For Type C / D Generation sites in the UK, one of the most important studies to carry out in the early stages of a project is a reactive power capability assessment. This study is a specific type of load flow case, used, to confirm that the equipment is correctly sized, and the site can deliver the amount of active power (MW) and reactive power (MVAr) at the system entry point. Unfortunately many clients have problems at this point as they find there is a shortfall of reactive power (MVAr), and they must constrain their active power (MW) output to remain compliant.
The basic requirement for reactive power flow capability of a generating site, is the ability to produce reactive power when the host DNO / TSO system voltage is low, and to absorb reactive power when the host DNO / TSO system is high. Within the UK the requirement is to be able to operate at 0.95pf lag to 0.95pf lead, at the Connection Point, across a range of different DNO / TSO system voltages.
To demonstrate compliance there are a series of loadflow studies that must be carried out, at different loading, powerfactor and voltage. In many software packages that can be automated with a script. The standard shows two main sets of requirements: A VQ (Voltage-Reactive Power) diagram that defines the reactive capability when the site is operating at maximum power in relation to the DNO / TSO system voltage. A PQ (Active Power – Reactive Power) diagram that defines the reactive power capability across a range of different site power outputs. The VQ diagram is intended to show compliance at full power for different network voltages and the PQ diagram is to show compliance at intermediate loading levels.
Many companies get themselves into some difficult as they wrongly equate Registered Capacity with the amount of installed inverter capacity, forgetting that kW and kVA are not the same and that the site will have both active power losses, but also some significant reactive power consumption. Problems can then arise if the site cannot produce enough reactive power when operating at the required active power. This may need to inclusion of additional inverters, or MVAr compensation equipment.
Where things are made more complicated, is that the DNO connection agreements are specified at a given operating power factor (like 0.98 lag), so the need for reactive power capability across an active power range and voltage range can be confusing. The key thing to remember here is that the DNO can change the specified operating PF, so although it may be 0.98 lag now, they could change at a future date to 0.95, and the shortfall could be a problem. It is also possible that the DNO could request the site to operate in voltage control mode and thus the site RC would have to be restricted as it would be unable to meet the MVAr obligation.