Commercial buildings commonly have a 208/120 V transformer in a delta-wye configuration to feed receptacles. Single-phase, non-linear loads connected to the receptacles produce triple harmonics, which add up in the neutral. When this neutral current reaches the transformer, it is reflected into the delta primary winding where it causes overheating and transformer failures.
Another transformer problem results from core loss and copper loss. Transformers are normally rated for a 60 Hz phase current load only. Higher frequency harmonic currents cause increased core loss due to eddy currents and hysteresis, resulting in more heating than would occur at the same 60 Hz current.
Transformers supplying nonlinear loads should be checked periodically to verify operation within acceptable limits. Transformers are also critical to the integrity of the grounding system.
- Transformer loading (kVA)
- Harmonic spectrum
- Total Harmonic Distortion
- a. Voltage THD
- b. Current THD
- Ground currents
The proper grounding of the transformer is critical. (Table 3.3.) NEC Article 250 in general and 250-26 in particular address the grounding requirements of the SDS.
SolutionsThere are a number of solutions for transformer-related PQ problems:
- Install additional distribution transformers (Separately Derived Systems)
- Derate transformers
- Install K-rated transformers
- Used forced air cooling
1. Separately Derived System (SDS)
The distribution transformer is the supply for a Separately Derived System (SDS), a term which is defined in the NEC (Article 100). The key idea is that the secondary of this transformer is the new source of power for all its downstream loads: this is a powerful concept in developing a PQ distribution system. The SDS accomplishes several important objectives, all beneficial for PQ:
- It establishes a new voltage reference.
- It lowers source impedance by decreasing, sometimes drastically, the distance between the load and the source.
- It achieves isolation.
- A new ground reference is established.
2. K-rated transformers
Harmonics cause heating in transformers, at a greater rate than the equivalent fundamental currents would. This is because of their higher frequency. There are three heating effects in transformers that increase with frequency:
- Eddy currents.
- Skin effect.
Application issues with K-factor transformers
K-rated transformers have been widely applied, but there are certain issues with them. Many consultants do not see the need for using transformers with a rating higher than K-13 although K-20 and higher might be supplied as part of an integrated Power Distribution Unit (PDU).
3. Derating standard transformers
Some facilities managers use a 50 % derating as a rule-of-thumb for their transformers serving single-phase, predominantly nonlinear loads. This means that a 150 kVA transformer would only supply 75 kVA of load. The derating curve, taken from IEEE 1100-1992 (Emerald Book), shows that a transformer with 60 % of its loads consisting of SMPS (switched-mode power supplies), which is certainly possible in a commercial office building, should in fact be derated by 50 %.
4. Forced air cooling
If heat is the problem, cooling is the solution. Break out the fan, turn it on the transformer and use forced air cooling. Some experienced hands figure that's worth 20-30 % on the up side. In any case, it can only help. Read the complete paper to get detailed information, illustrations and instructions on how to correctly take the measurements.