DYNAMIC VOLTAGE RESTORER FOR VOLTAGE SAG MITIGATION IN OIL & GAS INDUSTRY

Department of Electrical Engineering Senior Design Project ELEC 499 DYNAMIC VOLTAGE RESTORER FOR VOLTAGE SAG MITIGATION IN OIL & GAS INDUSTRY Student Names: Chresteen Baraket Marina Messiha Supervised by: Dr. Ahmed Massoud Dr. Atif Iqbal Spring 2015

2 Acknowledgment We deeply would like to thank the Department of Electrical Engineering. We would like to thank Dr.Ahmed Massoud and Dr.Atif Iqbal for their support and encouragement throughout the semester. We also would like to thank Eng.Hamid Azani, Eng.Ayman Ammar and Eng. Mohammed Ayad for their help. We also thank Eng. Ramadan, Senior Engineer from Qatar Petroleum industry.

Outline Introduction Voltage Sag Definition Problem Definition Objectives and Constraints of the Design Voltage Sag Compensation and Detection Methods DVR Compensation Methods Design and Implementation of the System Experimental Results Conclusion and Future work 3

5 Introduction Customer s concern is to receive a constant and well-regulated voltage level. Power disturbances and Interruptions reduce many companies and industries (customers) profitability. The majority of power quality problem is Voltage Sag event.

Voltage Sag Definition According to IEEE standard 1159-1995. - Voltage sag is: The drop of the RMS value of the voltage from 0.1 to 0.9 p.u for a short duration that last for 8 millisecond to 1 minute. Line 1 Line Impedance Fault AC supply Source Impedance Line 2 400 200 200 Line Impedance 100 0 400-100 200 0-200 0 0.2 0.4 0.6 0.8 1 0-200 -200 Voltage Sag zone -400 0 0.2 0.4 0.6 0.8 1 Line 3, 4, 5... -400 0 0.2 0.4 0.6 0.8 1 7

Problem Definition Most of the voltage dips happens in the range of 90% to 80% for duration less than 1 second, although the under voltage relays shall be adjusted to avoid such small dips which the motors can withstand, but some of motors contactor may be released depending on the dip value. Line 230V Start Stop Contactor Coil Neutral 9

Objectives of the Design The Design of DVR shall meet the following objectives: The DVR will operate for low voltage network application. The DVR will compensate for voltage up to 50% of the nominal voltage and current of10a. Protect loads in oil and gas facilities from undesired shutdown. Efficient and reliable DVR will be implemented with minimized energy storage requirements. 11

12 Design Constraints Cost Environmentally Used for low voltage application in QP (230V). Considered a techno-economical solution, compensating for 50% and a current of 10A. Efficient and reliable DVR will be implemented with minimized energy storage requirements. Using Energy optimization method for DVR compensation by using capacitors in order to overcome the use of battery in UPS. Manufacturability The DVR will be implemented in the power electronics lab at Qatar University depending on the capabilities available in the lab. Health and Safety The project is mainly for protection and power quality applications, and no health concerns.

14 Compensation Methods of Voltage Sag Uninterruptible Power Supplies Dynamic Voltage Restorer

15 Compensation Methods of Voltage Sag UPS DVR

16 Uninterruptible Power Supplies UPS technology is essential for continuous protection against any power variation or disturbance. UPS using the battery-based inverter, protect loads by replacing the grid.

17 Compensation Methods of Voltage Sag UPS DVR

18 Dynamic Voltage Restorer Its function is to detect the voltage sag, and injecting the voltage difference between the pre-sag and post-sag voltages. Unlike UPS, DVR supplies only part of waveform that has been reduced due to voltage sag and not the whole waveform. Supply Voltage By-pass switch Load DC energy storage device Inverter Low pass filter Voltage injection transformer

19 Method UPS Advantages High speed data processing to protect the electronic control system of an industry. Disadvantages Require large battery bank when operating for a long time. Periodic maintenance. Not economical solution. DVR Highly efficient. Economic solution based on its size and capabilities. DVR can't mitigate interruptions.

20 Voltage Sag Detection Techniques RMS Peak Voltage Discrete Fourier Transform Missing Voltage

Technique Advantages Disadvantages 21 Root-Mean- Square Simple method. Needs at least quarter cycle. Detection of magnitude is delayed if voltage sag occurred by phase shifting. Requires three separate modules for the three-phase. Peak voltage Doesn t pick up noise. Slow. Requires three separate modules for the three-phase. Fourier transform Preferred for sag and harmonic calculation requirements. Massive computations with estimation techniques takes up to 1 cycle for detection. Missing voltage Observe the real-time variation of the recorded waveform. Precise indication of the duration of sag events. Has problems with the zero-crossing point that could mishandle the compensator device.

22 Root-Mean-Square technique L AC supply PCC

Simulation of RMS technique 400 Voltage sag PCC Voltage (V) 200 0-200 -400 0 0.05 0.1 0.15 0.2 Time (sec) The voltage sag is of 50% drop from nominal value with phase shift of 30 from pre-sag condition. 23

Simulation of RMS technique (cont.) Triggering Signal 1.5 1 0.5 0 Voltage Sag Occurrence Voltage Sag Detection -0.5 0.5 0.502 0.504 0.506 0.508 0.51 Time(s) 24

26 DVR Compensation Methods Pre-sag Compensation In-phase Compensation Energy Optimization

27 Single line diagram fro Voltage Sag compensation Grid Y Load Y Critical Load DVR Energy storage

28 DVR Compensation Methods Pre-sag Compensation In-phase Compensation Energy Optimization

29 Pre-sag Compensation method Advantage: It restores the voltage magnitude and phase angle to the nominal pre-sag condition. Disadvantage: It requires a high capacity energy storage device along with a high voltage injection capability.

30 DVR Compensation Methods Pre-sag Compensation In-phase Compensation Energy Optimization

31 In-phase Compensation method Advantage: The injected voltage is minimized, so require small capacity energy storage device along with a small voltage injection capability. Disadvantage: The DVR will generate and inject with the same phase shift of the post sag voltage.

32 DVR Compensation Methods Pre-sag Compensation In-phase Compensation Energy Optimization

33 Energy optimization method Advantage: Minimize the use of real power. Hence, the DVR will compensate only the reactive power. The basic idea of this method is to draw as much as possible active power from the grid in order to minimize the amount of active power drawn from the DC-link of the DVR. Disadvantage: The voltage injected will be higher. I' I

Energy optimization method (cont.) Magnitude of DVR voltage vector: V postsag 2 = V modified 2 + V DVR 2 2 V modified V DVR cos β Phase of DVR voltage vector: V DVR 2 = V modified 2 + V postsag 2 2 V modified V postsag cos α This is a relationship must be maintained, in order for the DVR to compensate perfectly as required by the energy optimization approach. x > y cos φ where x is the p.u value of the voltage drop from the nominal voltage in range of 0.5< x <0.9, where y is the p.u value of the restored (modified) voltage of the nominal voltage in range of 0.9 < y< 1. 34

Energy optimization method (cont.) Characteristic curve of Energy-optimization strategy Drop voltage level in per unit 1 0.9 0.8 0.7 0.6 0.5 0.95 0.4 0.5 0.6 0.7 0.8 0.9 1 0.9 Displacement factor 1 Restored voltage level in per unit 35

Energy optimization method (cont.) Zero Active Power Minimum Active Power A B Allowable restored voltage according to IEC 60038-1983 standard. 36

Simulation for Energy optimization method 400 300 The distribution voltage waveform Voltage Sag 200 Voltage [V] 100 0-100 -200-300 -400 0 0.05 0.1 0.15 0.2 Time [sec] The voltage sag is of 80% drop from nominal value with phase shift of 30 from pre-sag condition. 37

Simulation for Energy optimization method (cont.) Load current [A] 200 150 100 50 0-50 -100-150 100 75 50 25 0-25 -50-75 DVR injected voltage [V] -200 0 0.03 0.06 0.09 0.12 0.15 0.18-100 0.2 Time [sec] 38

Comparison The comparison will be presented for the maximum compensation time for different sags drop (ε). The comparison is based on a low-voltage DVR system with a constant DC-link capacitor. A well suited solution for DVRs considering energy saving, is to use DC-link capacitors as a storing energy device. Vin DVR inverter The output voltage of the inverter: V out = 4V in π m sin θ Vout The stored energy of the capacitor: v m E C = C v c dv c Where C is the capacitance value, V o is the initial charged voltage in the capacitor and v m is the minimum voltage across the capacitor that is allowed for compensation. V o 39

Comparison (cont.) The stored energy in the capacitor is related to DVR power and sag duration: E c = P DVR t max Peak DVR voltage magnitude foreach compensation method: t max = C π V DVR 4 2 α πv nom 8 2 P DVR 2 Pre-sag compensation V DVR = 2 V nom 1 2(1 ε)cos(δ) + 1 ε 2 In-phase compensation V DVR = 2 V nom ε Energy-optimized compensation V DVR = 2 V nom sin φ (1 ε) 2 cos 2 φ Where ε is the sag drop %, δ is the sag phase jump and φ is the displacement factor angle 40

41 Comparison (cont.) Active power supplied by DVR for each compensation method: P DVR = P load P supply Pre-sag compensation P DVR = V nom I nom (cos φ 1 ε cos(φ δ)) In-phase compensation P DVR = ε V nom I nom cos φ Energy-optimized compensation P DVR = V nom I nom (cos φ 1 + ε)

42 Comparison (cont.) Finally, the maximum compensating time for different compensation method could be obtained and the graph is presented: Nominal Voltage 60 VDC Phase jump 12 o Apparent power at the load 1 KVA Displacement factor 0.9 Compensation time (s) 0.05 0.04 0.03 0.02 0.01 Pre-sag In-phase Energy-Optimized 0 10 20 30 40 50 60 Sag Drop (%)

Block Diagram of Overall System Sensing Unit PWM1 Control unit DSP F28335 PWM2 VSG inverter RL load DVR inverter 44

45 Detailed Design Stages Design of VSG and DVR inverters stage The inverter and passive filter stages of VSG and DVR was implemented. 1 2 SiC MOSFET V rated I rated DC supply DC capacitor 3 4 Inductor AC Cap. Passive filter Sinusoidal output 1200 V 24 A Switching frequency 50 khz Corner frequency Inverter 1 khz

Detailed Design Stages (cont.) Band-pass Digital Filter Bode plot Magnitude (db) 10 0-10 -20-30 Bode Diagram System: G Frequency (rad/sec): 315 Magnitude (db): 0.156 Phase (deg) -40 90 45 0-45 System: G Frequency (rad/sec): 315 Phase (deg): 0.54-90 10 0 10 1 10 2 10 3 10 4 10 5 10 6 Frequency (rad/sec) 46

Overall System Demonstration System Prototype Isolated DC DVR inverter Biasing supply Digital oscilloscope Hosting PC DVR LPF VSG inverter Voltage transducer VSG LPF RL load 47

48 Overall System Demonstration (cont.) System Demonstration

Experimental results The DVR ratings are taken into consideration, to not exceed 50% of V nominal. The condition of Energy Optimization method, that is the relationship between displacement factor, p.u. level of restored and sag voltage should be maintained. The connected load was of displacement factor of 0.79. Experimental Cases Zero Active power Minimum Active power Restoring voltage to 1 p.u Sag of 80% Restoring voltage to 0.9 p.u Sag of 75% Restoring voltage to 1 p.u Sag of 75% 50

51 Experimental results (cont.) Experimental Cases Zero Active power Minimum Active power Restoring voltage to 1 p.u Sag of 80% Restoring voltage to 0.9 p.u Sag of 75% Restoring voltage to 1 p.u Sag of 75%

53 Experimental results (cont.) Experimental Cases Zero Active power Minimum Active power Restoring voltage to 1 p.u Sag of 80% Restoring voltage to 0.9 p.u Sag of 75% Restoring voltage to 1 p.u Sag of 75%

55 Experimental results (cont.) Experimental Cases Zero Active power Minimum Active power Restoring voltage to 1 p.u Sag of 80% Restoring voltage to 0.9 p.u Sag of 75% Restoring voltage to 1 p.u Sag of 75%

Experimental results (cont.) 57

Conclusion The DVR system to be implemented for Qatar Petroleum is to overcome voltage sag events, hence secure the power supply to the facility. The presented DVR system is considered as an appropriate techno-economical solution, particularly using Energy optimization technique and RMS detection method. The results of the system were presented and the operation principles were addressed to validate the proposed voltage control scheme DVR. As the results for the zero active power and minimum active power was achieved and ensured by measuring the phase angle between DVR voltage and PCC current waveforms. 59

60 Future Work The DVR could be a closed-loop control scheme that will minimize the steady-state error. The system can be enhanced to compensate voltage sags for dynamic loads. The DVR could operate with DC-link capacitor connected, instead of DC power supply.