by Bill Rose
Last October, an important milestone was reached when the power grids in Texas and Mexico were integrated. The Sharyland asynchronous interconnection, located on the banks of the Rio Grande River, now supports energy trading between the two countries and emergency power exchanges via its unique black start capability–the first large-scale asynchronous interconnection to support both.
The Sharyland Utilities’ high voltage direct current (HVDC) interconnection connected at a point near the border cities of Mission, Texas, and Reynosa, Tamaulipas, Mexico. The project was completed in a tight schedule of just 20 months and is considered a breakthrough success for both countries.
The $40 million HVDC back-to-back tie connects the state power grid of Texas and the national power grid of Mexico, operated by the Electric Reliability Council of Texas (ERCOT) and ComisiÃ³n Federal de Electricidad (CFE) respectively. It enables 150 MW of power to be transferred in either direction and allows each grid to support the other during peak demand and grid emergencies. It is also equipped with short-term overload capacity in excess of its continuous rating of 150 MW.
The Sharyland interconnection is an essential installation because even though the American and Mexican power networks have the same fundamental power frequencies (60 Hz), they operate independently and thus could have different frequency and voltage variations. An AC link could create severe operational difficulties, such as uncontrolled power flow and power swings leading to grid collapse. The power moving in either direction must be synchronized with the power network it is moving into, which is what Sharyland’s power converters do.
As an open-access tie, Sharyland allows CFE and any energy provider in ERCOT to use the facility for commercial energy transactions between the two countries.
Unique black start capability
The HVDC solution includes a unique “black start” emergency assistance capability that provides a safe supply of power during a blackout in either AC grid. This is an important reliability-enhancing feature with which normal operations can be suspended and a safe flow of power provided to help restore affected areas.
This is the world’s first application using a conventional back-to-back HVDC interconnection and an AC bypass arrangement to provide black start capability. The bypass arrangement facilitated extremely smooth conduction of commissioning tests in power circulation mode with only the ERCOT grid.
A firewall to prevent propagation of disturbances
The Sharyland HVDC solution also acts as a “firewall” that isolates disturbances and prevents them from spreading from one grid to the other.
Aerial view of Sharyland HVDC back-to-back station built by ABB on the Texas-Mexico border. Photo, ABB. Click here to enlarge image
Major blackouts in recent years have shown how relatively minor malfunctions in interconnected grids can have repercussions over wide areas. As one link overloads, it’s tripped, increasing the strain on neighboring links that in turn disconnect, cascading blackouts over vast areas.
The solution is a “firewall,” permitting the interchange of power but preventing the spread of disturbances. This is readily accomplished using an HVDC link since it can fully control transmission but does not overload or propagate fault currents.
HVDC helps during contingencies
One reason a fault condition spreads to a wide area is that AC transmission links become overloaded. This leads to their disconnection, which in turn overloads other lines.
The Sharyland HVDC transmission tie is engineered to take specific remedial actions in case of a disturbance. These actions are often smooth and continuous–in contrast to the hard switching of AC links. The most important feature of HVDC is that it can never become overloaded.
If there is a sudden outage of generation in the ERCOT or CFE networks that leads to an abnormal frequency and/or voltage, the Sharyland tie can automatically adapt its power flow to support the troubled grid. The power flow is limited so it won’t jeopardize the integrity of the sending network.
One of the major advantages of HVDC transmission is its controllability. The basic power control is achieved through a system where one of the converters controls its DC voltage and the other converter controls the current through the DC circuit.
Sharyland HVDC Benefits
- Energy trading — open to any power supplier in ERCOT and CFE
- Helps stabilize both grids through active and reactive power variation
- Emergency support–immediate transfer of power in the event of supply/demand imbalances
- Black start capability — safe supply of power during blackouts
- Firewall function — prevents disturbances from spreading between grids
- Unmanned operation — designed to operate remotely from a dispatch center on the ERCOT grid
Bill Rose is manager of corporate communications for ABB Power Products and Power Systems divisions, based in Raleigh, NC. Bill can be reached at firstname.lastname@example.org.