{"pinia":{"ui":{"prefs":{"theme":"system","textScale":"md","fontFamily":"sans","reducedMotion":false,"highContrast":false}},"academy":{"universities":[],"universitySummaries":[{"slug":"rtu","name":"RTU","fullName":"Rajasthan Technical University","branches":[{"slug":"first-year-common","name":"First Year (Common)"},{"slug":"computer-science","name":"Computer Science"},{"slug":"mechanical","name":"Mechanical"},{"slug":"civil","name":"Civil"},{"slug":"ai-data-science","name":"AI & Data Science"},{"slug":"ee-ec-eee","name":"EE / EC / EEE"}]},{"slug":"aktu","name":"AKTU","fullName":"Dr. A. P. J. Abdul Kalam Technical University","branches":[{"slug":"first-year-common","name":"First Year (Common)"},{"slug":"computer-science","name":"Computer Science"}]}],"activePapers":[{"id":"power-quality-and-facts-2021-7E1733","title":"Power Quality and FACTS (Feb-March 2021)","subjectSlug":"power-quality-and-facts","subjectName":"Power Quality and FACTS","universitySlug":"rtu","branchSlug":"ee-ec-eee","year":4,"semester":7,"paperCode":"7E1733","questions":[{"id":"power-quality-and-facts-2021-7E1733-pA-q1","number":1,"marks":2,"tags":["Part A","Q.1"],"text":[{"type":"paragraph","content":"Q.1. What are various power quality issues? [2]"}],"answer":{"explanation":[{"type":"paragraph","content":"Power quality issues refer to deviations of voltage, current, or frequency from their ideal sinusoidal, constant-magnitude, constant-frequency waveform, which cause failure or mis-operation of customer equipment."},{"type":"list","items":["Voltage sag and swell (short duration RMS variations)","Harmonics and waveform distortion (THD, notching, DC offset)","Transients (impulsive and oscillatory)","Voltage flicker (fluctuation from arc furnaces, welders)","Voltage imbalance in three-phase systems","Interruptions (momentary/sustained) and frequency variation"]}]}},{"id":"power-quality-and-facts-2021-7E1733-pA-q2","number":2,"marks":2,"tags":["Part A","Q.2"],"text":[{"type":"paragraph","content":"Q.2. What do you mean by harmonics? [2]"}],"answer":{"explanation":[{"type":"paragraph","content":"Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the fundamental power system frequency (e.g., 100 Hz, 150 Hz for a 50 Hz system). They combine with the fundamental to produce a non-sinusoidal, distorted waveform."},{"type":"paragraph","content":"Harmonics are mainly generated by non-linear loads such as rectifiers, inverters, arc furnaces, and switch-mode power supplies. Their severity is quantified by Total Harmonic Distortion (THD), and they cause overheating of transformers/cables, capacitor bank resonance, and mal-operation of protective relays."}]}},{"id":"power-quality-and-facts-2021-7E1733-pA-q3","number":3,"marks":2,"tags":["Part A","Q.3"],"text":[{"type":"paragraph","content":"Q.3. Write short note on TCSC. [2]"}],"answer":{"explanation":[{"type":"paragraph","content":"TCSC (Thyristor Controlled Series Capacitor) is a series-connected FACTS controller consisting of a fixed series capacitor in parallel with a thyristor-controlled reactor (TCR)."},{"type":"list","items":["By varying the firing angle of the anti-parallel thyristors controlling the reactor, the effective reactance of the TCSC can be smoothly varied between capacitive and inductive regions","It provides fast, continuous control of line series compensation, improving power transfer capability, damping power oscillations, and mitigating sub-synchronous resonance"]}]}},{"id":"power-quality-and-facts-2021-7E1733-pA-q4","number":4,"marks":2,"tags":["Part A","Q.4"],"text":[{"type":"paragraph","content":"Q.4. What do you mean by fault current limiter? [2]"}],"answer":{"explanation":[{"type":"paragraph","content":"A fault current limiter (FCL) is a device connected in series with a power system that presents very low impedance under normal operation but rapidly increases its impedance to limit the prospective fault current magnitude when a short circuit occurs."},{"type":"paragraph","content":"FCLs may be superconducting, solid-state (thyristor-based), or reactor-type. They reduce the stress on circuit breakers and switchgear, allow use of lower-rated equipment, and improve system reliability during fault conditions."}]}},{"id":"power-quality-and-facts-2021-7E1733-pA-q5","number":5,"marks":2,"tags":["Part A","Q.5"],"text":[{"type":"paragraph","content":"Q.5. What is voltage swell? [2]"}],"answer":{"explanation":[{"type":"paragraph","content":"A voltage swell is a short-duration increase in RMS voltage to between 1.1 pu and 1.8 pu of nominal, lasting from 0.5 cycles up to 1 minute, as per IEEE 1159 classification."},{"type":"list","items":["Caused by single-line-to-ground faults on adjacent phases, sudden large load switch-off, or energization of large capacitor banks","Can stress insulation and cause overvoltage tripping of sensitive equipment"]}]}},{"id":"power-quality-and-facts-2021-7E1733-pA-q6","number":6,"marks":2,"tags":["Part A","Q.6"],"text":[{"type":"paragraph","content":"Q.6. What is flicker meter? [2]"}],"answer":{"explanation":[{"type":"paragraph","content":"A flicker meter is an instrument used to measure voltage flicker, i.e., rapid, repetitive variations in RMS voltage magnitude that cause perceptible fluctuation in lamp luminosity and visual annoyance to the human eye."},{"type":"paragraph","content":"It is standardized under IEC 61000-4-15 and outputs short-term (Pst, over 10 minutes) and long-term (Plt, over 2 hours) flicker severity indices. Flicker is typically caused by arc furnaces, welding machines, and rapidly varying industrial loads."}]}},{"id":"power-quality-and-facts-2021-7E1733-pA-q7","number":7,"marks":2,"tags":["Part A","Q.7"],"text":[{"type":"paragraph","content":"Q.7. What do you mean by Reactive Power Compensation? [2]"}],"answer":{"explanation":[{"type":"paragraph","content":"Reactive power compensation is the process of controlling reactive power (VARs) flow in a power system, either by generating or absorbing it locally, to regulate voltage, improve power factor, and reduce transmission losses."},{"type":"list","items":["Achieved using shunt capacitors, shunt reactors, synchronous condensers, SVC (Static VAR Compensator), or STATCOM","Improves voltage profile, system stability, and power transfer capability while reducing I2R losses"]}]}},{"id":"power-quality-and-facts-2021-7E1733-pA-q8","number":8,"marks":2,"tags":["Part A","Q.8"],"text":[{"type":"paragraph","content":"Q.8. Define the term Notching. [2]"}],"answer":{"explanation":[{"type":"paragraph","content":"Notching is a periodic voltage disturbance caused by the normal commutation process of power electronic converters (such as line-commutated rectifiers and inverters) when current is transferred from one phase to another."},{"type":"paragraph","content":"During commutation, a momentary short circuit occurs between two phases, producing a notch in the voltage waveform. Since it occurs during every cycle, notching is classified as a steady-state waveform distortion and its severity is analyzed using its harmonic spectrum rather than as a transient."}]}},{"id":"power-quality-and-facts-2021-7E1733-pA-q9","number":9,"marks":2,"tags":["Part A","Q.9"],"text":[{"type":"paragraph","content":"Q.9. Define the term voltage imbalance. [2]"}],"answer":{"explanation":[{"type":"paragraph","content":"Voltage imbalance (or unbalance) is a condition in a three-phase system in which the RMS magnitudes of the phase voltages or the phase angles between them are not equal."},{"type":"paragraph","content":"It is quantified either by the NEMA definition (maximum deviation from average voltage divided by average voltage, expressed as percentage) or by the symmetrical-component method (ratio of negative-sequence to positive-sequence voltage). It is caused by unbalanced single-phase loads, untransposed lines, or blown fuses on capacitor banks, and can cause overheating of induction motors."}]}},{"id":"power-quality-and-facts-2021-7E1733-pA-q10","number":10,"marks":2,"tags":["Part A","Q.10"],"text":[{"type":"paragraph","content":"Q.10. What do you mean by voltage source converters? [2]"}],"answer":{"explanation":[{"type":"paragraph","content":"A voltage source converter (VSC) is a power electronic converter, typically built with self-commutating switching devices such as IGBTs along with anti-parallel diodes, that converts a DC bus voltage into a controllable AC voltage of desired magnitude, phase, and frequency."},{"type":"list","items":["Uses PWM switching to synthesize the AC output waveform from a DC capacitor voltage source","Forms the core building block of modern FACTS and custom power devices such as STATCOM, DVR, UPQC, and VSC-based HVDC links"]}]}},{"id":"power-quality-and-facts-2021-7E1733-pB-q1","number":11,"marks":8,"tags":["Part B","Q.1"],"text":[{"type":"paragraph","content":"Q.1. Define series compensation at the mid-point of an AC Transmission line. [8]"}],"answer":{"explanation":[{"type":"subheading","content":"Series Compensation Concept"},{"type":"paragraph","content":"Series compensation involves inserting a capacitor (or a controllable series device such as TCSC/SSSC) in series with a transmission line to reduce its effective inductive reactance. Since the maximum power transfer over a line is inversely proportional to its series reactance, reducing the effective reactance directly increases the power transfer capability of the line for a given sending and receiving end voltage and angle."},{"type":"subheading","content":"Why Mid-Point Location"},{"type":"paragraph","content":"For a long transmission line, compensation can be placed at the sending end, receiving end, or at the mid-point. Analysis using the transmission line equivalent circuit shows that when a series capacitor is placed at the electrical mid-point of the line, it symmetrically divides the line into two equal halves, each carrying half of the total voltage drop. This symmetric division produces the largest possible increase in the effective power-angle curve compared to any other location for the same amount of compensation, i.e., mid-point compensation gives the maximum improvement in power transfer capability and transient stability margin."},{"type":"paragraph","content":"The mid-point voltage also receives maximum support, since compensation forces the voltage profile along the line to bulge upward near the middle rather than sagging as it would in an uncompensated line. This flattening of the voltage profile improves voltage regulation for the connected loads and reduces reactive power demand from the sources at both ends."},{"type":"subheading","content":"Benefits and Practical Use"},{"type":"list","items":["Increases steady-state and transient stability limits of the transmission corridor","Improves voltage profile along the line, reducing need for additional shunt compensation","Enables better load sharing among parallel transmission lines","Practically implemented via fixed series capacitors, or dynamically via TCSC/SSSC for continuous control and to damp sub-synchronous resonance and power oscillations"]}]}},{"id":"power-quality-and-facts-2021-7E1733-pB-q2","number":12,"marks":8,"tags":["Part B","Q.2"],"text":[{"type":"paragraph","content":"Q.2. What is DVR? What are the component of DVR? Explain its operation. [8]"}],"answer":{"explanation":[{"type":"subheading","content":"Definition of DVR"},{"type":"paragraph","content":"A Dynamic Voltage Restorer (DVR) is a series-connected custom power device installed between the supply and a sensitive load, which injects a controlled compensating voltage in series with the incoming supply voltage to correct voltage sags, swells, and imbalances before they reach the protected load."},{"type":"subheading","content":"Components of DVR"},{"type":"list","items":["Voltage source converter (VSC): IGBT-based converter that generates the compensating AC voltage waveform from a DC source","Injection (series) transformer: couples the VSC output in series with the line and provides galvanic isolation and voltage matching","Energy storage / DC-link capacitor: supplies the real power needed during voltage sag compensation (battery, capacitor bank, or flywheel)","Harmonic filter: removes high-frequency PWM switching harmonics from the injected voltage","Control and monitoring system: continuously senses supply voltage, detects sag/swell events, and computes the required injection voltage magnitude and phase"]},{"type":"subheading","content":"Operation"},{"type":"paragraph","content":"During normal operation, the DVR injects little or no voltage and the injection transformer is essentially transparent to load current. When the control system detects a voltage sag on the supply side, it rapidly (within a few milliseconds) commands the VSC to generate a compensating voltage of appropriate magnitude and phase so that, when added in series with the sagged supply voltage through the injection transformer, the resultant voltage delivered to the load is restored to its nominal rated value."},{"type":"paragraph","content":"The required real power for this injected voltage is drawn from the DC-link energy storage. Once the sag or swell condition clears, the DVR ramps down its injected voltage smoothly to avoid transients and returns to standby mode. Because the DVR only needs to compensate for the difference (missing) voltage rather than supply the full load power, it can be rated at a fraction of the load rating, making it a cost-effective solution for protecting sensitive industrial loads from short-duration voltage disturbances."},{"type":"svg","content":"","alt":"DVR block diagram showing supply, series injection transformer, VSC with DC link, and load"}]}},{"id":"power-quality-and-facts-2021-7E1733-pB-q3","number":13,"marks":8,"tags":["Part B","Q.3"],"text":[{"type":"paragraph","content":"Q.3. Explain the principal of operation of STATCOM. [8]"}],"answer":{"explanation":[{"type":"subheading","content":"STATCOM Overview"},{"type":"paragraph","content":"A STATCOM (Static Synchronous Compensator) is a shunt-connected FACTS device built around a voltage source converter (VSC) that generates a controllable AC voltage from a DC capacitor. It is connected to the power system bus through a coupling transformer and functionally behaves like an ideal synchronous condenser, exchanging reactive power (and, with storage, real power) with the network."},{"type":"subheading","content":"Principle of Operation"},{"type":"paragraph","content":"The STATCOM's VSC generates a fundamental-frequency voltage V_conv that is synchronized in phase with the system bus voltage V_sys but whose magnitude is controllable. Reactive power exchange between the STATCOM and the system is determined by the magnitude difference between V_conv and V_sys across the coupling transformer's leakage reactance. If V_conv is greater than V_sys, the STATCOM supplies (generates) reactive power to the system, acting as a capacitor. If V_conv is less than V_sys, the STATCOM absorbs reactive power from the system, acting as an inductor. When the two voltages are equal in magnitude, no reactive power flows."},{"type":"paragraph","content":"Since the reactive current exchanged depends on the converter's own generated voltage rather than on the system voltage magnitude directly, the STATCOM can maintain its rated reactive current output even during low-voltage conditions, unlike a purely passive SVC. This gives it superior dynamic performance during faults and voltage sags, faster response time, and a smaller physical footprint compared to conventional synchronous condensers."},{"type":"subheading","content":"Applications"},{"type":"list","items":["Dynamic voltage regulation and support at transmission and distribution buses","Power factor correction and reactive power compensation","Damping of power system oscillations and improvement of transient stability","Mitigation of voltage flicker from fluctuating loads such as arc furnaces"]}]}},{"id":"power-quality-and-facts-2021-7E1733-pB-q4","number":14,"marks":8,"tags":["Part B","Q.4"],"text":[{"type":"paragraph","content":"Q.4. What are the causes and effects of power frequency variation? [8]"}],"answer":{"explanation":[{"type":"subheading","content":"Causes of Power Frequency Variation"},{"type":"paragraph","content":"System frequency is directly governed by the balance between total generation and total load (including losses) in an interconnected power system, since generator rotor speed and frequency are proportional. Frequency variation occurs whenever this generation-load balance is disturbed."},{"type":"list","items":["Sudden loss of a large generating unit, causing frequency to drop as remaining generators supply the deficit","Sudden loss of a large load or line, causing frequency to rise momentarily","Faults and switching operations that disconnect generation or transmission lines","Poor governor response or inadequate spinning reserve during load changes","Islanding of a section of the grid, where local generation-load mismatch is more severe"]},{"type":"subheading","content":"Effects of Power Frequency Variation"},{"type":"paragraph","content":"Frequency deviations affect nearly all equipment that is designed to operate at a fixed synchronous speed or timing reference. Induction and synchronous motors experience speed changes proportional to frequency, which affects the output of connected pumps, fans, and production machinery. In generating stations, sustained under-frequency operation increases mechanical stress and vibration on steam turbine blades, and severe under-frequency can trigger automatic load-shedding or even unit trip to protect turbine blades from resonance-induced fatigue damage."},{"type":"paragraph","content":"Frequency-sensitive protection relays (such as under-frequency and rate-of-change-of-frequency relays) may operate incorrectly or trigger unwanted load shedding if frequency deviates significantly from nominal. Clock-driven and timing-synchronized equipment that derives time reference from mains frequency can lose accuracy. In extreme cases, sustained frequency excursions can lead to cascading trips and system-wide blackouts if corrective action (governor response, automatic generation control, load shedding) is not fast enough."},{"type":"subheading","content":"Corrective Measures"},{"type":"list","items":["Automatic Generation Control (AGC) and governor droop control to restore frequency","Spinning reserve maintained to cover sudden generation loss","Under-frequency load shedding schemes to arrest severe frequency decline"]}]}},{"id":"power-quality-and-facts-2021-7E1733-pB-q5","number":15,"marks":8,"tags":["Part B","Q.5"],"text":[{"type":"paragraph","content":"Q.5. Explain different types of FACTS controllers. [8]"}],"answer":{"explanation":[{"type":"subheading","content":"Classification of FACTS Controllers"},{"type":"paragraph","content":"FACTS (Flexible AC Transmission Systems) controllers are power-electronic-based devices used to enhance the controllability and power transfer capability of AC transmission systems. Based on their connection to the network, they are broadly classified into series controllers, shunt controllers, combined series-series controllers, and combined series-shunt controllers."},{"type":"subheading","content":"Series Controllers"},{"type":"paragraph","content":"Series controllers inject a voltage in series with the transmission line, effectively modifying the line's impedance or inserting a controllable voltage source. TCSC (Thyristor Controlled Series Capacitor) varies the effective series reactance using a thyristor-controlled reactor in parallel with a fixed capacitor, providing continuous compensation control. SSSC (Static Synchronous Series Compensator) uses a VSC to inject a controllable voltage in quadrature with line current, independent of line current magnitude, giving it superior dynamic control of power flow and damping."},{"type":"subheading","content":"Shunt Controllers"},{"type":"paragraph","content":"Shunt controllers inject current into the system at the point of connection, effectively behaving as a controllable reactive power source or sink. The SVC (Static VAR Compensator) uses thyristor-switched/controlled capacitors and reactors (TSC/TCR) to provide variable reactive power. STATCOM uses a VSC to synthesize a controllable voltage source, providing faster and more compact reactive support with better performance during low-voltage conditions than SVC."},{"type":"subheading","content":"Combined Series-Series and Series-Shunt Controllers"},{"type":"paragraph","content":"Combined series-series controllers, such as the Interline Power Flow Controller (IPFC), consist of two or more series VSCs sharing a common DC link, connected to different transmission lines, enabling active power transfer between lines for overall power flow management across a multi-line corridor. Combined series-shunt controllers, most notably the Unified Power Flow Controller (UPFC), integrate a shunt VSC and a series VSC on a common DC bus. The shunt converter provides reactive power support and DC bus voltage regulation, while the series converter injects a voltage that controls active and reactive power flow on the line, giving UPFC simultaneous control of voltage, impedance, and phase angle."},{"type":"list","items":["Series: TCSC, SSSC - control line impedance/power flow","Shunt: SVC, STATCOM - control bus voltage and reactive power","Series-series: IPFC - coordinated power flow control across multiple lines","Series-shunt: UPFC - comprehensive independent control of P, Q, and voltage"]}]}},{"id":"power-quality-and-facts-2021-7E1733-pB-q6","number":16,"marks":8,"tags":["Part B","Q.6"],"text":[{"type":"paragraph","content":"Q.6. What could be impact of 'poor power quality' on system efficiency, reliability and operation? [8]"}],"answer":{"explanation":[{"type":"subheading","content":"Impact on System Efficiency"},{"type":"paragraph","content":"Poor power quality directly reduces the efficiency of both the supply network and connected equipment. Harmonic currents cause additional I2R losses in conductors, transformers, and cables due to skin effect and increased RMS current, while eddy current and hysteresis losses rise in transformer cores. Reactive power flow caused by poor power factor increases line current for the same real power delivered, further raising transmission and distribution losses. Voltage imbalance causes negative-sequence currents in induction motors, producing additional heating and reduced motor efficiency."},{"type":"subheading","content":"Impact on Reliability"},{"type":"paragraph","content":"Voltage sags, swells, and transients can cause nuisance tripping of adjustable-speed drives, PLCs, and contactor-controlled equipment, leading to unplanned production stoppages. Harmonic resonance between line inductance and power factor correction capacitor banks can cause capacitor failure or fuse blowing, reducing overall system reliability. Repeated overheating from harmonics and imbalance accelerates insulation aging in transformers, cables, and motors, shortening equipment life and increasing failure rates and unplanned outages."},{"type":"subheading","content":"Impact on Operation"},{"type":"paragraph","content":"Protective relays and metering equipment that rely on accurate voltage/current waveforms may mis-operate under harmonic distortion or notching, causing false tripping or failure to trip when needed. In sensitive electronic and computing equipment, sags and transients can cause data loss, memory corruption, and processor resets. In industrial processes, momentary interruptions can cause loss of entire batches, requiring costly restart procedures. Overall, poor power quality translates into higher operating costs, reduced plant availability, increased maintenance expenditure, and financial losses from lost production and equipment damage."},{"type":"list","items":["Efficiency loss: increased I2R and core losses, reduced motor efficiency","Reliability loss: nuisance tripping, capacitor/fuse failure, accelerated insulation aging","Operational impact: relay mis-operation, data loss, production downtime, financial loss"]}]}},{"id":"power-quality-and-facts-2021-7E1733-pB-q7","number":17,"marks":8,"tags":["Part B","Q.7"],"text":[{"type":"paragraph","content":"Q.7. Explain current-control techniques is used for DSTATCOM. [8]"}],"answer":{"explanation":[{"type":"subheading","content":"Role of Current Control in DSTATCOM"},{"type":"paragraph","content":"A DSTATCOM (Distribution Static Compensator) is a shunt-connected VSC-based device used at distribution level to inject a compensating current into the point of common coupling so that the source current becomes sinusoidal, balanced, and in phase with the supply voltage, thereby compensating for harmonics, reactive power, and load unbalance. The performance of a DSTATCOM depends heavily on the current control technique used to force the actual injected current to track the reference compensating current."},{"type":"subheading","content":"Hysteresis Band Current Control"},{"type":"paragraph","content":"In hysteresis control, the actual injected current is continuously compared with the reference current, and a fixed tolerance band is defined around the reference. The VSC switches are turned on/off whenever the actual current crosses the upper or lower boundary of the hysteresis band, forcing the current to track the reference within the band. This method is simple, robust, and provides fast dynamic response, but it produces a variable switching frequency, which complicates output filter design and can generate a wider harmonic spectrum."},{"type":"subheading","content":"PI-Based Control with PWM"},{"type":"paragraph","content":"In this technique, the error between reference and actual current is processed through a proportional-integral (PI) controller, whose output is compared with a fixed-frequency triangular carrier wave to generate PWM switching pulses. This gives a fixed switching frequency, which simplifies filter design and reduces electromagnetic interference, but the dynamic response is generally slower than hysteresis control and the PI gains must be tuned for the operating conditions."},{"type":"subheading","content":"Predictive Current Control"},{"type":"paragraph","content":"Predictive control uses a mathematical model of the VSC and system to predict the future value of the current for each possible switching state, and selects the switching state that minimizes the predicted error at the next sampling instant. This provides fast and accurate tracking with fixed sampling period, though it requires more computational resources and accurate system modeling."},{"type":"list","items":["Hysteresis control: simple, fast, variable switching frequency","PI + PWM control: fixed switching frequency, easier filtering, slower response","Predictive control: accurate tracking, needs system model and higher computation"]}]}},{"id":"power-quality-and-facts-2021-7E1733-pC-q1","number":18,"marks":15,"tags":["Part C","Q.1"],"text":[{"type":"paragraph","content":"Q.1. Explain Unified Power Quality Conditioner (UPQC) in detail. [15]"}],"answer":{"explanation":[{"type":"subheading","content":"Introduction to UPQC"},{"type":"paragraph","content":"The Unified Power Quality Conditioner (UPQC) is a versatile custom power device that combines the functionality of a series-connected voltage compensator (similar to a DVR) and a shunt-connected current compensator (similar to a DSTATCOM), sharing a common DC-link capacitor. This unified structure allows the UPQC to simultaneously mitigate both voltage-related and current-related power quality problems at the point of installation, making it one of the most comprehensive power quality mitigation devices available in distribution systems."},{"type":"subheading","content":"Basic Configuration"},{"type":"paragraph","content":"The UPQC consists of two voltage source converters (VSCs): a series VSC connected between the supply and the load through a series injection transformer, and a shunt VSC connected in parallel with the load through a shunt coupling transformer or directly at the load bus. Both VSCs share a common DC-link capacitor, which acts as an energy buffer, allowing real power to be exchanged between the two converters as required for compensation. Passive LC filters are used at both series and shunt sides to remove high-frequency switching harmonics generated by the PWM-based VSCs."},{"type":"subheading","content":"Series Compensator (Voltage Compensation)"},{"type":"paragraph","content":"The series converter of the UPQC operates similarly to a DVR: it injects a controlled voltage in series with the supply so that the voltage delivered to the load remains sinusoidal, balanced, and at rated magnitude regardless of disturbances present in the incoming supply voltage. It compensates for voltage sags, swells, voltage imbalance, and voltage harmonics present on the source side, effectively isolating the load from source-side voltage disturbances."},{"type":"subheading","content":"Shunt Compensator (Current Compensation)"},{"type":"paragraph","content":"The shunt converter operates similarly to a DSTATCOM: it injects a compensating current at the point of common coupling to cancel harmonic currents drawn by non-linear loads, correct the load power factor by supplying/absorbing reactive power, and balance any load-side current imbalance. It also regulates the DC-link voltage by drawing or injecting a small amount of real power from the source as needed to keep the common DC bus at its reference value."},{"type":"subheading","content":"Working Together"},{"type":"paragraph","content":"During normal operation, the shunt converter continuously compensates load harmonics and reactive power, ensuring the source only supplies fundamental, in-phase active current. When a source-side voltage sag or swell occurs, the series converter injects the missing (or excess) voltage component so that the load voltage remains undisturbed, drawing the necessary real power through the common DC link, which in turn is replenished from the source via the shunt converter. This coordinated operation allows the UPQC to simultaneously deliver clean voltage to the load and clean current to the source."},{"type":"svg","content":"","alt":"UPQC block diagram showing series VSC and shunt VSC sharing a common DC link between source and load"},{"type":"subheading","content":"Advantages, Limitations, and Applications"},{"type":"paragraph","content":"The main advantage of UPQC is its comprehensive compensation capability, addressing nearly all common power quality problems (sag, swell, imbalance, harmonics, poor power factor) with a single integrated device, unlike a DVR (voltage only) or DSTATCOM (current only) used individually. Its limitations include higher cost and control complexity due to two VSCs and coordinated control algorithms, larger physical size, and the need for accurate real-time detection and reference generation algorithms. UPQC is typically applied at the point of common coupling for sensitive industrial and commercial loads such as data centers, semiconductor fabrication units, and hospitals, where both clean voltage and clean current are simultaneously required for uninterrupted, high-quality operation."},{"type":"subheading","content":"Comparison with Standalone DVR and DSTATCOM"},{"type":"paragraph","content":"When compared with a standalone DVR, which only addresses voltage-side disturbances such as sag, swell, and voltage imbalance, and a standalone DSTATCOM, which only addresses current-side problems such as harmonics, reactive power, and load unbalance, the UPQC is distinguished by its ability to address both categories simultaneously through a single shared DC-link architecture. This eliminates the need to install two separate compensators with independent DC storage and control systems, reducing overall footprint and allowing coordinated power exchange between the series and shunt stages during compensation events. For a plant that experiences both harmonic-polluting non-linear loads and occasional supply-side sags, a UPQC is more cost-effective and technically superior to installing a DVR and a DSTATCOM as two independent units."},{"type":"subheading","content":"Rating and Sizing Considerations"},{"type":"paragraph","content":"The series converter of the UPQC is generally rated for the voltage injection capacity required to compensate the expected range of sags and swells (commonly designed for 50 percent voltage compensation range), while the shunt converter is rated based on the harmonic and reactive current drawn by the non-linear load. Because the two converters share the DC bus, careful control coordination is required to prevent one converter's compensation action from starving the DC-link voltage needed by the other; DC-link voltage regulation is generally assigned to the shunt converter, which draws or supplies small amounts of real power from the source to keep the DC bus at its set-point while the series converter handles the larger real power transient during a sag event using the energy temporarily stored in the DC capacitor."},{"type":"subheading","content":"Control Architecture"},{"type":"paragraph","content":"The overall UPQC control system typically uses instantaneous reactive power theory (p-q theory) or synchronous reference frame theory to separate the load current into fundamental active, fundamental reactive, and harmonic components for the shunt converter reference generation, while the series converter reference is generated by comparing the sensed source voltage against a synchronized sinusoidal template to determine the missing or excess voltage component that must be injected. Both converters use a fast inner current or voltage control loop (hysteresis, PI with PWM, or predictive control) to track their respective references with minimal delay, ensuring the compensation action completes within a few milliseconds of disturbance detection, which is essential to prevent nuisance tripping of protected sensitive loads."},{"type":"paragraph","content":"In terms of practical deployment, UPQC configurations are further classified by the placement of the shunt converter relative to the series converter: in a left-shunt UPQC, the shunt converter is placed on the source side of the series converter, while in a right-shunt UPQC, it is placed on the load side. The right-shunt configuration is generally preferred when the load current itself needs direct compensation, since the shunt converter directly senses and compensates the actual load current. The choice between configurations, along with the sizing of the DC-link capacitor and the design of the injection and coupling transformers, must be tailored to the specific combination of voltage and current quality problems present at the installation site."},{"type":"paragraph","content":"Maintenance and reliability considerations also differ between the two converter stages of a UPQC: the series converter and its injection transformer are exposed to line fault currents passing through to the load, so it must include a bypass mechanism (thyristor bypass switch) that short-circuits the series injection path during severe faults or internal converter failure, allowing the load to remain connected directly to the source rather than being interrupted. The shunt converter, being connected in parallel, does not carry the full load current and therefore does not require the same bypass protection, though it does need overcurrent protection to prevent damage during load-side short circuits. These protection and bypass features are essential parts of a practically deployed UPQC and must be included in any detailed system design in addition to the core series and shunt compensation functions."}]}},{"id":"power-quality-and-facts-2021-7E1733-pC-q2","number":19,"marks":15,"tags":["Part C","Q.2"],"text":[{"type":"paragraph","content":"Q.2. What are principle of voltage sag performance? Give solution at end user level. [15]"}],"answer":{"explanation":[{"type":"subheading","content":"Definition and Principle of Voltage Sag"},{"type":"paragraph","content":"A voltage sag (or dip) is a short-duration reduction in RMS voltage magnitude to between 0.1 pu and 0.9 pu of nominal voltage, lasting from half a cycle to one minute, as per IEEE 1159 classification. Voltage sags are among the most frequent and economically damaging power quality disturbances faced by industrial and commercial customers."},{"type":"subheading","content":"Causes of Voltage Sag"},{"type":"paragraph","content":"The most common cause of voltage sags is short-circuit faults (line-to-ground, line-to-line, or three-phase) occurring anywhere in the transmission or distribution network. During a fault, a large fault current flows through the system impedance, causing a temporary voltage drop that is felt by all customers electrically close to the fault, until the protection system clears the fault. The magnitude and duration of the sag experienced by a particular customer depend on the fault type, the fault location, the system impedance between the fault and the customer, and the fault-clearing time of the protective devices. Other causes include starting of large induction motors (which draw high inrush starting current), energization of large transformers, and sudden connection of heavy loads."},{"type":"subheading","content":"Characterizing Sag Performance"},{"type":"paragraph","content":"Voltage sag performance at a given location is characterized using two parameters: retained voltage magnitude (as a percentage of nominal) and duration (in cycles or milliseconds). Equipment sensitivity to sags is commonly represented using tolerance curves such as the CBEMA curve or its successor the ITIC (Information Technology Industry Council) curve, which plot the magnitude and duration combinations that equipment can withstand without malfunction. If the sag magnitude-duration point falls below the ITIC curve's lower boundary, the connected equipment is likely to trip or malfunction. Utilities and industries use sag statistics (SARFI indices - System Average RMS Variation Frequency Index) to quantify how often a given bus experiences sags below specified thresholds over a period."},{"type":"subheading","content":"Impact of Voltage Sags"},{"type":"paragraph","content":"Even brief sags of only a few cycles can cause adjustable-speed drives to trip on under-voltage protection, contactors to drop out, PLCs to reset, and computer/data-processing equipment to lose data or restart, leading to costly production interruptions in continuous-process industries such as semiconductor manufacturing, paper mills, and textile plants, where a single sag can cause losses far exceeding the cost of mitigation equipment."},{"type":"subheading","content":"End-User Level Solutions"},{"type":"paragraph","content":"Since utilities cannot economically eliminate all faults causing sags, mitigation is often applied at the end-user level, close to the sensitive load. A Dynamic Voltage Restorer (DVR) is one of the most effective solutions: it is installed in series with the incoming supply to the sensitive load and injects a compensating voltage during a sag so that the load sees a constant, undisturbed voltage. Uninterruptible Power Supplies (UPS), either online or line-interactive, use stored energy (batteries) to supply the load during a sag or interruption, completely decoupling the load from supply disturbances."},{"type":"paragraph","content":"Ferroresonant (constant-voltage) transformers exploit magnetic saturation to maintain a relatively constant output voltage despite input voltage variation, offering a low-cost passive solution for smaller loads. Motor-generator (M-G) sets use flywheel energy storage coupled to a motor-generator combination to ride through short interruptions and sags. Additionally, improving the inherent ride-through capability of end-use equipment itself, for example by adding hold-up capacitors in DC power supplies or programming adjustable-speed drives to ride through momentary disturbances, reduces sensitivity without requiring a separate compensating device. The selection among these options depends on load criticality, sag frequency and severity at the site, and the economic cost of downtime versus the mitigation equipment investment."},{"type":"list","items":["DVR: series voltage injection, corrects sag/swell in real time","UPS: battery-backed decoupling from supply disturbances","Ferroresonant transformer: passive, low-cost voltage regulation","Motor-generator set: flywheel-based ride-through","Improved equipment ride-through capability (hold-up capacitors, control settings)"]},{"type":"subheading","content":"Additional Consideration: Point-on-Wave and Phase-Angle Jump"},{"type":"paragraph","content":"Beyond magnitude and duration, a voltage sag caused by an unbalanced fault also typically introduces a phase-angle jump, i.e., a sudden shift in the phase angle of the voltage waveform at the instant the fault occurs, in addition to the change in magnitude. Sensitive equipment such as adjustable-speed drives that use phase-locked loops for synchronization can mis-operate due to this phase jump even if the magnitude drop itself is within tolerance. Point-on-wave characteristics, i.e., the instant within the cycle at which the sag begins, also affect the severity of impact on rectifier-front-end equipment, since a sag beginning near a voltage zero-crossing can cause a different transient response than one beginning near the peak. A complete voltage sag characterization for end-user mitigation studies must therefore consider magnitude, duration, phase-angle jump, and point-on-wave, not magnitude and duration alone."},{"type":"subheading","content":"Economic Perspective on Mitigation Selection"},{"type":"paragraph","content":"The choice of an appropriate end-user mitigation solution is ultimately an economic decision that weighs the capital and maintenance cost of the mitigation device against the expected cost of downtime avoided. For a small number of critical control circuits, a simple constant-voltage transformer or a small UPS may be adequate and inexpensive. For an entire sensitive production line drawing hundreds of kilowatts, a custom power device such as a DVR sized to the specific sag depth and duration profile recorded at the site (through power quality monitoring over several months) offers the most cost-effective solution, since its rating only needs to cover the missing voltage component rather than the full load power. Facilities with very high sensitivity to any interruption, such as data centers, typically deploy a combination of UPS for ride-through of very short events together with standby generation for sustained interruptions."},{"type":"subheading","content":"Standards and Industry Practice"},{"type":"paragraph","content":"Utilities and industries commonly reference IEEE 1159 for disturbance classification, IEEE 1346 for compatibility assessment between the equipment sensitivity (via ITIC/CBEMA curves) and the site's sag performance, and IEC 61000-4-11 for equipment immunity test procedures against short voltage interruptions and sags. Site power quality audits typically begin with a monitoring campaign using a power quality analyzer over a representative period (often three to twelve months) to statistically characterize the expected sag magnitude-duration distribution at the site, which then forms the technical basis for selecting and sizing the most appropriate end-user level mitigation equipment, whether a DVR, UPS, ferroresonant transformer, or improved equipment ride-through specification."},{"type":"paragraph","content":"Utility-side measures also contribute to reducing sag exposure even though the questions here focus on end-user solutions: faster fault clearing through improved relay coordination and current-limiting fuses reduces sag duration, while installing fault current limiters at strategic network points reduces the severity of voltage drop seen by nearby customers during a fault. Nonetheless, since utilities cannot guarantee zero sags at every customer connection point, the end-user level solutions described above (DVR, UPS, ferroresonant transformers, motor-generator sets, and improved equipment ride-through) remain the primary practical means by which a facility protects its own sensitive processes from the residual sag exposure that utility-side measures cannot fully eliminate."}]}},{"id":"power-quality-and-facts-2021-7E1733-pC-q3","number":20,"marks":15,"tags":["Part C","Q.3"],"text":[{"type":"paragraph","content":"Q.3. Explain the state of art on power quality: Classify the power quality problem along with its causes. [15]"}],"answer":{"explanation":[{"type":"subheading","content":"State of the Art in Power Quality"},{"type":"paragraph","content":"Power quality refers to the degree to which the voltage, current, and frequency of an electrical supply conform to defined limits so that equipment operates as intended without malfunction, damage, or reduced life. With the proliferation of non-linear, power-electronic-based loads (variable frequency drives, computers, LED lighting, EV chargers) and distributed/renewable generation, power quality has become a critical concern for utilities and end-users, driving standardization efforts by IEEE (notably IEEE 1159) and IEC to classify and quantify disturbances."},{"type":"subheading","content":"Classification Framework (IEEE 1159)"},{"type":"paragraph","content":"IEEE 1159 classifies power quality problems into distinct categories based on the spectral content, magnitude, and duration of the disturbance. The main categories are: transients, short-duration RMS variations, long-duration RMS variations, voltage imbalance, waveform distortion, voltage fluctuation, and power frequency variation."},{"type":"subheading","content":"Transients"},{"type":"paragraph","content":"Transients are sudden, undesirable changes in voltage or current that occur over a very short time, and are subdivided into impulsive transients (a sudden, unidirectional change in voltage/current, typically caused by lightning strikes) and oscillatory transients (a sudden change that includes both positive and negative polarity values, typically caused by capacitor bank switching, transformer energization, or ferroresonance). Transients can reach very high magnitudes over microseconds to milliseconds and can damage insulation and sensitive electronic components."},{"type":"subheading","content":"Short-Duration and Long-Duration RMS Variations"},{"type":"paragraph","content":"Short-duration variations include voltage sag (0.1-0.9 pu), voltage swell (1.1-1.8 pu), and interruption (less than 0.1 pu), all lasting from half a cycle up to one minute, caused mainly by faults, motor starting, and load switching. Long-duration variations, lasting longer than one minute, include sustained interruption, undervoltage, and overvoltage, generally caused by load variations and switching operations on the system rather than faults."},{"type":"subheading","content":"Waveform Distortion"},{"type":"paragraph","content":"Waveform distortion is a steady-state deviation from an ideal sine wave, primarily at the fundamental frequency component's periodicity, and includes five sub-categories: DC offset (presence of DC voltage/current in an AC system, caused by geomagnetic disturbances or half-wave rectification), harmonics (integer multiples of fundamental frequency caused by non-linear loads), interharmonics (frequencies not integer multiples of fundamental, caused by cycloconverters and arc furnaces), notching (periodic disturbance from converter commutation), and noise (unwanted broadband signals caused by power electronic devices, arcing, or improper grounding)."},{"type":"subheading","content":"Voltage Imbalance, Fluctuation, and Frequency Variation"},{"type":"paragraph","content":"Voltage imbalance is the condition where three-phase voltages differ in magnitude or are not phase-displaced by exactly 120 degrees, caused by unbalanced single-phase loads or asymmetrical impedances. Voltage fluctuation (flicker) is a series of rapid voltage changes, typically caused by arc furnaces or other rapidly varying loads, and is perceived as light flicker by the human eye. Power frequency variation is a deviation from the nominal system frequency, caused by imbalance between generation and load in the interconnected system."},{"type":"subheading","content":"Summary Table of Causes"},{"type":"list","items":["Transients: lightning, capacitor switching, transformer energization","Sag/swell/interruption: faults, motor starting, load switching","Undervoltage/overvoltage: sustained load or capacitor bank switching imbalance","Harmonics/notching/DC offset/noise: non-linear loads, power converters, arcing devices","Imbalance: unbalanced single-phase loading, untransposed lines","Flicker: arc furnaces, welders, rapidly varying industrial loads","Frequency variation: generation-load mismatch, loss of generation/load"]},{"type":"paragraph","content":"This classification framework allows utilities and consultants to systematically diagnose reported power quality complaints by measuring and comparing recorded disturbance signatures against these standard categories, and then select the appropriate mitigation technology (surge arresters for transients, DVR/UPS for sags, active/passive filters for harmonics, SVC/STATCOM for flicker and imbalance, and AGC for frequency variation)."},{"type":"subheading","content":"Emerging Trends in the State of the Art"},{"type":"paragraph","content":"The state of the art in power quality has evolved considerably with the growth of distributed generation, particularly grid-connected solar photovoltaic and wind installations, which introduce new power quality challenges such as voltage rise from reverse power flow, increased harmonic injection from inverter-based generation, and rapid voltage fluctuations caused by intermittent renewable output combined with cloud transients or wind gusts. Modern power quality standards and monitoring practices have therefore expanded to cover inverter-interfaced distributed energy resources, requiring compliance with grid codes for harmonic emission limits, ride-through capability during voltage disturbances, and reactive power support during faults."},{"type":"subheading","content":"Measurement and Monitoring Approaches"},{"type":"paragraph","content":"Modern power quality assessment relies on continuous monitoring using power quality analyzers that sample voltage and current waveforms at high resolution and compute the standard indices for each disturbance category defined by IEEE 1159: RMS magnitude for sag/swell/interruption events, THD and individual harmonic magnitudes for waveform distortion, Pst/Plt flicker severity indices per IEC 61000-4-15, and negative-sequence voltage ratio for imbalance. Statistical indices such as SARFI (System Average RMS Variation Frequency Index) are used by utilities to benchmark overall system-wide sag performance across many monitored buses and over extended periods, supporting infrastructure planning decisions such as where to prioritize protective equipment upgrades or FACTS device installation."},{"type":"subheading","content":"Mitigation Technology Mapping"},{"type":"paragraph","content":"Each classified power quality problem has a corresponding family of mitigation technologies that represents current engineering practice. Surge arresters and transient voltage surge suppressors address impulsive and oscillatory transients. Active and passive harmonic filters, along with phase-shifting transformers in multi-pulse converter arrangements, address waveform distortion. DVR and UPS address short-duration sags and interruptions at the end-user level, while utility-side solutions such as fault current limiters and improved protection coordination reduce the frequency and severity of sags at their source. SVC and STATCOM address flicker and voltage imbalance through fast dynamic reactive power control, and automatic generation control together with adequate spinning reserve addresses power frequency variation at the system level."},{"type":"paragraph","content":"This layered classification and mitigation mapping approach, moving from disturbance characterization (IEEE 1159 categories) through measurement (standardized indices) to mitigation technology selection, represents the current state-of-the-art methodology used by power quality engineers to diagnose complaints, design compatible equipment specifications, and plan targeted investments in compensation devices across both transmission and distribution networks."},{"type":"paragraph","content":"Grounding and wiring practices are also recognized within the state-of-the-art power quality framework as an underlying contributor to several disturbance categories, particularly noise and transient overvoltages; poor grounding can introduce ground potential differences that appear as noise on sensitive signal and data lines, and inadequate bonding can aggravate the severity of transients seen by connected equipment. Consequently, comprehensive power quality investigations, in addition to classifying the electrical disturbance itself, typically include a review of the facility grounding and wiring configuration against recommended practices such as IEEE 1100, since correcting wiring and grounding deficiencies is often a low-cost first step that resolves a meaningful fraction of reported power-quality-related equipment malfunctions before more expensive mitigation equipment is considered."},{"type":"paragraph","content":"In summary, a rigorous classification of power quality problems by category, cause, and standard measurement index, combined with a matching set of mitigation technologies, forms the complete modern engineering framework for addressing power quality in contemporary transmission and distribution networks."}]}},{"id":"power-quality-and-facts-2021-7E1733-pC-q4","number":21,"marks":15,"tags":["Part C","Q.4"],"text":[{"type":"paragraph","content":"Q.4. Explain the principle of operation of passive shunt and series compensators along with its classification. [15]"}],"answer":{"explanation":[{"type":"subheading","content":"Introduction to Passive Compensators"},{"type":"paragraph","content":"Passive compensators use fixed or mechanically/thyristor-switched combinations of inductors and capacitors (without active power electronic voltage synthesis) to provide reactive power compensation and improve voltage regulation and power transfer capability in AC transmission and distribution systems. They are classified primarily by their connection to the line - shunt (parallel) or series - and by function."},{"type":"subheading","content":"Shunt Compensators"},{"type":"paragraph","content":"Shunt compensators are connected in parallel across the bus, at the sending end, receiving end, or at intermediate points along a line. Shunt capacitor banks supply reactive power (behave as a source of leading VARs) to the system, improving the voltage profile at the point of connection, correcting lagging power factor caused by inductive loads, and reducing line current for the same active power transfer, thereby reducing I2R losses. Shunt reactors, conversely, absorb reactive power and are used on lightly loaded long transmission lines to counteract the capacitive charging (Ferranti) effect that causes voltage rise at no-load or light-load conditions."},{"type":"paragraph","content":"Shunt capacitors are further classified as fixed (permanently connected) or switched (mechanically or thyristor-switched capacitor banks, TSC, that can be connected/disconnected in steps to follow reactive power demand). The principle of operation relies on the fact that reactive power supplied by a shunt capacitor is proportional to the square of the bus voltage, so their effectiveness diminishes precisely when the voltage is low and support is most needed - a key limitation compared to active shunt devices like STATCOM."},{"type":"subheading","content":"Series Compensators"},{"type":"paragraph","content":"Series compensators are connected in series with the transmission line, typically in the form of fixed series capacitor banks, and function by cancelling a portion of the line's inherent inductive reactance. Since maximum power transfer capacity of a line is inversely proportional to its net series reactance (P = V1V2 sin(delta)/X), reducing the effective reactance by series capacitive compensation directly increases power transfer capability for the same sending and receiving end voltage magnitudes and angle."},{"type":"paragraph","content":"Series capacitors also improve the voltage profile along the line (particularly effective when installed near the mid-point), improve reactive power balance by reducing the line's own reactive power absorption, and improve the load-sharing among parallel circuits by reducing the impedance of the compensated line relative to uncompensated parallel paths. A key operational concern with fixed series capacitors is sub-synchronous resonance (SSR), which can occur when the compensated line's electrical resonant frequency interacts with the torsional mechanical modes of nearby turbine-generator shafts, potentially causing shaft damage; this is one reason thyristor-controlled series devices (TCSC) are often preferred for higher compensation levels."},{"type":"subheading","content":"Classification Summary"},{"type":"list","items":["Shunt: capacitor banks (fixed/switched) - supply reactive power, raise voltage, improve PF","Shunt: reactors (fixed/switched) - absorb reactive power, limit Ferranti voltage rise on long lines","Series: capacitors (fixed) - cancel line inductive reactance, raise power transfer limit, improve mid-line voltage profile","Combined/hybrid passive schemes: fixed capacitor banks combined with mechanically switched reactors for coarse stepped control"]},{"type":"svg","content":"","alt":"Passive series and shunt compensator placement along a transmission line with series capacitor at mid-line and shunt devices at both ends"},{"type":"subheading","content":"Advantages and Limitations"},{"type":"paragraph","content":"Passive compensators are simple, reliable, low-cost, and require minimal maintenance compared to active FACTS controllers, making them widely used across transmission and distribution systems. However, their compensation is either fixed or changes only in discrete steps (for switched banks), they cannot respond continuously or fast enough to rapidly varying system conditions, their reactive power output is voltage-dependent (falling exactly when most needed during low-voltage conditions), and fixed series capacitors introduce SSR risk. These limitations motivate the use of active, power-electronic-based FACTS devices such as SVC, STATCOM, TCSC, and SSSC where continuous, fast, and voltage-independent compensation is required."},{"type":"subheading","content":"Comparison with Active (FACTS) Compensators"},{"type":"paragraph","content":"Unlike passive compensators, which rely purely on the fixed or stepped reactive impedance of capacitors and reactors, active FACTS compensators such as SVC and STATCOM use power-electronic switching to synthesize a continuously controllable reactive power output that is largely independent of the instantaneous system voltage. This is the fundamental distinction driving the choice between passive and active compensation: passive devices are far cheaper per MVAR installed and have negligible standing losses, but their support capability is proportional to the square of the bus voltage and only changes in discrete switched steps, so during a severe voltage depression (exactly when reactive support is most needed) a passive shunt capacitor bank's output collapses along with the voltage, whereas a STATCOM continues to inject its rated reactive current down to very low voltage levels."},{"type":"subheading","content":"Practical Deployment Practice"},{"type":"paragraph","content":"In practice, most transmission and distribution utilities use passive compensation as the primary, base-level reactive power source because of its low capital and operating cost, reserving active FACTS devices for locations where fast dynamic response, continuous control, or voltage-independent support is specifically required, such as at weak grid interconnection points, near large fluctuating industrial loads, or at critical system buses prone to voltage collapse. A common hybrid practice is to install a large passive shunt capacitor bank to provide the bulk (steady-state) reactive power requirement economically, supplemented by a smaller-rated SVC or STATCOM to provide the fast, continuously variable trim compensation needed to handle transient disturbances and fluctuating loads, combining the cost advantage of passive devices with the dynamic performance of active devices."},{"type":"subheading","content":"Series Compensation Degree and Protection"},{"type":"paragraph","content":"The degree of series compensation, defined as the ratio of the series capacitor reactance to the total line inductive reactance, is a key design parameter; typical compensation levels range from 25 percent to 70 percent depending on the stability improvement required and the risk of sub-synchronous resonance with nearby generating units. Series capacitor installations require dedicated protection schemes, including metal oxide varistors (MOVs) connected in parallel with the capacitor to limit the voltage across it during fault conditions (bypassing excessive current around the capacitor), along with a bypass gap or fast bypass switch to protect the capacitor bank and MOV during severe through-faults, since the capacitor must not be exposed to the full fault current magnitude that could otherwise flow through the line during a nearby short circuit."},{"type":"paragraph","content":"From a voltage-regulation standpoint, passive shunt capacitor banks are frequently deployed at distribution substations and along feeders in fixed steps switched by mechanical circuit breakers or contactors, coordinated with load tap changers on substation transformers, to maintain the feeder voltage profile within statutory limits across the daily load cycle. This coordination between switched capacitor banks and tap changers, governed by local voltage/VAR control schemes, represents the classical (and still widely used) approach to distribution voltage regulation, complementing the transmission-level series and shunt passive compensation described above, and together these passive elements form the backbone of reactive power management in most power systems even as active FACTS devices are added at specific critical locations."}]}},{"id":"power-quality-and-facts-2021-7E1733-pC-q5","number":22,"marks":15,"tags":["Part C","Q.5"],"text":[{"type":"paragraph","content":"Q.5. Explain the operation of DSTATCOM used for Sag mitigation. [15]"}],"answer":{"explanation":[{"type":"subheading","content":"Introduction to DSTATCOM"},{"type":"paragraph","content":"A Distribution Static Compensator (DSTATCOM) is a shunt-connected, VSC-based custom power device installed at the distribution level to provide fast reactive and/or active power compensation, correcting power quality problems such as voltage sag, harmonic currents, poor power factor, and load unbalance at the point of common coupling (PCC)."},{"type":"subheading","content":"Structure of DSTATCOM"},{"type":"paragraph","content":"The DSTATCOM consists of a voltage source converter (built using IGBTs), a DC-side energy storage element (capacitor, or capacitor supplemented with battery/supercapacitor for sustained real power support), a coupling transformer or interfacing inductor connecting the VSC to the distribution bus, an output ripple filter, and a control system that senses the PCC voltage and load/source currents and generates the switching signals for the VSC."},{"type":"subheading","content":"Principle of Sag Mitigation"},{"type":"paragraph","content":"When a voltage sag occurs on the source side (due to a fault or large motor starting elsewhere in the network), the voltage at the PCC and hence at the sensitive load drops below its nominal value. The DSTATCOM's control system detects this drop by continuously monitoring the RMS or instantaneous PCC voltage and comparing it against a reference. To mitigate the sag, the DSTATCOM injects a compensating current into the PCC that is calculated to boost the local bus voltage back toward its nominal value. Specifically, the VSC is controlled to inject a current with a component in phase with (or leading) the PCC voltage, which increases the voltage drop across the source-side impedance in a direction that raises the voltage at the PCC - effectively, the DSTATCOM behaves as a controllable current source that regulates the local bus voltage by exchanging the necessary reactive (and if needed, real) power with the system."},{"type":"paragraph","content":"Unlike the DVR, which is a series device correcting voltage directly, the DSTATCOM is a shunt device and its ability to mitigate sags depends on the source impedance: it works best when the ratio of source impedance to the impedance up to the load is favorable, since its correction relies on modifying the voltage drop across that shared impedance by injecting current. For deep sags accompanied by significant active power support requirements, the DSTATCOM's DC-side energy storage must supply real power in addition to reactive power, since simply injecting reactive current may be insufficient to fully restore voltage magnitude."},{"type":"subheading","content":"Control Strategy"},{"type":"paragraph","content":"The reference compensating current is typically derived using instantaneous reactive power theory (p-q theory) or synchronous reference frame (d-q) theory, which decomposes the load current into fundamental active, fundamental reactive, and harmonic components. During normal operation, the DSTATCOM's shunt current cancels harmonics and reactive current drawn by non-linear/inductive loads. During a detected sag event, the reference current calculation is modified to include an additional voltage-support component, and a current control technique (hysteresis band, PI with PWM, or predictive control) forces the actual injected current from the VSC to track this reference with minimal delay, typically within a few milliseconds, fast enough to prevent sensitive load equipment from tripping."},{"type":"svg","content":"","alt":"DSTATCOM connected in shunt at the point of common coupling between source and load, injecting compensating current"},{"type":"subheading","content":"Advantages, Limitations, and Comparison with DVR"},{"type":"paragraph","content":"The DSTATCOM offers fast dynamic response, simultaneous compensation of harmonics, reactive power, imbalance, and (to an extent) voltage sags using a single device, and does not require a series injection transformer, reducing installation complexity compared to a DVR. However, its effectiveness for deep voltage sags is limited by the source impedance and by the current rating of the VSC, since correcting voltage via injected current requires higher currents as the required voltage boost increases. For very deep or long-duration sags, a DVR (series device) or a combination of DSTATCOM with sufficient energy storage is preferred, since the DVR directly injects the missing series voltage regardless of source impedance characteristics. In practice, DSTATCOM is widely applied for industrial and commercial distribution feeders where moderate sag mitigation, together with harmonic filtering and power factor correction, is required from a single compact device."},{"type":"subheading","content":"DC-Link Voltage Regulation During Compensation"},{"type":"paragraph","content":"A critical aspect of DSTATCOM operation during sag mitigation is maintaining the DC-link capacitor voltage at its required reference level, since the quality of the injected compensating current depends directly on having sufficient DC bus voltage available to the VSC. During a sag event, if the DSTATCOM must supply real power to help support the load voltage, the DC-link capacitor will tend to discharge, and the control system must include an outer DC-voltage regulation loop that adjusts the real power component of the reference current to replenish the DC-link from the source as soon as the source voltage recovers, while the inner current control loop simultaneously handles the fast reactive/active current injection needed for immediate sag support. Where sags are frequent, deep, or of extended duration, the DSTATCOM DC bus is typically augmented with a battery or supercapacitor bank to provide the sustained real power headroom that a capacitor alone cannot supply."},{"type":"subheading","content":"Interfacing and Filtering"},{"type":"paragraph","content":"The DSTATCOM connects to the distribution bus either through a step-up coupling transformer, which provides voltage matching and galvanic isolation between the VSC's lower DC-bus-derived voltage and the distribution voltage level, or directly through an interfacing inductor for lower-voltage applications. An LC or LCL output filter is placed between the VSC and the coupling point to attenuate the high-frequency PWM switching harmonics generated by the converter, ensuring that the injected compensating current itself does not introduce new high-frequency distortion into the network while still allowing the desired fundamental and selected harmonic compensation components to pass through with minimal attenuation."},{"type":"subheading","content":"Performance Comparison and Selection Guidance"},{"type":"paragraph","content":"When comparing sag-mitigation performance, the DSTATCOM is most effective for shallow to moderate sags (typically down to about 0.6-0.7 pu retained voltage) where the required current injection remains within the converter's current rating, and it offers the added benefit of simultaneously correcting harmonics and power factor during normal operation, which a DVR does not do. For deeper sags approaching complete voltage loss, or where the source impedance is too low to allow adequate voltage boosting through current injection alone, a series-connected DVR becomes the more reliable choice since its voltage injection capability is not fundamentally limited by the source-side impedance ratio in the same way. Selection between DSTATCOM, DVR, and UPQC for a given installation is therefore guided by a site-specific power quality audit that characterizes the expected sag depth, duration, and frequency, together with the load's sensitivity to both voltage and current quality issues, and the acceptable capital cost of the mitigation equipment."}]}}],"examYear":2021}],"loading":false,"error":null}}}

Q5Power Quality and FACTS

Question

Answer