are illustrated in Fig 4.2(a). It is clear that both magnitude and the phase of V, relative to the supply voltage E, are functions of the magnitude and phase of the load current namely the voltage drop depends on both the real and reactive power of the load. The component ∆V is rewritten as

∆V = I s Rs + jI s X s

Fig 4.2 A Simple Circuit for demonstrating the voltage regulation principle.

Fig 4.2 (a) Phasor diagram for uncompensated 18

2) Voltage regulation with DSTATCOM: Now consider a compensator connected to the system. It is as shown in Fig 4.2(b) shows vector diagram with voltage compensation. By adding a compensator in parallel with the load, it is possible to make E=V by controlling the current of the compensator.

Is =Ir + Il Where Ir is the compensating current.

Fig 4.2(b) Phasor diagram for voltage regulation with compensation

CHAPTER 5

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MODELING OF DSTATCOM 5.1 INTRODUCTION The Fig 5.1 shows the basic structure of a six-pulse DSTATCOM to a load bus in a power system where Rp represents the 'ON' state resistance of the switches including transformer leakage resistance, Lp is transformer leakage inductance and the switching losses are taken into account by a shunt dc-side resistance Rdc. A VSI resides at the core of the DSTATCOM. It generates a balanced and controlled three-phase voltage Vp. The voltage control is achieved by firing angle control of the VSI. Under steady state, the dcside capacitor possesses fixed voltage Vdc, and there is no real power transfer, except for losses. Thus, the ac-bus voltage remains in phase with the fundamental component of Vp. However, the reactive power supplied by DSTATCOM is either inductive or capacitive depending upon the relative magnitude of fundamental component of Vp with respect to

Vt. If |Vt| > |Vp| the VSI draws reactive power from the ac-bus whereas if |Vt| < |Vp|, it supplies reactive power to the ac-system. This is the basic principle of DSTATCOM.

Fig 5.1 Basic DSTATCOM connected to a load in a distribution system The sending end source is assumed to be a strong system with high short circuit ratio and low impedance. Thus, the source voltage is treated as a constant source irrespective of variations in load current. The equivalent circuit of the above system is shown is figure below:

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5.2 Equivalent circuit of the above system with DSTATCOM 5.2 Modeling of DSTATCOM in d-q frame The dynamic equations governing the instantaneous values of the three-phase voltages across the two sides of DSTATCOM [9] and the current flowing into it are given by:

d   R p + L p i p = Vt − V p dt   where i p = (ia Vt = (Vta

Vtb Vtc ) T

and

ib ic )T

,

V p = (V pa

V pb V pc ) T

.

Under the assumption that the system has no zero sequence components, all currents and voltages can be uniquely represented by equivalent space phasors and then transformed into the synchronous d-q-o frame by applying the following Park’s transformation (q is the angle between the d-axis and reference phase axis).

T=

2 3

  cos θ  − sin θ   1  2

2π 2π  ) cos(θ + ) 3 3  2π 2π  − sin(θ − ) − sin(θ + ) 3 3  1 1   2 2 cos(θ −

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This transformation is called as Park's Transformation. And after transforming to d-q axis the equations in two phases are given by,

Vd  Va  V  = T V   q  b V0  Vc 

,

id  ia  i  = T i   q  b i0  ic 

.

Thus the transformed dynamic equations are

di pd dt

=−

Rp Lp

di pq

and

dt

where

i pd + ω i pq +

=−

Rp Lp

1 (Vtd − V pd ) Lp

i pq − ωi pd +

-----------------(1)

1 (Vtq − V pq ) Lp

-----------------(2),

ω = (dθ dt ) , is the angular frequency of the source voltage.

For an effective dc-voltage control, the input power should be equal to the sum of load power (if any) and the charging rate of capacitor voltage on an instantaneous basis [9]. Thus, by power balance between the ac input and the dc output,

p=

3  2 2 V i + V i − ( i + i ) R pd pq td pd tq pq p  2  

dVdc V 2 dc = CVdc + dt Rdc Hence,

----------------(3),

2 2 dVdc 3 Vtd i pd + Vtq i pq − (i pd + i pq ) R p Vdc = − dt 2 CVdc CRdc ---------(4).

The above equation models the dynamic behavior of the dc-side capacitor voltage. The equation (1), (2) and (4) together describe the dynamic model of DSTATCOM. The Voltage regulation control strategy for DSTATCOM is concerned with the control of ac-

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bus and dc-bus voltage on both sides of DSTATCOM. The dual control objectives are met by generating appropriate current reference (for d- and q-axis) and, then, by regulating these currents in the DSTATCOM. PI controllers are conventionally employed for both the tasks while attempting to decouple the d- and q-axis current regulators. The DSTATCOM current (ip) is split into real (in phase with ac-bus voltage) and reactive components. The reference value for the real current is decided so that the capacitor voltage is regulated by power balance. The reference for reactive component is determined by ac-bus voltage regulator. As per the strategy, the original currents in d - q frame (ipd, ipq) are now transformed into another frame, d1-q1 frame, where d1-axis coincides with the ac-bus voltage (Vt), as shown in Fig4.5. Thus, in d1-q1 frame, the currents ipd1 and ipq1represent the real and reactive currents and they are given by,

i pd 1 = i pd cos ∂ t + i pq sin ∂ t i pq1 = i pq cos ∂ t − i pd sin ∂ t

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Fig 5.3 Phasor Diagram showing d-q and d1-q1 frame. Now for DSTATCOM current control, the equations (1), (2) and (4) are modified as,

di pd 1 dt

di pq1 dt

=−

=−

Rp Lp

Rp Lp

i pd 1 + ωi pq1 +

i pq1 − ωi pd 1 +

1 (Vt − V pd 1 ) LP

1 (−V pq1 ) LP

-----------(5)

------------------(6)

where V pd 1 = V pd cos ∂ t + V pq sin ∂ t V pq1 = V pq cos ∂ t − V pd sin ∂ t

----------------------------------(7)

The VSI voltages are controlled as follows,

V pq1 = −(ωL p i pd 1 + L p uq1 ) V pd 1 = ωL p i pq1 + Vt − L p ud 1 By using the equation (7) in (5) and (6), the equations will be modified to,

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di pd 1 dt di pq1 dt

=− =−

Rp Lp Rp Lp

i pd 1 + ud 1 i pq1 + uq1 -------------------(8)

Also the dc bus voltage dynamic voltage equation is given by 2 2 dVdc 3 Vt i pd 1 − (i pd 1 + i pq1 ) Rp Vdc = − dt 2 CVdc CRdc

-------

(9)

Now the control signals ud1 and uq1 are determined by selecting the proper values for the

Kp and Ki’s used in the control technique. 5.3 DSTATCOM VOLTAGE REGULATION TECHNIQUE The DSTATCOM improves the voltage sags and swell conditions and the ac output voltage at the customer points is improved, thus improving the quality of power at the distribution side In this thesis the voltage controller technique [14] (also called as decouple technique) is used as the control technique for DSTATCOM. The method is already discussed in the previous topic. This control strategy uses the dq0 rotating reference frame because it offers higher accuracy than stationary frame-based techniques [2]. In this VABC are the three-phase terminal voltages, Iabc are the three-phase currents injected by the DSTATCOM into the network, Vrms is the root-mean-square (rms) terminal voltage, Vdc is the dc voltage measured in the capacitor, and the superscripts indicate reference values. Such a controller employs a phase-locked loop (PLL) to synchronize the three phase voltages at the converter output with the zero crossings of the fundamental component of the phase-A terminal voltage. The block diagram of a proposed control technique is shown in Fig 4.6. Therefore, the PLL provides the angle φ to the abc-to-dq0 (and dq0-to-abc) transformation. There are also four proportionalintegral (PI) regulators.

25

Fig 5.4 Block Diagram of DSTATCOM Control The first one is responsible for controlling the terminal voltage through the reactive power exchange with the ac network. This PI regulator provides the reactive current reference Iq*, which is limited between +1pu capacitive and -1pu inductive. Another PI regulator is responsible for keeping the dc voltage constant through a small active power exchange with the ac network, compensating the active power losses in the transformer and inverter. This PI regulator provides the active current reference Id*. The other two PI regulators determine voltage reference Vd*, and Vq*, which are sent to the PWM signal generator of the converter, after a dq0-to-abc transformation. Finally, Vabc* are the three-phase voltages desired at the converter output.

26

CHAPTER 6 Test Systems and Simulation Results 6.1 Test System for Distribution system 6.1.1 Introduction Basically, DSTATCOM consists of PWM voltage source inverter circuit and a DC capacitor connected at one end.. In the distribution voltage level, the switching element is usually the integrated gate bipolar transistor (IGBT), due to its lower switching losses and reduced size. Moreover, the power rating of custom power devices is relatively low. Consequently, the output voltage control may be executed through the pulse widthmodulation (PWM) switching method. IGBT based PWM inverter is implemented using Universal bridge block from Power Electronics subset of Sim Power Systems. RC 27

snubber circuits are connected in parallel with each IGBT for protection. Such a model consists of a six-pulse voltage-source converter using IGBTs/diodes, a 3000Vdc capacitor, a PWM signal generator with switching frequency equal to 3 kHz, After modeling of DSTATCOM, It is applied to a simple radial distribution line consisting of different loads. The single line diagram of the radial distribution system to be tested is shown in Fig 61. Refer to Appendix A for the complete details of the system.

Fig 6.1 Single Line Diagram of the system used.

6.1.2 Test Details: The Test System details: Table 6.1 The test system details of single line diagram used Input Voltage 11kv, 50Hz. Source Impendence

0.968Ω, 0.03H

Line impedance

0.4 Ω, 0.003H

DC Voltage

3000V.

Capacitor Load1

2000µF. 0.5MW, 0.2MVAr

Load2

0.10MW, 0.05MVAr

28

6.1.3 Testing the DSTATCOM: To verify the performance of the DSTATCOM, a variable load is connected at bus 2 and the substation voltage is also changed during the simulation. The sequence of events simulated is explained as follows. Initially, there is no load connected at bus 2. At t=200ms, the switch S1 is closed so that load1 is applied and at t=500ms, the switch S2 is closed i.e load2 is applied too; both switches remain closed until the end of the simulation. During these events, the terminal voltage of bus 2 decreases showing the effect of sags and, at t=800ms, the substation voltage is increased to, the terminal voltage of bus 2 also rises, showing the swell condition.

6.1.4 Simulation Results: The DSTATCOM along with the Distribution System is simulated in MATLAB / SIMULINK Software of Version 7.0.1. and the diagram is shown in Fig 6.2 below. For the detailed circuit refer to Appendix A.

Fig 6.2.The Single line diagram implemented in MATLAB.

29

Fig 6.3 Terminal Voltage of Bus2 in per unit. The three-phase rms value of the line voltage Vab of bus2 for the events previously described is shown in Fig 6.3. In the absence of the DSTATCOM, the terminal voltage varies considerably, but such variations are minimized in the presence of the DSTATCOM. Furthermore, the reactive and active power injected by the DSTATCOM into the network is shown in Fig6.4, where the consumption of active and reactive powers by the DSTATCOM is represented by positive values and the generation by negative values.

30

Fig 6.4 Active (P) and Reactive (Q) Powers injected by the DSTATCOM The voltage and current waveforms of the phase A after connecting the DSTATCOM in to the network are shown in Fig 6.5 and Fig 6.6. Clearly we can observe from the figure that the voltage sag and swell conditions are compensated with DSTATCOM.

31

Fig 6.5 Terminal voltage of Bus2 Va in pu with DSTATCOM

Fig 6.6 Current Ia in pu injected by DSTATCOM into the network.

Fig 6.7 Dc capacitor voltage

32

Fig 6.8 Three phase currents injected by DSTATCOM in to the network. The 3 phase current injected by the DSTATCOM into the network is shown in Fig 6.8 where as the respective two phase currents i.e. Id and Iq are shown in Fig 6.9 and 6.10 respectively.

33

Fig 6.9 The Id (Active current) injected by DSTATCOM before converting to 3 phase

Fig 6.10 The Iq (Reactive current) injected by DSTATCOM before converting to 3 phase

6.1.5 Conclusion Voltage sag and swells has emerged as a major concern in the area of power quality. The voltage sag and swell problems in a 11 kV distribution system is investigated in this topic. The analysis and simulation of a DSTATCOM application for the voltage flicker mitigating are presented and discussed. The three-phase rms value of the line voltage of bus2 for the events previously described is shown in Fig6.3. In the absence of the DSTATCOM, the terminal voltage varies considerably, but such variations are minimized in the presence of the DSTATCOM. The dynamic behavior of the dc voltage is also shown in Fig 6.7. Furthermore, the reactive and active power injected by the DSTATCOM into the network is shown in Fig 6.4, where the consumption of active or reactive power by the DSTATCOM is represented by positive values and the generation by negative values.

34

Hence, by the application of DSTATCOM in to the network the voltage sag and swell conditions are improved and the voltage is recovered to approximately 1pu voltage.

6.2 Test System with Distributed Generation 6.2.1 Introduction In this section, the model is developed in MATLAB / SIMULINKS. All the network components were represented by three-phase models. The distribution feeders were modeled as series RL impedances. The three-phase transformers were simulated taking into account the core losses (T circuit). The single-line diagram of the test network is shown in Fig. 6.2.1. Such network comprises a 132 kV, 50 Hz, sub transmission system with short-circuit level of 1500 MVA, represented by a Thevenin equivalent (Sub), which feeds a 33 kV distribution system through two 132/33 kV, ∆ / Yg, transformers. This test system is taken from [16]. An AC generator with capacity of 30 MW is connected at bus 7, which is connected to the network through a 33 / 0.69 kV, ∆ / Yg, transformer. This machine can represent one generator [16] in a thermal generation plant as well as an equivalent of various generators in a wind or small hydro generation plant. There is a three-phase capacitor bank of 10 MVAr (selected as one third of the capacity of generator) connected at the generator terminal plant. In some cases, such a generator was simulated as an induction generator and in other one as a synchronous generator. Moreover, there is a DSTATCOM with a capacity of 5 MVAr connected at bus 5 through a 33/2- kV ∆ / Yg transformer. AC Generator Models: The dynamic behavior of the induction generator was represented by a sixth-order three-phase model (available in SIMULINK) in the d–q rotor reference frame [Appendix B]. In the cases simulated without a DSTATCOM, a three-phase capacitor bank was connected to the terminals of the induction generator, which was adjusted to keep the terminal voltage at 1 p.u. during steady state (usually the capacity of 3 phase capacitor bank is selected as one third of the capacity of the generator). The mechanical power was considered constant (i.e., the primer mover and governor effects were neglected). The synchronous generator was represented by an eighth-order three-phase model (available in SIMULINK) in the d–q rotor reference frame [Appendix B]. Such a generator was considered equipped with an 35

automatic voltage regulator (AVR) represented by the IEEE –Type 1 model. The mechanical power was considered constant i.e. the regulator and primary mover dynamics are neglected.

6.2.2 Test System: The test system for simulating the DSTATCOM along with AC generators is shown in Fig 6.2.1. It has, AC generator (G), which includes either of Induction generator, or synchronous generator.

Fig 6.2.1 Test System with DG and DSTATCOM The test system consists of a substation, which has the ratings of 132kV and the short circuit capacity of 1500 MVA. The voltage 132kV is stepped down to 32kV by two transformers of capacity 100MVA which are connected in ∆ / Yg configuration. All the transmission line parameters and transformer ratings are shown in Appendix B. The loads are always connected to the system. The DSTATCOM is connected in parallel to the bus5 with a transformer 33/2 kV. The capacity of DSTATCOM used is 10MVA. At bus 5 the system voltage is stepped down to 690v using a ∆ / Yg transformer. At bus6 the generator is connected. The capacity of generator is 30 MW. The detailed parameters of the generators are shown in Appendix B. For the simulation, first the Induction generator is connected and the system is analyzed and again the synchronous generator is connected and the system is analyzed.

36

Throughout the simulation the loads are connected to the system. The faults applied are (1) 3 phase to ground fault and single phase fault at bus 4 and fault is cleared by tripping the branch 2-4 and (2) 3 phase to ground fault at middle of branch 4-5 and fault is cleared without tripping the branch.

6.3

Simulation with DG

6.3.1 With fault at bus 4 and cleared by tripping line 2-4: Case (a): DSTATCOM with Induction Generator: The simulation with induction generator is divided in to two parts: one with a 3-phase fault and another with a single-phase fault. 1) With Three Phase fault and fault clearance time=0.15s: A three phase to ground short circuit is applied at bus4 at t=10. 5secs. and eliminated at t=10.65 by tripping branch 2-4 of the circuit shown in Fig 6.1. The generator terminal voltage responses for this fault are shown in Fig 6.3.1. It can be verified from the figures that the two simulations i.e. with DSTATCM and without DSTATCOM are stable since the voltage is at the acceptable level but with the lower value in without DSTATCOM.

Fig 6.3.1 The Induction Generator terminal voltage 37

Fig 6.3.2 The rotor speed of Induction Generator

Fig 6.3.3 The reactive power injected by DSTATCOM in to the network

38

The rotor speed responses are exhibited in Fig. 6.3.2 It can be seen that both the cases present a good damping, confirming the fact that the transients of induction generators are very fast. Note that the pre and posfault rotor speeds are different from each other due to the distinct values of the terminal voltage. Fig 6.3.3 shows the Reactive power injected by DSTATCOM in to the network at the fault moment. 2) With Three Phase fault and fault clearance time=0.20s This case is equal to the previous one except the fault clearance time =0.20s. The terminal voltage responses are presented in Fig 6.3.4. It can be observed that only the case with the DSTATCOM voltage controller is stable. Without the DSTATCOM, the system becomes unstable due to a lack of reactive power. In the other situations, the DSTATCOM acts as a variable reactive power source. However, the reactive power injections are shown in Fig. 6.3.6, where the consumption of reactive power by the DSTATCOM is represented by positive values and the generation by negative values. In the voltage-control mode, the DSTATCOM increases the injection of reactive power during and after the short-circuit interval.

Fig 6.3.4 The terminal voltage response of Induction Generator 39

Fig 6.3.5 The rotor speed of Induction generator

Fig 6.3.6 Reactive Power Injected by DSTATCOM

40

It can also be seen from the Fig 6.3.5, without DSTATCOM the rotor speed of the generator exceeds the stability limit where as without DSTATCOM the system remains stable. 3) With Single Phase fault and fault clearance time =0.2s This case was simulated by applying a phase-A-to-ground fault bus 4 at t=10.50 second, which was cleared at t=10.70s by isolating line 2–4.

Fig 6.3.7 The terminal Voltage of induction generator The terminal voltage responses are shown in Fig.6.3.7. With or Without DSTATCOM the system is stable, but only with the DSTATCOM, the voltage quickly recovers to approximately 0.95 p.u. Otherwise, the postfault terminal voltage is equal to 0.85 p.u. This is due to the fact that the other phases b and c remains excited by the source.

CASE (b): DSTATCOM with Synchronous Generator: 1) With Three Phase fault and fault clearance time=9 cycles: The same fault as described in above section was simulated (i.e., a three-phase-toground short circuit at bus 4 cleared by tripping line 2–4). Now, the induction generator was substituted by a synchronous generator. The dynamic behavior of the generator

41

terminal voltage is presented in Fig 6.3.8. It can be seen that the responses for all situations are very similar. According to Fig. 6.3.9, which presents the rotor speed responses, the initial damping is slightly worse without the DSTATCOM.

Fig 6.3.8 The Terminal voltage response of Synchronous Generator.

Fig 6.3.9 The rotor speed of Synchronous generator.

42

This is due to the fact that the Synchronous Generator has a separate excitation where as the Induction Generator does not has the separate excitation. And this excitation takes care of the required reactive for the generator. 2) With Three Phase fault and fault clearance time =0.20s The rotor speed responses and the terminal voltage responses are presented in Fig. 6.3.11and in 6.3.10. Moreover, it can be observed that similar damping is obtained from the cases without and with a DSTATCOM voltage controller. It can be easily observed that without and with the DSTATCOM, the Synchronous Generator provides the same results. This is due to the fact that the Synchronous Generator has the Automatic voltage regulator (AVR). This AVR takes care of the required reactive power for the generator.

Fig 6.3.10 The terminal voltage response of Synchronous Generator.

43

Fig 6.3.11 The rotor speed of Synchronous generator

3) With Single Phase fault and fault clearance time is equal to 0.20 s A phase-A-to-ground short-circuit at bus 4 was applied at t=10.5s, which was cleared at t=10.70s by tripping the line 2–4.The dynamic behavior of the terminal voltage is shown in Fig. 6.3.12. Note that similar behavior is obtained from both the cases. There is not any enhancement in the system dynamic performance due to the presence of the DSTATCOM.

44

Fig 6.3.12 Terminal Voltage of Synchronous Generator

6.3.2 With Fault in between buses 4-5 and cleared without line tripping: All simulations presented in this section were obtained using Matlab/Simulink. The short circuits simulated were applied in between buses 4-5 from t=10.5s to t=10.7s and cleared without line tripping. The system loads remained connected to the network during the contingencies analyzed. The objective of this study is to determine the influence of a DSTATCOM on the short-circuit currents supplied by ac generators during faults.

A) With Three Phase to ground fault: The installation of ac generators may elevate the values of the short-circuit currents, becoming mandatory to update the protection and/or the network devices. Thus, in this section, the short-circuit currents supplied by the ac generators during faults are determined by using simulations. The fault and ground resistances were set equal to 0.001 45

ohm. Fig. 6.3.13 and Fig 6.3.14 presents the dynamic behavior of the currents supplied by both the induction and synchronous generators (stator current) during a three-phase-to ground short circuit applied at bus 5 at t=10.5s. It can be seen that the current response is different for each generator.

Fig 6.3.13 Phase A stator currents of Synchronous generator with and without DSTATCOM In the case of the induction generator, although initially the magnitude of the currents is high, they decrease quickly because this machine has no capacity to provide sustained short-circuit currents unloaded. Consequently, there is no external excitation source for the generator, and it becomes unable to produce voltage. In the case of synchronous generators, it can be observed that the usage of the excitation system as a voltage regulator permits that the generator supplies sustained short-circuit current. It can be verified from the Fig 6.3.13 and 6.3.14 that the presence of the DSTATCOM has no influence on the short-circuit currents provided by the ac generators. Furthermore, comparing Figs.6.3.13 and 6.3.14, it can be noted that even though the

46

initial value of the short-circuit current provided by the synchronous generator is larger than the current supplied by the induction generator, the latter decays very quickly.

Fig 6.3.14 Phase A stator current of Induction Generator with and without DSTATCOM Therefore, an induction generator with a DSTATCOM controlled by voltage may be a good solution for distribution networks. And if synchronous generators are used, then there is no need to install a separate DSTATCOM.

47

6.4 Case Study of Agasthyamuzhy substation 6.4.1 Introduction The following figure shows the structural layout of the substation. For the case study, Agasthyamuzhy substation is selected. The incoming feeder for the substation consists of 2lines of 110kV from kunnamangalam. These 110kV lines are stepped down to 33kV using a 16MVA Yg/∆ Transformer. Again these 33kV bus is stepped down to 11kV using 10MVA transformer for supplying to local load centers. The outgoing feeder form the substation consists of two feeders of 11kV each going to Manassery and Omassery.

Fig 6.4.1 Structural layout of Agasthyamuzhy Sub-Station

48

Also shown in the Figure a 6MW Hydel Power generation located at Chembukadavu village. The distance from Chembukadavu village to Agasthyamuzhy station is 25kms. This is a Pico Hydel station since it generates only a small amount of power. This Hydel station is operated only during rainy season since it has plenty of water in that season. After the rainy season this hydel station is turned off due to lack of water. A power of 6MW is generated at 11kv at this station and transmitted at 33kV. In rainy season the power generated is used for local feeders and also it supplies for the grid if extra power is generated. The generator used for electricity generation is Synchronous Generator. However the Induction Generator is also taken and simulated for the comparison purpose. The single diagram of Fig 6.2.1 is taken and the complete system is replaced by the data from Agasthyamuzhi substation. A model is developed in MATLAB/SIMULINK with Synchronous Generator and DSTATCOM.A 3phase-fault is applied at near to Generator and cleared with out line tripping. See Appendix(C) for complete data. 6.4.2 Simulation Results A three phase to ground fault is applied at t=10.5s in between buses 2-4 and cleared at t=10.65s without line tripping. Since the hydel power station consists of Synchronous generators, it will have an exciter system, such that the required reactive currents are obtained from the excitation system. Hence there is not much any improvement in the terminal voltage of the generator. In general, for power generation the Induction generators are also used, mainly in Wind Generation systems. In this study, the synchronous generator is replaced by the Induction Generator of the same capacity and the same fault is applied and removed without line tripping. The terminal voltage of the Induction generator is shown in Fig 6.4.3 below. By observing the Fig 6.4.2 and 6.4.3, the Induction Generators installed with DSTATCOM, recovers the voltage to 1pu in less time compared to without DSTATCOM.

49

Fig 6.4.2 The Terminal voltage of the Synchronous Generator

Fig 6.4.3 The terminal Voltage of the Induction Generator Hence, In Wind generations where the Induction generators are installed, the DSTATCOM improves the performance of the generator during sag conditions.

50

CHAPTER 7 CONCLUSIONS 7.1 Conclusion Custom power devices like DVR, D-STATCOM, and UPQC can enhance power quality in the distribution system. Based on the power quality problem at the load or at the distribution system, there is a choice to choose particular custom power device with specific compensation. Distribution Static Synchronous Compensator (DSTATCOM) can compensate the voltage sag and swells conditions. A simple control technique called as Voltage Regulation Technique is simulated for DSTATCOM control and the same is applied to the radial distribution system. The Simulation results shows that the DSTATCOM can compensate the voltage sag and swell conditions caused due to sudden switching of loads. The DSTATCOM voltage controller can significantly improve the voltage stability performance of induction generators without increasing the short-circuit currents provided by them. A DSTATCOM voltage controller does not introduce significant improvements in the transient stability of synchronous generators. In fact, the AVR system of these machines can provide voltage control. In a distribution system suffering from short-circuit level and stability constraints, the installation of an induction generator combined with a DSTATCOM voltage controller may be a good choice for distributed generation expansion since the fault currents are minimized in the case of Induction generators. Hence, in the cases of Wind Generations where the Induction Generators are majorly used, it is a good choice to install a DSTATCOM since it can provide the required reactive support for the system 7.2 Scope for future work In this thesis work, it is shown that the DSTATCOM can mitigate the voltage sag and swell conditions. The work can be extended to reduce the source voltage and source current harmonics supplied due to the non-linear loads. This thesis can also be extended for multilevel inverters to reduce the harmonic current at the supply side due to

51

loads. This thesis is done for only single generators and can be extended to multiconnected generators with multi level inverters for DSTATCOM.

REFERENCES [1] N.G.Hingorani “ Introducing custom power”, IEEE spectrum, vol.32, June 1995, PP.41-48. [2] N.G.Hingorani and L.Gyugyi, Understanding FACTS: Concepts and Technology of flexible ac transmission systems, IEEE Press, New York, 1999. [3] Ray Arnold” Solutions to Power Quality Problems “ power engineering journal, Volume 15; Issue: 2 April 2001,pp: 65-73. [4] Anaya-Lara Olimpo, E.Acha “Modeling and Analysis of custom power systems by PSCAD/EMTDC”, IEEE Transactions on Power Delivery, Volume: 17, Issue: 1, Jan 2002 pp: 266-272. [5] Ambra Sannino, Jan Svensson, Tomas Larsson,” Power-Electronic Solutions to power quality problems”, Electric Power Systems Research, Science direct, 2003, pp: 71-82. [6] Juan A. Martinez,” Modeling of Custom Power Equipment Using Electromagnetic Transients Programs” IEEE 2000, pp: 769-774. [7] Soo-Young Jung, Tae-Hyun Kim, Seung-II Moon, Byung- Moon Han,” Analysis and control of DSTATCOM for a Line Voltage Regulation “ Power Engineering Society Winter Meeting, 2002.IEEE, volume: 2, Jan 2002, pp- 729-734. [8] Sung- Min Woo, Dae- wook kang, Woo-Chol Lee, Dong-Seok Hyun, “ The Distribution DTSTAOCM for Reducing the effect of Voltage Sag and Swell” IECON’01: The 27th Annual Conference of the IEEE Industrial Electronics Society, 2001, pp: 1132-1137. [9] N.C.Sahoo, B.K.Panigrahi, P.K.Dash, G.panda,” Application of a multivariable feedback linearization scheme for STATCOM control” Electric Power Systems Research, 2002, pp 81-91. [10]Walmir Freitas, and A Morelato,” Comparative Study Between PSB and PSCAD/EMTDC for Transient Analysis of Custom Power Devices Based on Voltage Source Technology” International conference on Power System Transients-IPST 2003.

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[11]Elanady, M. M. A. Slalma,”An Efficient Control Technique for DSTATCOM used for Voltage Flicker Mitigation “ EPCS, Volume 33, June 2005, pp: 233-246. [12]B.Singh, A.Adya, A.P.Mittal, J.R.P.Gupta,

“Analysis, Simulation and control of

DSTATCOM in Three-phase, Four –wire Isolated Distribution Systems” IEEE, Power India conference, April 2006. [13]Su Chen, Geza Joos “ Direct Power Control of DSTATCOM’s for voltage Flicker mitigation” IEEE, industry Applications conference, October-2001, Volume-4, pp 2683-2690. [14]Walmir Freitas, Eduarado Asada, Andre Morelato, Wilsun xu,” Dynamic Improvement of Induction Generators Connected to Distribution Systems Using a DSTATCOM”, IEEE 2002, pp 173-177. [15]Nitus Voraphonpiput and Somchai Chatratana, “STATCOM Analysis and Controller Design for Power System Voltage Regulation “ IEEE / PES, 2005, pp: 1-6. [16]Walmir Freitas, Andre Morelato, Wilsun Xu, Fujio Sato,” Impacts of AC Generators and DSTATCOM devices on the Dynamic Performance of Distribution Systems” IEEE Transactions on power delivery, vol. 20,No:2, pp: 1493-1501, April-2005. [17]K. H. Sobrink, N. Jenkins, F. C. A. Schettler, J. Pedersen, K. O. H. Pedersen, and K. Bergmann, “Reactive power compensation of a 24 MW wind farm using a 12-pulse voltage source converter,” in Proceedings CIGRE International Confeerence on Large High Voltage Electric Systems, 1998. [18]Prof.Mrs. P.R.Khatri, Prof.Mrs. V.S.Jape, Prof.Mrs. N.M.Lokhande, Prof.Mrs. B.S.Motling,” Improving Power Quality by Distributed Generation “ IPEC 2005, vol 2 pp: 675-678, Dec-2005. [19]Kari Alanne, Arto Saari, “Distributed energy generation and sustainable development” Renewable and Sustainable Energy Reviews, Science direct, Nov2004, pp: 539- 558. [20]International Energy Agency (IEA). Distributed Generation in Liberalised Electricity Markets. OECD/IEA, Paris, France, 2002.

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[21]Ahmed M. Azmy and Istvan Erlich, “Impact of distributed generation on the stability of electrical power system”, IEEE Power Engineering Society, vol 2, June – 2005,pp: 1056-1063 [22]Proceedings of AICTE summer school on “Power Quality Issues and Remedial measures”. [23]Sim Power Systems User’s Guide, Trans Energy Technologies Inc., 2002. [24]http://www.nalanda.nitc.ac.in/ [25]http://tdworld.com/mag/power_custom_power_choices/.

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APPENDIX Appendix - A (Data of Distribution System) Source Details: Voltage ph-ph (rms)

: 11kV

Frequency

: 50 Hz

Short Circuit MVA

: 12.5

Source Resistance

: 0.968Ω

Source Inductance

: 0.03H

Distribution Line data: Length of Line

: 1 km.

Positive and Zero Sequence Resistance (in ohms/km): R1 : 0.1903

R0 : 0.4359

Positive and Zero- Sequence Inductance (in H/km): L1: 1.249mH

L0 : 5.9mH

Loads Data: Load1

: 0.5 MW and 0.2 MVAr

Load2

: 0.1 MW and 0.05 MVAr

Coupling Transformer: (Yg / ∆) Rated Power

: 100 KVA

Rated Voltage

: 11kV / 2kV

Resistance

: 0.01 pu

Inductance

: 0.02pu

DSTATCOM details: Power Device used

: IGBT

DC side Capacitor

: 2000 µF

Inverter DC Voltage

: 3000V

Switching Frequency

: 3000Hz

Appendix – B (Data of Distributed Generation) Source Details:

55

Voltage ph-ph (rms)

: 132kV

Frequency

: 50 Hz

Short Circuit MVA

: 1500MVA

Source Resistance

: 0

Source Inductance

: 0.0369H

Source Transformer details: Rated Power Rated Voltage R1, R2 L1, L2

: 100MVA : 132 / 33kV : 0.005pu : 0.02pu

Feeder Details: Table A Feeder Details Resistance in Inductance in Branch Ohms Henry 2 -4

2.34

9.9e-3

2 -3

0.486

5.54e-3

3 -4

2.6

12e-3

4 -5

1.3

6e-3

Load Details: Load at branch

Table B Load Details P (MW) Q (MW)

2

58

12

3

6

2

4

24

5

5

12

3

Generators Data: A) Induction Generator: Power

: 30MVA

56

Voltage (rms) : 0.69kV Stator Resistance

: 0.01pu

Stator Inductance

: 0.1pu

Rotor Resistance

: 0.014pu

Rotor Inductance

: 0.098pu

Inertia constant

: 1.5s

Mutual Inductance: 3.5 pu B) Synchronous Generator: Power

: 30MVA

Voltage (rms) : 0.69kV Stator Resistance

: 0.0014pu

Reactances

:

Xd=1.4pu, Xd1=0.231pu, Xd11=0.118 Xq= 1.372pu, Xq1=0.8, Xq11=0.118, Xl=0.05pu Inertia constant

: 1.5s

Appendix C (Case study data) Substation details Voltage incoming = 110kV(2lines from kunnamangalam) Transformer ratings: HV

= 110kV

LV

= 33kV

MVA = 16MVA Out going feeders = 11kV lines (2 nos) 1) Manassery 2) Omassery Hydel Power from Chembukadavau Power generated = 6MW at 11kV Distance from chembukadavu to Agasthuamuzhi = 25kms.

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