T & D - 6) Flexible AC Transmission System – What and Why?

Flexible AC Transmission System (FACTS) have been evolving to a mature technology with high power rating. This technology has wide spread application, became a top rate, most reliable one, based on power electronics. The main purpose of these systems is to supply the network as quickly as possible with inductive or capacitive reactive power that is adapted to its particular requirements, while also improving transmission quality and the efficiency of the power transmission system.

With the progression and development in power electronics application not only improved the performance of AC systems but also make it feasible for long distance.

Facts can also help solve technical problems in the interconnected power systems.

Facts are available in:

1. Parallel connection
• Static Var Compensator (SVC)
• Static Synchronous Compensator (STATCOM)
  .
2. Series connection
• Fixed Series Compensation (FSC)
• Thyristor Controlled/Protected Series Compensation (TCSC/TPSC)

Parallel Compensation

Any type of reactive power compensation employing either switched or controlled units that are connected in parallel to the transmission network at a power system node.

Mechanically Switched Capacitors/Reactors (MSC/MSR)

Most economical reactive power compensation devices are mechanical switched devices:

Mechanical switched capacitors are a simple but low speed solution for voltage control and network stabilization under heavy load condition. Their utilization has almost no effect on the short circuit power but it increases the voltage at the point of connection. Mechanical switched reactors have exactly the opposite effect and are therefore preferable for achieving stabilization under low load conditions.

An advanced form of mechanically switched capacitor is the MSCDN. This device is an MSC with an additional damping circuit for avoidance of system resonances.

a) Mechanically switched capacitors (MSC) and mechanically switched reactors (MSR) connected to the transmission system; b,c) Static Var compensator (SVC) with three branches (TCR, TSC, filter) and coupling transformer

Static Var Compensator (SVC)

Static Var compensators are a fast and reliable means of controlling voltage lines and system nodes. The reactive power is changed by switching or controlling reactive power elements connected to the secondary side of the transformer. Each capacitor bank is switched ON and OFF by thyristor valve (TSC). Reactor can be either switched (TSR) or controlled (TCR) by thyristor valves.

SIEMENS - Turnkey Static Var Compensator (SVC) Project

When system voltage is low, the SVC supplies capacitive reactive power and raises the network voltage. When system voltage is high, the SVC generates inductive reactive power and reduces the system voltage.

Static Var Compensators perform the following tasks:

1. Improvement in voltage quality
2. Dynamic reactive power control
3. Increase in system stability
4. Damping of power oscillations
5. Increase in power transfer capability
6. Unbalance control (option)

The design and configuration of an SVC, including the size of the installation, operating conditions and losses, depend on the system condition (weak or strong), the system configuration (meshed or radial) and the tasks to be performed.

Static Var Compensator (SVC) Plus

The modular SVC PLUS is equipped with an IGBT multilevel converter and a storage capacitor on the DC side. From approximately +/- 25 MVA to +/- MVAr, all of the main equipment, including the IGBT converter, the control and protection system and the converter cooling system of the SVC PLUS, is installed in a container and factory pretested so that it is ready to be installed outdoor at the site.

For indoor installations, converter modules with approximately +/- 100 MVAr are available.

Siemens - Static Var Compensator (SVC) PLUS

Parallel operation of converter modules is also possible, resulting in higher ratings. The footprint of an SVC PLUS installation is smaller than a conventional SVC installation of the same rating.

Series Compensation

Series compensation is defined as insertion of reactive power element into transmission lines.

The most common application is the fixed series capacitor (FSC). Thyristor-valve controlled systems (TCSC) and thyristor-valve protected systems (TPSC) may also be installed.

Fixed Series Capacitor (FSC)

The simple and most cost effective type of series compensation is provided by FSCs. FSCs comprise the actual capacitor banks, and for protection purposes, parallel arresters (metal oxide varistors, MOVs), spark gaps and a bypass switch for isolation purpose.

Fixed series compensation provides the following benefits:

1. Increase in transmission capacity
2. Reduction in transmission angle

Thyristor-controlled Series Capacitor (TCSC)

Reactive power compensation by means of TCSCs can be adapted to a wide range of operating conditions. It is also possible to control the current and thus the load flow in parallel transmission lines, which simultaneously improves system stability. Further applications for TCSC including power oscillation damping and mitigation of sub synchronous resonance (SSR), which is a crucial issue in case of large thermal generators.

Additional benefits of thyristor-controlled series compensation:

1. Damping of power oscillations (POD)
2. Load-flow control
3. Mitigation of SSR (sub synchronous resonances)
4. Increase in system stability

a) Fixed series compensation (FSC) connected to the network; b,c) Thyristor-controlled series capacitor (TCSC) connected to the network

Thyristor-Protected Series Capacitor (TPSC)

When high power thyristors are used, there is no need to install conventional spark gaps or surge arresters. Due to the very short cooling down times of the special thyristor valves, TPSCs can be quickly returned to service after a line fault, allowing the transmission lines to be utilized to their maximum capacity.

View of a TCSC system

TPSCs are the first choice whenever transmission lines must be returned to maximum carrying capacity as quickly as possible after a failure.

Short-Circuit Current Limitations (SCCL)

Extension of HV AC networks, coupling of independent grids and adding of new generation increase the existing short-circuit power in many cases. If the designed short-circuit level of the existing equipment is exceeded, and extension of the network, without extremely costly replacement of the existing equipment, is not possible. This no-go criteria can be avoided by using the Siemens short-circuit current limiter.

Fast short-circuit current limitation (SCCL) with high-power thyristor

By combining the TPSC with an external reactor, this combination can now be used as short-circuit current limiter (SCCL).

In case of a system fault, the thyristor valve will be fired, by passing the series capacitor. The corresponding short-circuit current will be limited by the reactor to the design values.