SMJE 2103 Power Quality, Reliability and Availability

Power quality, reliability and availability Topics discussion 1)Power performance 2)Quality, Reliability, Availability 3)Type of PQ problems 4)Solutions

Power Performance Due to advance power system loads (ASD, PLC, EEL), costumers need very high power quality instead of modernization of the system. PQ is focusing on clean voltage waveform, but due to scheduled and unscheduled disturbances, generated voltage is on signal interferences. Both parties should responsible for preventing the problems by respecting respective rules.

Quality, Reliability, Availability PQ – Any electric supply condition that causes appliances to malfunction (CP). A PQ problem might be perceived as noncompliance with various standards (UP). PR – Primarily concerned with all aspect of customer interruptions, including sustained (> few minute) and momentary (< few minute) interruptions. PA – The percentage of time a voltage source is interrupted.

Type of PQ problem D. A decrease of the normal voltage level between 10 to 90% of the nominal rms voltage at the power frequency for durations of 0.5 cycle to 1 minute C. Fault on the transmission or distribution network. Faults in consumer’s installation. Connection of heavy loads and start-up of large motor. Co. Malfunction of infor technology equipment, namely micropr- based control system (PCs. PLCs, ASDs, etc) that may lead to a process stoppage. Tripping of contactors and electro relays. Disconnection and loss of efficiency in electrical rotating machine.

D. Total interruption of electrical supply for duration from few ms to one or two seconds. C. Mainly due to the opening and automatic reclose of protection devices to decommission a faulty section of the network. The main fault causes are insulation failure, lightning and insulator flashover. Co. Tripping of protection devices, loss of information and malfunction of data processing equipment. Stoppage of sensitive equipment such as PC, PLC, ASD, if they are not prepared to deal with this condition. Type of PQ problem

D. Total interruption of electrical supply for duration greater than 1 to 2 seconds. C. Equipment failure in the power system network, storms and objects (trees, cars, etc) striking lines or poles, fire, human error, bad coordination or failure of protection devices. Co. Stoppage of all equipment. Type of PQ problem

D. Very fast variation of the voltage value for durations from a several ms to few ms. These variations may reach thousands of volts, even in low voltage. C. Lightning, switching of lines or power factor correction capacitors, disconnection of heavy loads. Co. Destruction of components (electronics equip) and of insulation materials, data processing error or data loss, electromagnetic interference. Type of PQ problem

D. Momentary increase of the voltage at the power frequency, outside the normal tolerance, with duration of more than one cycle and typically less than a few seconds. C. Start/stop of heavy loads, badly dimensioned power sources, badly regulated transformers (mainly during off-peak hours) Co. Data loss, flickering of lighting and screens, stoppage or damage of sensitive equipment, if the voltage values are too high. Type of PQ problem

D. Voltage or current waveform assume non-sinusoidal shape. The waveform corresponds to the sum of different sinewaves with different magnitude and phase. Having frequency that are multiples of power-system frequency. C. Classic sources: electric machine working above the knee of the magnetization curve (magnetic saturation), arc furnaces, welding machines, rectifiers and DC brush motors. Modern source: all non- linear loads, such as ASDs, data processing equip. Co. Increased probability in occurrence of resonance, neutral overload in 3-phase system, overheating of all cables and equipment, loss of efficiency in electric machines, electromagnetic interference with communication system, errors in measures when using average reading meters, nuisance tripping of thermal protections. Type of PQ problem

D. Oscillation of voltage value, amplitude modulated by a signal with frequency of 0 to 30Hz. C. Arc furnaces, frequent start/stop of electric motors (elevator), oscillating loads. Co. Most of them are common to under-voltages. The most perceptible problem is flickering of lighting and screens, giving the impression of unsteadiness of visual perception. Type of PQ problem

D. Superimposing of high frequency signals on the waveform of the power system frequency. C. Electromagnetic interferences provoked by Hertzian waves such microwaves, television diffusion, and radiation due to welding machines, arc furnaces, and electronic equipment. Improper grounding may also be a cause. Co. Disturbances on sensitive electronic equipment, usually not destructive. May cause data loss and data processing errors. Type of PQ problem

D. A voltage variation in a three-phase system in which the three voltage magnitudes or the phase angle differences between them are not equal. C. Large single-phase loads (induction furnaces, traction loads), incorrect distribution of all single-phase loads by the three phases of the system (this may be also due to a fault). Co. Unbalanced system imply the existence of a negative sequence that is harmful to all three-phase loads. The most affected loads are three-phase induction machines. Type of PQ problem

Solution

Solution (Grid Adequacy) Many PQ problem have origin in transmission and distribution grid. Adequate planning and maintenance is essential to minimize the occurrence of PQ problem.

Solution (Distributed Resources – ESS) It has potential to provide increased power reliability. Most practical in electrochemical battery instead of flywheels, super-capacitors and superconducting magnetic energy due to low price.

Flywheels

A supercapacitors provides power during short duration interruptions or voltage sag

SMES Due to the high inductance, off switch, magnetic coil behaves as a current source. SMES systems are large and generally used short duration, such as utility switching events

As a back-up generator to assure energy supply to critical loads during sustained outages. Also, can be used for load management purposed to decrease the peak demand. Solution (Distributed Resources – DG)

Dynamic Voltage Restorer (DVR) – voltage source connected in series with the load to kept approximately constant voltage by using Tx. Transient Voltage Surge Suppressors (TVSS) – to damp the transient before it reaches the load. Constant Voltage Transformer (CVT) – to mitigate the effect of voltage sag and transient. Noise Filters – to avoid unwanted frequency current or voltage by using a combination of capacitors and inductances. Solution (Enhanced Interface Devices)

Isolation Transformer – to isolate sensitive loads from transients and noise deriving from the main. The Tx has a grounded shield made of nonmagnetic foil located between the primary and secondary. Static VAR Compensators – it use a combination of capacitors and reactors to regulate the voltage quickly. Harmonic Filters – it is used to reduce undesirable harmonics. Solution (Enhanced Interface Devices)

The utilities have to provide some measures to consumers for the purpose of regulating the minimum PQ problem level and the immunity level that equipment should have to operate properly when the power supplied is within the standard. Such the CBEMA curve – the standard specifies the minimum withstanding capability of computer equipment to voltage sag, micro- interruptions and overvoltage. Solution (Develop Codes and Standard)

Adding a capacitor with a larger capacity to power supplies. Using cables with larger neutral conductors. Derating transformers. Adjusting under-voltage relays. Solution (Make End use Devices Less Sensitive)