Photovoltaic (PV) - Utility Power Grid Interface 

By on

Image(32)
ABB String Inverter (3.3 to 8 kW)
- IEC 61727 Complaint
Photovoltaic (PV) systems are typically more efficient when connected in parallel with a main power gird.  During periods when the PV system generates energy this can be utilized and the grid energy used at other times. 

For large PV systems, any connection interface is likely to need discussion with the power network operator.  For smaller systems (around 10 kVA), there are existing standards which will facilitate making a functional and safe grid connection. This note looks at  some of the standards.

IEC 61727 Utility Interface

IEC 61727 - Photovoltaic (PV) systems – Characteristics of the utility interface

As an international standard, IEC 61727 specifies the main requirements of a grid interface which will ensure that it is both functional and safe for PV connections of 10 kVA or less.  Most the required functionality to comply with the standard is implemented within the PV systems inverter. 

When the utility limits move outside normal operational bounds the inventor should disconnect itself from the grid.

Inverter Output Power Quality - the output of any inverter should operate within the following limits:

  • the inverter must limit the injection of any d.c. current into the utility to less than 1% of its rated output
  • the total harmonic current distortion should be less than 5%
  • harmonic current distortion for each individual harmonic should be less than those given in the standard
  • the inverter must control flicker to be in line with IEC 61000
  • when the inverters output is greater than 50%, the power factor must be greater than 0.9

Note: in most countries there would be local regulations related to connection to an utility power gird.  It is incumbent that users wishing to connect to a utility grid understand and adhere to local regulations.

Non-Islanding

If the grid power is lost for any reason then for safety reasons the inverter should stop supplying power.  Also if the utility voltage or frequency goes out of limits the inverter should stop supplying power.  The maximum disconnection times laid down in the standard to achieve this are:

  • V < 0.5 x Vn - 0.1 s
  • V ≤ 0.85 x Vn -2.0 s
  • V > 1.1 x Vn - 2.0 s
  • V ≥= 1.35 x Vn - 0.05 s
  • Frequency ±1 Hz - 0.2 s

     Vn is the nominal voltage

Once any grid power is restored, the inverter should not connect for some time.  This is typically between 20 seconds to 5 minutes and should be agreed with the utility provider.

Local Implementation

ENADesisionTree
ENA Decision Tree For The
Distributed Generation Connection Guide

Image Source: Energy Networks Association (ENA)
Local authorities may have their own guides or standards which need to be applied.  These could be in line with IEC 61727 or have a completely different set of connection requirements.  Anyone installing a grid connected PV system should follow all local standards.

As an example, the Energy Network Association (ENA) in the UK produces connection guides which are adopted by  authorities in the UK.  The image (click for a larger version), shows the ENA decision tree for selecting the correct guide.

Installation Requirements

Any PV system will fall under the relevant regulations and standards governing electrical installations. 

Within the IEC framework this would be IEC 60364 "Electrical installations of buildings".  While any installation needs to fully comply with the standard, part 7-712 deals with particular requirements related to PV systems. 

Local regulations will often be in force, for example in the UK, BS 7671 (the IEE Wiring Regulations) is used.  If in the US, then NFPA 70 (National Electric Code) would be used.

Image(33)
Typical PV Connection Single Line Diagram
In designing the PV system and it's grid connection, the designer would typically need to consider the following:

  • d.c. protective devices on the PV array output
  • a.c. protective devices on the inverter output
  • connection of the inverter into the installations main electrical systems
  • PV array over voltage protection
  • junction boxes for connecting multiple arrays
  • devices for isolation of the PV array from the inverter
  • devices for isolation of the inverter
  • earthing and equipotential bonding of the equipment

Hopefully the note has provided an overview of both utility grid interface requirements and some necessary installation considerations.  If you have an comments or would like to discuss, please do so.

See Also



Steven McFadyen's avatar Steven McFadyen

Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

myElectrical Engineering

comments powered by Disqus

  1. Notes's avatar Notes says:
    7/11/2013 2:46 PM

    Trackback from Notes

    Photovoltaic (PV) cells (sometimes called solar cells) convert solar energy into electrical energy.  Every year more and more PV systems are installed.  With this growing application, it's a good idea for every practicing professional to have an understanding... ...


Comments are closed for this post:
  • have a question or need help, please use our Questions Section
  • spotted an error or have additional info that you think should be in this post, feel free to Contact Us



How to Check a Circuit is Dead

If you want to check a circuit is dead (not live), you should always use the three point method. First check a known live circuit, then check the dead...

Meeting room of the future

The IET site has a video of a visit showing of a high tech meeting room developed at Napier University in Edinburgh. It a good demonstration of innovative...

What is an Open Delta Transformer

In three phase systems, the use of transformers with three windings (or legs) per side is common.  These three windings are often connected in delta or...

8 Motor parts and common faults

Straight forward list of some common motor faults.  If I have missed any other common faults, please take a bit of time to add them in as a comment below...

Maximum Demand for Buildings

Estimating maximum demand is a topic frequently discussed. Working out how much power to allow for a building can be very subjective . Allowing too much...

Capacitors - Energy Storage Application

Capacitors have numerous applications in electrical and electronic applications.  This note examines the use of capacitors to store electrical energy....

IEC 60287 Current Capacity of Cables - Rated Current

In the previous note we looked at the approach taken by the standard to the sizing of cables and illustrated this with an example.  We then looked at one...

IEC Document Designation

Often document control is dictated by project requirements, for example a particular organisation may have an existing numbering system. Existing company...

Periodic Electrical Installation Inspection – What to Inspect?

This is the second post in a series of two on periodic electrical inspections. In the first post, I discussed how often inspections should be carried out...

Surface Treatment – Ladders, Trays and Baskets

Steel ladders, trays and baskets form the backbone of cable containment systems. Often these items need some form of surface treatment to prevent corrosion...

Have some knowledge to share

If you have some expert knowledge or experience, why not consider sharing this with our community.  

By writing an electrical note, you will be educating our users and at the same time promoting your expertise within the engineering community.

To get started and understand our policy, you can read our How to Write an Electrical Note