Anyone wishing to generate electricity for connection to the grid at 11KV/33KV should undertake a formal design based on the connection proposal, then a power quality study. The Distribution Network Operator (DNO) has restrictions and regulations that must be adhered to at the planning stage of any such project is of paramount importance prior to an approved Connection Agreement being awarded.

Here at MPN, our engineers can undertake this process on your behalf and conduct a full study into your proposals and even a redesign if required or beneficial. The following factors are briefly explained and taken into account:

1. G5/4 Assessment: This is required to evaluate the level of harmonic voltage distortion against the planning levels for the connection of non-linear equipment. Examples of these are types of load that draw non-linear current when excited by a sinusoidal voltage, such as a generator. Synchronous machines are not classed as such. A G5/4 assessment is not required unless a source of non-linear current is identified. Items such as 6 or 12 pulse drives and switch mode power supplies will provide a non-linear contribution through system auxiliaries. Synchronous and induction machines may contribute to the fault level as harmonics sink and rise.

A Stage 3 assessment may be required for a 33KV system due to the non – linear properties of the impedance at the DNO point of connection. An 11 KV system will need a Stage 2 assessment as the DNO point of connection can be considered as linear, as at this point the impedance increases in proportion to the frequency.

 

2. P28 Assessment: This is a process for analysing voltage fluctuation which occurs as loads are connected to the system causing voltages to dip or flicker and limits are applied to these events. Demonstration of compliance is required under a condition of connections offer. Short Term Operating Reserve (STOR) types of generation have the largest effect on these events such as full load rejection and energisation of the main connecting transformer at 33KV and the auxiliary transformer for connection at 11KV. It is recommended that a transient simulation is included to illustrate the effect that the start-up of the STOR has on the DNO connection of load ramping. A transient fault infeed study will provide the DNO with the fault contribution from the plant for making and breaking duties of its switchgear.

 

3. Voltage Regulation and PF Envelope Review: Although not a condition of connections, a review of the operational constraints of the STOR is recommended. Its objective is to examine the operational range of the plant at the point of connection and to determine whether the operational requirements in the Connection Offer can be achieved under all operating conditions as per the generator capability. (Fig 1)

Fig 1 – Generator Capability Chart.

 

4. Transient Stability Studies: Depending on DNO requirements, additional design works are required. Western Power Distribution (WPD) has confirmed a requirement to carry out a transient stability study when connecting a synchronous generation scheme to the distribution network.  This  is  a  new  requirement  and  is  being requested due to the view that recent changes (mainly related to the extent of embedded generation) exposes the distribution network to the possibility of much larger disturbance load swings The concern is that a synchronous machine could introduce instability into the network for identifiable fault conditions before being cleared  by  protection.

This scheme will examine the behaviour of the synchronous machines under system disturbances agreed with the DNO for clearance times provided by the DNO.  It has been indicated that three scenarios on average would be a reasonable expectation for these studies.  In order to complete a stability study it is necessary to model the dynamic behaviour of the prime mover and its governor, the generator, AVR and associated exciter. We will develop block diagram representations of these devices using the ERACS Universal Dynamic Modeller (UDM) and test the machines on simplified networks introducing step changes and faults to confirm the stability and dynamic performance of the model is a reasonable representation of the real machine.  These models will then be used to simulate the dynamic performance of the machines  on the target DNO  network simulating the faults  and behaviour  of  other  plant in  that  network  in a manner agreed with the DNO. (Fig 2)

Fig 2 – 3 Phase Fault Simulation

Fig 3 – Voltages and Tap Positions for 20MW Export at the PCC

 

Scope of Works for Customers Generation Site (Example)

1. Construct an ERACS model of the STOR system and the DNO network. The DNO network will be represented by an IPSA+ model for the WPD Midlands sites, and a PSS/e model for WPD South Wales. Where this is an 11kV site and only a Stage 2 assessment is required this will not require a representation of the DNO network.
2. Run a load flow to verify the mod
3. Carry out Stage 2 assessment for the 11kV site, and multiple G5/4 assessments for the various agreed configuration and load cases for the Stage 3 model.
4. Calculate the transfer gains between the pre-connection and post connection cases.
5. Produce the appropriate G5/4 Stage 2 or Stage 3 report.
6. Carry out an inrush calculation of the largest connected transformers for each of the sites.
7. Investigate the voltage change at the DNO connection for a starting ram
8. Calculate the voltage change for a full load rejection.
9. Produce a P28 assessment report.
10. Produce an overall summary of the site capability and any mitigating factors that may be addressed.

 

Information Required from Customer

1. Single line diagrams of the STOR schemes (Fig 4).
2. Power cable details: cable type (data sheet), dimensions and leng
3. Transformer ratings and inrush d
4. DNO connection offer
5. Background distortion data from DNO including allowance for installatio
6. DNO fault level details at the Point of Common Coupling (PCC).
7. DNO model informatio
8. Generator data sheets including saturated and unsaturated reactance and resistance values and associated time cons
9. Generator open and short circuit characteristi
10. Prime mover and generator individual and combined inertia constants or moments of iner

 

MPN will be pleased to offer advice and guidance on any of your power generation requirements.

 

Fig 4 – Example Load Flow /SLD

 

Other Considerations?

Not only do we have to consider the vast array of electrical design requirements and regulations but we also offer our assistance to study the layout of the buildings and the safety of those who operate or work near High Voltage electrical switchgear.

MPN can arrange to survey, design and install your earthing and provide advice and guidance on access to and around switchgear, taking into account manufacturers data. (Fig 5)

 

Fig 5 – Manufacturers Recommendations.

 

MPN will be pleased to offer advice and guidance on any of your Power Generation projects.