This article defines requirements and procedures for electrical system and equipment deign calculations. Main keywords for this article are Electrical Power System Design Calculations. Short Circuit Study. Voltage Drop Calculations. Harmonic Analysis. Power Factor Improvement. load flow study, Power System Design References, ETAP for Power System Design, Electrical Load Summary, Transient Stability Studies, Re-acceleration Studies, Load Shedding Study, Power Factor Improvement.
Power System Design References
Reference is made in this standard to the following documents.
American National Standards Institute (ANSI)
C57.12.10 Transformers – 230 kV and Below 833/958 Through 8333/10,417 kVA, Single- Phase, and 750/862 Through 60,000/ 80,000/100,000 kVA, Three-Phase Without Load Tap Changing; and 3750/ 4687 Through 60,000/ 80,000/100,000 kVA with Load Tap Changing – Safety Requirements
Institute of Electrical and Electronic Engineers (IEEE)
141 Recommended Practice for Electric Power Distribution for Industrial Plants
399 Recommended Practice for Industrial and Commercial Power System Analysis
519 Recommended Practice and Requirements for Harmonic Control in Electric Power Systems
National Fire Protection Association (NFPA), 2008
70 National Electrical Code
Electrical Power System Design Calculations in Process Industry
It is the intent of this practice to define requirements for electrical system calculations and calculations required for selection and application of power system equipment.
ETAP for Power System Design
- System calculations shall be made using a commercially available analysis program such as ETAP (Electrical Transient Analysis Program) by OTI (Operational Technology Incorporated).
- Existing plants will normally have an established electrical system data base. In most cases the calculations shall be made using the analysis program previously used for the plant.
- Calculations shall be made in accordance with IEEE 141 and 399.
- Calculation methods are related to the testing requirements for the equipment. Calculations for systems using IEC are different than for systems using ANSI/NEMA equipment. Some programs are capable of performing both type calculations but is important to select the appropriate mode of calculation.
Electrical Load Summary
- There are actually three different types of load summaries required: (1) Main (or utility) Load, (2) Essential or Stand-By Power and (3) UPS Power. Preparation of the main load summary and maintenance of an accurate data base is the major task. The other load summaries do not have as many loads or bus summaries.
- The load summary can be produced using a custom data base or spread sheet developed specifically for the project or the load summary tools contained in the master calculation package such as ETAP, EDSA, or PTW can be used. The load summary shall be electronically linked to the calculation data base such that single entry of data is required. It is preferred that the load summary be electronically linked with the mechanical equipment list and the electrical one line diagrams to permit single point data entry and insure consistency between these documents.
- The load summary shall contain the information and assumptions outlined below:
a. Equipment Number
b. Service
c. Status (operating or spare)
d. Mechanical Equipment Rated maximum kW and normal brake kW
e. Motor rated voltage, kW, efficiency and power factor at full, 3/4 and 1/2 load
f. Calculated motor load for mechanical equipment maximum and normal bake
loads (kW, kVAR and kVA)
g. Utility factor
h. Heating load kW
i. Lighting Load kW, kVAR and kVA - Preliminary load estimates shall use motor nameplate rating in kW and shall assume a power factor of 0.85. As actual driven equipment loads and motor data become available, the motor loads shall be calculated from this data.
- Spare motors shall not be included in the overall total load. However, spare motors shall be included in the loads of buses, transformers, and feeders to which they contribute, in order to obtain the maximum operating load on individual equipment. When both the operating motor and its spare would contribute to the same load, if both were operating, only the operating motor shall be included in the load.
- Lighting and receptacle transformer operating loads shall be included at 80% of the transformer kVA rating. Substation transformers and load bus shall be sized to accommodate future load growth thus it is appropriate to include these loads at the initial design value.
- Intermittent short time loads such as motor operated valves shall not be included.
Load Flow Study
- A load flow study shall be run immediately after completion of data entry as a first check on the electrical system and to debug data in case errors were made. Once the system model has been confirmed, system load flow cases can be run or other calculations such as short circuit analysis can be started.
- The load flow study is used to confirm that voltages and loading on different elements of the system are within project parameters. For redundant systems the load flow shall be performed with one supply interrupted to insure that the system will operate satisfactorily in this condition.
- The load flow study provides information on the power factor of the circuits and buses of the electrical system. Refer to Section (Power Factor Improvement) for information on how to use power factor information to improve the power factor.
Short Circuit Study
- Short circuit studies shall be run to determine preliminary equipment rating and confirm required equipment ratings for purchase.
- The secondary of the 480 volt transformers shall be modeled with motor load equivalent to the kVA rating of the transformer.
- Buses with medium voltage motors shall be modeled with the actual motors at their nameplate rating plus additional motors to represent the capacity provided for future additions.
- Fault calculations for equipment selection shall be based on the system operating configuration that will result in the maximum available fault current.
- Standard transformers impedances shall be used; deviations must be approved by. Transformers shall be modeled with maximum allowed negative impedance tolerance.
Voltage Drop Calculations
- Voltage drop calculations shall be made for both steady state and motor starting. As indicated above, the load flow study shall indicate steady state voltage at all buses.
- The allowable voltage drops are based on the use of transformer taps to maintain 100% voltage at the secondary terminals under normal loading conditions. Permissible steady state voltage drops shall be as follows: a. Medium Voltage Distribution System. The total voltage drop to motors or unit substation transformers shall not exceed 5% under normal loading conditions.
b. Systems Rated 480 Volts and Below. The total voltage drop from the unit substation secondary terminals to the utilization equipment shall not exceed 5%. - ‘Snap-shot’ voltage drop calculations are acceptable for most motors. Most calculation programs provide for this type calculation as an adjunct to the load flow; It calculates the voltage at the instant the switch is closed with out regard to any affect this lowering of the bus voltage might have on any other operating loads.
- For very large motors started on the bus with other motors, a transient stability program or dynamic motor starting program shall be employed to recognize the effect on and from all loads on the bus.
- The maximum acceptable voltage drops during motor starting are shown below. However in all cases must approve the permissible voltage drop limits for each project and approve individual calculations for motors larger than 2000 kW.
a. For motors on 480volt bus with other loads, the maximum drop on the bus shall be 10%. The maximum drop to motor terminals shall be 15%.
b. For medium voltage motors on the same bus with other motors the maximum drop on the bus shall be 15%. The maximum drop to motor terminals shall 20%. If solid state control devices are used and the devices cannot accept 15% voltage drop, then the bus voltage drop shall be limited to 10%. The maximum voltage drop at an LV bus due to motor starting on an MV bus shall be restricted
to 10%.
c. For motors on captive transformers, the voltage drop at the bus supplying the captive transformer shall not exceed 10%. The voltage at motor terminals shall be at least 10% above the minimum value required to accelerate the load.
Transient Stability Studies
- In general transient stability studies are not required for plants supplied from the utility grid unless in plant generation is to be operated in parallel with the utility, or as discussed for starting large motors. A transient stability study shall also be performed if a fast bus transfer scheme is to be utilized.
- Plants powered by one or more generators will require transient stability studies to insure that fault can be cleared quickly enough to maintain system stability.
Re-acceleration Studies
- Reacceleration studies shall be purchased to determine the impact of automatically restarting designated process drive motors after a brief power interruption. Typically this control would permit the motors to restart over some time range. The minimum for the range would be the time required for the internal voltage to decay and the maximum would depend upon process considerations.
- The re-acceleration study is made to determine whether the motors can restarted simultaneously. If not, additional studies shall be made to establish group sizes and required time delay between groups.
- This study is a specialized motor starting study with the motors starting simultaneous, modeled as an equivalent single motor.
Load Shedding Study
- The electric utility requires automatic load shedding in some regions to maintain stability for the power grid in the event of a major power system upset condition.
- A load shedding study shall be preformed to determine how to provide the required load reduction with the least impact on production. Normally the reduction is done by under-frequency relaying in 2 or more steps. The utility will define the frequency for each step and the percentage reduction for each step.
Harmonic Analysis
- Harmonic currents and voltages have undesirable affects on operation of the electrical power system including overheating of equipment and overvoltage failures. Aside from the undesirability from the owners standpoint, the utility generally has strict limits on harmonics since they flow back into the power grid.
- Harmonics are produced by rectifiers and frequency converters. The larger the loads are, the greater problem they create, so special attention must be given to large adjustable frequency drives and electrolytic process equipment.
- The normal practice is to require the supplier of the largest harmonic producing load to perform a harmonic study for the plant including all harmonic producing loads and calculating the affect on all buses.
- This study shall be done in accordance with IEEE 519.
- Harmonic filters can be installed to reduce the harmonics on the system. The design and supply of the filters if required are normally included with the harmonic study.
- Harmonic filters are of two types.
- One or more tuned reactor/capacitor filters tuned to just below the harmonics with the highest values. These are used mainly where there harmonic magnitudes are large.
- For small levels of harmonics, such as variable speed drives on 480 V motor control centers, programmable static harmonic filter are available. Placing the VFD at the 480 V level protects the equipment at that level, and reduces the harmonic impact on the step-down transformer.
- Harmonic Analysis study shall form an integral part of the studies when VFD is used with Power factor improvement/VAR compensation.
- Harmonic content calculations shall be performed for the entire distribution system and cancellation units shall be provided when equipment, building or plant exceeds the limits of IEEE Std 519.
Power Factor Improvement
- Power factor correction is important because low power factors increase voltage drop and sizes of cables, breakers, transformers, etc.; and may violate utility requirements or contracts.
- Early load flow studies can indicate if power factor correction may be needed. As soon as the need for correction is indicated, correction can be added to the study in the form of capacitors or utilization of synchronous motor operating with leading power factor.
- For detailed discussion of the application of capacitors for power factor improvement refer to Click Here.
- As soon as synchronous motors are identified, their effect at both unity and leading power factor should be studied.
- It should be understood that the existence of synchronous motors does not necessarily guarantee that all power factor problems are resolved. The location and the size of the motor(s) will determine where and how much the motor improves the power factor.
- Once the system load has been firmed up, the load flow study will confirm that the desired correction has been obtained and the effect on bus voltages and equipment loading. The computer programs referenced in section 5.1 shall be used to run the load flow studies.
- Power factor improvement can be studied by changing the following system parameters:
a. Adding capacitors to the system at different locations
b. Changing an induction motor to synchronous motor
c. Changing the power factor on a synchronous motor
d. Changing the loading on a motor
e. Changing the taps of a transformer
f. Changing the impedance of a transformer