Network controls for electrical systems

Networking of electrical systems has a number of aspects that should be considered.

03/11/2013


Many network failures have left residents in the dark. Courtesy: CFE MediaThere is a movement in the engineering design community toward networking controls for electrical power systems. While according to popular opinion this is a good thing, there are some issues that should be considered before implementation.

Networking of electrical systems has a number of aspects that should be understood before we delve into these aforementioned issues. One element of a network could be the basic monitoring of system conditions such as an elementary type of a building automation system (BAS). Monitoring of voltage, current, and electrical usage using the BAS can have benefits for building management and energy conservation. This data can be used to allocate electrical energy usage for individual tenants or specific functions and can aid in systems maintenance. For example, a pump with a constant load has a trend toward increased electrical usage over time. There could be restrictions in the lines downstream or increased bearing friction that could cause an increase in motor current, which can be investigated and the cause corrected before major equipment damage occurs. 

Special protection schemes (SPSs) can also have “weak” ties between systems or components. Weak ties are those that are intended to accommodate minor changes in the systems without manual intervention. An example of a weak tie would be a power system that would allow a preset energy transfer between one distribution system and another. Thus, as the loads for each distribution vary, the diversity of generators in each of the distribution systems can allow generators of each respective system to share the total load. For example, the generators in system B can carry some of the extra load of system A when the system A generators do not have the spinning reserves to serve the short-term load increases. When there is a generator failure or system disturbance on system B, the network control will disconnect the tie between systems A and B to protect the operations of system A, thus, it can be called a weak tie.

In SPSs, there are also “strong” ties between systems or components. Strong ties operate differently than weak ties in that they have the ability to carry a much larger portion of the load. When a generator fails on system B, the generators on system A attempt to ramp up to assume the load and keep system B on line. 

All three SPS configurations are determined by the control settings and may be changed through software modifications.

Pros and cons of networked SPS controls 

When network SPS control systems work properly, many operational benefits can result: 

  • A single operator can control the flow of energy into and out of his network.
  • The operator can, at a glance, determine the system status and make system adjustments to maintain the stability of his network.
  • Operators can use predictive software to determine whether there might be future system capacity or overloads that should be addressed immediately. The operator can shed generating capacity, bring additional generating capacity online to increase his spinning reserves, or shed selective loads.
  • Networked SPS systems can have an inherit increase in reliability.
  • If the system data are accurate and properly used, the network can be quite useful. 

On the con side, the negatives can outweigh the positives, particularly when it comes to maintaining strong ties between systems: 

  • Inaccurate data or a failure of the operator to properly use the data can precipitate a system-wide failure. Data collection can range from $500 to $5000 per point, so an extensive data collection network can be very expensive. If data are only collected and not used in the network operation, the expense is difficult to justify.
  • Strong ties between systems can be disastrous when system B fails and system A does not have the capacity or stability to assume the load. When this happens and the operator for system A does not sever his connection to system B, then both systems A and B can fail.
  • Multiple systems, all connected with strong ties, can result in a widespread failure that can ripple across multiple counties or states. Under worst-case conditions, major sectors of a country or adjacent countries can be affected.  

Networking 

Networking is a very good way to improve the reliability and monitoring of a system. This statement needs some qualification, given the examples where monitoring of controls and network status failed. Also, the instances where a network of SPS systems with strong ties between them created a cascading failure must be considered. 

Obviously, having more information on a system is always beneficial. As long as the information is accurate, the operator acts on the information in the prescribed manner, the system automatically adjusts itself as intended, and/or there are means to make sure that the information gathering system is operational, the electrical system will remain online. All too often, one or more of these links in the system break down and either the system or operator does not make the correct decision based on the available information or the information is wrong. 

The strength of the ties between elements of the networked SPSs can be either very beneficial or very detrimental to system operation. In the above examples, the effects of strong ties are shown to create the environment for a massive network failure. The failure could be throughout a single facility or a major portion of the country, according to the network extent. 

While all of the examples show the possible scope of a network failure due to strong ties, there are underlying clues to the benefits of a network. In areas where there were weak ties between sections of the network, one can see that there were many noted instances in which the ties were broken and the weak-tied section of the network remained in operation. 

Also, note that there were no examples of a network composed on many sections having strong ties between each of the sections remaining up and running because of these strong ties. Widespread blackouts are always newsworthy and are carefully analyzed and dissected to determine the root cause. But there are virtually no records of networks that stayed up and fully operational after a system disturbance.


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