Study regarding the characteristics of an adjustement equipment for active power on lines and tenssion in the National Power System Stations (NPS) – phase I and II

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Using PST (Phase Shifting Transformer) allows, independ from the power market or economical issues, the avoidance of local overloading (on certain lines) and the fulfilment of the conditions imposed by the interconnection lines (limiting the exchange to an imposed value, continuously doing a certain exchange). Using PST offers the change to do certain power circulation scheme leading to reducing loses, augmenting transport capacity, doing the desired tranzactions with international partners.

RET Developing studies for connectiong to NPS an important power in renewable energy sources (especially CEE) placed especially in the east area (Dobrogea, Moldavia) revealed the opportunity to introduce in Romania SVC both for respecting –in all kinks – the tenssion permitted range and for assuring the tenssion constancy (statically).

Services offered:

In Romania, the problem is very up-to-date as the OHTL 400 kV interconexion of the Dobrogea and Bulgaria area and the power transmitted are limited by the engineering control in Bulgaria (850 MW for the Isaccea –Dobrudja 400 kV and 900MW for the Isaccea-Varna 400 kV line).
The functioning conditions analysis in the NPS – the 2016-2021 stage- showed actually the opportunity to introduce in the NPS a minimum radioactive power compensation and also to place in the incident areas by a SVC with power between 100MVAr – 300MVAr.

Phase I pointed out the following aspects:

  • Functioning characteristics analysis for phase changer transformers (SVC)
  • Technical characteristics for phase changer transformers
  • Analysis of the existent types on the market (PST/SVC)
  • Usage exemples in the ENTSO-E (PST/SVC) systems
  • Main characteristics establishment for PST/SVC meant to RET
  • Study regarding the characteristics necessary for PST, designed for a power limit on OHTL 400 KV Medgidia South – Dobrudja, meaning Deleni – Varna.

The second phase pointed out the following aspects:

  • Interconditionality between the phase changer transformer parameters:
    • Between the major power which can circulate and the phase changers angle
    • Between the level of tenssion in the network and the transformaion report of PST
  • A permanent policy using phase changer transformers according to the installed power and that evacuated from CEE in the Dobrogea area
  • Necessary phase changer for power confinement
  • SVC choosing method
  • Analysis done for the necessity of using SVC: reducing loses by coming down the reactive power circulation and the tenssion timing in the NPS.

Technical estimated performances:

Elaborated calculation considered as network strenghtening ( interconexion lines interface in and out the Medgidia South station and building the 400 kV Deleni station), shows the necessity of the phase changer starting with an installed power in the Dobrogea area of 3645 MW, evacuated from CEE 2552 MW.

For a power value included in the spacing 2552MW and 3724 MW (according to the installed power in CEE 3645MW – 5319MW) it is necessary a phase changer transformer limiting the power evacuated on OHTL 400 KV Deleni – Dobrudja at 850 MW. The necessary angle adjustement from the preliminary calculation presented is between (0o) and (-30.07o).

All the estimations made have done an analysis of the export from Romania to Bulgaria, having as role the phase changers transformers for limiting the transited power. In these conditions the phase changer angle turned out to be negative. In case of changing the import power , taking into consideration this exchange dimension, it could be necessary only the use of PST on a positive adjustement angle, making a spheral bunch.

Taking into consideration the studies done until now, according to NPS, when the renewable energy sources appear in the 4000MW network (priority CEE and CFV), the necessity to introduce SVC only in case of incidents with an element retired from exploitation (N-1-1 functioning elements) turnes out to be compulsory. In case of an extra power installed in the East area of the NPS – more than 4000MW- appears as desirable the neccesity to prevent some SVC according to the RET development level – both to situate the tenssion in the normal limits as for mantaining the tenssion stability allowed limits.


Romania, NPS

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