Transformer Loading for DER Customers

PPL Electric Utilities (PPL Electric) has established thresholds on utility-owned secondary transformer sizing. Transformers are important pieces of electric equipment directly connecting the electric utility and our customers. Therefore, maintaining the health and reliability of this key asset is critically important. Based on third-party findings and available data illustrating an increase of Distributed Energy Resources (DERs) and corresponding two-way power flow on the grid, these third-party researchers have found that overloading of transformers leads to significant life-span reductions. This potential overloading directly affects reliability, safety, and cost impacts for customers.

Transformer - Secondary - Loading for DER Customers

Traditional transformer loading standards are derived based on the internal oil temperature and the breakdown of the paper insulation on the transformer windings. Typically, a transformer is anticipated to experience two peaks within a 24-hour load period. It is similarly assumed the transformer will experience an overnight cooling period. When interconnecting an inverter-based generation customer to PPL Electric’s distribution system, a different load profile must be considered. A transformer interconnected to such an installation is anticipated to be in a constant reverse loading throughout the daytime generation period, with minimal load present at night. The following is an example of a solar loading pattern, as confirmed by a third-party engineering firm:

GS3 rate customers taking secondary service connection may size their DER generation to 100% of the PPL Electric installed, owned, and maintained transformer nameplate with a 90% power factor.

- i.e., 2500 kVA (nameplate rating) X 0.9 (PF) = 2250 kW (Interconnected Generation)
- This formula will be applied for any transformer size that PPL allows (up to a 2500 kVA maximum transformer size).
If an applicant wishes to connect more than the allowable generation, they may apply for a LP4 rate primary service connection in which the customer installs, owns, and maintains its’ transformation.
If a solar customer exceeds the maximum allowable capacity of a transformer, they shall not be permitted to install multiple, parallel transformers, to increase the interconnected generation kW allowance.
- Individual transformers and meters must be used.
- The customer is required to include visible and mechanical isolation points to prevent any unintentional paralleling, back-feeding, or overloading of a single transformer.

PPL Secondary Transformer maximum name plate connected generation.

2500 kVA X 0.9 (PF) = 2250 kW (Interconnected Generation)
2000 kVA X 0.9 (PF) = 1800 kW (Interconnected Generation)
1500 kVA X 0.9 (PF) = 1350 kW (Interconnected Generation)
1000 kVA X 0.9 (PF) = 900 kW (Interconnected Generation)
750 kVA X 0.9 (PF) = 675 kW (Interconnected Generation)
500 kVA X 0.9 (PF) = 450 kW (Interconnected Generation)
300 kVA X 0.9 (PF) = 270 kW (Interconnected Generation)
150 kVA X 0.9 (PF) = 135 kW (Interconnected Generation)
75 kVA X 0.9 (PF) = 67.5 kW (Interconnected Generation)
50 kVA X 0.9 (PF) = 45 kW (Interconnected Generation)
25 kVA X 0.9 (PF) = 22.5 kW (Interconnected Generation)
15 kVA X 0.9 (PF) = 13.5 kW (Interconnected Generation)