UNIT 4: SPECIAL MACHINES AND POWER QUALITY FOR SCADA SYSTEMS

I. FOUNDATIONS OF MAGNETIC CIRCUITS

A. Soft vs Hard Ferromagnetic Materials

Soft materials minimize core losses (hysteresis & eddy current) in AC applications. Hard materials retain high residual flux density ($$\displaystyle B_r $$), essential for PM motors.

B. B-H Relationship

• Definition: Plot of magnetic flux density (B) vs. magnetic field intensity (H).

• Key Regions:

  1. Linear Region: $$\displaystyle B = \mu H $$ (μ = permeability). Initial, low H.

  2. Saturation: B increases minimally with increasing H. Core material fully magnetized.

  3. Hysteresis: Path dependence. Energy loss per cycle = area of loop.

  4. Remanence ($$\displaystyle B_r $$): Flux density at H=0 after magnetization.

  5. Coercive Force ($$\displaystyle H_c $$): Reverse H required to reduce B to zero.

$$ \text{Hysteresis Loss } P_h \propto f B_m^n \quad (\text{Steinmetz equation}) $$

C. Leakage Flux and Fringing

• Leakage Flux: Magnetic flux that does not follow the intended path through the core and air gap. It links only the winding that produces it.

  • Effect: Reduces effective magnetizing flux, increases leakage inductance, causes voltage regulation issues.

• Fringing: Flux bulging out at the air gap edges due to lower reluctance of air.

  • Effect: Increases effective air gap area, reduces average flux density in the gap.

  • Correction: Effective air gap length $$\displaystyle g_{eff} = g + \frac{4}{\pi} \cdot \frac{\text{slot opening}}{\text{air gap length}} $$ (approx.).

D. Stacking Factor

• Definition: Ratio of effective cross-sectional area of the magnetic core to the total physical area (including insulation between laminations).

$$ \text{Stacking Factor (}k_{si}\text{)} = \frac{A_{core}}{A_{gross}} \quad (0.8 < k_{si} < 0.95) $$

• Purpose: Accounts for insulation and gaps between laminated steel sheets, which reduce net conducting area for flux.

E. Energy Conversion via Electric Field

Conversion using electric field (capacitive or piezoelectric) rather than magnetic field.

• Examples:

  1. Capacitive Micromotors: Use electrostatic attraction between stator and rotor plates. Requires high voltage, small air gap.

  2. Piezoelectric Motors: Use inverse piezoelectric effect (electric field → mechanical strain). High precision, low speed, no magnetic fields.

F. Magnetic Reluctance

• Definition: Opposition to magnetic flux creation, analogous to electrical resistance. $$\displaystyle \mathcal{R} = \frac{l}{\mu A} $$ (l = path length, A = cross-section, μ = permeability).

• Principle: Flux follows path of least reluctance. Basis for Reluctance Motors (SRM) and Variable Reluctance Steppers. Torque is produced by tendency to align rotor to position of minimum reluctance.

II. STEPPER MOTORS

A. Types

1. Variable Reluctance (VR): Toothed rotor (no PM). Aligns with energized stator pole to minimize reluctance. Simple, low cost, low torque.

2. Permanent Magnet (PM): Rotor has PM. Detent torque when unpowered. High torque, lower resolution.

3. Hybrid (HV): Combines VR and PM principles. Toothed rotor with PM. Highest resolution and torque. Most common for precision applications.

B. Construction and Working Principle (Hybrid Stepper - Example)

• Stator: Two (or more) phases with salient poles, wound with coils. Often castellated.

• Rotor: Toothed, with a permanent magnet axially magnetized (or radially for some designs).

• Working: Energize stator phases sequentially. Magnetic attraction between rotor teeth (with PM flux) and energized stator teeth causes rotation. Step angle determined by number of stator/rotor teeth.

C. Key Features

D. Static and Dynamic Characteristics

• Static: Torque vs. rotor position at fixed current (detent curve, holding torque curve). Shows torque ripple.

• Dynamic: Torque-speed envelope. Pull-in and Pull-out curves define safe operating area. Speed decreases as load torque increases.

E. Torque Equation and Load Angle Control

• Torque Equation (approx.): $T \propto I \cdot \sin(\beta)$

  • $I$ = Phase current

  • $\beta$ = Load Angle (angular displacement between rotor's equilibrium position and actual position under load).

• Load Angle Control: For a given load torque $$\displaystyle T_L $$, the motor develops torque by advancing $\beta$. Maximum torque at $$\displaystyle \beta = 90^\circ $$. $\beta$ must be kept within stable region ($$\displaystyle < 90^\circ $$) to avoid loss of synchronism.

F. Driver Circuits

1. Single Voltage (Unipolar): Center-tapped winding. Current flows in one direction per half-winding. Simple, lower torque.

2. Single Voltage (Bipolar): Whole winding used. Current reverses direction. Higher torque, requires H-bridge.

3. Dual Voltage: Uses two voltage levels (high for fast current rise, low for holding). Improves performance at high speed.

4. Bi-level and Chopper Drives: Maintain constant current by chopping supply voltage (PWM). Provides high torque over wide speed range. Most common for performance.

G. Microstepping

• Principle: Divide full step into smaller increments by proportionally controlling current in two simultaneously energized phases.

• Implementation: Use sine-cosine current references. Driver circuits (usually chopper type) precisely regulate phase currents to follow these references.

• Benefit: Smoother motion, reduced resonance, increased resolution. Does not increase inherent accuracy (depends on motor linearity).

H. Speed Control Methods

1. Step Rate Control: Vary pulse frequency. Primary method.

2. Voltage Control: Vary supply voltage (affects current rise time, max speed).

3. Current Control: Use constant current drives (choppers) to maintain torque at higher speeds.

I. Single-Stack vs Multi-Stack Configuration

• Single-Stack: All stator poles on one axial section. Standard design.

• Multi-Stack: Multiple identical stator/rotor sections stacked axially. Increases torque capacity and length. Used in high-torque, low-speed applications.

J. Comparison: Permanent Magnet vs Hybrid Stepper Motors

K. Step Angle Calculation (Hybrid Stepper)

$$ \theta_s = \frac{360^\circ \times (N_r - N_s)}{N_r \times N_s} \quad \text{or} \quad \theta_s = \frac{360^\circ}{N_r \cdot N_s} \quad (\text{if } N_r \gg N_s) $$

Where:

• $$\displaystyle N_r $$ = Number of rotor teeth

• $$\displaystyle N_s $$ = Number of stator teeth per phase (or number of pole pairs)

Example: 50 rotor teeth, 8 stator poles (4 poles/phase) → $$\displaystyle N_s = 4 \times \text{teeth/pole} $$. If each stator pole has 5 teeth, $$\displaystyle N_s = 20 $$. $$\displaystyle \theta_s = 360 / (50 \times 20) = 0.36^\circ $$.

L. Applications

• Printers/Plotters (paper feed, head positioning)

• CNC machines & Robotics (open-loop positioning)

• Disk drives (head positioning)

• Valve control, medical devices, camera lenses.

III. SWITCHED RELUCTANCE MOTORS (SRM)

A. Construction

1. Stator: Salient poles, concentrated windings. Simple, robust.

2. Rotor: Salient poles, no windings, no PMs. Made of laminated steel (or solid for some low-cost applications).

3. Pole Arcs: Stator and rotor pole arcs are designed to ensure only one pair of poles is aligned at a time (for torque production). Typically, rotor pole arc > stator pole arc.

B. Principle of Operation

• Reluctance Minimization: Torque is produced by tendency of rotor to move to position of minimum magnetic reluctance (aligned position: rotor pole fully under stator pole).

• Doubly Salient: Both stator and rotor have salient poles.

• Sequence: Stator phases energized in sequence (A→B→C→A). As phase A is energized, rotor pole aligned with A experiences attraction. Before alignment, phase A is turned off and phase B is turned on.

C. Torque Production

1. Expression: Instantaneous torque for one phase (neglecting mutual inductance):

$$ T = \frac{1}{2} I^2 \frac{dL(\theta)}{d\theta} $$

- $I$ = Phase current

- $L(\theta)$ = Phase inductance as function of rotor position $\theta$.

- $$\displaystyle \frac{dL}{d\theta} > 0 $$ (inductance increasing with $\theta$) produces **motoring torque**.

2. Angle-Torque Characteristics:

   • Aligned Position ($$\displaystyle \theta_a $$): $$\displaystyle L = L_{max} $$. $$\displaystyle dL/d\theta = 0 $$. Torque = 0.

   • Unaligned Position ($$\displaystyle \theta_u $$): $$\displaystyle L = L_{min} $$. $$\displaystyle dL/d\theta = 0 $$. Torque = 0.

   • Torque is positive (motoring) between $$\displaystyle \theta_u $$ and $$\displaystyle \theta_a $$ where $$\displaystyle dL/d\theta > 0 $$. Peaks at mid-position.

3. Energy Conversion Cycle:

   • When phase is ON: Electrical energy input $$\displaystyle W_e = \int_0^I \psi \, dI = \int_0^I L(\theta) I \, dI = \frac{1}{2} I^2 L(\theta) $$.

   • Mechanical work done: $$\displaystyle W_m = \int T \, d\theta $$.

   • Co-energy: $$\displaystyle W'_{f} = \int_0^I \psi \, dI $$ (same as $$\displaystyle W_e $$ for linear magnetics). $$\displaystyle T = \frac{\partial W'_{f}}{\partial \theta} \big|_I $$.

   • Instantaneous Torque: As given by formula above.

4. Calculation of Instantaneous Torque (Given $L(\theta)$, I):

   • Determine $L(\theta)$ for the specific rotor angle.

   • Compute $dL/d\theta$ (slope of L-θ curve at that angle).

   • Plug into $$\displaystyle T = \frac{1}{2} I^2 \frac{dL}{d\theta} $$.

5. Maximum Energy Conversion per Stroke:

   • Energy converted per phase per stroke (from unaligned to aligned):

$$ W_{max} = \frac{1}{2} I_{max}^2 (L_{max} - L_{min}) $$

- **Average Torque** over one stroke (electrical cycle):

$$ T_{avg} = \frac{W_{max}}{\text{stroke angle}} = \frac{\frac{1}{2} I_{max}^2 (L_{max} - L_{min})}{\theta_{on}} $$

where $$\displaystyle \theta_{on} $$ = conduction period (rotor travel during which phase is ON).

D. Shaft Position Sensing

1. Sensor-based:

   • Hall Effect Sensors: Mounted on stator, detect rotor pole passage. Robust, common.

   • Optical Encoders: High precision, more expensive.

2. Sensorless Methods:

   • Inductance Sensing: Measure phase inductance (or its derivative). Inductance is minimum at unaligned, maximum at aligned. Detect zero-crossing of $dL/dt$.

   • Back EMF Detection: During phase OFF period, back EMF ($$\displaystyle e = -d\psi/dt $$) is induced. Its polarity/zero-crossing indicates rotor position. More common for higher speeds.

E. Advantages & Disadvantages

F. Rotor Designs: Solid vs Laminated

• Solid Rotor: Simple, cheap. Used in very low-cost, low-performance applications (e.g., some appliances). High eddy current losses, limited speed.

• Laminated Rotor: Standard. Reduces eddy current losses, allows higher speeds. Slightly more expensive. Performance choice.

G. Applications

• Traction drives (EVs, locomotives - robust, high starting torque)

• Industrial drives (pumps, fans, compressors - wide speed range)

• Appliances (washing machines, vacuum cleaners - cost-effective, robust)

• Safety-critical applications (no PM means no risk of demagnetization at high temp).

IV. BRUSHLESS DC MOTORS (BLDC)

A. Construction

1. Stator: Three-phase, slotted or slotless, with concentrated or distributed windings. Similar to PMSM stator.

2. Rotor: Surface-mounted PMs (SPM) on steel core. PM materials: NdFeB (high energy, temp sensitive), SmCo (high temp), Ferrite (cheap, low energy).

3. PM Materials: Selection based on required $$\displaystyle B_r $$, coercivity, cost, temperature coefficient.

B. Torque Production

1. Back EMF: Trapezoidal waveform (ideally). $$\displaystyle e(\theta) = k_e \cdot \omega \cdot f(\theta) $$, where $f(\theta)$ is trapezoidal.

2. Permeance Coefficient ($$\displaystyle P_c $$): For SPM rotor, ratio of magnet volume to air gap volume. Derives air gap flux density.

$$ P_c = \frac{\mu_0 \cdot A_m \cdot h_m}{A_g \cdot g} \quad \text{or} \quad B_g = \frac{B_r}{\frac{\mu_0 \cdot A_g \cdot g}{\mu_r \cdot A_m \cdot h_m} + 1} \approx \frac{B_r}{1 + \frac{g}{\mu_r h_m}} \text{ (if } A_g \approx A_m) $$

Where: $$\displaystyle A_m, h_m $$ = PM area & thickness; $$\displaystyle A_g, g $$ = air gap area & length; $$\displaystyle \mu_r $$ = PM relative permeability.

3. Commutation Sequence: Six-step (120° electrical). Two phases ON at a time (conducting), one phase OFF. Hall sensors provide 60° electrical commutation signals.

C. Winding Patterns

• Series (Delta): Phase ends connected in delta. Higher phase voltage, lower phase current for same power.

• Parallel (Star/Wye): Phase ends connected in star. Lower phase voltage, higher phase current. Most common for BLDC.

• Star connection is standard; delta used for specific high-voltage applications.

D. Commutation and Armature Reaction

1. Electronic Commutation: Based on rotor position (Hall sensors or sensorless). Switches phases to keep torque vector aligned with rotor magnet flux.

2. Armature Reaction: Distortion of main PM flux by stator MMF.

   • Effect: Flux weakening under load, distortion of back EMF waveform (from ideal trapezoidal), potential for demagnetization if PM is weak.

   • Mitigation: Magnetic circuit design (e.g., rotor shaping), control strategies.

E. Speed Control Methods

1. Voltage Control: Adjust DC bus voltage (linear regulator - inefficient).

2. PWM (Pulse Width Modulation): Primary method. Vary duty cycle of DC bus voltage applied to inverter. Controls average voltage/current.

3. Current Control (Torque Control): Inner current loop regulates phase current. Directly controls torque ($T \propto I$). Used in high-performance drives.

F. Position Sensing

1. Hall Sensors: Three sensors spaced 120° electrical apart. Provide six commutation states per electrical cycle. Simple, robust, used up to moderate speeds.

2. Sensorless Control:

   • Back EMF Detection: Monitor unenergized phase voltage during PWM off-time. Zero-crossing indicates position. Works only at speeds > ~10% rated (back EMF > threshold).

   • Methods: "Terminal Voltage Sensing", "Inductance Sensing" (for low speed).

G. Torque-Speed Characteristics

• Constant Torque Region: Up to base speed ($$\displaystyle \omega_b $$). Torque constant ($$\displaystyle K_t $$). Current controlled, back EMF < DC bus voltage.

• Constant Power Region: Above $$\displaystyle \omega_b $$. Field weakening achieved by advancing commutation angle (current phase shift). Torque decreases as $1/\omega$.

H. Comparison with Conventional Brushed DC Motors

I. Applications

• Fans & blowers (computers, HVAC)

• Pumps (circulator, booster)

• Electric Vehicles (traction, power steering)

• Aerospace (fuel pumps, actuators)

• Home appliances (refrigerator compressors, washing machines)

• Robotics & CNC (direct drive).

V. PERMANENT MAGNET SYNCHRONOUS MOTORS (PMSM)

A. Construction

1. PM Types: Same as BLDC (NdFeB, SmCo, Ferrite).

2. Rotor Structures:

   • Surface-Mounted PM (SPM): PMs on rotor surface. $$\displaystyle L_d \approx L_q $$ (saliency ratio ~1). Simpler, lower cost. Permeance coefficient applies.

   • Interior PM (IPM): PMs embedded inside rotor. $$\displaystyle L_d < L_q $$ (saliency ratio >1). Reluctance torque component ($$\displaystyle T_{rel} \propto (L_d - L_q) I_d I_q $$). More robust, higher torque density, better for field weakening.

B. Principle of Operation

• Synchronous Speed: $$\displaystyle n_s = \frac{120 f}{P} $$ (P = number of poles). Rotor locks to rotating stator magnetic field.

• Sinusoidal Back EMF: Due to distributed winding and sinusoidal PM flux distribution. $$\displaystyle e(\theta) = k_e \omega \sin(\theta) $$ (per phase).

• Requires Sinusoidal Current for smooth torque (unlike BLDC's square-wave).

C. EMF Equation (Derivation)

$$ E_{ph} = 4.44 \cdot f \cdot N \cdot \Phi \cdot k_w $$

• $$\displaystyle E_{ph} $$: RMS phase back EMF.

• $f$: Frequency (electrical) = $$\displaystyle \frac{P \cdot n}{120} $$.

• $N$: Turns per phase.

• $\Phi$: Flux per pole (Wb) from PM.

• $$\displaystyle k_w $$: Winding factor (accounts for distribution & pitch).

• Derivation Basis: $$\displaystyle E_{peak} = N \frac{d\psi}{dt} = N \omega \psi_{max} = 2\pi f N \Phi $$. RMS = Peak/√2. Combine with $$\displaystyle k_w $$ → $$\displaystyle 4.44 f N \Phi k_w $$.

D. Torque-Speed Characteristics

1. Phasor Diagram:

   • $$\displaystyle \vec{V} = \vec{E} + jI_a X_s + I_a R_a $$ (simplified).

   • $\delta$ = Power angle (angle between $\vec{E}$ and $\vec{V}$).

   • Torque $$\displaystyle T \propto \frac{E V}{X_s} \sin \delta $$ (for small R).

2. Circle Diagram:

   • Plot of constant current loci in d-q plane.

   • Torque $$\displaystyle T = \frac{3}{2} \frac{P}{2} \left[ \psi_f I_q + (L_d - L_q) I_d I_q \right] $$.

   • For SPM ($$\displaystyle L_d = L_q $$): $$\displaystyle T \propto I_q $$ (torque proportional to q-axis current).

   • For IPM: Torque has Magnet torque ($$\displaystyle \propto I_q $$) + Reluctance torque ($$\displaystyle \propto I_d I_q $$). Maximum torque at specific $$\displaystyle I_d, I_q $$ angle.

E. Speed Control Methods

1. Variable Frequency Control (V/f control): Maintain constant $V/f$ ratio to keep flux constant. Simple, open-loop. Used for pumps/fans.

2. Vector Control (Field-Oriented Control, FOC):

   • Transform 3-phase currents to d-q rotating reference frame.

   • Control $$\displaystyle I_d $$ (flux) and $$\displaystyle I_q $$ (torque) independently like a DC motor.

   • Decouples torque and flux. High dynamic performance.

3. Direct Torque Control (DTC):

   • Directly control torque and flux by selecting optimal voltage vectors from inverter.

   • No PWM modulator, fast torque response. Higher torque ripple than FOC.

F. Torque Pulsation Reduction Techniques

• Skewing: Rotor or stator skewed by one slot pitch. Reduces cogging torque & ripple.

• Winding Distribution: Use distributed windings (higher $$\displaystyle k_w $$, lower harmonics).

• Control Strategies: FOC with precise current control, DTC with optimized switching tables, harmonic injection.

G. Sensorless Control

• Back EMF Detection: Similar to BLDC, but requires sinusoidal integration. Works above ~5% speed.

• High-Frequency Injection: Inject high-frequency signal into stator, detect rotor position from resulting impedance variation (saliency-based). Works at zero speed (for IPM).

• Observer-Based: Use mathematical model (Kalman filter, sliding mode observer) to estimate rotor position from voltages/currents.

H. Power Controllers and Drives

• Inverters: Two-level (standard), Multilevel (NPC, flying capacitor - for high power/voltage).

• PWM Schemes:

  • SPWM (Sinusoidal PWM): Sinusoidal reference vs. triangular carrier. Simple, good fundamental control, higher harmonics.

  • SVPWM (Space Vector PWM): Controls inverter voltage vector directly. Better DC bus utilization (15% more), lower harmonics, more complex.

I. Comparison with BLDC Motors

J. Applications

• High-performance servo systems (CNC, robotics)

• Electric & Hybrid Vehicles (traction motor)

• Wind turbine generators (direct-drive)

• Aerospace (actuators, fuel pumps)

• Industrial drives requiring high efficiency & precision.

VI. PERMANENT MAGNET DC MOTORS (BRUSHED)

• Construction: PM field (stator), armature (rotor) with commutator & brushes.

• Principle: Armature current interacts with PM field → torque. Commutator reverses current direction to maintain unidirectional torque.

• Features: Simple control (vary armature voltage), low cost, high starting torque, maintenance required (brush/commutator wear).

• Applications: Low-power adjustable speed (toys, small tools), automotive (windshield wipers, seat adjusters), cost-sensitive applications where efficiency/maintenance not critical.

VII. AC SERVOMOTORS

• Construction: Typically a two-phase or three-phase synchronous motor with damper windings (like squirrel cage) on rotor. Often uses PM or DC excited field.

• Principle: Two-phase (or three-phase with vector control) stator winding produces rotating magnetic field. Rotor (with PM or field winding) locks to it. Damper winding provides damping and starting torque.

• Torque-Speed Characteristics: Linear over wide speed range. High torque at low speed, fast acceleration/deceleration. Excellent position/speed control.

• Applications: High-performance positioning (machine tools, robotics, antenna positioning, radar).

VIII. POWER QUALITY IN SCADA SYSTEMS

A. Definition and Importance

• Definition: Characteristics of electricity at a given point on a power system, including voltage, current, frequency, and waveform fidelity. (IEEE 1159)

• Importance for SCADA:

  • Prevents malfunction/damage to sensitive industrial control equipment (PLCs, RTUs, drives).

  • Ensures data integrity and communication reliability.

  • Reduces downtime, maintenance costs, and production losses.

  • Critical for process continuity in manufacturing, oil & gas, water treatment.

B. Causes of Power Quality Disturbances

• Natural: Lightning, weather.

• Man-made:

  • Faults (short circuits): Cause sags/swells.

  • Switching Operations: Capacitor bank switching, load switching → transients, inrush.

  • Non-linear Loads: Draw non-sinusoidal current → harmonics.

  • Large Motor Starting: Voltage sags.

  • Arc Furnaces, Welders: Fluctuations, flicker, harmonics.

C. Classification of Disturbances (IEEE 1159)

1. Voltage: Sags, Swells, Interruptions, Imbalance, Waveform Distortion (harmonics), Flicker, Transients.

2. Current: Overcurrent, undercurrent, imbalance.

3. Frequency: Deviation from nominal (50/60 Hz).

4. Temporal:

   • Short Duration: < 1 min (sags, swells, transients).

   • Long Duration: > 1 min (sustained overvoltage, undervoltage, imbalance).

D. Power Quality Indices

1. Total Harmonic Distortion (THD):

$$ \text{THD}_V = \frac{\sqrt{\sum_{h=2}^{40} V_h^2}}{V_1} \times 100\% \quad \text{(Voltage)} $$

$$ \text{THD}_I = \frac{\sqrt{\sum_{h=2}^{40} I_h^2}}{I_1} \times 100\% \quad \text{(Current)} $$

- $$\displaystyle V_h, I_h $$: RMS value of h-th harmonic.

- $$\displaystyle V_1, I_1 $$: RMS fundamental.

- **Limits**: IEEE 519 recommends THD_V < 5% for systems < 69 kV.

2. Total Demand Distortion (TDD):

$$ \text{TDD} = \frac{\sqrt{\sum_{h=2}^{40} I_h^2}}{I_{demand}} \times 100\% $$

- $$\displaystyle I_{demand} $$: Peak demand current (typically 15-min or monthly). **Normalizes** harmonic current to system size. More fair for comparing different users.

3. Crest Factor (CF):

$$ \text{CF} = \frac{\text{Peak value}}{\text{RMS value}} $$

- Indicates "peakedness" of waveform. High CF (>2) suggests non-sinusoidal or switching transients. Can stress equipment.

4. Interharmonics:

   • Definition: Frequencies not integer multiples of fundamental (e.g., 150 Hz on 60 Hz system). $$\displaystyle f_{inter} = m f_1 \pm n f_2 $$ (from cycloconverters, arcing).

   • Sources: Cycloconverters, arcing loads (welders, furnaces), static frequency converters.

   • Effects: Flicker, control system interference, torque pulsations in motors.

IX. VOLTAGE DISTURBANCES

A. Voltage Sags and Swells

1. Causes:

   • Sags (Dips): 0.1 pu to 0.9 pu, 0.5 cycles to 1 min. Faults (remote or local), large motor starting, load switching.

   • Swells: 1.1 pu to 1.8 pu, 0.5 cycles to 1 min. Fault clearing (single-line-to-ground), large capacitor bank switching, sudden load drop.

2. Effects:

   • Sags: Tripping of sensitive equipment (computers, drives, contactors), data loss, motor stalling.

   • Swells: Overvoltage stress on insulation, overheating in magnetic components, false triggering.

3. Mitigation Techniques:

   • Dynamic Voltage Restorer (DVR): Series-connected voltage source converter. Injects voltage to compensate sag/swell. Fast.

   • Uninterruptible Power Supply (UPS): Provides backup during sags/interruptions. Double-conversion type also cleans harmonics.

   • Solid-State Transfer Switch (SSTS): Fast switching to alternate source.

   • UPQC: Can also compensate sags/swells (series part).

B. Voltage Fluctations (Flicker)

• Cause: Rapid, repetitive voltage variations from fluctuating loads (arc furnaces, welders, large motor cycling).

• Measurement:

  • Pst (Short-term Flicker): 10-minute observation. Perceptibility threshold = 1.0.

  • Plt (Long-term Flicker): 2-hour average of Pst. Threshold = 1.0.

  • Measured by flicker meters per IEC 61000-4-15.

• Mitigation: Static Var Compensator (SVC/STATCOM) for fast reactive power support, dedicated flicker mitigation plants, soft starters for motors.

C. Transient Overvoltages

1. Sources:

   • Lightning (direct/indirect).

   • Switching: Capacitor bank switching (worst - high frequency), line/cable switching, fault clearing.

   • Ferroresonance: Nonlinear inductance (transformer) with capacitance → sustained overvoltage.

2. Transient Recovery Voltage (TRV):

   • Voltage across breaker contacts after current interruption.

   • Factors: System grounding (effectively grounded → lower TRV), fault type (3-phase > SLG), source impedance, load.

   • Critical for breaker rating (TRV capability).

3. Mitigation:

   • Surge Arresters (LA): Clamp overvoltage to safe level (V-I characteristic).

   • RC Snubbers: Across breaker contacts or switching devices (damp oscillations, limit dV/dt).

   • Closing Resistors: Pre-insertion resistors in breakers to limit inrush/TRV.

D. Surges and Impulse Voltages

• Surge: Transient wave of voltage/current (microseconds to milliseconds). Caused by lightning, switching.

• Protection Methods:

  • Surge Protective Devices (SPD) / TVSS: Shunt-connected devices (gas discharge tubes, MOVs, spark gaps). Divert surge current to ground.

  • Shielding & Grounding: For lightning protection (Faraday cage, low-impedance ground).

  • Proper Wiring Practices: Avoid loops, separate power/control cables.

E. Voltage Sag Protectors

• Devices: Solid-state transfer switches (SSTS), dynamic voltage restorers (DVR), some UPS systems.

• Protection Scheme Need: Critical loads cannot tolerate sags. Need fast (< 1/4 cycle) compensation to avoid tripping. SCADA systems themselves need protection to maintain monitoring/control.

F. Estimation of Voltage Sag Performance

• Indices:

  • SARFI (Sag Average RMS Frequency Index): Average number of sags per year below a threshold.

  • Sag Magnitude vs. Duration Plot: Probabilistic (density) plot.

• Monitoring: Use power quality monitors at critical buses. Log events with magnitude, duration, phase.

• Statistical Analysis: Fit sags to distributions (e.g., Beta) for reliability studies. Estimate expected number of sags for equipment immunity specification.

X. HARMONICS

A. Sources

1. Industrial:

   • Arc Furnaces, Welders: Major source of interharmonics & harmonics.

   • Rectifiers (DC drives, electrolysis): 6-pulse (5th, 7th), 12-pulse (11th, 13th).

   • Variable Speed Drives (VSDs): PWM inverters (switching frequency & sidebands).

   • Choppers, Cycloconverters.

2. Commercial:

   • Computers, UPS, LED Drivers: 3rd harmonic dominant (due to single-phase rectifier with capacitive filter).

   • Electronic Ballasts (fluorescent, HID).

   • Switch-mode Power Supplies (SMPS).

B. Effects on Power Systems

• Heating: $$\displaystyle I^2R $$ losses in conductors, transformers, motors (due to harmonic currents).

• Resonance: Harmonic source excites system L-C resonance → amplified voltages/currents → overvoltages, overheating.

• Nuisance Tripping: Protective relays misoperate on harmonic-distorted waveforms.

• Interference: With communication lines, control signals (SCADA), telemetry.

• Neutral Overloading: In 3-phase 4-wire systems, triplen harmonics (3rd, 9th, 15th) add in neutral.

• Torque Pulsations: In motors (5th, 7th cause 6x fundamental frequency ripple).

C. Harmonic Indices

• Individual Harmonic Distortion (IHD):

$$ \text{IHD}_h = \frac{I_h}{I_1} \times 100\% \quad \text{or} \quad \frac{V_h}{V_1} \times 100\% $$

• THD (see VIII.D.1).

• TDD (see VIII.D.2). TDD is the index used in IEEE 519 for current distortion limits.

D. Interharmonics

• Sources: Cycloconverters, arcing loads, static frequency converters, PWM drives with non-integer carrier ratio.

• Effects:

  • Flicker: Modulation of light output (especially < 25 Hz).

  • Control Issues: In motor drives, cause torque ripple, speed oscillation.

  • Relay Misoperation: Distort waveform, affect RMS measurement.

E. Non-linear Loads as Primary Sources

• Definition: Loads where current is not sinusoidal for sinusoidal voltage. I-V characteristic is non-linear.

• Examples: All devices with rectifier front-end (diodes/thyristors), saturation in transformers, arc devices.

• Characteristic: Draws current in pulses near voltage peaks → rich in odd harmonics (3rd, 5th, 7th...).

XI. POWER QUALITY MITIGATION TECHNIQUES

A. Passive Filters

1. Series Filters (Notch Filters): Block specific harmonic frequencies (e.g., 5th, 7th). Series LC tuned to harmonic. Low impedance at harmonic freq.

2. Shunt Filters:

   • Tuned Filters (Single/Double-tuned): LC circuit tuned to specific harmonic (e.g., 5th). Provides low-impedance path to ground.

   • C-Type Filter: For 2nd harmonic (in 3-phase rectifier systems). Uses capacitor in series with L and parallel R.

   • High-Pass Filter: Broadband, attenuates high-frequency harmonics. Uses R-L in series with C in parallel.

3. Advantages: Simple, low cost, high power handling.

4. Disadvantages: Fixed tuning (cannot adapt), can cause resonance with system impedance, large size, aging of components.

B. Active Power Filters (APF)

1. Shunt-Active Filter:

   • Principle: Connected in parallel with load. Injects compensating current ($$\displaystyle i_{comp} $$) that is equal and opposite to load harmonic current.

   • Reference Current Generation:

     • p-q Theory: Instantaneous power theory. Separates instantaneous active/oscillating power. $$\displaystyle i_{comp} = i_{load} - (active\ current\ component) $$.

     • Synchronous Detection: Measure fundamental current component, subtract from load current.

   • Advantages: Dynamic, selective compensation (can target specific harmonics), does not cause resonance, can also compensate reactive power.

   • Disadvantages: High cost, complex control, power rating limited by semiconductor switches.

2. Series-Active Filter: Connected in series with source. Injects voltage to cancel voltage harmonics/imbalances. Protects load from source disturbances.

3. Comparison: Shunt APF is more common for harmonic current compensation.

C. Flexible AC Transmission Systems (FACTS)

1. SVC (Static Var Compensator):

   • Components: Thyristor-Controlled Reactor (TCR) + Thyristor-Switched Capacitor (TSC) ± Fixed Capacitor (FC).

   • Operation: TCR provides continuously variable inductive reactance (phase angle control). TSC switches capacitor banks in/out (stepped). FC provides basic capacitive support.

   • Response: ~1-2 cycles.

2. STATCOM (Static Synchronous Compensator):

   • Components: Voltage Source Converter (VSC) + DC capacitor + coupling transformer.

   • Operation: Acts as a controllable voltage source behind a reactance. Injects/absorbs reactive power by controlling output voltage magnitude.

   • Advantages over SVC:

     • Faster response (< 1 cycle, typically 1-2 ms).

     • Better performance at low voltage (reactive current capability linear with voltage, vs. SVC's quadratic).

     • Smaller footprint (no large inductors/capacitor banks).

     • Less harmonic generation (PWM can be controlled).

     • Higher cost.

3. DSTATCOM (Distribution STATCOM):

   • Operation: Same as STATCOM but rated for distribution voltage levels (typically < 69 kV). Used for voltage support, flicker mitigation, harmonic compensation at distribution level.

   • Benefits: Improves voltage profile, increases hosting capacity for DG, mitigates flicker from large loads.

4. Comparison: SVC vs STATCOM

   | Feature | SVC | STATCOM | |---------|-----|---------| | Response Time | 1-2 cycles | < 1 cycle (ms) | | Reactive Power vs. Voltage | Quadratic (drops with V²) | Linear (drops with V) | | Harmonic Generation | Generates harmonics (TCR) | Minimal (PWM controlled) | | Size/Cost | Larger, lower cost | Smaller, higher cost | | Low Voltage Support | Poor | Excellent |

D. Unified Power Quality Conditioner (UPQC)

1. Principle: Back-to-back connection of series APF and shunt APF sharing a common DC link capacitor.

   • Shunt part: Compensates load current harmonics, reactive power, unbalance.

   • Series part: Compenses supply voltage sags/swells, harmonics, unbalance.

2. Classification (based on compensation strategy):

   • UPQC-Q: Series part focuses on voltage sag/swell compensation (voltage injection). Shunt part does current compensation.

   • UPQC-P: Series part focuses on power flow control (phase angle). Shunt part does current compensation.

   • UPQC-S: Both parts share compensation (voltage & current) based on load/supply conditions.

3. Working & Operation:

   • Voltage Sag Mitigation: Series APF injects voltage in phase with supply to boost voltage to load.

   • Harmonic Compensation: Shunt APF injects harmonic currents; series APF can block harmonic voltages from reaching load.

   • Reactive Power Support: Shunt APF can supply/absorb reactive power locally.

4. Advantages over DSTATCOM: Comprehensive - Mitigates both voltage disturbances (sags, swells, harmonics, flicker, imbalance) and current harmonics. DSTATCOM (shunt only) only handles current-side issues.

5. Power Quality Problems Mitigated:

   • Voltage sags, swells, interruptions (series).

   • Voltage harmonics, unbalance (series).

   • Current harmonics, reactive power, unbalance (shunt).

   • Flicker (both).

E. Power Factor Correction

1. Benefits:

   • Reduce I²R losses in distribution system (lower current for same real power).

   • Improve voltage regulation (less voltage drop in lines).

   • Avoid utility penalties (most utilities charge for PF < 0.9 lagging).

   • Increase system capacity.

2. Capacitor Bank Installation Locations:

   • Load-side: At individual inductive loads (motors). Most effective, reduces current at source.

   • Feeder-side: At distribution panel. Compensates for group of loads.

   • Substation-side: At main transformer bus. Compensates for entire facility, but does not reduce feeder currents.

3. Reactive Power Compensation Penalty: Utilities impose penalty because low PF (lagging) increases their system current, requiring larger conductors, transformers, and higher generation capacity. Penalty incentivizes customers to correct PF.

F. Zero-Voltage Crossing Switching

• Principle: Switch capacitor banks (or other reactive elements) when supply voltage is at zero crossing.

• Why Optimal for Transient Mitigation:

  • Capacitor Inrush Current: When switching a capacitor, initial condition is zero voltage across capacitor. If switched at voltage peak ($$\displaystyle V_{peak} $$), maximum inrush current $$\displaystyle I_{inrush} = V_{peak} / Z_{circuit} $$ (limited only by circuit impedance, can be very high).

  • Switching at zero voltage means capacitor voltage starts at zero, matching line voltage → no sudden voltage step → minimal inrush current (only limited by circuit resistance).

  • Prevents capacitor switching transients (overvoltages, harmonic generation, nuisance tripping of breakers).

• Implementation: Use synchro-check relays or zero-crossing detectors with thyristor/TRIAC switches.

XII. APPLICATIONS IN SCADA-CONTROLLED SYSTEMS

A. Electric Vehicles (EVs)

1. Motor Drives: BLDC and PMSM are dominant due to:

   • High efficiency (extends range).

   • High power density.

   • Good speed-torque control (regenerative braking).

   • PMSM (FOC) slightly higher performance, BLDC slightly lower cost.

2. Control Strategies:

   • Torque Control (FOC for PMSM, 6-step/torque control for BLDC).

   • Regenerative Braking: Motor acts as generator, feeds energy back to battery.

   • Energy Management: Optimizes motor efficiency, battery SOC, thermal management.

3. SCADA Role:

   • Battery Management System (BMS) Integration: Monitors cell voltage, temperature, SOC.

   • Motor Monitoring: Temperature, vibration, current, efficiency.

   • Fault Diagnosis: Detects motor/generator faults, inverter faults, sensor failures.

   • Charging Station Management: Coordinates charging, monitors grid interaction.

B. PV Water Pumping Systems

1. Motor Types: BLDC or PMSM preferred for:

   • High efficiency (maximize water output per solar watt).

   • Wide speed range (matches variable solar irradiance).

   • No gearbox often (direct drive).

2. Drive Control:

   • MPPT Algorithm: Adjusts motor speed to extract maximum power from PV array (perturb & observe, incremental conductance).

   • Speed Control: Varies frequency to match available power.

   • Soft Start: Limits inrush current, protects motor/pump.

3. SCADA Monitoring:

   • Solar Irradiance & PV Voltage/Current.

   • Pump Status (running/fault), Water Flow Rate, Tank Level.

   • Fault Detection: Dry run, over/under voltage, motor stall.

   • Remote Control: Start/stop, scheduling.

C. Industrial Process Control

1. Motor Monitoring:

   • Vibration Analysis: Bearing faults, imbalance, misalignment.

   • Temperature Monitoring: Stator/ bearing temps (thermocouples, RTDs).

   • Current Signature Analysis: Detect broken rotor bars, airgap eccentricity.

   • Efficiency Monitoring: Input/output power measurement.

2. Power Quality Monitoring and Mitigation:

   • Why: Prevent unexpected downtime, equipment damage (drives, PLCs), product quality issues.

   • SCADA Integration: PQ monitors (harmonic analyzers, sag/swell recorders) feed data to SCADA historian.

   • Alarm Management: Alerts on PQ events exceeding thresholds.

   • Mitigation Coordination: Triggers mitigation devices (UPQC, DVR) or load shedding.

XIII. POWER QUALITY MONITORING AND ANALYSIS

A. Harmonic Analyzers

• Operation: Sample voltage/current waveforms at high rate (e.g., > 10 samples/cycle). Apply FFT (Fast Fourier Transform) to extract harmonic magnitudes/phases up to 50th/40th harmonic.

• Standards: IEC 61000-4-7 specifies measurement methods, grouping (harmonic groups for interharmonics), windowing, accuracy.

• Output: THD, individual harmonic magnitudes, harmonic spectrum, phase angles.

B. Sag/Swell Recorders

• Operation: Continuously monitor RMS voltage (over 1-cycle or half-cycle window). Detect when RMS deviates beyond preset thresholds (e.g., 0.9 pu for sag, 1.1 pu for swell).

• Capture: Record waveform pre-event (pre-trigger) and post-event. Measure magnitude, duration, point-on-wave.

• Standards: IEC 61000-4-11 for testing equipment immunity, also guides monitoring.

C. Power Quality Meters

• Comprehensive: Measure voltage, current, frequency, power (P, Q, S), PF, THD, harmonics up to 50th, interharmonics, sags/swells, transients, flicker, unbalance.

• Features: Data logging, trend analysis, event capture, communication (Modbus, Ethernet) for SCADA integration.

D. SCADA Integration

• Remote Monitoring: PQ meters communicate via RS485, Ethernet, or wireless to SCADA RTUs/PLC.

• Data Logging: Historical storage in SCADA historian for trend analysis, reporting.

• Alarm Management: Configure alarms for PQ events (e.g., THD > 5%, sag < 0.8 pu). Alarms trigger operator alerts or automated responses.

• Dashboard/Visualization: Real-time PQ trends, event logs, statistical summaries on HMI.

XIV. SPECIAL TOPICS (RECURRING THEMES)

A. Torque Pulsation in PMSM and Reduction Methods

• Causes::

  1. Cogging Torque: Interaction between PMs and stator slots (even with no current). Due to variation in magnetic reluctance with rotor position.

  2. Torque Ripple: From current commutation (in BLDC), imperfect sinusoidal current, harmonics in back EMF, saliency effects (IPM).

• Reduction:

  • Skewing: Rotor or stator skewed by one slot pitch (most effective for cogging).

  • Fractional-Slot Winding: Non-integer slots per pole per phase.

  • Optimized PM Arc/Pole Shape: Shaping to smooth airgap flux.

  • Distributed Windings: vs. concentrated.

  • Control Strategies: FOC with precise current control, d-axis current injection (for IPM to smooth torque).

B. Sensorless Control Techniques for PMSMs and BLDCs

C. Advanced Driver Circuits for Stepper Motors

• Chopper Drives: Standard for performance. Use PWM to regulate current to constant value regardless of speed. Provides constant torque.

• Resonant (LC) Drives: Use LC circuit to create sinusoidal current waveform. Smoother motion, less vibration, but complex and less common.

• Bi-level Drives: Use two voltage levels (high for acceleration, low for holding). Simple improvement over single voltage.

D. Comparative Analysis of Motors for Specific Applications

E. Magnetic Materials for PM Motors

Selection Trade-off: Energy product vs. cost vs. temperature stability. NdFeB dominates high-performance; ferrite for cost-sensitive.

UNIT 4 - EXAM FOCUS SUMMARY:

• Steppers: Types, step angle calc, driver circuits (chopper), microstepping, pull-in/pull-out.

• SRM: Torque formula $$\displaystyle T = \frac{1}{2} I^2 \frac{dL}{d\theta} $$, aligned/unaligned, sensorless (back EMF/inductance), solid vs laminated rotor.

• BLDC vs PMSM: Back EMF shape (trap vs sin), commutation (6-step vs FOC), winding (120° vs sinusoidal), applications.

• PQ Disturbances: Define sags/swells, flicker (Pst/Plt), transients, harmonics (THD/TDD).

• Mitigation: UPQC (series+shunt, comprehensive), STATCOM vs SVC (response, V-dependency), Active vs Passive Filters, DVR for sags.

• Applications: EV motors (BLDC/PMSM), PV pumping (MPPT + BLDC/PMSM), SCADA monitoring role.

• Recurring: Torque ripple reduction, sensorless control, zero-voltage crossing switching, magnetic materials.