Load Shedding

Load shedding is what electric utilities do when there is a huge demand for electricity that exceeds the generation available. The alternative is to have a brown-out where the voltage is reduced. 

It costs a lot to have generators standing by just in case there is a surge of demand, and the operators of those generators expect to be paid whether they run the generators or not. An alternative is if there is a large consumer of electricity (say, a factory) that could suddenly turn off all its electricity demand, they could agree to do that on request, and it has the same benefit as adding that amount of generation to the electric grid. In fact, it's better -- as there is less demand on the wires which are often saturated at the same time. 

That factory has losses from shutting down its equipment and idling its workers, but if the money it gets paid is enough, then it's worth it. This is an example of load shedding. 

There are many other cases where lots of smaller consumers agree to reduce demand on hot summer days, such as by reducing air conditioning or lighting. 

Someone who aggregates all these smaller cases can have the same effect as one big generator and so get the same money, and the operators of the electric grid are happy as it effectively solves the peak demand problem. 

When there is a shortfall in the electricity supply, there can be a need to reduce demand very quickly to an acceptable level, or risk the entire electricity network becoming unstable and shutting down completely. This is known as a “cascade” event, and can end in a total or widespread network shutdown affecting very large areas of a country. Some recent examples include the blackouts in northeast America and Canada in 2003 and across Italy in the same year and in India in 2012.

As the system frequency approaches the normal 60 Hz or 50 Hz ,a frequency relay can be used to automatically begin the restoration of the load that has been shed. The amount of load that can be restored  is determined by  the ability of the system to serve it .The criteria is that   the available generation must  always exceed  the amount  of  load being restored so that the system frequency will continue to recover . Any serious decrease in system frequency at this point could lead to undesirable load shedding repetition, which could start a system oscillation between shedding and restoration. This would be a highly undesirable condition. The availability of generation, either locally or through system interconnections ,   determines whether or not the shed load can be successfully restored.

Therefore, a load restoration program usually incorporates time delay, which is related to the amount of time required to add generation or to close tie- lines during emergency conditions. Also, both the time delay and the restoration frequency set points should be staggered so that all of the load is not   reconnected at the same time.  Reconnecting loads on a distributed basis also minimizes power swings across the system and thereby minimizes the possibility of initiating a new disturbance.

The drop in frequency may endanger generation itself .While a hydro-electric plant is   relatively unaffected by even a  ten percent  reduction in frequency, a thermal generating plant is quite sensitive  to even a five percent reduction.  Power output of a thermal plant depends to a great extent  on  its motor driven auxiliaries such as boiler feed water pumps, coal pulverizing and feeding equipment, and draft fans. As system frequency decreases, the power output to the auxiliaries begins to fall off rapidly which in

turn further reduces the energy input to the turbine generator. The situation thus has a cascading effect with a  loss of frequency leading to a loss of power which can cause the frequency to deteriorate further and the entire plant is soon in serious trouble. An additional major concern is the possible damage to the steam  turbines due  to prolonged operation at reduced frequency during this severe overload

condition .

Load shedding normally happens in two ways:

Automatic Load Shedding

This is a result of concurrent failures of major element(s) in the national grid (e.g.co-incidental generator or key transmission line failures), resulting in protection schemes initiating the automatic isolation of additional parts of the national grid, to protect the entire grid from cascading to a total blackout. Automatic load shedding always occurs on the transmission system level, with the result being large

amounts of electricity and large blocks of customers taken off supply in a very short time. Typical load reduction amounts can be in the order of 1000MW – 2000MW, affecting hundreds of thousands of customers.

Manual (Selective) Load Shedding

This occurs where time is available (typically up to 30mins) to make selective choices on what customers are shed. Selective load shedding often occurs on the distribution system level, and typically requires medium to small amounts of electricity to be “shed” in a short time (rolling blackout). Typical load reduction amounts can be in the order of 50MW – 100MW, affecting tens of thousands of customers at a time. 

In order to minimise the impact on individual customers and share the burden, rotational load shedding (rolling blckout) will occur on the low priority feeders if the load shedding duration extends for several hours. Typically the first group of customers who were shed will be restored after one or two hours, at the expense of the next group of customers to be taken off supply. This can continue until the supply/demand equation is balanced again and load shedding is no longer required.

Magnetic Inrush

When a transformer is initially connected to a source of AC voltage, there may be a substantial surge of current through the primary winding called inrush current.

We know that the rate of change of instantaneous flux in a transformer core is proportional to the instantaneous voltage drop across the primary winding. Or, as stated before, the voltage waveform is the derivative of the flux waveform, and the flux waveform is the integral of the voltage waveform. In a continuously-operating transformer, these two waveforms are phase-shifted by 90^o. Since flux (Φ) is proportional to the magnetomotive force (mmf) in the core, and the mmf is proportional to winding current, the current waveform will be in-phase with the flux waveform, and both will be lagging the voltage waveform by 90^o 

Let us suppose that the primary winding of a transformer is suddenly connected to an AC voltage source at the exact moment in time when the instantaneous voltage is at its positive peak value. In order for the transformer to create an opposing voltage drop to balance against this applied source voltage, a magnetic flux of rapidly increasing value must be generated. The result is that winding current increases rapidly, but actually no more rapidly than under normal conditions. Both core flux and coil current start from zero and build up to the same peak values experienced during continuous operation. Thus, there is no "surge" or "inrush" or current in this scenario.

Alternatively, let us consider what happens if the transformer's connection to the AC voltage source occurs at the exact moment in time when the instantaneous voltage is at zero. During continuous operation (when the transformer has been powered for quite some time), this is the point in time where both flux and winding current are at their negative peaks, experiencing zero rate-of-change (dΦ/dt = 0 and di/dt = 0). As the voltage builds to its positive peak, the flux and current waveforms build to their maximum positive rates-of-change, and on upward to their positive peaks as the voltage descends to a level of zero.

In an ideal transformer, the magnetizing current would rise to approximately twice its normal peak value as well, generating the necessary mmf to create this higher-than-normal flux. However, most transformers aren't designed with enough of a margin between normal flux peaks and the saturation limits to avoid saturating in a condition like this, and so the core will almost certainly saturate during this first half-cycle of voltage. During saturation, disproportionate amounts of mmf are needed to generate magnetic flux. This means that winding current, which creates the mmf to cause flux in the core, will disproportionately rise to a value easily exceeding twice its normal peak:

This is the mechanism causing inrush current in a transformer's primary winding when connected to an AC voltage source. As you can see, the magnitude of the inrush current strongly depends on the exact time that electrical connection to the source is made. If the transformer happens to have some residual magnetism in its core at the moment of connection to the source, the inrush could be even more severe. Because of this, transformer overcurrent protection devices are usually of the "slow-acting" variety, so as to tolerate current surges such as this without opening the circuit.

Surge Impedance Loading

The surge impedance loading or SIL of a transmission line is the MW loading of a transmission line at which a natural reactive power balance occurs.  

Transmission lines produce reactive power (Mvar) due to their natural capacitance. The amount of Mvar produced is dependent on the transmission line's capacitive reactance (XC) and the voltage (kV) at which the line is energized.  In equation form the Mvar produced is:  

                                                         

Transmission lines also utilize reactive power to support their magnetic fields (inductive).  The magnetic field strength is dependent on the magnitude of the current flow in the line and the line's natural inductive reactance (XL).  It follows then that the amount of Mvar used by a transmission line is a function of the current flow and inductive reactance.  In equation form the Mvar used by a transmission line is:

  

A transmission line's surge impedance loading or SIL is simply the MW loading (at a unity power factor) at which the line's Mvar usage is equal to the line's Mvar production.  In equation form we can state that the SIL occurs when:  

If we take the square root of both sides of the above equation and then substitute in the formulas for XL (=2pfL) and XC (=1/2pfC) we arrive at:  

The term     in the above equation is by definition the "surge impedance.  The theoretical

significance of the surge impedance is that if a purely resistive load that is equal to the surge impedance were connected to the end of a transmission line with no resistance, a voltage surge introduced to the sending end of the line would be absorbed completely at the receiving end.  The voltage at the receiving end would have the same magnitude as the sending end voltage and would have a phase angle that is lagging with respect to the sending end by an amount equal to the time required to travel across the line from sending to receiving end.

The value of the SIL to a system operator is realizing that when a line is loaded above its SIL it acts like a shunt reactor - absorbing Mvar from the system - and when a line is loaded below its SIL it acts like a shunt capacitor - supplying Mvar to the system.

Frequency

The primary reason for accurate frequency control is to allow the flow of alternating current power from multiple generators through the network to be controlled. The trend in system frequency is a measure of mismatch between demand and generation, and so is a necessary parameter for load control in interconnected systems.

Frequency of the system will vary as load and generation change. Increasing the mechanical input power to a synchronous generator will not greatly affect the system frequency but will produce more electric power from that unit. During a severe overload caused by tripping or failure of generators or transmission lines the power system frequency will decline, due to an imbalance of load versus generation. Loss of an interconnection, while exporting power (relative to system total generation) will cause system frequency to rise. Automatic generation control (AGC) is used to maintain scheduled frequency and interchange power flows. Control systems in power plants detect changes in the network-wide frequency and adjust mechanical power input to generators back to their target frequency. This counteracting usually takes a few tens of seconds due to the large rotating masses involved. Temporary frequency changes are an unavoidable consequence of changing demand. Exceptional or rapidly changing mains frequency is often a sign that an electricity distribution network is operating near its capacity limits, dramatic examples of which can sometimes be observed shortly before major outages.

Frequency protective relays on the power system network sense the decline of frequency and automatically initiate load shedding or tripping of interconnection lines, to preserve the operation of at least part of the network. Small frequency deviations (i.e.- 0.5 Hz on a 50 Hz or 60 Hz network) will result in automatic load shedding or other control actions to restore system frequency.

Smaller power systems, not extensively interconnected with many generators and loads, will not maintain frequency with the same degree of accuracy. Where system frequency is not tightly regulated during heavy load periods, the system operators may allow system frequency to rise during periods of light load, to maintain a daily average frequency of acceptable accuracy.

Frequency affects the power system in following ways;

 

Lighting

The first applications of commercial electric power were incandescent lighting  (normal bulb) and commutator-type electric motors. Both devices operate well on DC, but DC could not be easily changed in voltage, and was generally only produced at the required utilization voltage.

If an incandescent lamp is operated on a low-frequency current, the filament cools on each half-cycle of the alternating current, leading to perceptible change in brightness and flicker of the lamps.

Rotating machines

Commutator-type motors do not operate well on high-frequency AC because the rapid changes of current are opposed by the inductance of the motor field; even today, although commutator-type universal motors are common in 50 Hz and 60 Hz household appliances, they are small motors, less than 1 kW. The induction motor was found to work well on frequencies around 50 to 60 Hz but with the materials available in the 1890s would not work well at a frequency of, say, 133 Hz. There is a fixed relationship between the number of magnetic poles in the induction motor field, the frequency of the alternating current, and the rotation speed; so, a given standard speed limits the choice of frequency (and the reverse). Once AC electric motors became common, it was important to standardize frequency for compatibility with the customer's equipment.

Generators operated by slow-speed reciprocating engines will produce lower frequencies, for a given number of poles, than those operated by, for example, a high-speed steam turbine. For very slow prime mover speeds, it would be costly to build a generator with enough poles to provide a high AC frequency. As well, synchronizing two generators to the same speed was found to be easier at lower speeds. While belt drives were common as a way to increase speed of slow engines, in very large ratings (thousands of kilowatts) these were expensive, inefficient and unreliable.

Transmission and transformers

With AC, transformers can be used to step down high transmission voltages to lower customer utilization voltage. The transformer is effectively a voltage conversion device with no moving parts and requiring little maintenance. The use of AC eliminated the need for spinning DC voltage conversion motor-generators that require regular maintenance and monitoring.

Since, for a given power level, the dimensions of a transformer are roughly inversely proportional to frequency, a system with many transformers would be more economical at a higher frequency.

Electric power transmission over long lines favors lower frequencies. The effects of the distributed capacitance and inductance of the line are less at low frequency.

 

System interconnection

Generators can only be interconnected to operate in parallel if they are of the same frequency and wave-shape. By standardizing the frequency used, generators in a geographic area can be interconnected in a grid, providing reliability and cost savings.

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.