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 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 expected to experience two peaks within a 24-hour load period. It is similarly assumed the transformer will experience an overnight cooling period. When interconnecting a solely generating solar customer to PPL’s distribution system, a different load profile must be considered. A transformer interconnected to a solar installation is anticipated to be in constant reverse loading throughout the daytime generation period, with minimal load present at night. The standard below, confirmed by a third-party engineering firm, considers the loading pattern of these solar DER customers:

GS3 rate (3PH), GS1 rate (1PH) and RS rate (1PH) customers taking secondary service connection may size to 95% of the PPL installed, owned, and maintained transformer nameplate of the transformer kVA rating equal to the total inverter kW nameplate rating, rounded down to the nearest whole number value.
- i.e., 2500 kVA (nameplate rating) X 0.95 %= 2375 kW (Connected Generation)
- This formula will be applied for any transformer size that PPL allows (up to a 2500 kVA maximum pad-mounted 277/480V wye-wye transformer size).
If an applicant wishes to connect more than the allowable generation, they may apply for an LP4 rate (3PH) primary service connection in which the customer installs, owns, and maintains their own transformation.
If a solar customer exceeds the maximum allowable capacity of a transformer, they shall not be permitted to install multiple, paralleled transformers, to increase the connected generation kW.
- 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.
- No delta 120/240V 3 PH configuration is allowed for DER connection.

PPL Secondary Transformer maximum name plate connected generation.

2500 kVA (PAD) (3PH) X 0.95 % = 2375 kW (Connected Generation)
2000 kVA (PAD) (3PH X 0.95 % = 1900 kW (Connected Generation)
1500 kVA (PAD) (3PH) X 0.95 % = 1425 kW (Connected Generation)
1000 kVA (PAD) (3PH) X 0.95 % = 950 kW (Connected Generation)
750 kVA (PAD) (3PH) X 0.95 % = 712 kW (Connected Generation)
500 kVA (PAD/OH) (3PH) X 0.95 % = 475 kW (Connected Generation)
300 kVA (PAD/OH) (3PH) X 0.95 % = 285 kW (Connected Generation)
167 kVA (PAD/OH) (1PH) X 0.95 % = 158 kW (Connected Generation)
100 kVA (PAD/OH) (1PH) X 0.95 % = 95 kW (Connected Generation)
75 kVA (PAD/OH) (1PH) X 0.95 %= 71 kW (Connected Generation)
50 kVA (PAD/OH) (1PH) X 0.95 %= 47 kW (Connected Generation)
25 kVA (PAD/OH) (1PH) X 0.95 %= 23 kW (Connected Generation)
15 kVA (OH) (1PH) X 0.95 %= 14 kW (Connected Generation)
10 kVA (OH) (1PH) X 0.95 %= 9 kW (Connected Generation)