The Energy Networks Association have released a new document G99 titled “Requirements for connection of generation equipment in parallel with public distribution networks after 27th April 2019” it replaces the existing G59. The new G99 makes significant changes to the way generation can be connected and operated. The document is expected to have further amendments as there are some discrepancies throughout the document and a need for more clarity on some issues. There is also and update to G83 to G98 which is concerned with small scale generation.
There is a host of EU regulations involved but this is outside the scope of this short document. This document gives a high-level view of some of the main changes and how G99 will affect connections after the deadline.

Not just a relay
Many people believe that G59 is concerned only with the protection relay that stops the generation feeding back into an islanded DNO network. However, the new document runs to 362 pages (over twice the size of G59) and includes detailed information on the connection process from initial design through type testing of equipment to the performance of generating units particularly under fault conditions. The “G59” relay is now only a very small part of the connection process
Impact of the changes

Under G59 the application process for getting generation connected to the network was straight forward with an ENA generator application form being needed. After the site was energised, the G59 relay test document was signed. In G99 much more planning and information interchange is required at different times of the process and the final installation must undergo much more rigorous testing and includes a requirement to have a full commissioning plan agreed with the DNO before work commences.

The largest impact is in the way that generation modules need to help support the Transmission system when there is a fault which reduces the voltage on the DNO network. Modules must remain connected and support the grid. They must also be able to operate at a wide range of power factors and to adjust their output dynamically in relation to frequency changes to help reduce overvoltage and undervoltage.

Under G99 the generation is split into four types of generation devices; Types A,B,C and D. G99 lists these between 800W and 1MW, between 1MW and 10MW, between 10MW and 50MW and over 50MW respectively. There are also additional requirements for connection above 110kV.

The technical requirements and the tests needed for each type of generation are cumulative and type D require a lot more documentation and testing than a type B.
There are explanations in G99 which describe each type, but an essential issue to grasp is that a solar farm with 63kW inverters all connected to the grid at 33kV with a total output power of 20MW is a single C type whereas a gas peaking plant with 25x 2MW generators is treated as multiple B type generators.

Rotating plant generators have a characteristic that naturally allows them to perform better during voltage drops on the grid caused by transmission system faults. Some wind turbines and all inverter equipment do not have the same potential to support this as the power available at the time of a fault is limited to the power source, the wind or the solar radiation. Diesel and gas generators can increase their fuel supply and have a large amount of inertia which allows them to remain connected to the Grid. The risk therefore of a solar site not being able to continue to operate is higher than for a synchronous site.

What are the requirements for the plant?
Any new generation connected after 27th April that has not already been purchased and notified to the DNOP by 17th November must be G99 compliant.
An existing site that has significant changes or any expansion may, subject to DNO approval, also have to be G99 compliant.
It is not clear if all the Distributed Generation (DG) plant currently in use, such as inverters and synchronous generators, would be able to support the new requirements. Fault ride through capability, the need to operate at power factor of 0.95 lagging and leading will cause many existing generators to fail the requirements for G99. The ability to operate at 95% and 105% of voltage and to be able to dynamically adjust output with rise and fall of frequency can be onerous.

Connection process
For G59 the connection process was relatively simple. One document was issued and the DNO then made a connection offer and little further formal exchange of data took place. Under G99 the process is more complicated and is iterative. The main document will be submitted a number of times with different version numbers, each version will either detail changes to the previous data submitted or will contain more information. It will detail the results of the tests that have been carried out on the site.

This will result in additional work for the DNO and customers but it will at least mean that the energisation plan will be agreed at an early stage and there should not be any last minute surprises about the DNO requirements.

In G99 the size of the generation plant that can be type tested is unlimited. However G99 defines the requirements for type tested equipment and in summary anything that is built on site or has settings that can be changed on site or is not connected using special dedicated connections can not be type tested. The feeling by many is that for large generation sites there will be very little type tested equipment. The impact on this is that all of the performance requirements of the plant must be tested on site. Of these the easiest test to do will be what use to be known as the G59 relay injection.

The plant must be shown to provide all of the voltage, frequency power injection and response details.

Take for example a site of 10x 2MW 11kV synchronous machines. Each machine is a type B unit and must be tested independently as they are very unlikely to be type tested. Therefore the test must be carried out on site either before synchronising with the grid or shortly after it. Some tests can be carried out such as operation at leading and lagging power factors. But the performance at high and low voltages is unlikely to be possible unless he DNO change the connection voltage outside of the permitted limits. Certainly the performance at different frequencies will be impossible. Fault ride through is another issue that cannot be demonstrated on site.
It is therefore acceptable in G99 to use data supplied by the manufacturers to demonstrate compliance. This can be either tests at their works or simulation studies using time series elements to determine the power voltage and current during a system issue. Something similar is already done when carrying out P28, P29 voltage rise and fall studies on the grid in different states of outage and generation and for the G5/4 harmonic studies on the network.

Whether the generators will be able to provide this information is not yet known. It is the duty of the customer to find and supply this information to the DNO, a generator manufacturer with this information and with units that will provide compliance will be in a possibly favourable position in the market and they may increase costs.

There is also a duty to provide a full commissioning plan to the DNO. Most DNO’s are unable to specify exactly what they will require to see in the plan as it is as new to them as it is to the customers. It is the customers responsibility to inform the DNO of all of the data and the plan for testing at least 28 days before energisation.

At each stage of the connection application documents must be sent between the DNO and the customer and in some cases will need National Grid involvement. Documents must be agreed for energisation, synchronisation, export of power and final operational compliance and G99 explains these stages.

Financial Impact
There will be a financial impact to all of this. It is possible that the price of the generating units could rise as the plant requirements are technically more demanding. The data collection and form completion will take much longer and site testing will be extended by a few days. For example the operation at different points in the generator performance chart will have to be demonstrated over about a six hour period for each generator and if a site has twenty units that’s probably at least ten days that will require a commissioning engineer and a DNO witness engineer. Getting the DNO’s to find people to witness the tests will be a challenge and they are all carrying out internal training courses. Our contact with DNO’s has however been positive and we believe that they may be flexible in their approach. By demonstrating one unit passes the tests and ensuring that all 20 sets are identical they may relent and allow just one engine to be tested, or they may allow testing of all sets together. The connection form process will help to establish the needs and lead to an agreed plan. It is also likely that G99 specialist consultants will be needed to assist with the energisation planning.

DNO to Customer Interface
The requirements for interfacing under G59 were much less onerous. In G99 for example the Active Network Management systems introduced two years ago by some DNO’s are now a formal requirement and require a control loop from the DNO. In G99 there is a need for the transfer of data between the customer and the DNO. The actual requirements are sketchy and not exactly certain but data regarding the fault currents and power flows from each generating unit are needed. Exactly what the DNO is going to do with this data is again not certain or if it has to be in real time.
There is also a requirement for ongoing data exchange and the DNO can ask for tests to be repeated on plant to ensure it remains compliant.

Conclusion
The increase in distributed generation, particularly with inverter fed equipment has seen a massive change in the complexity of the Grid. The Grid is now reliant on energy contributions from customers to allow stability of operation, with a risk of cascade tripping if the generation is lost. It is therefore essential that the customers connecting to the Grid are made aware of their responsibilities. They must provide equipment that not only supplies power, that would once have come from nuclear or coal fired stations, but also to ensure that their generation is able to remain connected during the natural issues that occur in large interconnected system.

Whether the equipment available is capable of doing this will be a challenge for some manufacturers particularly in the time scales we have.

For more information on how 33kV Limited can assist you in updating to the new requirements. Contact info@33kv.com or Kevin Hann on +44 (0) 1444 226 365