Rebati Dass, ABB Power Systems, Sweden
In October 2007 an historic step towards the integration of the Texan and Mexican power grids was completed with the official opening of Sharyland Utilities’ high voltage direct current (HVDC) interconnection located along the Rio Grande border, connecting at a point near the cities of Mission, Texas, and Reynosa, Tamaulipas.
Based on ABB’s HVDC Classic thyristor-based technology, Sharyland is the first large-scale asynchronous interconnection to support both emergency power exchange and commercial energy trading between the United States and Mexico and the growing economies of the Rio Grande Valley. The contract was awarded to ABB in the last quarter of 2005 and the project was completed within 20 months.
The $40 million HVDC back-to-back tie connects the state power grid of Texas and the national power grid of Mexico, operated by Electric Reliability Council of Texas (ERCOT) and Comisiàƒ³n 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.
Sharyland is an essential installation, because even though the USA and Mexican power networks have the same fundamental power frequencies (i.e. 60 Hz), they operate independently and therefore 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. Furthermore, as an open-access tie, Sharyland allows CFE and ERCOT to use the facility for commercial energy transactions between the two countries.
Unique black start capability
With strengthened grid reliability a priority for the customer, Sharyland Utilities, ABB designed an HVDC solution that 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 in 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.
The Sharyland HVDC solution also acts as a ‘firewall’ that isolates disturbances and prevents them spreading from one grid to the other. 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 is tripped, increasing the strain on neighboring links which in turn disconnect, cascading black-outs over vast areas and causing huge productivity losses for the economy.
The solution is a firewall permitting the interchange of power, but preventing the spread of disturbances. This is readily accomplished using an HVDC link because it can fully control transmission but does not overload or propagate fault currents.
When a temporary fault occurs in the AC system connected to the rectifier (AC to DC), the HVDC transmission may suffer a power loss. Even in the case of close single-phase faults, the link may transmit up to 30 per cent of the pre-fault power. As soon as the fault is cleared, power is restored to the pre-fault value. When a temporary fault occurs in the AC system connected to the inverter (DC to AC), a commutation failure can occur interrupting power flow. Power is restored as soon as the fault is cleared. A distant fault with little effect on the converter station voltage (less than around 10 per cent) does not normally lead to a commutation failure. A capacitor commutated converter (CCC) HVDC converter can tolerate about twice this voltage drop before there is a risk of commutation failure.
Another advantage of HVDC transmission is that it does not contribute to the fault current à‚— the impact on the fault-free side of the DC transmission is smaller, and on the side with the fault, the fault current is lower than it would be with an AC link. The fault-free network experiences an interruption of power flow in the DC transmission but no fault current.
Transmission adaptability and controlability
The main reason why a fault condition spreads to a wide area is often that AC transmission links become overloaded. This leads to their disconnection which in turn overloads other lines and so on.
The Sharyland HVDC transmission tie is engineered to take specific remedial actions in case of a disturbance. Furthermore, such actions are often smooth and continuous, which is 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, leading 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 as not to jeopardize the integrity of the sending network.
A major advantage 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. The control system acts through firing angle adjustments of the thyristor valves and through tap changer adjustments on the converter transformers. Each pole in a bipolar ABB HVDC link has its own control system and each control system is duplicated.
The Sharyland tie is equipped with ABB’s MACH 2 fully digital control and protection system, which is designed specifically for converters in power applications. MACH 2 is the highest performance and most used control and protection system for HVDC and flexible AC transmission systems (FACTS) on the market, with more than 400 installations in operation worldwide.
Integrated with the MACH 2 control and protection equipment is the station control and monitoring (SCM) system. Work stations (PCs) are interconnected by a local area network. The distributed system for remote I/O, for control, as well as for process interfacing with the SCM system, is built up by a field bus network.
The most important part of the control system, the converter firing control, is built around a host 1.3 GHz Pentium III dual-processor system and six high performance digital signal processors (SHARC). This gives an unequalled calculation capacity that is used to fine-tune the performance of the converter firing control system during various system disturbances.
Anatomy of the Sharyland HVDC tie
The Sharyland back-to-back tie comprises:
- An incoming AC line from the CFE grid à‚— this line facilitates connection to the CFE grid, through which export or import of power to or from Texas takes place.
- Connection to railroad AC substation à‚— a feeder is connected to the adjacent AC substation that provides connection to the ERCOT grid.
- CFE side AC filter area à‚— this comprises AC filters for the CFE side converter, which includes capacitors, reactors, resistors, breakers, disconnect and current transformers.
- ERCOT side AC filter area à‚— this area houses AC filters for the ERCOT side converter. They are identical to the CFE side filters and serve the same function for the ERCOT converter.
- Converter transformers à‚— single 3-phase and 3-winding step-down transformer on each side rated for 183 MVA and equipped with on-load tap-changers provide appropriate AC voltage connection to the converter valves.
- Valve enclosures à‚— one enclosure on each side houses the 12-pulse water-cooled thyristor valves, the heart of the HVDC station. They perform the conversion between AC and DC voltage. The design uses valves suspended from the ceiling.
- Cooling enclosure à‚— it houses the valve cooling equipment such as liquid pumps, expansion vessel, deioniser vessels, heaters and valves. An adequate level of redundancy is included in the system.
- Cooling towers à‚— air cooled liquid coolers control the temperature of the water circulated through the thyristor valves. It includes fans and other items.
- Service building à‚— this houses the brain of the HVDC link, the MACH 2 control and protection system. It also houses the auxiliary supply system and the operator control desk and office. The entire control, protection and interface system comprises only 14 cubicles. The control system is duplicated to ensure a high level of reliability.
- DC smoothing reactors à‚— there are two air-core smoothing reactors connected in the neutral points on the CFE side. They control ripples in the DC current and help limit the fault current through valves, transformers and other equipment under certain fault cases. These reactors also serve as installed spares for each other.
- Bypass arrangement à‚— The bypass arrangement facilitates black-start on either side of the grids. It consists of one circuit breaker, two disconnectors, two current transformers and a short flexible bus.
- Spare converter transformer à‚— A fully prepare spare converter transformer is provided, which enables a faulty transformer to be replaced quickly, minimizing outage time for the link.