ADAPTIVE PIEZOELECTRIC ENERGY HARVESTING TECHNIQUE

 The need for a wireless electrical power supply has spurred an interest in piezoelectric energy harvesting, or the extraction of electrical energy using a vibrating piezoelectric device. Examples of applications that would benefit from such a supply are a capacitively tuned vibration absorber ,a foot-powered radio” tag and a Pico Radio .A vibrating piezoelectric device differs from a typical electrical power source in that its internal impedance is capacitive rather than inductive in nature, and that it may be driven by mechanical vibrating amplitude and frequency. While there have been previous approaches to harvesting energy generated by a piezoelectric device there has not been an attempt to develop an adaptive circuit that maximizes power transfer from the piezoelectric device. The objective of the research described herein was to develop an approach that maximizes the power transferred from a vibrating piezoelectric transducer to an electromechanical battery. The paper initially presents a simple model of piezoelectric transducer. An ac-dc rectifier is added and the model is used to determine the point of optimal power flow for the piezoelectric element. The paper then introduces an adaptive approach to achieving the optimal power flow through the use of a switch-mode dc-dc converter. This approach is similar to the so-called maximum power point trackers used to maximize power from solar cells. Finally, the paper presents experimental results that validate the technique. 

 

2. DESIGN

2.1. OPTIMAL POWER FLOW OF PIEZOELECTRIC DEVICE

fig.1  piezoelectric element model dc-dc converter

To determine its power flow characteristics, a vibrating piezoelectric element is modeled as a sinusoidal current source i_p (t) in parallel wit its internal electrode capacitance Cp. This model will be validated in a later section. The magnitude of the polarization current I_P varies with the mechanical excitation level of the piezoelectric element, but is assumed to be relatively constant regardless of external loading. A vibrating piezoelectric device generates an ac voltage while electromechanical batteries require a dc voltage, hence the first stage needed to be the output harvesting circuit is an ac-dc rectifier connected to the output of the piezoelectric device, as shown in the Fig. 1. In the following analysis, the dc filter capacitor Crect is assumed to be large; enough so that the output voltage Vrect is essentially constant; the load is modeled as a constant current sourceload ; and the diodes are assumed to exhibit ideal behavior.

The voltage and current waveforms associated with the circuit are shown in Fig 2. These waveforms can be divided into two intervals. In interval 1, denoted as u, the polarization current is charging the electrode capacitance of the piezoelectric element. During this time, all diodes are reverse-biased and no current flows to the output. This condition continues until the magnitude of the piezoelectric voltage vp (t) is equal to the output voltage Vrect. At the end of the communication interval, interval 2 begins, and output current flows to the capacitor Crect and the load.

				| sin (Tt) |.
							(2)
		+ io (t) ,   =
							(3)

By assuming C_rect  >> C_p , the majority of the current will be delivered as output current

 

The dc component of i_o (t) can be shown to be

The output power can be shown to vary with the value of the output voltage V_rect as follows

	+ P (t) ,   =
					(4)

It can then be shown that the peak output power occurs when

			(5)

Or one-half the peak open circuit voltage of the piezoelectric element

Fig.2 Voltage, current waveforms of a piezoelectric device

 

2.2. ENERGY HARVESTING CIRCUITRY

The magnitude of the polarization current I_p generated by the piezoelectric transducer, and hence the optimal rectifier voltage, may not be constant as it depends upon the vibration level exciting the piezoelectric element. This creates the need for flexibility in the circuit. i.e., the ability to adjust the output voltage of the rectifier to achieve maximum power transfer. To facilitate the attainment of the optimal voltage at the output of the rectifier, a dc-dc converter is shown in Fig. 3. Typically the controller of such a converter is designed to regulate the output voltage [11]; however, in this circuit the converter will be operated to maximize power flow into the battery. If effective, the piezoelectric element would be at peak power, which corresponds to the output voltage of the rectifier V_rect being maintained at its optimal value, approximately one-half the open circuit voltage, as described previously.

         The purpose of this circuit is to maximize the power flowing into the battery. As the battery voltage is essentially constant or changes very slowly, this is equivalent to maximizing the current into the battery, I_battery. By sensing this current, the duty cycle can be adjusted to maximize it. A control scheme such as this is general enough to be effective for many dc-dc converter topologies. To illustrate the theoretical principle of maximum power transfer and the control of the converter will be discussed in this paper. Fig. 4 shows a representation of the steady state battery current-duty cycle relationship using a step-down converter.

         In order to achieve peak battery current, an appropriate method of controlling the duty cycle is to incrementally increase or decrease the duty cycle as determined by the slope of the battery current curve JI/JD. The duty cycle is now the sum of the present duty cycle and the increment

D_i+1 = D_i  + K _sgn (JI/JD)

Where K is the assigned rate of change of the duty cycle and sgn() is the signum function which returns the sign of the quotient JI/JD.

         Note a few features of this control: First, as the control algorithm is based upon the sign of a rate of change, the duty cycle must continuously change in practice. Ideally, once the controller has settled, this will amount to small perturbations about the optimal operating point. Furthermore, as the control algorithm is based upon steady-state behavior of piezoelectric element and the dc-dc converter, a two time scale approach must be used when designing the controller [12]. Using two-time-scale analysis techniques, convergence of the controller can be assured provided the dynamics of the control algorithm are set to be “slow” enough such that the piezoelectric device and converter can be assumed to always be operating under steady state conditions. However, this also places limitations on the bandwidth of the controller.

 

2.3. CONTROL IMPLEMENTATION

         fig.5

The adaptive controller is implemented using a dSPACE DS1102 controller board. The board includes a Texas Instruments TMS320C31 floating point digital signal processor(DSP), analog-to-digital (ADC) converter for sampling measurements, and pulse-width modulated (PWM) signal outputs for controlling the converter. The control algorithm was developed in MATLAB 5.3 using the graphical interface Simulink 3.0 and the Real-Time Workshop to generate the controller code for the DSP.

         Fig. 5 shows a block diagram of the controller implementation. The initial duty cycle is set at 10% for circuit startup. The resulting battery current is evaluated using a current sense resistor in series with the battery and sampled by an A/D converter. The current signal is then low-pass filtered to attenuate noise and reduce the current ripple effect caused by the switching of the MOSFET. The derivative of the signal is then taken and divided by the derivative of the duty cycle. Dividing the derivative of the current by the derivative of duty cycle provides JI/JD. Which is used to determine the controller’s position n the battery current-duty cycle curve shown in Fig. 4.

                          

                                                       fig.4

          The sign of the quotient, JI/JD. is used by a 0-threshold block to increment the duty cycle by a set rate, in our case 21 mill percent /s (21-m%/s). This rate was determined to produce a measurable change in the battery current that could be used to evaluate the effectiveness of the new duty cycle. The resulting sign(+/-) of the division block, not its numerical magnitude, is all that is used by the 0- threshold block to increase or decrease the duty cycle. If either input signal would be zero, resulting in a zero or undefined quotient, the threshold block will decrease the duty cycle as a default. This default decrease allows the control to migrate to lower duty cycle values when the battery current might not be measurably changing, as is the case of circuit startup. Experimentation showed that, at a switching frequency of 1kHz, the current changes little at duty cycles above 10%, whereas optimal duty cycles occurred around 3-5%.

        

The duty cycle is then filtered and used to generate the PWM signal for the driver circuitry of the step-down converter. The additional filtering of the PWM signal is necessary to slow the rate of change of the duty cycle so the change in current can be measured and evaluated. Without the LPF, the controller is prone to duty cycle oscillations, as the perturbing signal reacts faster than the finite settling time of the battery current signal.

3. EXPERIMENTAL SETUP

         A Quickpack® QP20W purchased from Active Control eX-perts(ACX), Cambridge, MA, was used as the piezoelectric energy source. It is a two layer device that generates an ac voltage when vibrated in a direction perpendicular to its mid-plane. Device specifications and diagram are shown along with the piezoelectric element properties.

Fig.7

            The experimental setup is shown in Fig 7. The piezoelectric device is secured to an electric-powered shaker, which provides variable mechanical excitation in response to a sine wave input. The magnitude of the mechanical excitation of the piezoelectric element will be characterized by the open-circuit voltage that is measured across the unloaded rectifier capacitor, Voc. A small mass was added to the free tip of the bimorph to enhance the external stress and increase the tip deflection, thus providing a larger open-circuit voltage.

         The step-down converter consists of a MOSFET switch with a high breakdown voltage rating, a custom wound inductor with inductance of 10.03 mH, a Schotty diode, and a filter capacitor. The voltage across the current-sense resistor is amplified with a precision op-amp (powered by he 3V battery), and then sampled by the A/D converter on the controller card. The controller card then generates the PWM signals at the calculated duty cycle that is fed to a high side MOSFET driver. The driver was powered by an external dc power supply. Due to the low power levels expected from the piezoelectric element [2] – [4],[6], it is assumed that the converter will operate in discontinuous current condition mode at the chosen because switching losses in the experimental setup comprised a significant fraction of the power flow from the element.

4. RESULTS

         Experimental data were taken to illustrate the theories presented in this paper and to demonstrate the performance of the adaptive control algorithm. The first experiment was conducted in order to determine the validity of the piezoelectric model presented in the Fig.1. Various resistive loads were placed across the output of the excited piezoelectric element, as shown in fig. 8., and the output voltage was measured. The frequency of the excitation was adjusted to the resonant mode of the system for each resistor. This was done to ensure a relatively constant mechanical excitation level of the element throughout the experiment as the resistive load has a dampening effect on the amplitude of the mechanical vibrations. The output voltage for the circuit is given by

Ö1+ ( w CpR )2

 

(7)

R

Ip

Vo =

   

         A least squares fitting of the data to (7)resulted in I_p  equal to 2.2 mA_rms and C_p  equal to 0.184mF. Substituting these values into(7), the theoretical output voltage of the circuit can be compared to the measured output voltage over a range of load resistance as shown in Fig. 9.

         The next experiment was performed to validate the piezoelectric element-rectifier circuit optimal power transfer theory. Fig. 10 shows a plot of the output power versus the voltage maintained at the output rectifier for a vibrating piezoelectric element. The piezoelectric device was driven at a constant frequency of 53.8Hz and resistors of various values were inserted across the rectifier capacitor (see Fig. 1) to provide the load. At open-circuit condition, a voltage of 45.0 V was measured across the rectifier V_rect. The plot of power dissipated in the resistor at various voltages shows that the maximum power of 18.0mW is available with a 24.0kW resistor at a voltage of 20.57 V. this represents the maximum power available for a set level of execution and shows that maximum power occurs at a specific output voltage. The optimal rectifier voltage of the piezoelectric element. A possible reason for this discrepancy is unmodeled loss mechanisms in the piezoelectric device and/or rectifier.

         Using the same circuit and conditions, the output current io(t) of the piezoelectric element was measured using a 10W current sense resistor between the rectifier and the capacitor. The waveforms for load resistor of 430 kW to 0.51 kW across the rectifier capacitor. As the resistance is decreased, the communication interval u becomes smaller and the current waveform is closer to a rectified sine wave.

         To demonstrate that a dc-dc converter is capable of attaining the point of maximum power transfer, the step-down converter was operated with manually varied duty cycle. Fig. 10 revels that the battery current has a definite maximum with respect to the duty cycle. With a 45.0 V open-circuit voltage, the maximum current of 4.3 mA was measured at a duty cycle of 3.18%. at this point, the voltage at the rectifier bus capacitor was measured at 20.4 V(2V below one-half the open-circuit voltage). The current remained above 4mA for duty cycles between 2.5 and 4.5% and quickly decreased outside this range. The power stored by the 3 V batteries was 13.0 mW at the optimal duty cycle as compared to the previous experiment, which showed 18.0mW of power available with a resistive load. Power converter losses are therefore estimated to be 5mW. For comparison, direct charging of the battery across the rectifier capacitor yielded.5mA or 4.5 mW of power harvested.

         The adaptive controller was then used to show that the algorithm could find and maintain the maximum power into the battery at circuit startup and adjust itself aas the excitation varied. The initial duty cycle was set at 10%and the controller decreased the duty cycle linearly as the current increases. With an open-circuit voltage of 45.8 V, the controller settled to the maximum current of 4.3 mA. The controller then maintained maximum power transfer, while pertubing the duty cycle slightly.

         The settling time illustrates the duty cycle rate of change, 21-m%/s, and its effects. The value allows meaningful changes to the current to be measured without large oscillations around the maximum power point. This value does limit the controller speed at startup, taking almost 6 min to achieve maximum current, but once the optimum duty cycle is determined, it limits the oscillations that would increase the time away from the optimum duty cycle. Smaller rates of change that were investigated did not allow changes in the current to be reliably measured and larger rates cost inefficient harvesting due to the increased duty cycle oscillations.

           

5.APPLICATION

SHOE-POWERED RF TAG SYSTEM

         To demonstrate the feasibility and utility of scavenged shoe power, we developed a simple application circuit. The design is a self-powered, active radio frequency (RF) tag that transmits a short-range, 12-bit wireless identification (ID) code while the bearer walks. This system has immediate application in a smart environment, in which multiple users transmit their identities to the local surroundings. The IDs, for example, can enable a central server to make dynamic, near-real-time decisions to personalize the environment or route appropriate information to mobile users. Most previous work in this area relied on battery-powered infrared (IR) badges.9 Our RF-based design, however, requires no line of sight to the reader and therefore can be mounted in a shoe, operating without a battery under the power of a piezoelectric insert. Figure 11 shows a functional prototype pair of self-powered RFID sneakers.

Figure 11. Piezoelectric-powered RFID shoes with mounted electronics.

Figure 12 shows the RF tag system schematic. This design uses scavenged energy from either the PVDF or PZT source to encode and transmit a periodic, On/Off-keyed RFID signal using devices developed for automotive keyless entry systems. A local base station receives the transmission and emits an audible chirp upon identifying the transmitter. The signal from the piezoelectric source is full-wave rectified through 500-mA diode bridge D1. As the source signal ramps up, charge transfers to electrolytic bucket capacitor C1 whenever the source voltage overcomes the voltage already supported by this capacitor (plus two diode drops). As C1 charges beyond 12.6 V (the Z1 breakdown voltage plus the diode drop across the base-emitter junction of Q1), Q1 is forced into conduction, in turn activating Q2 and latching Q1. With Q1 on, the high side of C1 now has a current return path to ground and discharges through the Maxim MAX666 low-dropout (LDO) linear regulator U1. 

Figure 12

         The regulator is biased to provide a stable +5 V to the serial ID encoder U2 and RF transmitter U3, as long as C1 has sufficient charge to produce a valid regulator output voltage (Vout). Note that Vout exhibits some ripple when supplying the transmitter during the ID code’s On periods. When Vout swings below approximately 4.5 V (as set by R5 and R6), the low-battery in pin (LBin on U1) is pulled below its threshold, driving the low-battery out pin (LBout) to ground momentarily. This negative pulse through C3 turns Q1 Off, thus deactivating Q2 and renewing the C1 charging cycle. Note that R1, R2, and R3 bias Q1 and Q2 to show C1 a very high load impedance when the Q1-Q2 latch is deactivated. Finally, we included R4 and C2 to better match the load stage to the charging circuit and source impedance; the remaining resistors support the load stage components in other ways.

         Figure 13 is a representative graph of signals from the power-conditioning circuitry with the PZT source during a walk. The upper trace shows the voltage across C1 (in this case, 47 F), and the lower trace shows the MAX666 linear regulator’s output. Charge accumulates on the bucket capacitor, increasing with each step until the capacitor stores enough energy to power the transmitter for roughly half a second, generally after three to five steps with the current system. Substituting a high-frequency switching regulator for the MAX666 would further improve the efficiency of this circuit; this line of inquiry led to the results summarized in the following section.

Figure 13. Stored voltage (top) and regulated power output (bottom) waveforms for shoe-powered RFID transmitter while walking.

DISTRIBUTION SYSTEM EQUIPMENT

1  OUTDOOR CIRCUIT BREAKER (15 TO 230 KV)

            A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.

Types of circuit breaker

1. Low voltage circuit breakers

Low voltage (less than 1000 V_AC) types are common in domestic, commercial and industrial application, and include:

• MCB (Miniature Circuit Breaker)—rated current not more than 100 A, Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category.

• MCCB (Molded Case Circuit Breaker)—rated current up to 2500 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings.

• Low voltage power circuit breakers can be mounted in multi-tiers in LV switchboards or switchgear cabinets.

2. Magnetic circuit breaker

3. Thermal magnetic circuit breaker

4. Common trip breakers

5. Medium-voltage circuit breakers

Medium-voltage circuit breakers can be classified by the medium used to extinguish the arc:

• Vacuum circuit breaker—With rated current up to 3000 A, these breakers interrupt the current by creating and extinguishing the arc in a vacuum container. These are generally applied for voltages up to about 35,000 V,^[4] which corresponds roughly to the medium-voltage range of power systems. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.

• Air circuit breaker—Rated current up to 10,000 A. Trip characteristics are often fully adjustable including configurable trip thresholds and delays. Usually electronically controlled, though some models are microprocessor controlled via an integral electronic trip unit. Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.

SF₆ circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride gas.

6. High-voltage circuit breakers

High-voltage breakers are broadly classified by the medium used to extinguish the arc.

• Bulk oil
• Minimum oil
• Air blast
• Vacuum
• SF₆

2 GENERATORS AND MOTORS

               In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. The reverse conversion of electrical energy into mechanical energy is done by a motor; motors and generators have many similarities. An electric motor converts electrical energy into mechanical energy. Most electric motors operate through interacting magnetic fields and conductors to generate force, although a few use electrostatic forces. The reverse process, producing electrical energy from mechanical energy, is done by generators such as an alternator or a dynamo. Many types of electric motors can be run as generators, and vice versa. For example a starter/generator for a gas turbine, or traction motors used on vehicles, often perform both tasks. Electric motors and generators are commonly referred to as electric machines.

Comparison of motor types

Comparison of motor types
Type	Advantages	Disadvantages	Typical Application	Typical Drive
AC polyphase induction squirrel-cage	Low cost, long life, high efficiency, large ratings available (to 1 MW or more), large number of standardized types	Starting inrush current can be high, speed control requires variable frequency source	Pumps, fans, blowers, conveyors, compressors	Poly-phase AC, variable frequency AC
Shaded-pole motor	Low cost Long life	Rotation slips from frequency Low starting torque Small ratings low efficiency	Fans, appliances, record players	Single phase AC
AC Induction (split-phase capacitor)	High power high starting torque	Rotation slips from frequency Starting switch required	Appliances Stationary Power Tools	Single phase AC
Universal motor	High starting torque, compact, high speed	Maintenance (brushes) lifespan Only small ratings economic	Drill, blender, vacuum cleaner, insulation blowers	Single phase AC or DC
AC Synchronous	Rotation in-sync with freq - hence no slip	More expensive	Industrial motors Clocks Audio turntables tape drives	Poly-phase AC

3 MEDIUM VOLTAGE SWITCHGEARS (5 TO 15 KV)

                 The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is important because it is directly linked to the reliability of the electricity supply.

4 CURRENT LIMITING REACTORS

                       

              Current Limiting Reactors reduce short circuit levels to meet the system needs and reduce stresses on busses, insulators, circuit breakers and other high voltage devices. They are connected between the neutral of a system  and earth for limiting the line to ground current under system fault conditions. They are also used as load sharing  reactors for balancing the current in parallel circuits.

5 TRANSFORMERS

         A transformer is a static device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction.A distribution transformer (also called a pole mount transformer, pylon transformer, or colloquially a pole pig) is a transformer mounted on a utility pole. If the distribution lines are located underground, distribution transformers are mounted on concrete pads and locked in steel cases; these are known as pad mount transformers. The transformer provides the final voltage transformation in the electric power distribution grid, stepping down the voltage used in the overhead distribution wires to the level used by the customer. Because of weight restrictions transformers for pole mounting are only built for primary voltages under 30kV. Distribution  transformers  are  generally  used  in  electrical  power  distribution  and  transmission systems.  This class of transformer has the highest power, or volt-ampere ratings, and the highest continuous voltage rating.    The  power  rating  is  normally  determined  by  the  type  of  cooling methods the transformer may use.   Some commonly-used methods of cooling are by using oil or some other heat-conducting material.  Ampere rating is increased in a distribution transformer by increasing the size of the primary and secondary windings; voltage ratings are increased by increasing the voltage rating of the insulation used in making the transformer. Distribution transformers are used for lower voltage distribution networks as a means to end user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V, 230V).

            A delta-wye (Δ-Y) transformer is a transformer that converts three-phase electric power without a neutral wire into 3-phase power with a neutral wire. It can be a single three-phase transformer, or built from three independent single-phase units. The term Delta-Wye transformer is used in North America, and Delta-Star system in Europe.

Delta Wye Transformer

Delta-wye transformers are common in commercial, industrial, and high-density residential locations, to supply three three-phase distribution systems.

An example would be a distribution transformer with a delta primary, running on three 11kV phases with no neutral or earth required, and a star (or wye) secondary providing a 3-phase supply at 400 V, with the domestic voltage of 230 available between each phase and an earthed neutral point.

6 LOAD BREAK SWITCHES

         

                In power applications, switches perform function to energize or no energized an electric load. On the high end of the scale are load-break switches and disconnecting switches in power systems at the highest voltages (several hundred thousand volts).

Load-break switches are required to maintain the capability of interrupting the load current. The load break switch in a circuit with several hundred thousand volts, designed to carry a large amount of current without overheating the open position, having enough insulation to isolate the circuit in closed position, and equipped with arc interrupters to interrupt the load current.

Load break switches of air break are of versatile switch gear for transformer control & protection. They can also be used for Motor feeder in conjunction with Vacuum contactors. They are highly useful for Ring feeders for isolation of faulty section either manually or through remote control if fitted with motor operation. Load break switch in conjunction with HRC fuse can tackle high fault current and offer very good protection against dead short circuit capacity up to 40KA.The fault clearance and isolation through this combination will be achieved in a few milli seconds provided a proper selection of LBS and fuse is done. HRC fuses are manufactured with silver strips/silver coated Cu. Strips wires surrounded by granular quartz. When short-circuit occurs the metal element melts and the molten metal melts the surrounding quartz, making the combination an insulating material, called fulgarite. There will be a number of sections in the element which will create similar insulating media in quick succession and thus the arc will be extinguished in a matter of few milliseconds, say 5 to 8 milli-seconds, i.e., less than 1/2 a cycle.

7 MEDIUM VOLTAGE STARTERS

   A small motor can be started by simply plugging it into an electrical receptacle or by using a switch or circuit breaker. A larger motor requires a specialized switching unit called a motor starter or motor contactor. When energized, a direct on line (DOL) starter immediately connects the motor terminals directly to the power supply. A motor soft starter connects the motor to the power supply through a voltage reduction device and increases the applied voltage gradually or in steps.

Methods of Starting Three Phase Induction Motors

Direct Online Starting

                Direct online starting also known as across the line starting and full voltage starting, involves connecting each terminal of a three-phase induction motor to a separate line of a device. In this arrangement, the motor current is the same as the line current and the terminal voltage of the induction motor equals the line voltage.

Wye and Delta Connections

                   In a wye or star connection, the windings of the induction motor connect from the supply phases to the neutral. In a delta or mesh connection, the windings connect between the supply phases. A wye connection creates higher voltage to the windings of the three-phase induction motor than a delta connection. A starter with the ability to utilize both star and delta connections, also known as a wye start delta run connection, initializes the three-phase motor using a wye connection then transfers to a delta connection when the motor reaches a set speed.

Series Reactor Connection

           A reactor in series with the terminals of the motor decreases the terminal voltage of the induction motor, decreasing the initial current. The impedance decreases as the induction motor accelerates until a bypass method makes the motor run at full speed and full voltage.
Variable Frequency Drive

             A variable-frequency drive starts a three-phase induction motor at a frequency low enough to initialize a full-rated torque without an inrush of current. The low frequency increases the torque because it increases the impedance of the rotor circuit with slip frequency.

Methods of starting synchronous motor

Basically there are three methods that are used to start a synchronous motor:

• To reduce the speed of the rotating magnetic field of the stator to a low enough value that the rotor can easily accelerate and lock in with it during one half-cycle of the rotating magnetic field’s rotation. This is done by reducing the frequency of the applied electric power. This method is usually followed in the case of inverter-fed synchronous motor operating under variable speed drive applications.

• To use an external prime mover to accelerate the rotor of synchronous motor near to its synchronous speed and then supply the rotor as well as stator. Of course care should be taken to ensure that the direction of rotation of the rotor as well as that of the rotating magnetic field of the stator are the same. This method is usually followed in the laboratory- the synchronous machine is started as a generator and is then connected to the supply mains by following the synchronization or paralleling procedure. Then the power supply to the prime mover is disconnected so that the synchronous machine will continue to operate as a motor.

• To use damper windings or amortisseur windings if these are provided in the machine. The damper windings or amortisseur windings are provided in most of the large synchronous motors in order to nullify the oscillations of the rotor whenever the synchronous machine is subjected to a periodically varying load.

8. LOW VOLTAGE SWITCHGEAR

                The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. This type of equipment is important because it is directly linked to the reliability of the electricity supply. The very earliest central power stations used simple open knife switches, mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making open manually-operated switches too dangerous to use for anything other than isolation of a de-energized circuit. Oil-filled equipment allowed arc energy to be contained and safely controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed structure with electrically-operated switching elements, using oil circuit breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF₆ equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls, protection, metering and communications.

9. CONDUCTORS

         The wires and cables over which electrical energy is transmitted are made of copper, aluminum, steel, or a combination of copper and steel or aluminum and steel. A conductor is a material that readily permits the flow of an   electric   current.   Materials,   other   than   those mentioned, that conduct. In the early days conductor used on transmission lines were usually Copper, but Aluminium Conductors have completely replaced Copper because of the much lower cost and lighter weight of Aluminium conductor compared with a Copper conductor of the same resistance. The fact that Aluminium conductor has a larger diameter than a Copper conductor of the same resistance is also an advantage. With a larger diameter the lines of electric flux originating on the conductor will be farther apart at the conductor surface for the same voltage. This means a lower voltage gradient at the conductor surface and less tendency to ionize the air around the conductor. Ionization produces the undesirable effect called corona.

The symbols identifying different types of Aluminium conductors are as follows:-
AAC     : All Aluminium conductors.
AAAC   : All Aluminium Alloy conductors
ACSR   : Aluminium conductors, Steel-Reinforced
ACAR   : Aluminium conductor, Alloy-Reinforced

Aluminium alloy conductors have higher tensile strength than the conductor of EC grade Aluminium or AAC; ACSR consists of a central core of steel strands surrounded by layers of Aluminium strands. ACAR has a central core of higher strength Aluminium Alloy surrounded by layer of Electrical-Conductor-Grade Aluminium. Recently AAAC are being used in some SEBs to overcome menace of pilferage of ACSR and AAC conductors, particularly lower voltage lines. AAAC cannot be re-cycled and it does not have any common use for other purposes, as that in case of pure Aluminium.
AAAC is made out of heat treated Aluminium-Magnesium-Silicon Alloy designed as 64401 T 81 covered under IS: 9997:1991 containing 0.6-0.9% Magnesium and 0.5-0.9% Silicon. Besides use of AAAC on lower voltage lines from the point of view of avoiding its pilferage, it is also better for use in coastal areas to avoid corrosion problem prevalent in Steel core of ACSR conductors.

10. RECTIFIERS

                              A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which is in only one direction, a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components. The primary application of rectifiers is to derive DC power from an AC supply. Virtually all electronic devices require DC, so rectifiers find uses inside the power supplies of virtually all electronic equipment.

11. CAPACITORS

Shunt and series reactive compensation using capacitors has been a widely recognized and powerful method to combat the problems of voltage drops, power losses and voltage flicker in power distribution networks.

1.   Why Use Shunt Capacitor compensation in Distribution systems?

Fig. 1 represents an a.c. generator supplying a load through a line of series impedance (R+jX) ohms. Fig. 2(a) shows the phasor diagram when the line is delivering a complex power of (P+jQ) VA and Fig. 2 (b) shows the phasor diagram when the line is delivering a complex power of (P+jO) VA i.e. with the load fully compensated. A thorough examination of these phasor diagrams will reveal the following facts.

1. Current in the line, generator and intervening transformers, if any, is higher by a factor of  in the case of uncompensated load compared to compensated load. This results in a power loss, which is higher by a factor of () ² compared to the minimum power loss attainable in the system.
2. The loading on generator, transformers, line etc is decided by the current flow. The higher current flow in the case of uncompensated load necessitated by the reactive demand results in a tie up of capacity in this equipment by a factor of . i.e. compensating the load to UPF will release a capacity of (load VA rating X ) in all these equipment.
3. The sending-end voltage to be maintained for a specified receiving-end voltage is higher in the case of uncompensated load. The line has bad regulation with uncompensated load.
4. The sending-end power factor is less in the case of an uncompensated one. This due to the higher reactive absorption taking place in the line reactance. 
5. The excitation requirements on the generator are severe in the case of uncompensated load. Under this condition, the generator is required to maintain a higher terminal voltage with a greater current flowing in the armature at a lower lagging power factor compared to the situation with the same load fully compensated. It is entirely possible that the required excitation is much beyond the maximum excitation current capacity of the machine and  in that case further voltage drop at receiving-end will take place due to the inability of the generator to maintain the required sending-end voltage. It is also clear that the increased excitation requirement results in considerable increase in losses in the excitation system.

It is abundantly clear from the above that compensating a lagging load by using shunt capacitors will result in 

1. Lesser power loss everywhere up to the location of capacitor and hence a more efficient system 
2. Releasing of tied-up capacity in all the system equipments thereby enabling a postponement of the capital intensive capacity enhancement programmes to a later date.
3. Increased life of equipments due to optimum loading on them 
4. Lesser voltage drops in the system and better regulation 
5. Less strain on the excitation system of generators and lesser excitation losses. 
6. Increase in the ability of the generators to meet the system peak demand thanks to the released capacity and lesser power losses. 

Shunt capacitive compensation delivers maximum benefit when employed right across the load. And employing compensation in HT & LT distribution network is the closest one can get to the load in a power network. However, various considerations like ease of operation and control, economy achievable by lumping shunt compensation at EHV stations etc will tend to shift a portion of shunt compensation to EHV & HV substations. Power utilities in most countries employ about 60% capacitors on feeders, 30% capacitors on the substation buses and the remaining 10% on the transmission system. Application of capacitors on the LT side is not usually resorted to by the utilities. 

Just as a lagging system power factor is detrimental to the system on various counts, a leading system pf is also undesirable. It tends to result in over-voltages, higher losses, lesser capacity utilisation, and reduced stability margin in the generators. The reduced stability margin makes a leading power factor operation of the system much more undesirable than the lagging p.f operation.  This fact has to be given due to consideration in designing shunt compensation in view of changing reactive load levels in a power network.

Shunt compensation is successful in reducing voltage drop and power loss problems in the network under steady load conditions. But the voltage dips produced by DOL starting of large motors, motors driving sharply fluctuating or periodically varying loads, arc furnaces, welding units etc cannot be improved by shunt capacitors since it would require a rapidly varying compensation level. The voltage dips, especially in the case of a low short circuit capacity system can result in annoying lamp-flicker, dropping out of motor contactors due to U/V pick up, stalling of loaded motors etc and fixed or switched shunt capacitors are powerless against these voltage dips. But Thyristor controlled Static Var compensators with a fast response will be able to alleviate the voltage dip problem effectively.

2.   Why Use Series Capacitor Compensation in Distribution Systems? 

Shunt compensation essentially reduces the current flow everywhere upto the point where capacitors are located and all other advantages follow from this fact.But series compensation acts directly on the series reactance of the line. It reduces the transfer reactance between supply point and the load and thereby reduces the voltage drop. Series capacitor can be thought of as a voltage regulator, which adds a voltage proportional to the load current and there by improves the load voltage. 

Series compensation is employed in EHV lines to 1) improve the power transfer capability 2) improve voltage regulation 3) improve the load sharing between parallel lines. Economic factors along with the possible occurrence of sub-synchronous resonance in the system will decide the extent of compensation employed. 

Series capacitors, with their inherent ability to add a voltage proportional to load current, will be the ideal solution for handling the voltage dip problem brought about by motor starting, arc furnaces, welders etc. And, usually the application of series compensation in distribution system is limited to this due to the complex protection required for the capacitors and the consequent high cost. Also, some problems like self-excitation of motors during starting, ferroresonance, steady hunting of synchronous motors etc discourages wide spread use of series compensation in distribution systems.

12. STATIC VARIABLE SPEED DRIVE

Variable speed drives are equipment that is designed to regulate the torque output, or speed and rotational force, of an electric motor. The use of these drives significantly increase the efficiency of an electric motor by controlling the power fed into the machine based on the current demand or work required of the motor and are sometimes referred to as variable frequency drives. As a result, the motor is not driven at full capacity except when needed which can result in a significant increase in energy savings and a resulting prolonged life of the motor.

Types of Variable Speed Drives?

There are a number of types of variable speed or frequency drives in use throughout industry. Most of the drive types are designed to aid motors that use alternating current to achieve multiple speeds of operation.

AC Variable Speed Drive – Also known as an AC variable frequency drive (VFD). These drives control motors that use alternating current and are the most prevalent in industry.

DC Variable Speed Drive – These drives are designed to control a shunt would direct current (DC) motor which has separate shunt and armature circuits.

Pulse Width Modulated Frequency Drive (PWM) – The most complex of the variable speed drives, but results in the most efficiency through the use of transistors. These transistors are used to switch the DC at different frequencies which deliver voltage pulses to the motor.

Flux Vector Pulse Width Variable Speed Drives – These are one of the newer types of drives to be used in industry. The drive makes use of a control system that is similar to those used in a DC motor and have a microprocessor which is used to regulate motor operation.

Variable Voltage Input Speed Drive (VVI) – Also known as a VVI frequency drive. These drives are the most basic of the variable speed drives and make use of an output switching device which creates a new sine wave for the motors voltage through inputting a sequence of square waves at varying voltages.
Current Source Input Variable Speed Drive (CSI) – These drives are closely related to the VVI drives except that they force a square wave of current instead of voltage and require a large inverter in order to maintain a stable current.