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

The introduction of an on-line short circuit estimation process that automatically enable the operation mode (bipolar or monopolar of the HVDC link, or total disconnection, as the one developed in ASPAS), allows to reduce the operating margins maintaining the same reliability level and so optimize the operation, allowing substantial increase of the energy transferred through the link, with the corresponding economic benefits to REE.
ASPAS also facilitates companies in migrating to a more digital model based fundamentally on three pillars: Communications, Big Data and Artificial Intelligence. The right combination of these three pillars will optimize the operation of the electrical system, reducing costs and polluting emissions, accelerating the adoption of non-polluting energies and renewable energies, while maximizing economic performance and a closer way of interacting with users.

The Problem
The Spanish transmission power system owned by Red Eléctrica de España (REE) includes a submarine high voltage direct current (HVDC) link laid in the Mediterranean Sea that connects the Mallorca island (Santa Ponsa substation) with the Spanish mainland (Morverdre Substation).
This link is a Line Commutated Converter (LCC) in bipolar configuration with return conductor with a capacity of 2x200MW equipped with optical signal activated thyristors at ±250kV and six filters 2x100 + 4x41MVAr.
Figure 2 shows where HVDC connects to AC island grid. Normally, the energy flows from Spanish mainland to Mallorca island, being fundamental the total amount of short circuit power in Santa Ponsa substation for a correct HVDC operation.
Values of short circuit power in Santa Ponsa from 1300 to 500 MVA require changing the operational topology and the operation mode of the HVDC from a bipolar configuration to a monopolar one, which also implies a reduction in the flow of Active Power. Values lower than 500 MVA require a complete disconnection of the HVDC link, to avoid commutation failures or dangerous oscillatory behavior that could seriously affect the operation of the entire grid.

In Figure 3 we observe a current oscillation due to a trip on Santa Ponsa – Valldurgent circuit (DC1 on Figure 2) due to this trip the HVDC link is working only with 220/66kV transformers in Santa Ponsa, the ones in service condition after trip, this grid situation produces a current oscillation (power) unacceptable for a safe grid operation.
Figure 3 shows the oscillation from one phase of one HVDC pole, event that provoke the trip of the link by subsynchronous resonance protection. This oscillation is harmful for electrical system, not only for the affection to generation and loads on island due to grid voltages oscillation, also, for overvoltage produced in close trip instants of HVDC link due to filters operation to low short circuit power currently in grid.

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

Solution Proposed

A redundant system developed for two different manufacturers has been chosen to send to HVDC the command to monopole operation or disconnect the link, both systems must agree on disconnection and only one of them can command to monopole operation.

Manufacturer A: The solution implemented is based on synchrophasor technology, IEC61850 communications for commands, Phasor Data Concentrator (PDC) using GE_Power software Package which it has embedded the protocols IEC61850 Edition 2 Client/Server and IEEE C37.118 collecting raw data from the IEDs, a powerful Soft PLC processing the data and calculating the remedial actions sending the results in fast GOOSE messages to IEDs, and an operator HMI to visualize system condition and change the threshold levels without system disruption. The system architecture implemented is as seen in Figure 4, and its deployment can be observed in Figure 1.
In each substation is installed as Phasor Measuring Units (PMU) the device GE UR N60 to provide a synchrophasor information with analog data (voltage and current) and digital data (switchgear position), anomaly signals and bay alarms, and as IEEE C37.118 establishes, all synchronized through IEEE 1588 PTP provided by GE REASON RT430 clocks using as reference GPS and GLONASS constellations.
In the HVDC substation is installed a PDC where the information is processed, the calculation run, and finally a decision is taken and transmitted through IEC61850 GOOSE message to a GE UR N60 placed in this substation (PNSEC), which actuates to HVDC control system.  Also, this PDC repacks all synchrophasor information, changing its sample rate and sending to REE phasor control center in Madrid (Spain) in different synchrophasor frames. All the operation data, system status, alarms, events and trends can be seen in HMI placed in the same substation.

All the communication is in a dedicated LAN on each substation and interconnected to others with a dedicated SDH network with a maximum bandwith of 10Mbps creating a dedicated and redundant ASPAS grid for each manufacturer.
For a better maintenance purposes, and as a GOOSE communication is populated over the ASPAS grid, all GE UR N60 has signal systems on its front display leds. (Figures 1 and 5).

Manufacturer B:  The solution implemented is based on PMU (Phasor Measurement Unit), PDC (Phasor Data Concentrator) and PDP (Phasor Data Processor) technology, using the standard transmission protocol IEEE C37.118 to transmit the phasor values of voltages, phasor values of currents and binary signals from the PMU level, collect them in the PDC level, and process and use them in the PDP level with the required logics and application, having a powerful WAM (Wide Area Monitoring) and WAC (Wide Area Control) application with its inherent HMI to monitor the whole system. Furthermore, the solution uses control units as the interface between the PDP and the HVDC control system, using the standard protocol IEC 60870-5-104 to finally execute the required control commands (open commands, indications and alarms) in HVDC. 
All the collected information from IEDs, and the processed signals on the PDC are displayed on the local HMI to show the application in a friendlier and customized view.
The system architecture implemented is as seen in Figure 6.
As shown in the system architecture, an independent PMU is installed in each measurement point location, having used one of the devices of SIPROTEC 5 platform to measure the voltage and current values, collect the topology of the bays where the PMU are installed, and transmit the phasor values of voltage and currents as synchrophasors with the transmission protocol IEEE C37.118, as well as the binary information related to the topology of the bays, status of circuit breakers, and relevant alarms. The use of this platform as PMU allowed using only one IED for one or several PMU functions, instead of using as many IEDs as PMU functions were needed.

The PMUs are locally time synchronized with IRIG-B (IRIG-B 005(004) with extension according to IEEE C37.118) to get the PMU data streams time synchronized in the PDC.
In the upper level of this application, close to the HVDC control system, a PDC-PDP system is installed to collect, process and use the PMU values and carry out the required WAM-WAC application, having used SIGUARD PDP for such application.
Digital communication networks are used to establish the communication link between the PMU and PDC-PDP system, having used a dedicated SDH communication link for each PMU.
As an end user interface, a local HMI is built to get a global view of the WAM-WAC application, as well as its status, alarms and the sequence of events of the system, having used SICAM PAS and SCC for such customized HMI.
The standard protocol IEC 60870-5-104 is used to establish communication between the WAM-WAC system and the SICAM PAS, and then all the information is displayed on SICAM SCC.
Therefore, the whole system is based, as said before, in different PMU measurement point locations, a central PCP-PDC system with its inherent HMI to evaluate the system, three control units to transfer the control commands to the HVDC control system, and a customized HMI. (Figure 7).

Factory and Site Acceptance Tests

A complete truth table checking was done for each system, with satisfactory results on both systems (Figure 8).
Several tests have been carried out to simulate every event and every power level affecting the HVDC stability, checking at the same time the response time of the complete system is below the specified operation time. Time obtained was in the range of 30 to 40 ms.
Site tests were carried out by synchronizing secondary injection sets in 4 substations by means of using IEEE1588 PTP synchronization protocol available in one ASPAS network.
In this way were more than 450 automated tests were carried out, with all possible conditions and combinations, to both redundant systems, all with satisfactory results giving an average actuation time of 70ms (Figure 9).

Solution Analysis
After the successful commissioning of the ASPAS System we can highlight the following:
Benefits produced by the ASPAS System in the Balearic Electric System:  
Economic: it is not needed to increase the generation in the Balearic system (greater cost than the peninsular one) due to the limitation of the HVDC link.
Environmental: prevents conventional generation in Balearic system to cover the limitations imposed by the HVDC, which contributes to a more sustainable Balearic energy mix.

Technical: after contingencies, prevents the HVDC link can be kept operating with low short-circuit power, which can cause damage to the converter station and the transmission network close to it.
Improvements added during the normal operation of the link that the ASPAS system: It increases the security in the operation of the link, by discriminating several cases with potential risk in the stability of the system, which the old stability function would have erroneously evaluated. It facilitates the operation of the system, by sending all the on-line status of the system from the PDC of Santa Ponsa to the CECOIB, which helps the operators to anticipate the performance of the HVDC link on critical contingencies and allows to take actions before the event occurs. It improved the knowledge of the protection and control system of the HVDC link.
The unavailability of the system and unscheduled interventions are reduced, along with the facilitation of the execution of maintenance activities.

Conclusions:  Electrical islanded AC systems which may be prone to be weak, where a main-land links is delivering power, needs to optimize the energy imported which is cleaner and cheaper compared with the energy self-generated in the Island.
This optimization is based on solutions that allow optimizing the possibilities offered by the insular network. It will be crucial to be able to quickly modify the operating conditions, and the economic benefits. The ASPAS System is a new development using synchrophasors in the operation of an electrical system, facilitating the decision making in emergency situations, where the speed of action can be a critical factor to maintain the grid integrity. The new application achieved with the ASPAS System based on phasor technology opens a large field of research and development to provide the system with the appropriate tools for the energy transition. We evolved from a network based on conventional generation with synchronous machines, to a network based on renewable generation dominated by power electronics.



Javier Martín Herrera received his bachelor Electrical Engineering degree from the University of Madrid (Carlos III) and joined RED ELECTRICA in 2003. He works as a "protection man" always related to the protection's coordination of high voltage on the Spanish power system. He has successfully completed many R&D projects and participates in CIGRE B4 / B5 working groups. Additionally, Javier is an electrical engineering professor at the Universidad Pontificia Comillas-ICAI of Madrid, where he is responsible for electrical protection issues for 12 years.

Camilo de Arriba received the bachelor's in electrical engineering degree from the UPV in Bilbao in 1994 and joined GE in 1996. He has worked in different areas. Now he is Senior Product Manager for Advanced Automation Applications. He has great experience as Project and Technical Manager leading the execution of many challenging Projects worldwide. He is PMP certified and holds an MBA master's degree. He was one of the Authors of the paper generated by the CIGRE WG B5.42.

Joaquin Rodriguez Andres received his bachelor's Degree in Engineering from Universitat Politècnica de València, Spain in 2001, and obtained his master's degree in 2005, while working. Since then, he has continued working in companies which are involved in electrical substation construction and commissioning. In 2008 he started working on digital substations, participating also in several pilot projects with this technology. In 2011 he joined General Electric as a protection engineer and he is currently working as product manager in the advanced automation applications department and cooperating with CIGRE B5 group.

Esteban León Quiros works on Substation Automation and Digital Grid at Siemens Spain.  Esteban has 15 years of experience on design, configuration and commissioning of Substation Automation Systems.  He has developed projects in several countries in North, Central and South America, Europe, Africa and Asia.  Currently living and working on Madrid with his wife, and with their first son (on the way), Esteban moved to Spain in 2013, from his native country, Costa Rica.