Power Swing

Power Swing which is basically caused by the large disturbances in the power system which if not blocked could cause wrong operation of the distance relay and can generates wrong or undesired tripping of the transmission line circuit breaker.

Power swings can cause the change in load impedance which under steady state conditions, whereas within the relay’s operating characteristic, to induce unwanted relay operations at different network locations. These undesirable measurements may aggravate the power-system disturbance and cause major power outages, or even power blackout. Particularly, distance relays should not trip unexpectedly during dynamic system conditions such as stable or unstable power swings, and allow the power system to return to a stable operating condition.  Thereby, a Power Swing Block (PSB) function is adopted in modern relays to prevent unwanted distance relay element operation during power swing . The main purpose of the PSB function is to differentiate between power faults and power swings, and block distance or other relay elements from operations during a power swing.

Out-of-Step (OOS) phenomena, which is same as an unstable power swing .  Uncontrolled tripping of circuit breakers during an OOS condition could cause equipment damage, pose a safety concern for operating personnel, and further contribute to cascading outage and shutdown of larger areas of the power system. So, the main purpose of the Out-of-Step Trip (OST) function should be taken into account to accomplish differentiation stable from unstable power swings, and separation to system areas at the predetermined network locations and at the appropriate source-voltage phase-angle difference between systems, in order to maintain power system stability and service continuity. 

The power system disturbances cause big oscillations in active and reactive power, low voltage, voltage instability and phase or angular instability between the generated and consumed power which results in loss of generation and load which effected both the power generation and the end customers.  During the steady state condition, power systems operate on the nominal frequency (50Hz or 60Hz). The complete synchronism of nominal frequency and voltage at the sending and receiving ends cause complete balance of active and reactive power between generated and consumed active and reactive powers. In steady state operating condition Frequency= Nominal frequency (50 or 60 Hz) +/– 0.02 Hz and Voltage=Nominal voltage +/– 5% [1].

Power system faults, line switching, generator disconnection, and the loss or application of large blocks of load result in sudden changes to electrical power.

Whereas the mechanical power input to generators remains relatively constant.

The electrical power, Pg transferred from the generator, an electric machine, to the load is given by the equation:

where:

Eg = Internal voltage and is proportional to the excitation current

El = Load Voltage

X = Reactance between the generator and the load

Angle that the internal voltage leads the load voltage

                                                                    

Pm = Mechanical Turbine Power of the generating unit

Pg = Electromagnetic Power output of the generating unit

Pa = Accelerating Power

The mechanical power, Pm, is provided by the turbine and the average mechanical power must be equal to the average electrical power. When a system disturbance occurs there is a change in one of the parameters of the electrical power equation.  For faults, typically the reactance between the generator and the load (X), the load voltage (El), or some combination of these two parameters causes the electrical power to change. For example, for a short circuit the load voltage is reduced, for a breaker opening the reactance increases. When a generation unit trips, the required electrical power from the remaining generators increases. In this case, the instantaneous mechanical power provided by the turbine is no longer equal to the instantaneous electrical power delivered or required by the load. When the load on a unit is suddenly increased, the energy furnished by the rotor results in a decrease in the rotor angular velocity . And this decrease in rotor velocity will cause oscillations in rotor angle and can result in severe power flow swings.

 Generator disconnection due to fault

Suppose we have two generators G1&G2 in parallel, and both the generators are sharing load. On the sudden disconnection of G2, there will be an increase in load on G1 and due to this there will be the oscillations in the rotor angle of G1, which is represented in Fig.

In Fig, d is the steady state rotor angle and d’ is the change in rotor angle due to oscillations which will result in

the oscillation of nominal voltage, and this oscillation in the nominal voltage causes loss of synchronism between the generators in parallel or between the generation and load.

Depending on the severity of the disturbance and the actions of power system controls, the system may remain stable and return to a new equilibrium state experiencing what is referred to as a stable power swing. Severe system disturbances, on the other hand, could cause large separation of generator rotor angles, large swings of power flows, large fluctuations of voltages and currents, and eventual loss of synchronism between

groups of generators or between neighboring utility systems. Stable Power Swing: Small disturbances which can be control by the action of Power System and the system remain in its steady state condition. Unstable Power Swing: Severe disturbances can produce a large separation of System Generator Rotor angles, large swings of power flow, large fluctuations of voltages and currents, and eventually lead to lose synchronism.

 Power Swing Effect on the Distance Relay

Power swings can cause the load impedance, which under steady state conditions is not within the relay’s operating characteristic,to enter into the relay’s operating characteristic. Operation of these relays during a power swing may cause undesired tripping of transmission lines or other power system elements, thereby weakening the system and possibly leading to cascading outages and the shutdown of major portions of the power system.

Distance or other relays should not trip during such as stable or unstable power swings, and allow the power system to return to a stable operating condition. Distance relay elements prone to operate during stable or transient power swings should be temporarily inhibited from operating to prevent system separation from occurring at random or in other than pre-selected locations. A Power Swing Block (PSB) function is available in modern relays to prevent unwanted distance relay element operation during power swings. The main purpose of the PSB function is to differentiate between faults and power swings and block distance or other relay elements from operating during a power swing. However, faults that occur during a power swing must be detected and cleared with a high degree of selectivity and dependability. Severe system disturbances could cause large separation of the rotor angles between groups of generators and eventual loss of synchronism between groups of generators or between neighboring utility systems. When two areas of a power system, or two interconnected systems, lose synchronism, the areas must be separated from each other quickly and automatically to avoid equipment damage and power blackouts. Ideally, the systems should be separated in predetermined locations to maintain a load-generation balance in each of the separated areas. System separation may not always achieve the desired load-generation balance. In cases where the separated area load is in excess of local generation, some form of load shedding is necessary to avoid a complete blackout of the area. Uncontrolled tripping of circuit breakers during an Out-of- Step (OOS) condition could cause equipment damage, pose a safety concern for utility personnel, and further contribute to cascading outages and the shutdown of larger areas of the power system.

Therefore, controlled tripping of certain power system elements is necessary to prevent equipment damage and widespread power outages and to minimize the effects of the disturbance.  The Out-of-Step Trip (OST) function accomplishes this separation. The main purpose of the OST function is to differentiate stable from unstable power swings and initiate system area separation at the predetermined network locations and at the appropriate source-voltage phase-angle difference between systems, in order to maintain power system stability and service continuity.

Dielectric Heating

A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarization. Because of dielectric polarization, positive charges are displaced toward the field and negative charges shift in the opposite direction. This creates an internal electric field which reduces the overall field within the dielectric itself. If a dielectric is composed of weakly bonded molecules, those molecules not only become polarized, but also reorient so that their symmetry axis aligns to the field.
The most obvious advantage to using such a dielectric material is that it prevents the conducting plates on which the charges are stored from coming into direct electrical contact. More significant, however, a high permittivity allows a greater charge to be stored at a given voltage.

Dielectric heating, also known as electronic heating, RF heating, high-frequency heating and diathermy, is the process in which a high-frequency alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is caused by molecular dipole rotation within the dielectric.RF dielectric heating at intermediate frequencies, due to its greater penetration over microwave heating, shows greater promise than microwave systems as a method of very rapidly heating and uniformly preparing certain food items, and also killing parasites and pests in certain harvested crops.

Molecular rotation occurs in materials containing polar molecules having an electrical dipole moment, with the consequence that they will align themselves in an electromagnetic field. If the field is oscillating, as it is in an electromagnetic wave or in a rapidly-oscillating electric field, these molecules rotate to continuously align with it. This is called dipole rotation. As the field alternates, the molecules reverse direction. Rotating molecules push, pull, and collide with other molecules (through electrical forces), distributing the energy to adjacent molecules and atoms in the material. Once distributed, this energy appears as heat.
Temperature is the average kinetic energy (energy of motion) of the atoms or molecules in a material, so agitating the molecules in this way increases the temperature of the material. Thus, dipole rotation is a mechanism by which energy in the form of electromagnetic radiation can raise the temperature of an object. Dipole rotation is the mechanism normally referred to as dielectric heating, and is most widely observable in the microwave oven where it operates most efficiently on liquid water, and much less so on fats and sugars. This is because fats and sugar molecules are far less polar  than water molecules, and thus less affected by the forces generated by the alternating electromagnetic fields.

Flyback Transformers

                                    

Flyback transformers, popularly known as the Line Output Transformers, is a special mechanism of converting the energy supply, both voltage and current, into electronic circuits. Although it is termed as a transformer, it works against the typical functions of a conventional transformer, and is exploited more as energy storage equipment. When the primary switch is on, the energy is stored on ferrite core that has the air gap in it. However, when the primary switch is off, the energy is not stored but transferred to the outputs. The current will flow either in the primary winding, or the secondary one, but not both at the same time. Thus, a flyback transformer is often misguided to be an inductor having the secondary windings.

                              

                              

The primary  winding of the flyback transformer  is wound first around a ferrite rod, and then the secondary is wound around the primary. This arrangement minimizes the leakage inductance of the primary. A ferrite frame is wrapped around the primary/secondary assembly, closing the magnetic field lines. Between the rod and the frame is an air gap, which increases the reluctance. The secondary is wound layer by layer with enameled wire.

The primary winding of the flyback transformer is driven by a switch from a DC supply (usually a transistor). During the switch on time, there wont be any power conversion from primary to secondary side, since secondary side diode will be reverse biased, hence energy is stored in the inductor itself. In order to store the energy in magnetic field we use airgap in the inductor. but if it is too large, leakage inductance problem will occur. So provide airgap with minmum as per calculation obtained. 
When switch is off, the stored energy in the inductor(primary) will be transfered to the secondary. Through diode the capacitor will get charged. and this stored energy in the capacitor will be discharged when switch is in ON. The cycle then can be repeated. If the secondary current is allowed to discharge completely to zero (no energy stored in the core) then it is said that the transformer works in discontinuous mode. When some energy is always stored in the core then this is continuous mode.

Once the voltage reaches such level as to allow the secondary current to flow, then the current in the secondary winding begins to flow in a form of a descending ramp signal.

The current does not flow simultaneously in primary and secondary (output) windings. Because of this the flyback transformer is really a loosely coupled inductor rather than classical transformer, in which currents do flow simultaneously in all magnetically coupled windings.

Cuk Converter

The Cuk converter is a step-down/step-up converter based on a switching boost-buck topology. Essentially, the converter is composed of two sections, an input stage and an output stage.

The input voltage vg is fed into the circuit via inductor L1. When transistor Q1 is on, current i1 builds the

magnetic  field of the inductor in the input stage. The diode CR1 is reverse biased, and energy dissipates

from the storage elements in the output stage. When Q1 turns off, inductor L1 tries to maintain the current flowing through it by reversing polarity and sourcing current as its magnetic field collapses. It thus provides energy to the output stage of the circuit via capacitor C1. R1 and R2 are parasitic or stray resistances of inductor.

The inductor currents are the input and output currents, therefore, if the principle of conservation of energy is applied:

where Ds is the duty cycle of the switch :

The voltage ratio of a Cuk converter is the same as that of a buck-boost converter, but its main advantage over other converters is that the input and output inductors result in a filtered current on both sides of the converter, while buck, boost, and buck-boost converters have a pulsating current that occurs on at least one side of the circuit i.e either on input side or output side.

This pulsation will increase the ripple in the circuit and due to this ripple , the efficiency of battery gets lowered. To ensure good efficiency ripple should be reduced.

By controlling the duty cycle of the switch , the output voltage vo  can be controlled and can be higher or lower than the input voltage vg. By using a controller to vary the duty cycle during operation, the circuit can also be made to reject disturbances ,as second part of circuit consists of parallel resonance circuit and it work as a tank circuit for specific frequency (resonant frequency) , and during resonance current will not be allowed to enter in the circuit.

Series Resonance

In a series RLC circuit there becomes a frequency point were the inductive reactance of the inductor becomes equal in value to the capacitive reactance of the capacitor. In other words, X_L = X_C. The point at which this occurs is called the Resonant Frequency point, ( ƒ_r ) and as we are analysing a series RLC circuit this resonance frequency produces a Series Resonance circuit.

As the frequency approaches infinity the inductors reactance would also increase towards infinity with the circuit element acting like an open circuit. However, as the frequency approaches zero or DC, the inductors reactance would decrease to zero, causing the opposite effect acting like a short circuit. This means then that inductive reactance is "Proportional" to frequency and is small at low frequencies and high at higher frequencies.

The major difference between series and parallel resonance is that due to the formation of Tank Circuit , large amount of circulating current exists and it will be exactly oppositely following series resonance current curve.   

As the frequency approaches infinity the capacitors reactance would reduce to zero causing the circuit element to act like a perfect conductor of 0Ω's. However, as the frequency approaches zero or DC level, the capacitors reactance would rapidly increase up to infinity causing it to act like a very large resistance acting like an open circuit condition. This means then that capacitive reactance is "Inversely proportional" to frequency for any given value of capacitance.

Electrical resonance occurs in an AC circuit when the two reactances which are opposite and equal cancel each other out as X_L = X_C and the point on the graph at which this happens is were the two reactance curves cross each other. 

Note that when the capacitive reactance dominates the circuit the impedance curve has a hyperbolic shape to itself, but when the inductive reactance dominates the circuit the curve is non-symmetrical due to the linear response of X_L. If the circuits impedance is at its minimum at resonance then consequently, the circuits  admittance must be at its maximum and one of the characteristics of a series resonance circuit is that admittance is very high. But this can be a bad thing because a very low value of resistance at resonance means that the circuits current may be dangerously high.

The frequency response curve of a series resonance circuit shows that the magnitude of the current is a function of frequency and plotting this onto a graph shows us that the response starts at near to zero, reaches maximum value at the resonance frequency when I_MAX = I_R and then drops again to nearly zero as ƒ becomes infinite. The result of this is that the magnitudes of the voltages across the inductor, L and the capacitor, C can become many times larger than the supply voltage, even at resonance but as they are equal and at opposition they cancel each other out. As a series resonance circuit only functions on resonant frequency, this type of circuit is also known as an Acceptor Circuit because at resonance, the impedance of the circuit is at its minimum so easily accepts the current whose frequency is equal to its resonant frequency. The effect of resonance in a series circuit is also called "voltage resonance"

Boost Converter

A boost converter also known as step-up converter is a power converter with an output DC voltage greater than its input DC voltage. It is a class of switching-mode power supply containing at least two semiconductor switches which includes a diode and a transistor, and at least one energy storage element. Filters made of capacitors (sometimes in combination with inductors) are normally added to the output of the converter to reduce output voltage ripple. Output voltage ripple can be defined as difference between maximum and minimum output current. Large ripple introduces large loses e.g. harmonics, negative sequence current etc..

 A boost converter is sometimes called a step-up converter since it “steps up” the source voltage. Since power  must be conserved, the output current is lower than the source current. The key principle that drives the boost converter is the tendency of an inductor to resist changes in current. When the switch is turned-ON, the current flows through the inductor and energy is stored in it. When the switch is turned-OFF, the stored energy in the inductor tends to collapse and its polarity changes such that it adds to the input voltage. Thus, the voltage across the inductor and the input voltage are in series and together charge the output capacitor to a voltage higher than the input voltage.

The other function of inductor is to avoid any sudden change in current due to the fact that as these converters are generally attached to battery therefore to  ensure the good efficiency of battery , current should be uniform for most of the time. The other function of capacitor is to absorb AC component of supply which generally originates due to switching.

Photodiode And Photovoltaic Mode (PV mode)

Silicon photodiodes are constructed from single crystal silicon wafers . The purity of silicon is directly related to its resistivity, with higher resistivity indicating higher purity silicon.  A cross section of a typical silicon photodiode is shown in the figure. N type silicon is the starting material. A thin "p" layer is formed on the front surface of the device by thermal diffusion or ion implantation of the appropriate doping material (usually boron). The interface between the "p" layer and the "n" silicon is known as a pn junction. Small metal contacts are applied to the front surface of the device and the entire back is coated with a contact metal. The back contact is the cathode, the front contact is the anode. The active area is coated with either silicon nitride, silicon monoxide or silicon dioxide for protection and to serve as an anti-reflection coating. The thickness of this coating is optimized for particular irradiation wavelengths. As an example, a Centro Vision Series 5-T photodiode has a coating which enhances its response to the blue part of the spectrum.

Photodiode junctions are unusual because the top "p" layer is very thin. The thickness of this layer is determined by the wavelength of radiation to be detected. Near the pn junction the silicon becomes depleted of electrical charges. This is known as the "depletion region". The depth of the depletion region can be varied by applying a reverse bias voltage across the junction. When the depletion region reaches the back of the diode the photodiode is said to be "fully depleted". The depletion region is important to photodiode performance since most of the sensitivity to radiation originates there. The capacitance of the pn junction depends on the thickness of this variable depletion region. Increasing the bias voltage increases the depth of this region and lowers capacitance until the fully depleted condition is achieved. Junction capacitance is also a function of the resistivity of silicon used and active area size.        

            

                                           

Due to concentration gradient, the diffusion of electrons from the N- type region to the P-type region and the diffusion of holes from the P- type region to the N-type region, develops a built-in voltage across

the junction. The inter-diffusion of electrons and holes between the N and P regions across the junction results in a region with no free carri- ers. This is the depletion region. The built-in voltage across the deple- tion region results in an electric field with maximum at the junction and no field outside of the depletion region. Any applied reverse bias adds to the built in voltage and results in a wider depletion region.The electron-hole pairs generated by light are swept away by drift inthe depletion region and are collected by diffusion from the undepleted region. The current generated is proportional to the incident light or radiation power. The light is absorbed exponentially with distance and is proportional to the absorption coefficient. The absorption coefficient is very high for shorter wavelengths in the UV region and is small for longer wavelengths . Hence, short wavelength photons such as UV, are absorbed in a thin top surface layer while silicon becomes transparent to light wavelengths longer than 1200 nm. Moreover, photons with energies smaller than the band gap are not absorbed at all. The boundaries of the depletion region act as the plates of a parallel plate capacitor. The junction capacitance is directly proportional to the diffused area and inversely proportional to the width of the depletion region. In addition, higher resistivity substrates have lower junction capacitance. In photoconductive mode (reverse biased), however, the drift current becomes the dominant current (dark current) and varies directly with temperature.

As time constant is directly proportional to capacitance, therefore, lower the capacitance, lesser will br time constant and hence lesser will be response time.