ENGINEERING ARTICLES

Principle: An electrical generator is a machine which converts mechanical energy into electrical energy. The energy conversion is based on the principle of the production of dynamically induced emf, where a conductor cuts magnetic flux, dynamically induced emf is produced in it according to Faraday’s Laws of electromagnetic Induction. This emf causes a current to flow if the conductor circuit is closed. Hence, two basic essential parts of an electrical generator are (i) a magnetic field and (ii) a conductor or conductors which can so move as to cut the flux. The following figure shows a single-turn rectangular copper coil rotating about its own axis in a magnetic field provided by either permanent magnets or electromagnets. The two ends of the coil are joined to two slip-rings ‘a’ and ‘b’ which are insulated from each other and from the central shaft. Two collecting brushes (of carbon or copper) press against the slip-rings. Their function is to collect the current induced in the coi...

(1) By using a starting motor. This motor is directly coupled to the motor. It may be an induction motor which can run on a synchronous speed closer to the synchronous speed of the main motor. (2) Starting as an induction motor. This is the most usual method in which the motor is provided with a special damper winding on rotor poles. The stator is switched on to supply either directly or by star delta/reduced voltage starting. When the rotor reaches more than 95% of the synchronous speed, the dc circuit breaker for field excitation is switched on and the field current is gradually increased. The rotor pulls into synchronism (A) Pull-in torque. It is the maximum constant load torque under which the motor will pull into synchronism at the rated rotor supply voltage and rated frequency, when the rated field current is applied (B) Nominal pull in torque. It is the value of pull in torque at 95 percent of, the synchronous speed with the rated voltage and frequency applied to th...

1. Shunt generators with field regulators are used for ordinary lighting and power supply purposes. They are also used for charging batteries because their terminal voltages are almost constant or can be kept constant. 2. Series generators are not used for power supply because of their rising characteristics. However, their rising characteristic makes them suitable for being used as boosters in certain types of distribution systems particularly in railway service. 3. Compound generators: The cumulatively-compound generator is the most widely used dc generator because its external characteristic can be adjusted for compensating the voltage drop in the line resistance. Hence, such generators are used for motor driving which require dc supply at constant voltage, for lamp loads and for heavy power service such as electric railways. The differential-compound generator has an external characteristic similar to that of a shunt generator but with large demagnetization armature react...

Following are the three most important characteristics or curves of a dc generator: 1. No-load saturation Characteristic (E 0 /I f ): It is also known as Magnetic Characteristic or Open-circuit Characteristic (O.C.C.). It shows the relation between the no-load generated MMF in armature, E 0  and the field or exciting current I f  at a given fixed speed. It is just the magnetization curve for the material of the electromagnets. Its shape is practically the same for all generators whether separately-excited or self-excited. 2. Internal or Total Characteristic (E/I a ): It gives the relation between the MMF E actually induces in the armature (after allowing for the demagnetizing effect of armature reaction) and the armature current I a . This characteristic is of interest mainly to the designer. 3. External Characteristic (V/I): It is also referred to as performance characteristic or sometimes voltage-regulating curve. It gives relatio...

The advantages of using FACTS devices in electrical transmission systems are described below. 1. MORE UTILIZATION OF EXISTING TRANSMISSION SYSTEM In all the countries, the power demand is increasing day by day to transfer the electrical power and controlling the load flow of the transmission system is very necessary this can be achieved by more load centers which can change frequently. Addition of new transmission line is very costly to take the increased load on the system; in that case FACTS devices are much economical to meet the increased load on the same transmission lines. 2. MORE INCREASED TRANSIENT AND DYNAMIC STABILITY OF THE SYSTEM The Long transmission lines are inter-connected with grids to absorb the changing the loading of the transmission line and it is also seen that there should be no line fault creates in the line / transmission system. By doing this the power flow is reduced and transmission line can be trip. By the use of FACTS devices high power t...

Pilot bus or weak bus is defined as the bus which, when supported, improves voltage profile at all the buses and also ensures additional security to the system, in terms of increased loading margin. Usually, placing adequate reactive power support at the weakest bus enhances static voltage stability margins. The bus which is close to experience voltage collapse is the weakest bus. Changes in voltage at each bus for a given change in system load is available from the tangent vector, which can be readily obtained from the voltage collapse proximity index prediction index (VCPI) is calculated at every bus. The value of the index determines the proximity to voltage collapse at a bus. The technique is derived from the basic power flow equation, which is applicable for any number of buses in a system. The power flow equations are solved by Newton Raphson method, which creates a partial matrix. By setting the determinant of the matrix to zero, the index at bus k is written as foll...

The basic Thyristor Controlled Series Capacitor scheme was proposed in 1986 by Vithayathil with others as a method of "rapid adjustment of network impedance". A TCSC can be defined as a capacitive reactance compensator which consists of a series capacitor bank shunted by a thyristor-controlled reactor in order to provide a smoothly variable series capacitive reactance. In a practical TCSC implementation, several such basic compensators may be connected in series to obtain the desired voltage rating and operating characteristics. However, the basic idea behind the TCSC scheme is to provide a continuously variable capacitor by means of partially canceling the effective compensating capacitance by the TCR. The basic conceptual TCSC module comprises a series capacitor, C, in parallel with a thyristor controlled reactor. Figure: Structure of TCSC PRINCIPLE OF OPERATION: A TCSC is a series-controlled capacitive reactance that can provide continuous control of power on t...

STATCOM is a Static synchronous generator operated as a shunt-connected static VAR compensator whose capacitive or inductive output current can be controlled independent of the ac system voltage. STATCOM is one of the key FACTS Controllers. A STATCOM is a controlled reactive power source. It provides voltage support by generating or absorbing capacitors banks. It regulates the voltage at its terminals by compensating the amount of reactive power in or out from the power system. When the system voltage is low the STATCOM injects the reactive power to and when the voltage is high it absorbs the reactive power. The reactive power is fed from the Voltage Source Converter (VSC) which is connecting on the secondary side of a coupling transformer as shown in the Figure 1. By varying the magnitude of the output voltage the reactive power exchange can be regulated between the convertor and AC system. STATCOM is such a device in which the modern power electronic converters have been employe...

ADVANTAGES OF PHOTOVOLTAICS: Fuel source is vast and essentially infinite No emissions, no combustion or radioactive fuel for disposal (does not contribute perceptibly to global climate change or pollution) Low operating costs (no fuel) No moving parts (no wear) Ambient temperature operation (no high temperature corrosion or safety issues) High reliability in modules (>20 years) Modular (small or large increments) Quick installation Can be integrated into new or existing building structures Can be installed at nearly any point-of-use Daily output peak may match local demand High public acceptance Excellent safety record DISADVANTAGES OF PHOTOVOLTAICS: diffuse (sunlight is a relatively low-density energy) High installation costs Poorer reliability of auxiliary (balance of system) elements including storage Lack of widespread commercially available system integration and installation so far Lack of economical efficient energy storage

The shunt component is responsible for mitigating the power quality (PQ) problems caused by the consumer: poor power factor, load harmonic currents, load unbalance, DC offset, etc. The shunt active filter is responsible for power factor correction, compensation of load current harmonics and unbalances. It maintains constant average voltage across the dc storage capacitor C dc . The shunt part of UPQC consists of a VSI connected to the common dc storage capacitor C dc  on the dc side and on the ac side it is connected in parallel with the load through the shunt interface inductors L SH  and a star-connected three-phase shunt coupling auto-transformer T SH . The shunt interface inductors L SH  together with the shunt filter capacitors C SH  are used to filter out the switching frequency harmonics produced by the shunt VSI. T SH  is used for matching the network and VSI voltages. The series component of UPQC is responsible for mitigation of supply side distur...

HVDC links may be broadly classified into the following categories: Monopolar Links Bipolar Links Homopolar Links The basic configuration of a monopolar link is shown in figure. It uses one conductor, usually of negative polarity. The return path is provided by ground or water. Cost considerations often lead to the use of such systems, particularly for cable transmission. This type of configuration may also be the first stage in the development of a bipolar system. Instead of ground return, a metallic return may be used in situation where the earth resistivity is too high or possible interference with underground/ under water metallic structures is objectionable. The conductor forming the metallic return is at low voltage.

Figure: A schematic of a bipolar HVDC system identifying main components

1) CONNECTING REMOTE GENERATION Some energy sources, such as hydro and solar power, are often located hundreds or thousands kilometers away from the load centers. HVDC will reliably deliver electricity generated from mountain tops, deserts and seas across vast distances with low losses. 2) INTERCONNECTING GRIDS Connecting AC grids is done for stabilization purposes and to allow energy trading. During some specific circumstances, the connection has to be done using HVDC, for example when the grids have different frequencies or when the connection has to go long distances over water and AC cables cannot be used because of the high losses. 3) CONNECTING OFFSHORE WIND Wind parks are often placed far out at sea, because the wind conditions are more advantageous there. If the distance to the grid on land exceeds a certain stretch, the only possible solution is HVDC - due to the technology’s low losses. 4) POWER FROM SHORE Traditionally, oil and gas p...

Recent developments of FACTS research have led to a new device: the Interline Power Flow Controller (IPFC) . This element consists of two (or more) series voltage source converter-based devices (SSSCs) installed in two (or more) lines and connected at their DC terminals. Thus, in addition to serially compensating the reactive power , each SSSC can provide real power to the common DC link from its own line. The IPFC gives them the possibility to solve the problem of controlling different transmission lines at a determined substation . In fact, the under-utilized lines make available a surplus power which can be used by other lines for real power control. This capability makes it possible to equalize both real and reactive power flow between the lines, to transfer power demand from overloaded to underloaded lines, to compensate against resistive line voltage drops and the corresponding reactive line power, and to increase the effectiveness of a compensating system for dynamic disturbanc...

The FACTS technology is not represented by a single high-power controlling device, but it is a collection of all the controllers, these individually or in coordination with the others give the possibility to fast control one or more of the interdependent parameters that influence the operation of transmission networks. These parameters include e.g. the line series impedance, the nodal voltage amplitude, the nodal voltage angular difference, then the shunt impedance and the line current. The design of the different schemes and configurations of FACTS devices is based on the combination of traditional power system components (such as transformers, reactors, switches, and capacitors) with power electronics elements (such as various types of transistors and Thyristors). The development of FACTS controllers is strictly related to the progress made by the power electronics. Over the last years, the current rating of thyristors has evolved into higher nominal values making power electronics ...

Since the development of interconnection of large electric power systems, it has been the spontaneous system oscillations at very low frequencies in the range of 0.2–3.0 Hz. After starts, it would continue for a long period of time. In certain cases, it continues to develop causing system separation due to the lack of damping of the mechanical modes. In the past three decades, Power System Stabilizers (PSSs) have been extensively used to increase the system damping for low frequency oscillations. The power utility worldwide is currently implementing PSSs as effective excitation controllers to enhance the system stability. Yet, some problems are experienced with PSSs over the years of operation. Some of these were limited to the capability of PSS, due to damping in local modes and not in the inter-area modes of oscillations. In accumulation, it can cause huge variations in the voltage profile under severe disturbances and they may even result in leading power factor operation an...

The UPFC has many possible operating modes. In particular, the shunt inverter is operating in such a way to inject a controllable current, Ish into the transmission line. The shunt inverter can be controlled in two different modes: 1) VAR CONTROL MODE: The reference input is an inductive or capacitive VAR request. The shunt inverter control translates the VAR reference into a corresponding shunt current request and adjusts gating of the inverter to establish the desired current. For this mode of control a feedback signal representing the dc bus voltage, Vdc, is also required. 2) AUTOMATIC VOLTAGE CONTROL MODE: The shunt inverter reactive current is automatically regulated to maintain the transmission line voltage at the point of connection to a reference value. For this mode of control, voltage feedback signals are obtained from the sending end bus feeding the shunt coupling transformer. The series inverter controls the magnitude and angle of the voltage injected in series with the lin...

UPFC concept was proposed by GyuGyi in 1991. The UPFC was devised for real time control and dynamic compensation of ac transmission systems. It provides multifunctional flexibility to solve many of the issues facing the power delivery industries. UPFC is able to control synchronic or individually all the parameters (i.e. voltage, phase angle, and impedance) affecting power flow in the power system network. Thus this unique capability is announced by the adjective “unified” .the main reason behind the wide spreads of UPFC are its ability to power flow bi-directionally maintaining well regulated DC voltage, workability in the wide range of operating conditions. This is the second or latest generation of FACTS technology. This FACTs device combines the two features of two other FACTS devices STATCOM (static synchronous compensator) and SSSC (the static synchronous series compensator). Basically these devices are voltage source converters (VSC’s) .the UPFC is a generally synchronous ...

UPFC consist of two back to back converters named VSC1 and VSC2, are operated from a DC link provided by a dc storage capacitor. These arrangements operate as an ideal ac to ac converter in which the real power can freely flow either in direction between the ac terminals of the two converts and each converter can independently generate or absorb reactive power as its own ac output terminal. Figure: Basic UPFC scheme One VSC is connected to in shunt to the transmission line via a shunt transformer and other one is connected in series through a series transformer. The DC terminal of two VSCs is coupled and this creates a path for active power exchange between the converters. VSC provide the main function of UPFC by injecting a voltage with controllable magnitude and phase angle in series with the line via an injection transformer. This injected voltage act as a synchronous ac voltage source. The transmission line current flows through this voltage source resulting in reactive an...

Voltage Source Converter (VSC) technology has been selected as the basis for several recent projects due to its controllability, compact modular design, ease of system interface and low environmental impact. Following are few factors which increase use of Voltage Source Converter in transmission system. Independent control of reactive and active power. Reactive control independent of other terminal(s). Simpler interface with ac system. Compact filters. Provides continuous ac voltage regulation. No minimum power restriction. Operation in extremely weak systems. No commutation failures. No restriction on multiple in feeds. No polarity reversal needed to reverse power.  Black-start capability. Variable frequency. HVDC Light cable - economic extruded polymer.

A Synchronous Condenser is a device that control voltage on an electric utility’s transmission or distribution system. Voltage is the “pressure” needed to deliver electricity through such a system. Another device that controls voltage is a capacitor. Capacitors have no moving parts. Their simple design keeps their cost and maintenance requirements low. Synchronous Condensers have internal parts that spin a motor or generator. Their sophisticated design results in higher maintenance requirements and higher costs than those of capacitors. This higher cost may be justified because Synchronous Condensers are more effective than capacitors at controlling and stabilizing voltage. Synchronous Condensers are located in utility substations, inside buildings or protective enclosures. They tend to run for long periods, made significantly less noise than generators, and produce no smoke or emissions. Synchronous Condensers do not make electric power like a generator, so they ...

Use of thyristor control in series capacitors potentially offers the following little-mentioned advantages: 1. Rapid, continuous control of the transmission-line series-compensation level. 2. Dynamic control of power flow in selected transmission lines within the network to enable optimal power-flow conditions and prevent the loop flow of power. 3. Damping of the power swings from local and inter-area oscillations. 4. Suppression of subsynchronous oscillations. At subsynchronous frequencies, the TCSC presents an inherently resistive–inductive reactance. The subsynchronous oscillations cannot be sustained in this situation and consequently get damped. 5. Decreasing dc-offset voltages. The dc-offset voltages, invariably resulting from the insertion of series capacitors, can be made to decay very quickly (within a few cycles) from the firing control of the TCSC thyristors. 6. Enhanced level of protection for series capacitors. A fast bypass of the series capacito...