System Description

NETA World, Spring 2001

by Steven C. Reed, P.E.

Electric Power Systems

Padmount transformers are used frequently in industrial and commercial applications for distribution of power. Many of these transformers include primary fuse protection for system coordination and transformer protection. Th e manufac-turer has a typical acceptable range of fuse sizes for each size transformer. However, the manufacturer may supply a fuse in the company’s high range that may not protect the specifi c transformer-winding confi guration for all types of secondary faults. Engineers performing coordination studies and fi eld technicians need to be aware of common errors in sizing primary fuses for the appropriate protection of the transformer for various secondary faults.

Th e function of the transformer protective device is to provide system as well as transformer protection. System protection is the ability to isolate a faulted segment of the distribution system due to a damaging fault condition (for example, winding failure). System protection will allow for the remainder of the electrical system to continue to operate after removing the faulted section. Transformer protection includes the correct operation of the fuses due to a bus or cable fault located between the transformer and the nearest secondary side overcurrent protective device. Th e degree of transformer protection provided by the primary fuses should be checked for the level of fault current and the type of fault (three-phase, phase-to-phase, or phase-to-ground) produc-ing the most demandproduc-ing conditions. For certain secondary faults, the primary fuse may be exposed to a proportionally lower current than the windings. If this is the case a fuse must be selected to operate fast enough to avoid damage to the windings. Reference Figure 1 for the per unit fault currents on the primary, secondary, and internal windings.

As can be seen in Figure 1, there are conditions in a delta delta transformer for a phase-to-phase fault and in a delta wye transformer for a phase-to-ground fault where the per unit primary line side current is lower than the internal winding current. In particular, during a secondary ground fault in a delta wye transformer there is only .58 per unit of

current on the primary leg versus 1.0 per unit in the primary winding. In order to ensure correct transformer protection for the two cases mentioned, it is necessary to shift the transformer damage curve to the left in terms of per unit pri-mary-side line current to the transformer winding current.

Figure 1 —

Relationship between the per unit primary-side and secondary-side line currents and the associated per unit transformer winding currents for (a) grounded-wye grounded-wye, (b) delta delta, and (c) delta grounded-wye connected transformers for various types of second-ary faults. (Line current and winding current values are expressed in per unit of their respective values for a bolted three-phase secondary fault.)

As an example, we have used a 1500 kVA transformer, 12470 volt primary, 480 volt secondary with 5 percent impedance.

Th e winding confi guration will change for each example.

Th is type of transformer would be considered a category II transformer (501-5000 kVA, three-phase) in accordance with ANSI C57.12.00. category II transformers have a fault curve for both frequent faults (more than 10 faults in a lifetime) and infrequent faults (less than 10 in a lifetime).

Th e long curve is the through fault curve for the infrequent fault. Th e shorter angled curve is the frequent fault curve based upon fault currents from 70-100 percent maximum at I² t = K. Reference Figure 2 for a wye wye winding con-fi guration. Th ere is only one curve since all current on the secondary is refl ected to the primary and windings as 1.0 per unit. Reference Figure 3 for a delta delta transformer.

Th ere are two curves. Th e curve to the right represents the protection curve for a three-phase secondary fault. Th e curve to the left is the original curve shifted to the left by .87 times the current values (x-axis) to take into consideration a phase-to phase fault. Th is allows for correct transformer protection. No phase-to-ground fault exists for a delta delta transformer. Reference Figure 4 for a delta wye transformer.

Th e far right curve represents transformer damage curve for a three-phase and phase-to-phase (primary current actually higher than winding) fault condition. Th e curve to the left is the original curve shifted by .58 times the current value (x-axis) to take into consideration a phase-to-ground fault.

Figure 2 — Wye Wye Ref. Volt: 480 Current Scale X 2 Figure 3 — Delta Delta Ref. Volt: 480 Current Scale X 2

Th is allows for correct transformer protection. Various types of faults and transformer winding confi guration are critical in ensuring appropriate transformer protection.

In addition to ensuring the primary fuse operates prior to transformer damage, it is also possible to specify a fuse that will protect the secondary cable prior to the second-ary protective device. Certain engineering design schemes may allow for a padmount transformer to feed multiple secondary overcurrent devices with separate cable feeds.

Multiple feeds may allow for smaller sized cable feeds with lower rated cable damage curves. Although this type of coordination is not required it is good practice to re-view the possibility of specifying a small enough fuse to prevent a low-level fault from burning a large section of cable (prior to secondary protective device) versus blowing a primary fuse. It is always advisable to select the lowest possible fuse ratio that will allow for coordination of the highest ampere feeder protective device and still meet inrush standards. However, it is not always possible to select a small enough fuse to protect the secondary cables. Me-dium-voltage fuses are not intended to provide overload pro-tection, and ANSI C37.46 specifi es the minimum operating current to be signifi cantly greater than the ampere rating. As an example, “E” rated fuses operate at 200 to 220 percent of the ampere rating. Even the National Electrical Code specifi es in 240-3 (i) that where three-phase transformers are involved, overcurrent protective devices on the transformer primary do not protect secondary circuit conductors.

• Available fault current

• Peak loads

• Magnetizing inrush currents along with hot-load pickup current

• Transformer protection

• Coordination with primary and secondary protective devices

• Protection of downstream conductors

Following these seven steps and being aware of certain common errors should assist in correctly sizing the primary fuses of a padmount transformer.

Steven C. Reed has a BS in electrical engineering from Villanova University, a masters in business administration from the Olin School of Business at Washington University in St. Louis, and his professional engineering license in multiple states. Steve has worked at Electric Power Systems for 12 years and served as a fi eld engineer, system protection engineer, and now serves as regional manager. He is a NETA Certifi ed Technician Level III.

Figure 4 — Delta Wye Ref. volt: 480 Current Scale X 2