Reliability considerations in simple paralleling applications


A load shed scheme is required so that when generators are overloaded, the noncritical loads can be taken off line so that there will be sufficient capacity to serve the critical loads. Most paralleling generator set controls have a load shed or load dump output that can be connected to the load shed input of the transfer switches that serve the non-emergency loads (see Figure 2). This will take the non-emergency loads off line in the event that the generator sets are overloaded. Note that to properly shed load, transfer switches must be three position switches with center-off positions. Using two position switches to shed load is not recommended as this often results in connecting the load to a failed utility.

Figure 2. Implementing a load shed function: Connecting the load dump contacts of each generator set in parallel to the ATS load shed input will cause the ATS to shed load in the event that any of the generator sets activate their load dump output. Most gIsochronous vs. droop load sharing. Most load sharing systems today are isochronous, meaning that the voltage and frequency are held constant. However, there are still controls being produced that use droop load sharing, which allows voltage and frequency to vary with load. Droop load sharing controls were once popular because they allowed generator sets to parallel with each other without communicating with each other. Due to the variation of frequency and voltage with a droop paralleling system, the quality of power provided to the load typically is not very good and may not be suitable for some electrical equipment. Isochronous load sharing is the appropriate technology to use. 

Random access paralleling vs. exciter paralleling. Random access paralleling refers to a system in which the first generator at rated speed and voltage closes to the dead bus and then all the other generators actively synchronize and close to the bus. This type of paralleling is the most reliable method and is most commonly used in critical applications. A less expensive paralleling method known as exciter paralleling is used in some paralleling applications. In an exciter paralleling system, all of the generators start with their paralleling breakers closed and their excitation circuits de-energized. When the generators start, they are connected to the bus but produce no voltage. When all generators reach starter disconnect speed, their excitation circuits are energized and the generator bus voltage ramps up with the generators forcing each other into sync. Because exciter paralleling systems will not work until all generators either come up to speed or are locked out, they are not used in critical applications. Another disadvantage of an exciter paralleling system is that if one generator set shuts down it cannot be brought back on line unless the entire system is shut down and restarted. Random access paralleling with active synchronization should always be used when paralleling gensets in critical applications.

Fault tolerance—manual mode. A key consideration in assessing the reliability of a system is its ability to operate when certain components have failed. Having a predefined sequence of operations for a user to follow in a worst-case scenario to manually provide power to loads is often a requirement in critical applications. A user should be able to manually start generators, initiate synchronization, and close paralleling breakers. Manual operation does not mean that the generator control is not operating; it means that functions are user initiated rather than system initiated. All system protection functions will still be active. The control will not allow a paralleling breaker to close if the generator and the paralleling bus are not in phase with each other. 

Anonymous , 05/08/13 10:25 AM:

Excellent Article.
VISH , CA, United States, 05/08/13 12:53 PM:

Some of the increased cost of the parallel system can be off-set by Peak Load shaving technique. Illustration of Pros and cons of the system will greatly help the engineers to make the right decision.
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
A cool solution: Collaboration, chemistry leads to foundry coat product development; See the 2015 Product of the Year Finalists
Raising the standard: What's new with NFPA 70E; A global view of manufacturing; Maintenance data; Fit bearings properly
Sister act: Building on their father's legacy, a new generation moves Bales Metal Surface Solutions forward; Meet the 2015 Engineering Leaders Under 40
Cyber security cost-efficient for industrial control systems; Extracting full value from operational data; Managing cyber security risks
Drilling for Big Data: Managing the flow of information; Big data drilldown series: Challenge and opportunity; OT to IT: Creating a circle of improvement; Industry loses best workers, again
Pipeline vulnerabilities? Securing hydrocarbon transit; Predictive analytics hit the mainstream; Dirty pipelines decrease flow, production—pig your line; Ensuring pipeline physical and cyber security
Upgrading secondary control systems; Keeping enclosures conditioned; Diagnostics increase equipment uptime; Mechatronics simplifies machine design
Designing positive-energy buildings; Ensuring power quality; Complying with NFPA 110; Minimizing arc flash hazards
Building high availability into industrial computers; Of key metrics and myth busting; The truth about five common VFD myths

Annual Salary Survey

After almost a decade of uncertainty, the confidence of plant floor managers is soaring. Even with a number of challenges and while implementing new technologies, there is a renewed sense of optimism among plant managers about their business and their future.

The respondents to the 2014 Plant Engineering Salary Survey come from throughout the U.S. and serve a variety of industries, but they are uniform in their optimism about manufacturing. This year’s survey found 79% consider manufacturing a secure career. That’s up from 75% in 2013 and significantly higher than the 63% figure when Plant Engineering first started asking that question a decade ago.

Read more: 2014 Salary Survey: Confidence rises amid the challenges

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.