The consequences of the war in former Yugoslavia are still being remedied. In October the South-East European electricity markets were physically re-integrated with the markets of the UCTE zone. Throughout and after the resynchronization, wide area monitoring was used to monitor system stability.
Petra Reinhardt and Cadric Carnal, ABB and Walter Sattinger, ETRANS, Switzerland
Transmission system operators in Europe and all around the globe are being squeezed between two forces. On one hand, increasing demand, energy trading and economic pressures are pushing them to maximize the utilization of transmission assets. On the other hand, reliability concerns are forcing them to be more vigilant about just how far they push the grid’s infrastructure.
This situation presents a choice: build more lines, or do more with existing ones. The cost, time and siting issues associated with adding transmission capacity means it is seldom a viable solution. Instead, utilities are focusing on making better use of their existing systems.
Figure 1. 1st & 2nd UCTE synchronous zones, with the five reconnected 380 kV lines
With the recent advances in measurement, communications and analytic technologies, wide area monitoring systems (WAMS) have come to the fore as a means to address not just immediate reliability concerns but also operational issues and long-term system planning.
Critical busbars in transmission grids are usually monitored using static or quasi-dynamic data. Based on cyclical acquisition of non-synchronized RMS measurements by RTUs, traditional SCADA/EMS systems can provide only a limited picture of dynamic grid conditions. However, fast delivery of accurate snapshots can make a world of difference in maintaining system integrity under critical conditions.
A unique event in October 2004 saw WAMS used to support the reconnection of southeast Europe to the Union for the Co-ordination of Transmission of Electricity (UCTE), which coordinates the interests of transmission system operators (TSOs) in 23 European countries.
Resynchronizing the two UCTE zones that were split in 1991 after the war in the former Yugoslavia has been one of UCTE’s highest priorities, but preparations could not start until the beginning of this decade. In the first instance, the infrastructure had to be reconstructed, and 2003 finally saw the completion of major restoration and construction works on key transmission infrastructure, such as the Ernestinovo and Zerjavinec substations, managed by HEP (Hrvatska Elektroprivreda). The so-called Adriatic line between Mostar and Gacko and other important lines were recommissioned in Bosnia-Herzegovina by the Joint Power Control Centre (JPCC) in August 2004.
Meanwhile, since 2002 all the region’s TSOs have been coordinating measures to reconnect the UCTE synchronous zones into a single system. A UCTE Executive Team for Reconnection (ET), made the detailed plans and obtained members’ approvals for the resynchronization programme.
As a prerequisite, all previously agreed mandatory technical and organizational conditions for reconnection had to be completed by 22 September 2004 (including grid infrastructure, readiness of control blocks, measures to preserve steady-state and dynamic stability of the whole reconnected system and on-line transfer of data).
On 23 September a phase re-arrangement was successfully performed in the Croatian system.
The UCTE Steering Committee met in Sarajevo on 23 September and agreed to start the resynchronization process on 10 October and to coordinate the operations from Zagreb, Croatia.
During the reconnection, the new release of ABB’s PSGuard wide area monitoring system, installed both at HEP in Croatia and ETRANS in Switzerland, was required to provide reliable and powerful on-line wide area monitoring of critical corridors and give operators enhanced observability of the power system. PSGuard was used to record both the UCTE-1 and UCTE-2 frequencies as well as the differences in the voltage phase angles during the resynchronization process, when UCTE’S second zone, comprising most of the Balkan countries, Romania, Bulgaria and Greece and representing a load of 21 GW, was re-integrated with the remaining UCTE area.
PSGuard continuously records the dynamics of power networks with a time resolution down to 20 ms and a high time and angular accuracy (< 1 à‚µs, < 0.1à‚º) over wide areas. This accurate and on-line information allows the operator to load transmission corridors taking into account the security limits, as computed by PSGuard, and to be continuously advised of power margins to safely determine appropriate remedial actions under critical conditions. All of the data can be exported for off-line analysis and grid modeling enhancement.
As the UCTE South coordinator, ETRANS, in Switzerland, made an important contribution to the resynchronization. It also provided high-value on-line measurements of frequencies and voltage phase angles in Switzerland and in Greece, as well as the computed phase angle differences, and thus monitored the power system dynamics permanently.
Figure 2. UCTE-1 and UCTE-2 voltage phase angle difference and frequencies during the historical resynchronization process recorded by PSGuard
Thanks to daily follow-up from the Greek TSO and support from ETRANS’ communication department, a week before the reconnection one PMU in Greece was connected to the PSGuard system in ETRANS’s Network Control Center in Laufenburg. It provided important data. Like ETRANS, Croatia’s HEP, also used PSGuard, to provide on-line grid data from its Zerjavinec and Tumbri substations.
At 9:34 am (CET) on 10 October the second UCTE synchronous zone was finally re-integrated with the first one. First, the Arad-Sandorfalva line was reconnected. During the following 45 minutes, the other four 380 kV lines across the former interface between the first and second zones were taken into service (Sandorfalva-Subotica, Trebinje-Podgorica, Mukacevo-Rosiori and Ernestinovo-Mladost). With the closing of two 220 kV lines and one 110 kV line between Bosnia-Herzegovina and Serbia-Montenegro, the resynchronization was successfully completed by 10:58 am (CET).
The reunited UCTE transmission network now stretches across most of continental Europe, from Denmark to Greece, from Portugal to Poland and the shores of the Black Sea. The positive impact of the resynchronization will not only be felt in the countries previously in UCTE’s second zone (Albania, Bulgaria, Greece, the former Republic of Macedonia, Romania, Serbia and Montenegro), but will also physically integrate the regional markets of South-Eastern Europe with the markets of the UCTE zone going beyond EU frontiers.
WAMS in operation
Managing the reconnection of Southeast Europe was made easier because TSOs in Croatia and Switzerland were already using WAMS to help operate their networks.
The system acquires current, voltage and frequency phasor measurements collected by phasor measurement units (PMUs), from selected locations in the power system. The measured quantities include both magnitudes and phase angles, and are time-synchronized via global positioning system (GPS) receivers to a time resolution of one microsecond.
A Phasor Measurement Unit was installed in Greece in addition to those already placed in Switzerland and Croatia. These were all used throughout and after the resynchronization to monitor system stability during and after the reconnection process.
The availability of highly accurate synchronized phasor measurements has enabled a new level of monitoring capability. By comparing local phasor measurements, with the support of advanced applications, operators can observe not only the steady state, but also the dynamic state of critical busbars in their transmission and sub-transmission networks. This improvement facilitates better and faster analyses of grid conditions, which in turn provides operators with more time and options to preserve system stability.
Figure 3. Power plant outage in Greece (650 MW) recorded by PSGuard
It also improves the quality of data on which transmission planning and everyday operational decisions are based. Utility requirements such as intelligent load shedding, protecting the system against major disturbances, and counteracting cascaded line tripping can be broken down to physical phenomena that can be mitigated by a management scheme making use of information from WAMS.
Examples include angle, frequency or voltage instability and cascading outages.
To address each of these phenomena, a reliable system has to be designed with respect to input variables, decision criteria and output actions. PMUs allow for direct and fast angle measurement, instead of indirect power measurement, and enable the development of more accurate algorithms for emergency control or protective actions such as:
- phase angle monitoring with an early warning and emergency alarm
- line thermal monitoring with an early warning and emergency alarm
- voltage stability monitoring for transmission corridors and entire networks with indication of margin to stability limit
- power oscillation monitoring with amplitude and damping of the most dominant oscillation mode, warnings and alarms
- frequency stability monitoring with amplitude and damping of the most dominant frequency, warnings and alarms.
The online information on calculated stability and safety margins facilitates optimal utilization of the transmission capacity and helps to prevent wide area disturbances. This makes the system a viable choice for utilities required to run their power system in the most economic way whilst maintaining the desired levels of security.
The system further facilitates long-term system planning by providing data storage and export for off-line analysis and grid model optimization, as well as event-driven data archiving for subsequent analysis.
Future developments include wide area control and protection, where counter-measures are recommended and can even be initiated automatically e.g. by using Flexible AC Transmission System (FACTS). This offers a further possibility to optimize operations. WAMS also have the potential to benchmark and backup essential system stability calculations, eg in a state estimator.