Basic Design Consideration

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This is a Design Guide is based on IEEE standards.

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Any approach to the problems must include several basic considerations which will affect the overall design. These are

Safety: Safety takes two forms; safety to personnel and safety to property. The safety of personnel involves no compromise; only the safest system can be considered.; Safety to property may involve some compromise where safety to personnel is not jeopardized.
Reliability: The continuity of service required is dependent on the type of manufacturing or process operation of the plant. Some plants can easily tolerate momentary outages while others require a very high degree of service continuity. In view of this, the system should designed to isolate faults with a minimum disturbance to the system and should have features to give the maximum dependability consistent with the plant requirements.
First Cost: While the first cost is important, it is often minimized if the system is reliable and its operation is satisfactory. It should not be the determining factor in the design of the plant since distribution cost represents only 2 to 10 percent of the plant investment.
Simplicity of Operation: Simplicity of operation is a big factor in the safe and reliable operation of a plant. Avoid complicated and dangerous switching operations under emergency conditions.
Voltage Regulation: For some plant power systems, voltage spread may be the determining factor of the distribution design. Poor regulation is detrimental to the life and operation of electrical equipment.
Maintenance: A well-designed distribution system with properly chosen equipment will reduce emergency maintenance. In planning the system, the accessibility and availability for inspection and repairs should be given careful consideration.
Plant Expansion: Plant loads generally increase. Consideration of the plant voltages, ratings of equipment, pace for additional equipment and capacity for increased load must be given serious study.

Planning Guide for Distribution Design

With the above factors in mind, the following procedure is given to guide the Electrical Engineer in the design of an electric distribution system for any industrial plant.

Obtain a general layout and mark it with the major loads at various locations and determine the approximate total plant load in kilowatts, and kilovolt-amperes.
Estimate the lighting, air-conditioning, and other loads from known data and the system load preliminary information.
Determine the total connected load and calculate the maximum demand by using demand and diversity factors.
Investigate unusual loads, such as the starting of large motors, operation of arc furnaces or welders, and operating conditions such as boiler auxiliary motors, loads that must be kept in operation under all conditions, and loads that have a special duty cycle.
Investigate the various types of distribution systems and select the system or systems best suited to the requirement of the plant. Make a preliminary single line diagram of the power system.
If power is to be purchased from the utility, obtain such information concerning the supply system or systems as
1. voltage available and voltage spread,
2. type of systems available,
3. method of system neutral grounding, and
4. other data such as
  1. relaying,
  2. metering and
  3. the physical requirements of the equipment.
  4. interrupting rating and momentary ratings of power circuit breakers should be obtained as well as the present and future short-circuit capabilities of the utility system at the point of service to the plant.
    Investigate the utility’s power contract to determine if off-peak power at lower rates is available, and any other requirements, such as power factor and demand clauses, that can influence power cost.
If considering a generating station for an industrial plant, such items should be determined as: generat- ing kva required including standby loads, generating voltage, and such features as relaying, metering, voltage regulating equipment, synchronizing equip- ment and grounding equipment. If parallel operation is contemplated, be sure to review this with the utility and obtain its requirements.
A cost analysis ‘may be required of the different voltage levels and various arrangements of equip- ment to justify and properly determine the voltage and equipment selected. The study should be made on the basis of installed cost including all the com- ponents in that section of the system.
Check the calculations of short-circuit requirements to be sure that all breakers are of the correct rating. Review the selectivity of various protective devices to assure selectivity during load or fault disturbances.
Calculate the voltage spread and voltage drop at various critical points.
Determine the requirements of the various components of the electric distribution system with special attention given to special operating and equipment conditions.
Review all applicable national and local Codes for requirements and restrictions.
Check that the maximum safety features are incorporated in all parts of the system.
Write specifications on the equipment and include a single line diagram as a part of the specifications.
Obtain typical dimensions of equipment and make drawings of the entire system.
Determine if the existing equipment is adequate to meet additional load requirements. Check such ratings as voltage, interrupting capacity, and current-carrying capacity.
Determine the best method of connecting the new part of the power system with the existing system so as to have a minimum outage at minimum cost.

Naturally the sbove procedure will not automatically design the electric power system in itself; it must be used with good, sound, basic engineering judgment.

General Layout

A general layout of the plant should be available the Engineer can begin his study. This layout usually the location and the size of the proposed building in the initial particular project. The extent of the available layout gives the Engineer an idea of the possible expansion of the plant in the future, and must be considered by the Engineer in planning the electric distribution system.

Types of Circuit Arrangements

Most modern industrial plants make use of the load center system. This system generally consists of several small substations receiving power from a medium or high voltage system and stepping down at the substation to utilization voltages at the various load areas. The use of many small substations introduces the possibility of many different circuit arrangements. Since the type of circuit arrangement selected will have an important effect on power system performance, cost, and reliability, it is 5 factor which must be considered in the system planning stage.

The basic types of circuit arrangements together with their respective characteristics are discussed in detail under Selection of Circuit Arrangements

Plans for Future Expansion and Modernization

When plant facilities have to be expanded or modern- ized, the engineer is afforded an opportunity to design his ideal electric system. First, a one-line diagram should be prepared showing the type of system which would be used for a new plant of similar design. Having made the plan he should let it guide the future modernization and expan- sion. Existing equipment obviously cannot be retired at once, but as additions and replacements of equipment are required, they should be made on the basis of being inte- grated in the ideal plan and not as replacements for the old system.

Flexibility

Plants change manufacturing processes from time to time. Where castings are used today, welding may be use tomorrow. Both process and product may change as demands and styles change. The electric distribution system for any plant should be flexible enough so that complete new process layouts can be made without requiring major changes in the distribution system.

Flexibility for expansion should be considered. In line with this, the Engineer should strive for a system design that will permit reasonable expansion with minimum investment and minimum downtime to existing production.

Two great contributions toward flexibility are:

The load-center system with small substations which may be added in small blocks as required and
Pluing busway which permits the installation of flexible permanent power distribution systems on which machine tools and other devices are merely plugged in where necessary.

System Reliability

Service reliability in any plant is important. Most manufacturing processes are on a production-line basis line shut-down may hold up an entire plant. Also some processes in themselves require a very high order of service reliability. Many factors influence the reliability of electric supply in the plant. Among the most important factors are the following:

Reliability of bulk power supply from utility and/or local generation;
Plant distribution system arrangement;
Simplicity of system arrangement;
Simplicity of system operation and maintenance;
Reliability of equipment and installation

All of these factors should be considered in designing a system to meet service reliability requirements of a given system.

Selection of Equipment

The fundamental consideration in selecting equipment is to choose the optimum equipment consistent with the requirements of the plant. Frequently it costs no more in the long run to use the best equipment available as it pays dividends in service continuity and lower maintenance. Some widely accepted principles are:

Use of metal-clad and metal-enclosed equipment;
Choose non-flammable or dry type transformers for indoor installation;
Use factory assembled equipment for easier field installation and better coordination;
Ensure that equipment ratings are adequate in every respect such as voltage, current, momentary, and interrupting rating;

Economics

Economics is a very important part of power system engineering. The Engineer must complete systems on the basis of cost as well as other features. In making economic comparisons, it is important to include all parts of the system from the power source down to and including the utilization equipment.

Economic comparisons should also include installation as well as equipment costs. As an example, system A may require less transformer KVA than system B, but the connections of system A and other factors may require far more expensive switchgear and more cable than system B. The increased cost of switchgear and cable may more than offset savings in transformer equipment. A comparison on the basis of transformers only would give the wrong answer.

Operation and Maintenance

Proper maintenance is an important to successful performance of the system as selection of the system and its components. Long after the first cost is forgotten, the maintenance cost will continue and if foresight has not been used, this cost mau be considerable.

Although maintenance is largely in the hands of the operators, the system designer can aid in the problem by designing systems that provide alternate circuits.

One circuit can then be taken out for maintenance without dropping essential load. Likewise, draw-out equipment should be used wherever possible to enable maintenance on breaker elements to be done on a service shop.

Another important factor to consider in that maintenance costs often can be reduced by care in locating equipment and by providing convenient auxiliary services. As an example, adequate aisles should be provided and spare of vulnerable parts should be accessible. Provisions should be made for auxiliary power and light so that vital services can be continued during periodic maintenance works.

Other important consideration are location and routine operations and setting of devices. Switchboards, circuit breakers, panelboards, and similar equipment, should not be located where they are subjected to accumulations of dirt, dust, and other foreign materials or where they may be damaged by moving equipment. Operations and settings of relays should be checked regularly. The design engineer can help by including suggested maintenance schedules in the data given to the operator.

Utility Service

In designing the power distribution system, thought must also be given to the utility service. This service must be reviewed from the standpoint of voltage level and also voltage spread.

If the supply voltage is 15 kv or less, it is frequently used without transformation. Voltages above 15 kV are generally considered too high for use within buildings and, hence, transformation is required in these cases. When transtormation is necessary, there is a problem of selecting the proper primary voltage, i.e. 2.4, 4.16, 6.9 or 13.8 kV. The choice is usually based on economics and will greatly affect both the first cost and also the cost of future expansion of the distribution and utilization equipment. In general, 4.16 kV is chosen for loads less than 10,000 kva; 13.8 kV is chosen for loads above 20,000 kVA; and either might be selected for loads between 10,000 and 20,000 kVA, depending upon the possibility of expansion and concentration of the load. This subject is covered later under Selection of System Voltage.

Voltage spread in the utility system may be troublesome depending upon the type of utilization equipment involved. If the primary voltage variation is excessive, then feeder or bus voltage regulators may be required. These may be load tap-changing transformers, individual regulators, switched capacitors or secondary feeder regulators. The choice is based upon both economics and engineering considerations.

Industrial Plant Design/System Planning/Basic Design Consideration