Mexican grid uses FACTS for greater flexibility21 June 1999
With ongoing deregulation of the electricity supply industry in many parts of the world, the demand for flexibility in power transmission is growing continuously. This gives added momentum to established solutions for flexibility in power transmission, as well as opening up opportunities for new technologies in the field.
Facts stands for Flexible AC Transmission Systems. Ths flexibility in power transmission is achieved by a family of devices and systems more or less tailormade for various cases and applications.
There are both well established and novel technologies used in FACTS. The former includes devices like Static Var Compensators (SVC) and Phase Shifting Transformers (PST). The latter includes STATCOM and Thyristor-Controlled Series Capacitors (TCSC).
As an example, SVC has been installed in the Mexican 400/230/115 kV power transmission system, to offer extra flexibility in system utilisation under growing demands for transmission capability and availability.
Mexico: bulk user of SVC and series compensation
The Mexican power transmission system is vast, and in general, longitudinal in configuration. To safeguard the system's steady-state and dynamic stability under various operating conditions, SVC and Series Compensation has been successfully utilized in the grid for a considerable time. At present, additional large SVC installations are being commissioned in the network, and in the near future, still more SVC plants are going into operation.
By the end of 1999, a total of nine SVCs supplied by ABB will be in operation in various parts of the Mexican power grid, including the ones previously installed, along with several Series Capacitors. All in all, the usefulness of SVC and Series Compensation for increased benefits of the power transmission system is being demonstrated in the Mexican grid.
Mexican power supply system
Comisión Federal de Electricidad (CFE) has to assure supply of electricity at acceptable levels of quality, quantity and price, as well as providing good service, protecting the environment, and promoting social development.
A range of technologies are used to generate electricity in Mexico, including hydroelectric, thermal, wind and nuclear plants. Today, CFE has an installed capacity of about 35 000 MWe, in the following proportions:
The National Electrical Network (NEN) is divided into nine geographical areas. These areas are interconnected, in order to:
Power transmission within NEN is performed at 230 kV and 400 kV. Subtransmission is performed at various voltage levels from 69 kV to 161 kV.
Expansion of electrical systems is critically important to satisfy demand growth, needing proven technical and economic criteria.
CFE has a group of modern digital and analogue simulation models, which allow optimal planning of the electrical system by well-coordinated and hierarchical studies. These are based on well established technical and economic criteria. Due to world globalization, the Mexican electrical supply industry has new challenges to keep on fulfilling its objectives.
The Mexican electrical network has a longitudinal configuration in certain regions that makes it quite weak, and consequently its transmission capability is limited due to problems of voltage support or transient stability limits. This network configuration requires implementation of supplementary control schemes (Automatic Load Shedding, Automatic Trip Generation, etc) to meet the requirement of the first contingency criteria. On the other hand, application of compensation schemes based on SVC has been studied with the objective of increasing the limit of power transfer of the network, avoiding installation of supplementary control schemes.
SVC and Series Capacitors (SC) were first applied in the Mexican 400 kV grid in the early 1980s. The southeast part of the transmission grid is a weak system about 900 km long. A decision was made to improve the system's power transmission capacity very quickly by reactive power compensation. A total of seven SCs and two SVCs were ordered and installed. Soon after, two more SCs were ordered to replace existing units. The alternative, building a new 400 kV line, would have been more costly and time-consuming.
Several more SVCs are being installed and commissioned to accomodate expansion of the Mexican electricity industry.
Topilejo and Texcoco are both in the Mexico City area. They provide dynamic voltage stabilisation in the metropolitan grids, particularly in conjunction with network faults.
Güemez is north of Mexico City on a long interconnector between the northern and the southern 400 kV grid. Besides dynamic voltage control, the SVC also dampens active power oscillations on the interconnector. This is achieved by means of a Power Oscillation Damping (POD) control mode.
Nizuc is at the end of a longitudinal 115 kV system close to Cancun. The SVC provides dynamic voltage support to the system, particularly in conjunction with heavy loads.
Some main data for the SVC implemented in 1998 and to be implemented in 1999 are given in Table 1.
In the past 5 years, demand in the central control area has increased annually at 4.5 per cent, with a maximum demand of 6884 MWe in 1998. For 2008, demand is expected to rise to 10 000 MWe. To satisfy this, taking into consideration that the generating base of the area will not be increased, the Mexico City metropolitan area will be a great node of power import, needing proper voltage support and strategic reactive power reserves.
As part of CFE's short and medium term transmission programme for the area, it decided to reinforce the existing transmission grid in surrounding areas as well as dynamic compensation at the main receiving substations Topilejo and Texcoco. Inclusion of the SVCs allowed elimination of existing restrictions on importing energy to the central area and maintaining proper reliability of energy supply to the area.
In the past 5 years, demand has increased in the Yucatan peninsula at an annual rate of 4.6 per cent, with maximum demand of 800 MWe in 1998.
As part of CFE's short-term transmission programme for the Yucatan peninsular area, additional reinforcement of existing transmission facilities in the east of the country and dynamic compensation into the Cancun area were included in the programme. Inclusion of the SVC at Nizuc substation will prevent voltage collapses in the Cancun area as a consequence of grid fluctuation.
In the past 5 years, demand in the northeast control area has risen at 5.3 per cent annually, with a maximum demand of 4660 MWe in 1996. As part of CFE's medium term transmission programme for the area, additional internal transmission reinforcement and dynamic compensation of the North-South transmission link have been included. These projects will allow maintained reliability of supply to the Northeast area. The SVC at the Güemez substation will act to lift existing restrictions in the North-South link of the National Electrical Network.
SVC design highlight: Topilejo
In the case of the Mexico City Metropolitan Area (MCMA), the option of installing an SVC at Topilejo substation was analysed to provide dynamic voltage support at this bus for steady state and first contingency. This substation can be considered as one of the two frontiers of the MCMA to the south western part of the country and can be considered as one of the receiving points for the south western hydroelectric generation, and even for the nuclear generation at Laguna Verde.
The MCMA has a demand of about 6500 MWe, and 50 per cent of this has to be imported, so the receiving points have to be strengthened as much as possible. As a result, the Topilejo bus is important. On the other hand, the south west can be considered as the compensating bus for the National Interconnected System, so the transmission lines interconnecting the MCMA with that area experience strong load variations during the day.
The SVC being installed at Topilejo substation has the main objective of providing dynamic voltage support to the bus and offer continuous voltage control during the load variations in the transmission lines interconnecting this area with the south west. In addition, it should provide voltage support if a failure occurs in one of these 400 kV transmission lines (Temascal–Tecali–Topilejo or Temascul–Puebla–Texcoco). In general, no severe transient problems are expected to occur during any of these contingencies (angular oscillations, voltage, power, etc), so it was considered safe not to include any additional facilities in the SVC such as Power Oscillation Damping.
A single-line diagram of the Topilejo SVC, also valid for Texcoco and Güemez. Its overall rating of 90 Mvar (inductive) to 300 Mvar (capacitive) is provided by a 150 Mvar TCR (Thyristor Controlled Reactor), a 150 Mvar TSC (Thyristor Switched Capacitor) and two harmonic filter branches rated together 60 Mvar.
The harmonic filters are tuned to the 5th and 7th harmonics, and have the dual purpose of providing reactive power at network frequency (60 Hz), and at the same time filtering harmonics generated inside the SVC. The filtering function ensures that no unacceptable levels of harmonics generated in the SVC are allowed into the surrounding grid.
All reactors of the SVC (TCR reactor, TSC inrush current limiting reactors, harmonic filter tuning reactors) are of epoxy impregnated, fibre glass insulated, air core design, a technology totally adopted for SVC applications for many reasons. Capacitors are of the full film, non-PCB and internally fused type.
The rating of the SVC is defined at 1.0 pu system voltage (400 kV). At maximum continuous primary voltage of 1.05 pu (420 kV), the SVC can continuously produce 330 Mvar capacitive power. In the inductive operating range, the limit for continuous unrestricted operation is 109 Mvar at 1.1 pu voltage (440 kV). At voltages above 1.1 pu, the SVC reactive power absorbtion is limited by a TCR current controlling function, maintaining almost constant Mvar up to 1.3 pu voltage. Above 1.3 pu system voltage, the SVC will operate for about 1 second before limiting actions are taken to avoid overloading of the SVC.
The setting of the slope is controllable in the range 0-10 per cent.
The SVC is capable of operating continuously at voltages down to 0.8 pu (320 kV). At lower voltages, operation can be maintained for shorter periods of time.
On top of its dynamic range, the Topilejo SVC, and indeed, also the other three SVCs listed in Table 1 for commissioning in 1999 are equipped each with a Mechanically Switched Capacitor (MSC) branch, located at the intermediate SVC bus level. The MSCs have the purpose of accomodating slow variations in the reactive power balance of the systems and optimising the operating points of the SVC for steady-state conditions.
The major objective of the control system is to determine the SVC susceptance needed in the point of connection to the 400 kV system, to keep the system voltage close to the desired value. This function is realised by measuring the system voltage and comparing it with a set reference value. In case of a discrepancy between the two values, the controller orders changes in the susceptance until equilibrium is attained.
Controller operation results in a susceptance order from the voltage regulator which is converted into firing orders for each thyristor. The overall active SVC susceptance is given by the sum of susceptances of the harmonic filters, the continuously controllable TCR, and the TSC and MSC if switched into operation.
The control system also includes supervision of currents and voltages in different branches. In case of need, protective actions are taken.
There are two parallel operator interfaces, one via a mimic panel and one via a PC. The mimic is to monitor and manually operation the circuit breaker and disconnectors. The same functions can be performed on the PC, which is additionally used for changing control parameters or other settings.
The thyristor valves consist of single phase assemblies. Each valve comprises two stacks of antiparallel connected thyristors. The thyristors are electrically fired. The energy for firing is taken from snubber circuits, also being part of the valve assembly. The order for firing the thyristors is communicated via optical light guides from the valve control unit located at ground potential.
Heat sinks are located between thyristors. The heat sinks are connected to a water piping system. The cooling medium is a low conductivity mixture of water and glycol.
The TCR and TSC valves each comprise a number of thyristors in series, to obtain the blocking voltage capability needed for the valves. One thyristor is redundant, allowing the SVC to maintain operation with one thyristor level shortened. For the whole SVC, more than 150 single thyristors are in use, each with a voltage rating of 6.5 kV.
Bi-directional control thyristors
High power thyristors are normally able to conduct in one direction only. This is not a serious limitation in most applications. In the case of SVC, however, thyristors conducting in both directions would offer possibilities for cost and space savings. This is now a reality. In the Nizuc SVC, the thyristor valves are equipped with Bi-Directional Control Thyristors (BCT). In such devices, two thyristors are integrated into one wafer with separate gate contacts.
The two component thyristors in the BCT function completely independently of each other under static and dynamic operating conditions. Each component thyristor in the BCT has a performance equal to that of a separate conventional device of the same carrying capability.
The valves comprise only one thyristor stack in each phase instead of two, which enables considerable compacting of the valve design.