The Association of Manufacturers and suppliers of Power Systems and ancillary equipment


What needs to be considered when nominating an ac generator for Grid Code compliance applications?

Presently (January 2017) AMPS does not have a released document which offers straightforward advice regarding the nomination of either an ac generator, or indeed a complete generating set for a grid connected embedded generation application, often described as Distributed Generation equipment (DGs).

However, the AMPS Technical Committee has a number of sub-groups comprised of technical specialists assigned to Working Groups (WG) with each WG having a specific technical area for which they must gain and maintain the subject specific specialist knowledge, become active members of the relevant Codes & Standards bodies; thereby have some influence over the contents of regulations, requirements, and recommendation documents which are likely to affect power generation equipment destined for grid connected applications.

Responsibility for matter concerning embedded generators the AMPS working group is WG2. This group will be providing a brief report on their most recent activities and updates related to the development of Network Codes, at each AMPS TC meeting.

Now to considering specific points raised within the question by beginning with Grid Codes.

The UK Grid Code has been around for many years, last updated in 2012. Its influence over the selection of embedded generators has always been implied. The directness appears when considering the connection of a generator to UK grid/network. Now there becomes a clear need to comply with the rules and regulations being applied by the local Distribution Network Operator (DNO) responsible for the network at the point (Point of Common Coupling: PCC) to where the proposed generator will be connected.

The latest UK requirements are G59/3.

Now to the European scene and the changes being made in the EU parliament to ready the European grids for the very near future where the power transmitted (Transmission System Operators:- TSO) and distributed (Distribution System Operators:- DSO) by these system operators with the responsibility for network sectors which will be 'carrying power' from a multitude of low inertia generation systems (DGs) based on various topologies, but most likely dominated by renewable technologies ; wind, solar, wave, tidal, .....etc?

Todays experienced power system engineers are raising concerns about the national electrical systems’ operating stability with regards to the variance band-widths of network levels of V & Hz along with the security of maintaining continuous supply to all consumers. The concerns are aimed at the inevitable adverse effect of reliance on a multiple source of different power generation schemes dominating the power generation source with 'small' therefore low inertia generators.

This realisation of 'poor stability and security of supply' was the initiator for the creation of the EU Legislative family of Network Codes (NC) with subsets of ’Requirements’. The one most directly concerning power generation equipment design and performance needs is: Requirement for Generators (RfG).

The numerous technical performance requirements stipulated within RfG are yet to be rolled out to the EU Member States (MS). It is how each MS will adopt and enforce the requirements which are now of utmost interest to those engaged in the world of power generation equipment as either equipment manufacturers or suppliers and installers.

WG2 is closely watching both mainland European Member State (MS) reactions and comments regarding RfG implementation. WG2 is involved with various assemblies within the GB National Grid region and participating in short term and long term strategies for policy making processes.

There are some big decisions to be made by a number of appointed bodies. It is hoped that consultation and feedback will offer opportunities to influence, as well as provide an awareness of directions and so allow AMPS members a degree of product planning.

In the future, an embedded generator with an engine (or turbine) as the prime mover driving an ac generator will be expected to operate and contribute in a manner by which it will be able to assist with network voltage regulation Page 2 of 2 using ac generator excitation control and to assist with network frequency control (Hz) using prime mover power control and so electrical power contribution to the grid.

The question started with an inference of a simple answer regarding an application related selection of an ac generator. In accordance with that expectation the following aims to provide a simple answer.

In this example the application is Combined Heat and Power (CHP). There will be a contractual level of heat contribution, accepting this will be achieved from an electrical power contribution to the local grid, therefore an embedded generator, but likely to be subject to CHP related variances to the grid connection Codes, Standards and Requirements.

A by-product of this electrical contribution (load) to the grid, the generating package will need to be selected on a either a local DNO stated contractual requirement, or experienced based decisions for the selection of the ac generator to ensure capability to operate over:

  • Voltage range in the order of +/- 10%
  • Frequency range (Hz) of +/- 2%
  • Power factor which may extend to as far as 0.9 lead through to 0.85 lag (required by the DNO to assist with local network voltage regulation)

All the above conditions will apply whilst delivering the required heat and so kWe (P) over the specified voltage and Hz range

It seems that NC RfG may have an exception clause whereby a CHP equipment package will not be required to have Low Voltage Ride Though (LVRT) capability.

Within the UK the embedded generation equipment package will need to comply with ER G59/2 today; but maybe /3 will be applied, in terms of protection, and most significantly avoid nuisance tripping and the propagation and support of a network ‘island’; see FAQ 1005 for more information regarding Loss of Mains (LoM) detection.

The content of the prevailing version of G59 becomes academic; there will be a rule set to be observed for embedded generator protection equipment and settings. For certain the networks of the near future will operate with levels of instability which include Rates of Change of Frequency which have introduced the need for G59/3 with its much revised RoCoF settings.

Presently (January 2017) G59/3 has introduced a development period where DNOs and the likes of AMPS are discussing and validating RoCoF settings which have their tightest settings at 0.5Hz/s with the widest setting limit being considered at 1Hz/s.

The risk is that such levels of grid instability will impose severe stress levels on a typical synchronous generator's mechanical drive line and furthermore seriously challenge such a generators electro-magnetic design to remain in synchronism, i.e.: not pole slip.

A collection of technical aspects have been covered in the above explanation. Hopefully this provides an understanding of both AMPS activity and TC member’s specialist technical knowledge which is focussed on debating with other technical groups the complex considerations needed for compliance with the developing requirements for embedded generators.

FAQ 1012

 

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