ch16 - 16-1 Chapter 16 Powering Electric Drives: Power...

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Unformatted text preview: 16-1 Chapter 16 Powering Electric Drives: Power Quality Issues Exit 2001 by N. Mohan Print TOC " ! 16-2 Powering Electric Drives: Power Quality Issues ! Electric Drives can affect utility power quality ! Utility power quality can affect electric drive operation Electric Drive fixed form Electric Source (utility) Power Processing Unit Motor Load speed / adjustable position form Sensors Controller input command (speed / position) Exit 2001 by N. Mohan TOC " ! 16-3 Linear and Nonlinear Loads is + Vs vs φ − Is Linear Load P PF = = cosφ Vs Is Exit 2001 by N. Mohan TOC " ! 16-4 Linear and Nonlinear Loads (cont…) vs 0 idistortion (= is − is1 ) is1 is t φ1 / ω 0 t T1 Non-linear Load (PPU) ! Nonlinear loads reduce power factor Idistortion Total Harmonic Distortion: %THD = 100 x Is1 Displacement Power Factor: DPF = cos φ1 Exit 2001 by N. Mohan Is1 PF = (DPF) = Is DPF 1+THD 2 TOC " ! 16-5 Harmonic Currents Lower Power Factor 1 0.9 0.8 0.7 PF DPF 0.6 0.5 0.4 0 50 100 150 200 250 300 %THD ! Ratio of actual power factor to displacement power factor (PF based solely on angle of fundamental current) decreases with increasing THD Exit 2001 by N. Mohan TOC " ! 16-6 Harmonic Guidelines Total Harmonic Odd Harmonic Order h I SC / I1 h < 11 11 ≤ h ≤ 17 17 ≤ h ≤ 23 < 20 4.0 2.0 1.5 20 − 50 7.0 3.5 50 − 100 10.0 100 − 1000 > 1000 23 ≤ h ≤ 35 35 ≤ h Distortion(%) 0.6 0.3 5.0 2.5 1.0 0.5 8.0 4.5 4.0 1.5 0.7 12.0 12.0 5.5 5.0 2.0 1.0 15.0 15.0 7.0 6.0 2.5 1.4 20.0 ! IEEE – 519 ! Limits on allowable harmonic currents drawn by loads of various relative magnitudes ! Relative magnitude of load currents is based on Short Circuit Ratio (SCR) Isc SCR = Is1 Exit 2001 by N. Mohan Where Isc is the short circuit current and Is1 is the fundamental current of the load TOC " ! 16-7 Types of Electric Drive FrontEnds ! Diode-bridge rectifiers ! Switch-mode converters ! Thyristor converter Exit 2001 by N. Mohan TOC " ! 16-8 Single – Phase Diode – Rectifier Bridge vd idr is idr 0 t1 idr is t2 t3 is ωt + vs (t ) Ls 1 2 + Rs vd − vs 4 id − Cd Req 3 ! Power levels up to several kW ! Current drawn from utility in short pulses Exit 2001 by N. Mohan TOC " ! 16-9 Using Input Inductances to Lower THD increasing Ls increasing Ls ! Current pulses widen as Ls is increased Exit 2001 by N. Mohan TOC " ! 16-10 Three-Phase Diode-Rectifier Bridge idr − va − vb − vc + Ls + Ls + Ls 1 − va vb + + 5 vd 4 − 3 Ls Ls 6 + − Cd Req 2 1 3 5 idr P 4 − Exit 2001 by N. Mohan vc + Ls 6 2 N TOC " ! 16-11 Voltage and Current Without Cd va vb vc ia 0 0 t 120 o 60 o t ib 0 (a) vo t ic 2VLL 0 Vd t 0 t ! VP follows whichever phase voltage is most positive at any moment ! VN follows whichever phase voltage is most negative at any moment ! Without Cd, phase currents flow for a full 120° duration Exit 2001 by N. Mohan TOC " ! 16-12 Currents With Ls and Cd Included Ls = 0.1 mH THD = 123.33% Ls = 5 mH THD = 18.72% ! Showing characteristic “double pulse” in phase currents of 3-Φ bridge ! First pulse in ia occurs roughly when Vab is at its maximum. Second pulse comes 60° later when Vac is at its maximum ! Increasing Ls tends to smooth out the pulses lowering THD Exit 2001 by N. Mohan TOC " ! 16-13 Power Factor Corrected Interface (PFC) ! Passive methods to lower THD (using Ls) are generally unable to meet THD requirements ! PFC circuits use active switching to make the PPU front end look resistive to the utility ! Goal is sinusoidal input current in-phase with utility voltage (unity PF) Exit 2001 by N. Mohan TOC " ! 16-14 Single – Phase PFC iL is + + vs − + Vd vs − Load − vs is iL is t ! Boost converter is modulated to draw sinusoidal current from utility ! Vd is adjusted by setting ˆs to different values i ˆ ! Vd > Vs Exit 2001 by N. Mohan TOC " ! 16-15 Three-Phase PFC + Vd − is ! Combination of three boost converters ! Operation principle same as in single-phase PFC ˆ ! Vd > VLL Exit 2001 by N. Mohan TOC " ! 16-16 Avoiding Large Inrush Currents ! Resistor limits inrush current at startup ! Resistor switched out during operation Exit 2001 by N. Mohan TOC " ! 16-17 Bi-Directional Power Flow AC motor Rectifier P + P Inverter motoring mode regenerative braking mode i A (t ) va (t ) ia (t ) − e (t ) + A − n + c dc Exit 2001 by N. Mohan b db a da − Vd A C B dA dB !"#"" " $ dC ac motor TOC " ! 16-18 Bi-Directional Power Flow (cont…) i A , 1 (t ) ia ,1 (t ) + va (t ) + − − vao ,1 (t ) vao ,1 (t ) + + − − sinusoidal e A (t ) ! Front-end built as a switch-mode inverter Exit 2001 by N. Mohan TOC " ! 16-19 Phase-Controlled Thyristor Converters DC Motor Drives ! More economical when response is not critical ! Thyristors perform rectification and regulation Exit 2001 by N. Mohan TOC " ! 16-20 Thyristor Operation is + + vs − vd vs − vd is ! Thyristor is basically a diode that won’t turn on until it is gated on. Once on, it will conduct until external waveforms drop its current to zero ! Regulation comes by choosing when to turn the thyristor on (phase-control) Exit 2001 by N. Mohan TOC " ! 16-21 Single-Phase Phase-Controlled Thyristor Converters id is + vs Ls + 1 vd − 4 dc motor 3 2 − + ea − ! Similar to single-phase rectifier with diodes replaced by phase-controlled thyristors Exit 2001 by N. Mohan TOC " ! 16-22 Gating Action – Voltage and Current Waveforms 3 1 is − 4 vs vd + Vd 0 + vd 2 − Id vs t iG is α 0 α t Id −Id t ! Angle α is adjustable ! Increasing α lowers Vd Vd = 0.9 ⋅ Vs cos α ! Vd can be made negative with large values of α Exit 2001 by N. Mohan TOC " ! 16-23 Operating Modes Vd 0 Vd 90 o 150 o 180 o α 0 Rectifier P = Vd I d = + Id Inverter P = Vd I d = − ! Converter can pass power both directions Exit 2001 by N. Mohan TOC " ! 16-24 Power – Phasor Viewpoint vd Vd 0 − vs iG 0 is ωt vs φ1 = α ωt is1 I s1 Id 0 φ1 = α Vs −Id ωt !PAC = VsIs1 cos α Is1 = fundamental component of Id = 0.9 Id α = angle between vs and is1 Exit 2001 by N. Mohan !PDC = Vd Id Since Vd = 0.9 Vs cos α and Id = Is1 / 0.9, PDC = PAC TOC " ! 16-25 Commutation Interval 2ω Ls I d 1 4 3 i1 vs is Ls + − − vL + i4 2 vd Vd 0 i3 vd i2 − vs + − Id t vs iG t is Id 0 α − Id t u ! Time needed to reverse inductor current reduces output voltage 2 ∆ Vd = ω Ls I d π Exit 2001 by N. Mohan TOC " ! 16-26 Three-Phase Phase-Controlled Thyristor Converter id − − − va vb vc + ia 1 3 − 5 va + ia 1 P 3 Ls + 5 + + + 4 6 2 − 4 6 2 Id − vd N ! Vd = 1.35 VLL cos α (without commutation interval) ! Vd = 1.35 VLL cos α – (3 ω Ls Id) /π (with commutation interval) Exit 2001 by N. Mohan TOC " ! 16-27 Four-Quadrant Operation ω m ,Vd id conv1 + conv2 dc motor vd + − − regenerative braking CCW conv 2: inverter motoring CW conv 2: rectifier Tem , I d motoring CCW conv 1: rectifier regenerative braking CW conv 1: inverter ! Two anti-parallel thyristor converters are needed for four-quadrant operation Exit 2001 by N. Mohan TOC " ! Power Quality and Thyristor Converters 16-28 ! Poor DPF at low speeds (large angle α) ! Input voltage notching Point of Common Coupling (PCC) is + vs − Ls1 + vs v pcc Ls1 Ls1 + Ls2 Ls2 v pcc t − other loads Exit 2001 by N. Mohan TOC " ! Designing for Power System Disturbances 16-29 ! Many drives are designed to tolerate disturbances within the CBEMA limits shown (Computer and Business Equipment Manufactures Association) Exit TOC " ! 2001 by N. Mohan 16-30 Handling Outages and Voltage Sags Capacitor Inverter Rectifier Injection transformer AC source thyristor switch back-up switch L o a d ! UPS (Uninterruptible Power Supplies) ! DVR (Dynamic Voltage Restorer) Exit 2001 by N. Mohan TOC " ! 16-31 Summary/Review ! How is the power factor (PF) defined with linear loads? ! How does the definition of power factor change with loads which draw distorted currents? ! What is meant by the distortion component of the current? ! How is the Total Harmonic Distortion (THD) defined? ! What is the Displacement Power Factor (DPF)? What is it equal to in linear loads which draw sinusoidal waveform currents? ! What is the power factor equal to in terms of the displacement power factor? ! What are the deleterious effects of a high THD and a poor power factor? ! What are the various harmonic guidelines? Briefly describe the IEEE-519 and the rationale behind it. Exit 2001 by N. Mohan TOC " ! 16-32 Summary/Review ! What is the most common type of “front-end” used to interface drives with single-phase and three-phase utility systems? ! Briefly describe the operation of single-phase diode-rectifier bridges with a large capacitor at the dc output. Why is the current drawn by these highly distorted? What is the impact of the values of Ls and Cd on the input current waveform? ! Repeat the previous question for a three-phase diode-rectifier bridge. ! How do single-phase and three-phase rectifier bridges compare in terms of performance? At higher power levels, if there is a choice available, why is one much more favorable than the other? ! Describe the operating principle of the single-phase power-factorcorrected (PFC) circuit, where the power flow is unidirectional. Exit 2001 by N. Mohan TOC " ! 16-33 Summary/Review ! What is a good solution for a three-phase PFC in systems where the power is not fed back into the utility grid? ! In the case of a bi-directional power flow, assuming a three-phase utility input, draw the complete topology of the power-processing unit. How can we analyze it in steady state on a per-phase basis? ! For which type of motor drives are thyristor converters still used? ! Describe phase control and its effect on the simplest possible thyristor circuit. ! Describe the operation of a single-phase, full-bridge thyristor converter. How is the average value of the output voltage controlled? ! Describe current commutation and the effect of Ls on the operation of a single-phase, full-bridge thyristor converter. Exit 2001 by N. Mohan TOC " ! 16-34 Summary/Review ! How is it possible to achieve four-quadrant operation using thyristor converters? Describe the role of each converter, based on the quadrant in which the “front-end” is operating. ! What are the two worst power quality impacts of thyristor converter “front-ends”? ! Briefly describe the impact of utility disturbances on the proper operation of drives. Also, briefly describe various mitigation measures that can be applied. Exit 2001 by N. Mohan TOC " ! ...
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