Utility Source/Distribution Grid Power Quality Concerns for Solar Inverter Applications: Voltage

The publication of my thoughts here is being used as a primer for a full application and White Paper examination on the subject. I would welcome any and all feedback on any of the concepts discussed within this article. This discussion will focus on Utility/Source/Distribution voltage quality concerns. Specifically, voltage power quality. Please feel free to contact me at: [email protected]

Overview:

Solar energy is becoming an increasingly hot topic within the electrical industry. Between the public and the industrial markets becoming more environmentally conscious and the advent of governmental subsidies, the demand for more carbon neutral energy sources is growing and it has become more affordable. The concept of solar energy is fairly simple, the solar panels create DC energy which is then converted to AC energy by the use of an inverter. But the inverter within this application is acting as a current source for the distribution system versus a more traditional voltage source, which is the foundation of more traditional power generation and distribution grids. Present electrical standards are just now catching up to this surge in development of the technology and there is a lot of confusion about what system power quality parameters might compromise the successful application, deployment, and operation of the inverters.

All electrical equipment has qualifying environmental, source and load limitations as it relates to its proper operation. Solar inverters are no exception. In the case of a solar inverter, since it is a current source device, it is natural to understand that the distribution grid power quality as it relates to voltage must be considered. But this seems to be a subject where very little data or specification requirements are known, understood or communicated.

I’m working on a number of projects presently involving utility/distribution grid power quality concerns for both small solar installations and larger solar farm applications. The following points should help you understand three major considerations for the system into which the solar inverter system is being installed. Remember, the inverter is a current source device, so voltage distortion compliance to IEEE519-2022 (Table 1) and other power quality specification guidance on voltage distortion requirements becomes paramount, system voltage imbalance is important, and properly matching the inverter voltage rating to the source potential level is of concern. Surge and Transient voltage are also of significant consideration.

Utility/Source Voltage Total Harmonic Distortion (Vthd)

Questions:

·???????Does the solar inverter manufacturer require a limit on the Utility/Distribution system voltage distortion levels to which the inverter will be connected?

·???????What is that voltage distortion limit, i.e., Vthd at which the inverter manufacturer limits their claims for performance and proper operation?

·???????Does the manufacturer state within their instruction materials that the Utility/Distribution system must comply with IEEE519-2022 or other electrical standards relative to power quality?

These questions and research items are my first concern. Not too long ago, there was almost no mention by the manufacturer of any voltage harmonic distortion withstand limits, but now notations within installation manuals and warranty conditions are referencing these considerations as a qualification. Not providing a definitive limit as a percentage of fundamental voltage is common.

Source Voltage Imbalance

A Utility or Distribution Grid source imbalance of greater than 2 %, i.e., a prox. 12V imbalance phase to phase on a 600V system or prox. 9V on a 480V system may trigger false trips for Solar Inverters and compromise current injection balance. This situation if it develops directly impacts the integrity of the operation of the installation and can potentially damage the inverter equipment itself. There are no direct limitations for voltage imbalance within most reference standards, but it is well known that the higher the level of voltage imbalance the greater the current harmonic creation within non-linear load structures and the greater the challenge to any 3-phase load structures to proper operation. But very little is published on this topic in the Solar field.

Questions:

·???????Does the solar inverter manufacturer have a required installation source imbalance tolerance threshold within their specifications?

·???????If being fed from a utility source, are there any rate contract constraints on the voltage imbalance that can be provided for the interconnect coupling point or primary metering location?

The subject and concerns over voltage imbalance is more recognized within the general electrical engineering practices, but within the Solar field it seems to be overlooked as a concern. Yet, it is an important factor for any solar installation and specification.

Over-Excitation:

The potential of coordinating Utility/Source/Distribution voltage levels to the Inverter rated nominal voltage requirements is important. I have seen several installations where the Utility voltage levels meets the utility requirements for a voltage/service class (generally +/- 10% of nominal), but the solar inverters will not operate properly and experience nuisance trips. When tested and reviewed in detail, it is discovered that even a 2% over-excitation, i.e. a 2% higher than accepted nominal voltage rating of the inverter can trigger nuisance trips and operating challenges.

Question:

·???????What is the maximum nominal operating voltage specified by the inverter manufacturer and what are the short-term surges and transient voltage limits required by the manufacturer?

From my field experience and to resolve multiple applicational situations, access to the transformer tie was required to reset the taps to lower the nominal voltage to a level at or just below the nominal voltage nameplate rating of the inverter to resolve the nuisance trip condition.

An Applicational Solution:

Based on the parameters above, Mirus International is utilizing their Harmonic Mitigation technology to address this applicational challenge. By utilizing a tuned passive filter/auto transformer assembly, an engineer can design to resolve the challenges noted above.

·???????The AUHF series Mirus filter can significantly reduce the Utility/Grid voltage distortion to less than an IEEE 519-2022 Vthd limit by blocking the grid Vthd seen by the solar inverter via the filter tuning.

·???????With the capacitance reactance element of our LCL topology, we can reduce the grid source voltage imbalance of up to 3% to a 1% or less P-P voltage imbalance seen by the inverter in order to help resolve this issue.

·???????The same capacitance reactance assembly can assist with transient and surge protection, but when complimented with our Coordinated Surge Protection Option (CSP) a superior protection can be achieved.

·???????The addition of an Auto-transformer assembly to the AUHF filter can be provided by our AUHF-ATL model which then allows multiple 2-1/2% voltage correction steps above and below for nominal voltage correction should access to the transformer/transmission voltage correction taps not be available.

The Mirus AUHF is available in 480V, 600V and 690V voltage classes in both 50Hz and 60 Hz frequency ranges from 6kVA/4.5 kW through 3500kVA/3000kW, making this an effective solution for both small scale and large energy solar farm installations.

As stated, this communication is being used as a primer for a full-scale discussion presently being drafted. For more information, contact Mirus International or contact me directly to discuss this application.

Michael A McGraw - Cell: 713-208-8534 - [email protected]