Automatic System for the Control of Stability of an HVDC link - Implementation in Red Eléctrica de España (REE)

Authors: J Martin, Red Electrica de España (REE), C. de Arriba, J. Rodriguez, GE Grid Automation, and E. Leon, Siemens, Spain

HVDC Control system. Stability Function

From a grid operation point of view, the desirable operation of HVDC link consist in, based on real short circuit power (SCC) availability on Santa Ponsa 220kV Bus Bar, the control system changes the operation mode from bipolar to monopolar or disconnection on its own. As this measure of the real short circuit power is not possible; the Stability Function implemented in HVDC control system is an estimation of this quantity based in actual system topology and, also, has a prediction based on futures contingencies in electrical grid close to HVDC link.
The main factor to maintain enough SCC in Santa Ponsa is that at least one 220 kV line of the double circuit between Santa Ponsa and Valldurgent (DC1) is available and at least one 220 kV line of the double circuit between Valldurgent and Son Reus (DC2) is also available. If there was an outage of both DC1 lines or both DC2 lines the SCC in Santa Ponsa would not be sufficient to maintain the normal operation of the link. But, depending on the availability of the 66 kV grid of Mallorca Island and/or on the availability of enough generation in Ibiza Island, different control actions have to be taken.
Offline studies identified that in order to evaluate the availability of the 66 kV network in Majorca, it was sufficient to evaluate the ratio of active power flow in Valldurgent transformers and active power flow in Son Reus transformers. And, that the 8 transformers were available.
To assess the SCC supplied from the island of Ibiza, the PDC evaluates the balance of active power in the Ibiza substation, which is where synchronous generation is concentrated. Also, the system has to evaluate that Mallorca and Ibiza are connected.

Project Development and Implementation
All data collected and system behavior analysis lead to two operating conditions or steps:

  • Trip conditions, step 1:

a)  Trip/opening of DC Santa Ponsa - Valldurgent and active power balance from Ibiza less than a fixed threshold
b)  Trip/opening of DC Son Reus – Valldurgent, and active power balance from Ibiza less than a fixed threshold and ratio Son Reus – Valldurgent off certain limits

  • Monopole operation conditions, step 2:

a)  Trip/opening of DC1 Santa Ponsa - Valldurgent and active power balance (Pmin) from Ibiza bigger than a fixed threshold
b)  Trip/opening of DC2 Son Reus – Valldurgent, and ratio Son Reus – Valldurgent in certain limits
c)  Trip/opening of DC2 Son Reus - Valldurgent, and active power balance from Ibiza bigger than a fixed threshold and ratio Son Reus – Valldurgent off certain limits

The Son Reus – Valldurgent ratio is necessary to have this value between a minimum ratio and a maximum ratio (Ratiomin > RATIO > RatioMAX).
The thresholds given from studies are: Pmin = 30M; Ratiomin = 0,85;  RatioMA X= 1,2
Then, adding variables, timers for a safe operation, power restoration conditions and operational premises explained before, is developed next actuation logic.
But all of this remains in an accurate measurement of active power in several substations in Mallorca and Ibiza islands, to do so, it is necessary to reference all measures to a common time base becoming comparable over a wide area of measurement.
This data is obtained from synchrophasor which, a phasor value, is obtained from voltage or current waveforms and precisely referenced to a common time base. Simultaneous measurement sets derived from synchronized phasors provide a vastly improved method for tracking power system dynamic phenomena for improved power system monitoring, protection, operation, and control. For achieving the fastest actuation, the maximum data reporting rate specified by IEEE C37.118 in 100 frames per second is chosen.
To avoid fault conditions is clouding the actual system status, all the measurements taking for logic decision is 2s in past

BeijingSifang June 2016