Pfeiffer Engineering Co. - Industrial Automation

Design of Power Systems
by John C. Pfeiffer, P.E. & Timothy Striegel,
Pfeiffer Engineering Co., Inc.
September, 1996

The design of power systems is a multi-step process which is well known by those who specialize in this area of electrical engineering. Unfortunately these methods have not been part of the curriculum of many engineering schools for many years. Fortunately, the Institute of Electrical & Electronic Engineers (IEEE) has well documented the technology involved in their color book series. The following is the beginning of what is needed to understand this subject.

REFERENCES:

IEEE Std. 141, Recommended Practices for Electric Power Distribution Systems for Industrial Plants
IEEE Std. 142, Recommended Practices for Grounding of Industrial and Commercial Power Systems
IEEE Std. 241, Recommended Practices for Electrical Power Systems for Commercial Buildings
IEEE Std. 242, Recommended Practices for Protection and Coordination of Industrial and Commercial Power Systems
NFPA Std. 70, National Electrical Code

The sequence of steps in designing power distribution systems begin with a load study and power system layout and proceed as follows:

Quantify the various loads by type of load such as motor loads, lighting loads, resistive heat, etc.
Calculate the Estimated Maximum Demand, both for the present design and for future expansions.
Determine the power system demand and diversity as defined by the NEC.
Power system configuration is next determined by studying the types and physical locations of groups of loads. Define load centers.
Protection methods are next determined such as using fuses or circuit breakers and where each type will be used.
Preliminary power system components are defined and selected.
Electric single line diagram is produced.
Electric utility information is next obtained:
1. Utility supply voltage and connection diagram; delta/wye, delta/delta with grounded leg, etc.
2. Available short circuit current or kva
3. Utility reactance/resistance (X/R) ratio
4. Utility's power factor
Collect data for Short Circuit Analysis
1. Utility data (see above)
2. Motor Data - Calculate total motor full load current and HP at each load center by size groups, ie. all motors < 50 HP, 50 to 100 HP, etc. Determine short circuit X/R ratio for each motor group, Motor Lock Rotor Current, or NEC letter code
3. Large Motors
  1. Motor HP
  2. Motor full load current
  3. Motor power factor
  4. Motor locked rotor current
  5. Motor subtransient reactance (X"d)
4. Synchronous Motors
  1. Motor HP
  2. Motor full load current
  3. Motor locked rotor current
  4. Motor subtransient reactance (X"d)
5. Generators
  1. Generator full load current
  2. Generator locked rotor current
  3. Generator short circuit X/R current
  4. Generator power factor
  5. Generator RPM
  6. Generator subtransient reactance (X"d)
6. Transformers
  1. Voltage and wiring configuration (12,470 delta - 480/277Y)
  2. Size (KVA)
  3. Percent Impedance (%Z)
  4. Reactance/Resistance Ratio (X/R)
  5. Neutral Resistance
  6. Secondary tap percentages
7. Cable Runs
  1. Size of Cable
  2. Cable length
  3. Cable type
  4. Temperature rating
  5. Shielded/non- shielded (High Voltage)
  6. Zero-sequence impedance/positive sequence impedance ratio (Z0/Z1)
  7. Cable material (Copper or aluminum)
  8. Conduit type magnetic, non-magnetic, overhead distribution)
8. Power Distribution Equipment
  1. Main and secondary bus amp rating
  2. Main and secondary bus bracing -withstand current (Amps Symmetrical)
  3. Circuit Breaker and fuse data, see below
9. Panel Boards, Motor Control Centers
  1. Main and secondary bus amp rating
  2. Main and secondary bus bracing - withstand current (Amps Symmetrical)
  3. Circuit Breaker and fuse data (see below)
10. Circuit Breakers & Fuses - Type and size
  1. Interrupting capacity ( Amps Symmetrical)
  2. Current Limiting Ability
  3. Time-Current coordination curves
Perform Short Circuit Analysis
Apply current-limiting effects of current-limiting rated circuit breakers and fuses to the short circuit data to determine the level of fault current each device will see. Verify the short circuit interrupting rating of all switched, circuit breakers and fuses to insure that the device can safely interrupt expected bolted short circuit currents. Verify the short circuit withstand rating (bracing) of switchgear, panel boards, and motor control centers, etc. Verify that cables will not be damaged by the heating effects of a short circuit current (I squared x T). If the symmetrical short circuit current at a device is within 70% of the symmetrical short circuit interrupting rating of the device, then the asymmetrical rating of the device must be calculated based upon the device's test X/R ratio. The device's Iasym must be greater than the fault of the Iasym generated by the fault.
After the short circuit study is complete, finalize hardware specifications.
Develop coordination curves for groups of protection devices. The typical curve should have the following information:
1. Estimated Maximum Demand (current)
2. Available short circuit current
3. NEC maximum allowable currents, such as 125% of transformer FLA.
4. Transformer in-rush point
5. Transformer damage point and damage curve
6. Cable damage curves
7. Current characteristic curves for protective devices being studied
The purpose of these curves is to determine the setting of adjustable circuit breakers and protection relays and to insure that the items such as transformers and cables are properly protected. Finally, the sizes and settings of protective devices should be coordinated such that the protection device closest to the fault will open first, clearing the fault before any damage can occur and other protective devices open. Its very important to minimize the extent of a power outage due to a fault.
If the coordination study reveals problems where devices will not coordinate, equipment selection and specification may need to be revised.