Distributed Generation Consequences slides

Editor's Notes

• #1: Introduce myself Introduce yourselves Introduce this course Really the course is all about how to integrate new and renewable energy into electrical networks such that the power can be exported and sold.
• #2: OK so this is a list of the most important technical issues raised by the growth of DG. We will try to deal with each of these issues in turn. We dealt with voltage changes and fault levels on Wednesday
• #4: What is power quality ? Power quality = Voltage quality because this is what we can control we have less or little control over the current that is drawn by a load. It is a customer driven issue
• #5: Some power quality issues are caused by EG and affect load customers Some power quality issues are caused by loads or by system events/characteristics and affect EG. They can cause nuisance tripping of EGs. Sag is a decrease between 0.1 and 0.9 pu voltage for durations of from 0.5 cycles to 1min Swell an increase to 1.1 to 1.8 pu in voltage for durations of 0.5 cycle to 1 minute Both usually associated with fault conditions
• #6: Some power quality issues are caused by EG and affect load customers Some power quality issues are caused by loads or by system events/characteristics and affect EG. They can cause nuisance tripping of EGs. Sag is a decrease between 0.1 and 0.9 pu voltage for durations of from 0.5 cycles to 1min Swell an increase to 1.1 to 1.8 pu in voltage for durations of 0.5 cycle to 1 minute Both usually associated with fault conditions
• #7: There are many disturbances that cause power quality to be deteriorated, some transient some steady state. The issues we are going to focus on are Flicker, Harmonics and Unbalance as they are closely related to embedded generation.
• #8: Can be caused by loads which exhibit continuous, rapid variations in load current magnitude. Arc Furnace typical load that can cause flicker Sensitive in the range 6-10Hz On graph 10 Hz would be 20 changes per second as sinusoidal which would be 1200 voltage changes per minute. This would be the point the human eye is the most sensitive. Graph shows the percentage change that would cause annoyance. Also can be caused by EG if the output power fluctuates rapidly such that it causes voltage fluctuations of sufficient size and at the sensitive frequency range. Also if connecting and disconnecting IG. Tower Shadow 60 rpm, 3 blades, 180 changes per minute Obviously the impedance at the point of connection determines the magnitude of the voltage changes for a given power fluctuation.
• #9: Harmonic distortion is increasing Harmonics increase the losses in electrical machines and can interfere with the control systems of power electronic devices Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at which the supply system is designed to operate, the fundamental frequency. Distorted waveforms can be decomposed into a sum of the fundamental frequency and the harmonics. Harmonic distortion originates in the non linear characteristics of devices and loads on the power system. Often use THD as a measure
• #10: In the last 5 – 10 years the voltage source converter has become common prior to that line commutated thyristor based converters were used for some EG. The diagram shows a simple 6 pulse line commutated arrangement used on some PV systems. Vary firing angle 0 – 90 rectifier, > 90 acts as an inverter, used for HVDC High harmonics Low losses so people liked them EG may look to use this arrangement to keep losses low The modern equivalent is IGBT converters switching at kHz This technology produces less low order harmonics but do produce higher order harmonics
• #11: Unbalanced voltages are primarily caused by single phase loads Unbalance increases losses in electrical machines and can cause power converters to inject unexpected harmonic currents back into the network. Two points here 1 Unbalance can cause EG to nuisance trip SG,IG and PV 2 Up to now EG has been mainly three phase so hasn’t caused a problem, if micro generation grows this is likely to be single phase and will therefore increase imbalance as a house with generation will represent a smaller load than one without. P29 states that imbalance should be within 1.3% Unbalance most likely for LV networks. Use three wire systems for 11kV and above. Use four wire systems for LV networks to deal with the unbalance If an imbalance occurs a balancing current will flow in the neutral This helps to keep the unbalance down to a reasonable value However unbalance still occurs due to the network impedances and the non zero neutral impedance