Hesham E. Shaalan, Ph.D.
Electrical Engineering Technology
School of Technology
Georgia Southern University
This paper presents the idea of using automated switching techniques to enhance the capabilities of a power network as well as accommodate future load growth. A sample distribution system is used to illustrate the possible uti lization of automated switches to meet future load demand for a few years before it becomes necessary to build a new distribution substation. This material was provided as a handout to a class on Power Distribution to illustrate some of the benefits of di stribution automation.
I. An Example
An automatic switch can be defined as a switching device that opens and closes to connect two or more sections of a power distribution system, with the aid of computer control by remote communications. However, manual switches are c ommonly used in present power distribution systems. The following example is used to illustrate the operations of automated switches. The sample system represents an existing system which contains two distribution substations and two manual switches for power flow control as shown in Figure 1. Several types of loads may exist on the network such as residential, commercial, and industrial loads. It is assumed that load 1 and load 2 have stabilized and are no longer growing. However, the new load is assume d to be growing. Representative values for annual load growth factors may be in the range of 5% for residential, 3% for commercial, and 2% for industrial loads.
The new load may be supplied by either substation depending on the status of the two switches. But the system will be overloaded if the new loads grow beyond the limits of the existing equipment. Two alternative system designs may b e evaluated for comparison purposes. The first approach involves using only automated switching techniques to resolve circuit overloading. Automated switching can be performed on an hourly basis, which is certainly not feasible with manual switches. The s econd approach will be the classical approach of building a new substation to feed the new loads and prevent equipment overloading.
II. Automated Approach
The first approach allows for employing automated switches to maximize the capabilities of the existing substations, and to distribute the power flow in a more efficient manner. The idea behind automatic switching is that when load peaks change in different areas of the system, the switch opens or closes to direct power to the location where it is needed. This is an important factor due to the time varying nature of different load types (load diversity). Thus, automatic switching ca n be performed as frequently as needed in contrast with manual switching. Furthermore, an automatic switch can quickly isolate faulted sections of the network to minimize power interruption. The time required for these switching operations is reduced to a few minutes due to computer control and remote communications. By contrast, manual switches require a minimum of one to two man-hours. By replacing manual switches with the automated type, power flow can be altered much faster and cheaper.
The greatest benefit of the automated approach is that the need for a new substation and all of its associated costs are eliminated until future loads are much larger. This time extension factor is a significant benefit due to the l engthy regulatory and planning periods which must be endured before construction of a new substation can begin.
Figure 1. Example System
III. Classical Approach
The approach of building a new substation to meet future load growth has been the traditional solution in the past. Naturally, along with the substation, new lines must be set up for power flow, breakers must be installed for protec tion, and large step-down transformers must be installed. In addition to these circuit elements that must be purchased and installed, zoning for the new substation and high-voltage right of ways must be approved by local government agencies. Other sources of expenditures include maintenance costs of the equipment and high interest rates on loans for equipment and land.
Therefore, it is obvious that a great deal of money and planning time must be appropriated to construct a new substation. Regarding efficiency, the new and existing substations will be operating at only a fraction of their full cap acity until future load increases. However, with shorter power lines between each substation and the customer, circuit reliability will increase and line losses will decrease.
IV. Numerical Results
The costs associated with an expansion plan can be expressed as capital cost and operating expense. The capital cost is associated with the initial placement of equipment including purchase price and installation cost. The operating expense is associated with the ongoing operation of equipment. The yearly cost associated with capital cost is referred to as a carrying charge. The carrying charge factor is used to reduce capital costs to a yearly amount. Thus, the annual total cost is the sum of carrying charges and operating expenses. The annual operating cost is assumed to be 10% of the capital cost, and the carrying charge factor is assumed to be 20%.
The automated case is assumed to have two automatic switches installed to replace the two manual switches. The capital cost of the two automatic switches is assumed to be $12,000. The other necessary equipment include the following: voltage and current transducers, substation remote terminal units, and a central computer. The combined capital cost of these equipment is $49,800. Therefore, the total annual cost of the automated case is $18,540.
The classical approach of building a new substation requires a capital cost of $500,000. Therefore, the total annual cost is $150,000. As a result, there are significant savings associated with the automated approach compared to the classical approach.
If the cost of the new substation can be delayed for even one year, the savings could pay for the new automatic switching equipment. Furthermore, the additional time could be used to better plan for building a new substation and to see if load growth will actually outgrow the systemís capabilities. If building a new substation becomes inevitable, the network will be much more efficient if the automatic switches are also installed. Finally, it would be recommended that electric utili ties proceed with installation of automated switches to enhance the systemís present operation. This will also provide additional cost benefits in the future.
References1. H. Shaalan and R. Broadwater, "Using Interval Mathematics in Cost-Benefit Analysis of Distribution Automation," Electric Power Systems Research Journal, Vol. 27, No. 2, pp. 145-152, 1993.
2. W. Burke et al., "Trade off Methods in System Planning," IEEE Transactions, PWRS-3, pp. 1284-1290, 1988.
3. Distribution Automation: IEEE Tutorial Course No. 88EH0280-8-PWR, Piscataway, NJ, 1988.