& Coordination Studies
Short Circuit Study
Whenever a fault occurs in an electrical power system, relatively high currents flow, producing large amounts of destructive energy in the forms of heat and magnetic forces. When an electrical fault exceeds the interrupting rating of the protective device, or the fault withstands rating of equipment such as switchgear or panelboards, the consequences can be devastating, including injury, damaged electrical equipment, and costly downtime. A Short Circuit Study (SCS) is required to establish minimum equipment ratings for power system components to withstand these mechanical and thermal stresses that occur during a fault, and thus SCS’s are mandated by article 110.9 of NFPA NEC 70 Code. Also required by the NEC are markings and nameplates of the SCS ratings on equipment. These include industrial control panels (409.110), industrial machinery (670.3[A]), HVAC equipment (440.4[B]), meter disconnect switches (230.82[3]), and motor controllers (430.8).
The short‐circuit calculations that are the basis of an SPS involve the reduction of the electric power distribution system supplying current to each fault location to a Thevenin equivalent circuit of system voltage sources and impedances at each bus. (Refer to ANSI/IEEE Std 399 Recommended Practice for Power System Analysis (Color Book Series ‐ the Brown Book)). These calculations can be done by hand, but since they can be very tedious for mid‐size to large distribution systems, they are usually performed using the same specialized software that models the power system under normal load flow conditions to determine system power flows and voltage drop. When the software is used for a SCS, the model is used to calculate the resultant fault current at each bus in the system. Typically, results are presented for both three‐phase and line‐to‐ground faults. (Software used by electric utility companies also calculates line‐to‐line fault current values.)
In order for the electrical distribution system analysis software to produce reliable results, the electrical distribution system being analyzed must be modeled accurately. All current sources must be investigated and inputted into the model, such as:
The Electric Utility – As a minimum, the service voltage and available 3‐phase and single line‐to‐ground short circuit current (usually given in amps or MVA) and the three‐phase and line‐to‐ground equivalent circuit reactance/resistance ratio, referred to as the X/R ratio, are required. Many utilities also provide the equivalent system impedances at the point of service connection. The system short circuit contribution data must be requested from the electric utility.
On‐site Generation – If the worst‐case short circuit levels are to be calculated, all generations that can operate in parallel with the Utility supply must be included in the model. Rated voltage, kVA, power factor, and generator subtransient and transient impedances should be obtained from the equipment nameplate or manufacturer for input into the model.
In‐Service Motors – Rotating electric motors have stored kinetic energy, and under fault conditions, a motor can act as a generator for a short period and return some of this stored energy into the fault in the form of short circuit current. All across the line, induction motors and synchronous motors should be modeled. If induction motors are supplied by Variable Frequency Drives, only motors that are fed from drives that are regenerative should be included. Typically lumping contributing motors together on each bus provides reasonably accurate results. Software packages have default impedances and power factors that cover typical motors. Actual impedances should be used where known, and especially for very large motors.
UPS Systems – UPS systems often are limited as to the amount of short circuit current they can contribute to a downstream fault. Consult with the equipment supplier for characteristics of any such equipment on the system being analyzed.
All impedances in the system must be defined and inputted:
Transformers – Correct modeling of transformers is critical to an accurate model. Winding voltage ratings, kVA ratings, and impedances should be obtained from transformer nameplates or manufacturers. Winding connections must be inputted, as well as any ground impedances.
Cables – Wire and cable size, length, and routing type must be included in the model. Where existing cables sizes are not known use the NEC as a reference and include any assumptions that you made in the report as a reference for future investigations. Cable lengths can be estimated based on a site visit and inspection, or by making reasonable assumptions from building and site plans. Whether cable is routed in steel conduit/raceway or not can also make a difference in cable impedance.
Reactors – Where line reactors are included to limit short circuit current or minimize harmonics, they shall be included in the model. Obtain ratings from the equipment nameplate or the manufacturer.
Once the electrical distribution system model is created, the system can be analyzed for all operating scenarios. These might include generators operating or not, bus tie breakers closed or open, etc. The maximum short circuit fault current calculated from any of the scenarios should be recorded at each bus in the distribution system. Protective device manufacturers assign short‐circuit interrupting and fault withstand ratings to their equipment, signifying the maximum fault condition under which the device may be safely applied. The currents calculated in an SCS are used to specify the required ratings for new equipment and to evaluate the adequacy of existing system components to withstand and interrupt these high magnitude currents. Once the electrical distribution system is modeled, proposed changes in system configuration can be analyzed in order to determine what, if any, equipment must be upgraded to support the proposed changes. An up to date SCS is also a requirement for Protective Device Coordination Studies as well as Arc‐Flash Hazard Analysis, which is an assessment, conducted by a safety expert, to evaluate the electrical equipment and power systems to predict the potential for or incident energy of an arc flash. More information on this topic is included in Chapter 4.
Figure 3.2 Sample SCS Screenshot
(Courtesy of PMC Group One, LLC)
Coordination Study
The goal of a Protective Device Coordination Study is to determine overcurrent device settings and selections that maximize selectivity and power system reliability. In a well‐coordinated protective device, scheme faults are cleared by the nearest upstream protective device. This minimizes the portion of the electrical distribution system interrupted as a result of a fault or other disturbances. At the main distribution panel level, feeder breakers/fuses should trip before the main. Likewise, panel board branch breakers should trip before the feeder breaker/fuse supplying the panel.
As with Short Circuit Studies, a Protective Device Coordination is usually performed using the same specialized integrated software that is used for the Short Circuit, Load Flow, and Arc‐Flash calculations. Protective device types, ratings, and settings can be incorporated while the model is built or added after initial studies are completed. The use of computerized calculations allows the system protection engineer to evaluate a number of setting options in a short period of time, thereby allowing him (or her) to fine‐tune settings to achieve the best possible coordination.
Using the computer model, a Time Current Curve (TCC) is developed for each circuit fed from Service Switchboards and other critical buses. The curve includes the over current devices for the largest loads in series on the circuit – the worst case from a coordination aspect. All fuses, breakers, electromechanical or electronic relays, and trip devices are entered into the computer model, if not entered previously. Transformer and cable damage curves should be selected for inclusion
