Network controls for electrical systems
Control and monitoring: Before decisions are made to change the state of a network, the system status and the various parameters that would be used to decide what changes should be made must be verified. For high-reliability networks, there are two data collection systems that can be compared to make sure that the network is functioning properly and the data is correct. For very critical networks where there is automatic switching or other changes in the network state, three systems are used and the best two out of three data sets are used to make the correct decision. If a human operator is assessing the network condition, two sets of data are generally adequate since the operator can, from prior experience, make an educated decision on which data set more accurately reflects the actual network conditions.
Strong or weak ties: In SPS network design there must be a balance between strong and weak ties between sections. The network failure examples have a good mix of failures caused by strong and weak ties. For instance, the 1965 blackout was initiated by an overcurrent relay tripping out a transmission line well below its rated capacity—a weak tie. However, the failure cascade was the result of a series of strong ties attempting to maintain the network voltage and frequency. So does one design a network with all strong ties, so that the network will always try to keep itself operational? Or does one design a network with all weak ties, so that every small disturbance will segment the network into many sections, with each attempting to maintain its own operation while letting the adjacent sections fail?
Network stability (or instability) is the subject of many very long, involved analyses, which are well outside the scope of this article, but they have some nuggets of truth that we will extract. Networks are stable as long as the various elements are balanced and become unstable when the elements become unbalanced. This seems obvious, but in every example the cascaded failures were initiated by a large change in the network load or the network generating capacity. Let us suppose for a moment that we were able to recreate one of the aforementioned blackouts, say the 2003 Northeast Blackout. The issues with the control and monitoring problems were addressed already, so we can start with the first large step load change (the generating station shutdown). If this generating station was shut down slowly, over a period of time instead of all at once, there would have been less impact on the network and the initial step load disturbance would not have occurred. For example, if the generating station had four 1000 MW generators, the operators could have shut down one generator at a time, waited until the network was stable after assuming that load, and then shut down another generator. Another alternative would have been to notify the network controller so that he could bring peaking units on line to add to the spinning reserves, before the generating station was dropped offline.
We could second-guess nearly every event that occurred in the 2003 blackout or any other similar occurrence and still not solve the basic problem. The key to the stability of any network is to avoid network disturbances. When they occur, operating personnel must know in advance what actions they should take in order to restabilize the network. In particular, areas in which misunderstandings can occur should be clarified to prevent worsening the disturbance. For example, operating personnel reduced voltage to comply with the request to reduce load, when, in reality, the request to reduce load was meant for the operators to disconnect loads. While this was a minor misunderstanding, it had the opposite effect from what was intended and worsened the network disturbance, not mitigated it.
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