Power Quality, Reliability, and Security

Economies today are largely dependent on reliable electric power suggesting all time availability and dependability. Power outages can cause serious disruptions and damages such as failure of protection systems, shutting down of heating systems, shut down of life support systems, data loss, and so on. There are many problems faced by power systems that can cause outrages such as excessive loads affecting the quality of power making the electronic devices less reliable. Often service quality issues are considered as prime reasons for power disruption or quality degradation. However, in actual scenario, the power supply problems are the responsibilities of multiple parties including utility providers, equipment manufacturers, and system designers (GAPS).

The power service problems can be related to concerns of reliability and quality. Reliability suggests that a power supply is able to fulfil all basic design parameters at nominal voltage. These basic parameters are set by certain codes and standards defined including ANSI C84.1, std 141, and Std 446. These standards also define voltage profiles for distribution that include voltage requirements at power junctures and limits for power quality deviations. Basic consideration of reliability of power system is the voltage deviation that should be within defined limits for the power to be considered as reliable. Reliability issues in power systems can cause problems like blackouts, over-voltages, and low voltage abnormalities. Reliability problems can be solved to a great extent by deploying alternate power sources (McGranaghan).

Quality suggests waveform variations and voltage deviations that should be less. Quality of power can be reflected using two parameters including frequency and voltage. Voltage is an operational parameter which can differ with power nodes but frequency is a system-wise parameter such that it has to be same for the entire power grid (Tuma, Martinek and Tesarova).

 When power, voltage or current waveforms contain undesired components, the power can be called as dirty. This can cause distortions in power waveforms. Short term disturbances can cause problems like sags, swells, and drop outs. Voltage deviations can cause abnormal events such as frequency swings, voltage dips, and more. Common causes behind such distortions include electric switching, arcing devices, deteriorated wirings, overloaded or weak power supplies, saturated transformers and transformer saturation. Quality issues can be resolved mostly by either changing the waveforms or reshaping them. Electrical distortions can be reduced or eliminated using electromagnetic shields, voltage regulators, grounding, surge arresters, electrical maintenance, and low-impedance power conductors (Yang and Bollen).

Security is another area of concern when considering power systems and it refers to ability of a power system to withstand disturbances such as equipment failures, power outages, and so on (Osborn and Kawann). Security is not a single factor but a result of interaction of multiple that includes short term operational security and long term fuel access and adequacy of system or market. Long term security is about getting adequacy of fuels, generation, network and market, all at the same time. While decisions about generation are made considering business aspects, decisions about network are made as per the requirement of state regulatory bodies. Short term security refers to operational reliability of the power system which includes its components that can face short term failure (NEMA).

Besides the individual problems of quality, reliability, and security, there are also links between these constructs as illustrated in the figure below.

Improving Quality and Reliability

Solutions for improvement of quality and reliability of power systems can be categorized into pre and post-disturbance solutions. Pre-disturbance methods include use of protection instruments, VAR control with power factor correction, voltage optimization, system analysis, and maintenance. Post-disturbance methods include event analysis and fault location determination. These methods can reduce frequency of disturbances but do not provide system-wide coordination (Andersson).

Pre-disturbance reliability and quality: In order to maximize the reliability of power system, efficiency has to be improved and costs have to be reduced. Fault location, isolation, and service restoration schemes can be used to reduce system disturbances and those customers affected by permanent outages can be minimized. Single phase tripping can help reduce the number of customers who may be impacted by the disturbance. In past, utilities had to rely on electromechanical relay protection that had devices coordinating with each other which introduced inaccuracies at times (Glennon, Kusch and Nelson).  

However, with introduction of microprocessor-based relays, both protection and coordination have improved. These relays provide increased level of protection sensitivity using multiple protection levels which speeds up the fault clearance rate. This also results into an enhanced level of system-wide coordination. VAR control schemes can be applied to a system for improving quality of power and reducing losses in the system after the load characteristics and voltage profiles are understood. Power factor can be measured for understanding system losses. If the power factor had to be improved, voltage can be made to be controlled by switched and fixed capacitors. However, this approach is difficult because of the high instrumental costs involved for power factor measurement. Measurement is done using metering values throughout the system which can be useful in studying and planning for the system. If the measurement is not accurate, the coordination would be difficult to optimize (ABB).

There can also be other problems during pre-disturbance such as overvoltage and overcompensation. Voltage regulators can be used for controlling voltages such that the voltage losses can be compensated for. While capacitors and voltage regulators can improve voltage delivery efficiency, VAR can be enhanced with coordination of devices at the distribution feeder (Dugan, McGranaghan and Santoso).

Post-disturbance reliability and Quality: Reduction in time required for locating faults can have a positive impact on system reliability. Post-disturbance, first the incident is reported by a victim who can find fault and fix it. IEDs provide information about the distance from the fault using impedance based method of calculation. However, these methods may not give very accurate results when used for complex feeder configurations. Statistical analysis can use fault levels to identify locations. Some utilities may also make use of system models or software packages for estimation of location. These methods would still be as slow as data collection methods unless they are automated (Klimek and Baldwin).

Sequential Events recorders may be used for understanding the disturbances and system behaviour. Time stamped measurements of faults can be provided by Oscillographic events. SER reports along with Oscillographic event measurements help in understanding reliability and quality of power. There are software tools now available that can help automate these analysis processes during post-disturbance phase.

Improvement in System reliability

There can be different ways system reliability can be improved based on the program area to deal with from demand and supply. On the demand side, energy efficiency and alternative pricing have to be taken care of by meeting energy standards, managing demand, providing real time pricing, and load bidding. Standards are created and complied with by the equipment and appliances. Further, the access to information is improved for consumers such that the cost of the energy consumption is reduced. New regulations can be implemented to push suppliers minimize pricing for energy distribution. Low cost metering technologies can help serve small consumers of power (Strang and Pond).

On the supply side, power generation and transmission are involved. Power generation involves sitting and distribution. Resources can be upgraded and maximized for operating sites and new protocols or standards may be developed for managing interconnections. Transmission management for system reliability involves improvement of grid utilization, imports, planning and outage management. Grid utilization can be improved with network management and energy distribution through development of new optimization technologies and basing forecasting on weather trends.

Resource sharing can be improved with interconnected utilities. Planning can be done for setting standards, incentives and benchmarks by adjusting the regulatory framework for margins, generators for non-utility, and rate treatments, and by making efficiency and reliability information available publically.

Outage management involves maintenance, underground cabling management, and penalties. Economic trade-offs can happen between equipment replacement and equipment maintenance and these trade-offs have to be optimized. For underground cables, low cost development may be used so that a highly protected system of resources is created. Different levels of reliability can be achieved at different levels of customer needs in terms of cost/benefits.

Conclusions:

The literature review identified that there exists a connection between quality, security and reliability of a power system such that different components affect these factors in different ways but with coordination between them, the desired results could be achieved. Specific methods were identified that can be used for improving either of the three constructs or all together. For improving quality and reliability of the power system, both pre-disturbance and post-disturbance stages have to be handled separately while security can be made stronger by taking care of short-term and long term access and adequacy of the whole system.

References:

GAPS. "ELECTRIC POWER QUALITY AND RELIABILITY." 2015.

Osborn, Julie and Cornelia Kawann. "Reliability of the U.S. Electricity System: Recent Trends and Current Issues ." 2001.

Yang, Yongtao and Math Bollen. "Power quality and reliability in distribution networks with increased levels of distributed generation." 2008.

Tuma, Jiri, et al. "Security, Quality, and Reliability of Electrical Engineering." 2007.

Glennon, Bill, Christina Kusch and Elijah Nelson. Improve Reliability and Power Quality on Any System. Jeddah, Saudi Arabia: Schweitzer Engineering Laboratories, Inc, 2012.

McGranaghan, Mark. "POWER QUALITY STANDARDS." 2000.

NEMA. "American National Standard For Electric Power Systems and Equipment—Voltage Ratings (60 Hertz)." 2006.

Andersson, G¨oran. "Modelling and Analysis of Electric Power Systems." 2008.

ABB. "Capacitors and Filters Improving power quality for efficiency and reliability." 2010.

Dugan, Roger C., et al. Electrical Power Systems Quality. Mc Graw Hill, 2004.

Klimek, Andrew and Robert Baldwin. Benefits of Power Swing Recording. Vancouver, BC: NxtPhase T&D Corporation, 2004.

Strang, William and Jeff Pond. "Considerations for Use Of Disturbance Recorders ." 2006.