Isolation Transformers for PV+Storage

There may be numerous reasons for including a transformer in a design set. Maybe you are simply stepping PV voltage down to service voltage in a behind-the-meter context. Maybe your utility, inverter manufacturer, or authority having jurisdiction needs a specific type of grounding winding pair.

As the integration of battery energy storage systems (BESS)  with any new PV project is quickly becoming the norm rather than the exception,  it is important to know why and when to incorporate an isolation transformer in your next PV + BESS project.

The 2023 National Electrical Code defines an isolation transformer as follows:

Isolation Transformer. A transformer of the multiple-winding type, with the primary and secondary windings physically separated, that inductively couples its ungrounded secondary winding to the grounded feeder system that energizes its primary winding.

Why isolation?

• Galvanic isolation: reduce risk of ground faults, electric shocks, safety hazards.
  
• Mitigate signal noise: address harmonic distortion, voltage fluctuations, and other power quality issues.
  
• Coordinate operating voltage differences of BESS and PV: step voltage supplied by a PV array up or down to the operating voltage range of the BESS system.
  
• Offer flexibility for code compliance and safety requirements: meet neutral conductor requirements and/or grounding requirements,  while coordinating bidirectional power flow.

Galvanic Isolation

An isolation transformer transfers electrical energy through magnetic induction. Due to this physical separation of the primary and secondary windings, any fault in the primary circuit does not directly affect the secondary circuit. This separation effectively reduces the risk of ground faults and electric shocks, protecting users and sensitive equipment from surge currents or potentially lethal voltage levels.

This is especially important in the case of bidirectional power flow. The physical separation of the two circuits protects the BESS and inverter from possible overvoltage and/or overcurrent produced on the grid-side.

Noise Reduction and Total Harmonic Distortion (THD)

Non-linear loads can subject an interconnection point to voltage fluctuations or harmonic distortion. While many inverters intended for low-voltage projects, including residential PV, may be outfitted with filters capable of addressing harmonic distortion, medium-voltage projects with nonlinear loads may pose a more significant distortion risk.

The graph below visualizes the non-linear dynamics of voltage, current, and BESS state of charge (SOC) for a constant power demand:

During charge and discharge, lithium-ion-based BESS present current and voltage variation as  SOC varies. A quick answer to why this happens points us to electrolyte behavior. As SOC changes, the ion diffusion rates change, as do the internal resistances the electrolyte represents. Hence, in many MV projects that include battery storage, an isolation transformer is called for.

Total harmonic distortion (THD) is a means to quantify how distorted the load signal is from an ideal sinusoidal shape. The more harmonic distortion present in the system, the more non-sinusoidal the inverter output signal received will be, and the higher the THD percentage will be. THD can be understood  as a percentage ratio, that relates the

Sum of harmonic component amplitudes of the incoming signal to the amplitude of the incoming fundamental frequency.

There are electrical industry guides that define the maximum level of harmonic distortion allowed for transformers designed for usual service conditions (see ANSI/IEEE 519).  As a general rule of thumb, if the total harmonic distortion (THD) in the system is above 5%, you’ll need to either address the load(s) or choose a transformer that can handle the extra noise and heat. Thankfully, it’s common for BESS inverters to list a THD of less than 3%:

K-Factor

If your system THD exceeds that 5%, the industry has taken some of the thinking off your plate by including K-factor ratings or numbers on the transformer nameplate.  The K-factor number can give you an indication of the transformer’s ability to handle loads that produce harmonic currents and also how much a transformer must be derated or oversized to handle a non-linear load. 

One easy way to think about K-factors in practice is as a “heat-survival rating.”  Rather than rather taking measures to lower the load or signal’s THD, w Transformers with higher K-factor numbers—e.g.  20, 30, 50—are especially useful for circuits with known non-linear loads like computer servers, critical care facilities, and some hospital facilities.   In low-voltage cases, K13 may be adequate.

For more in-depth info from the pertinent standards on THD and K-factor, check out ANSI/IEEE C57.96 and  C57.110, as well as UL 1562 (medium-voltage, dry type transformers) and UL 1561 (low-voltage, dry type), and ANSI/IEEE 519. 

Mayfield Renewables is a technical consultancy specializing in commercial and industrial PV and microgrid engineering. Contact us today for a consultation.