Better Control

Role of HVDC and FACTS in power transmission

In India, the distribution of natural energy resources is uneven and concentrated in a few pockets. Without interstate as well as inter-regional links, these resources cannot be used optimally. There is also a need for a stable and secure grid. Technologies such as flexible alternating current transmission system (FACTS) and high voltage direct current (HVDC) ensure the stability and security of India’s complex grid system by reducing transmission losses; optimising the power flow; minimising cascading disturbances; preventing blackouts; supporting the integration of intermittent renewable energy resources into the grid; and increasing transmission capacity and system stability.

Power Line looks at the role of HVDC and FACTS in the Indian transmission network…

Overview of HVDC technology

HVDC technology is being increasingly deployed around the world for bulk power transfer. In India, some of the notable projects with ±800 kV HVDC systems are the Northeast Agra transmission link, the Champa-Kurukshetra link and the Raigarh-Pugalur-Tirichur link. In addition, HVDC technology is contributing to interconnections in the SAARC region. The 500 MW HVDC back-to-back India-Bangladesh interconnector has been operational since 2013.

HVDC technology enables power transmission over long distances in a more efficient and economical manner than alternating current (AC) transmission technology. In AC transmission, control and bulk power transfer have limitations of steady-state power transfer, voltage stability and transient stability. It also gives rise to inter-area oscillations and reactive power problems during bulk power transfers over long distances. These issues increase the risk of blackouts due to cascading effects.

The key benefits of HVDC systems include controlled power flow, bulk transfer and the absence of unrestricted line length. They also allow for asynchronous operations and stabilise HVAC systems by controlling power swings. HVDC is cheaper than HVAC technology when it comes to long distance and large quantum of power transmission since it requires fewer lines, and therefore less right-of-way (RoW) clearances for the same amount of power transmission.

An HVDC system can be divided into the conventional line commute converter (LCC) HVDC system and voltage source converter (VSC) HVDC system. Conventional HVDC transmission is based on LCC converter technology and uses high power thyristors. Such systems use semiconductors that can withstand voltage in either polarity and can be turned on by control action, but the turn-off and “commutation” rely on the external circuit. LCC HVDC presents several limitations vis-a-vis VSC HVDC as it requires stronger AC systems, filters for lower order harmonics, costly MI cables and 50 per cent reactive compensation. Further, its converter transformers are exposed to direct current.

HVDC transmission based on VSC uses high power insulated-gate bipolar transistors (IGBT). In these systems, active and reactive power is controlled independently. Compared to conventional HVDC, VSC HVDC is able to control AC voltage almost twenty times faster, thus eliminating the need for fast telecommunication between two stations. VSC operates in all quadrants of its capability curve and can be used as a static synchronous compensator (STATCOM). In addition, a standard transformer design can be used to withstand DC voltage or harmonic currents in a symmetrical monopole configuration. The disadvantages of VSC HVDC include low power capability, high station losses, high cost and low reliability.

Features of key HVDC projects in India

  • ±800 kV , 6,000 MW HVDC NER-Agra multi-terminal transmission link: This is the first ±800 kV HVDC project in the world with 12 pulse converter terminals. The project is designed with a continuous 33 per cent overload feature, which is also the first of its kind. Each pole of the multi-terminal link is designed for handling a power capacity of 2,000 MW. Earth electrodes are designed for 5,000 Ampere DC continuous current which is the first of its kind. This is also the first 800 kV project to have an indoor DC hall for DC yard equipment such as smoothing reactors, DC filters and DC disconnectors at the Agra terminal.
  • ±800 kV 6,000 MW Champa-Kurukshetra parallel bipole system: The project is the first 800 kV parallel bipole system in the world to use a dedicated metallic return (DMR) conductor. A key advantage of this conductor is that it removes uncertainty regarding the functionality of the earth electrode station. It also eliminates the need to acquire a separate land for each earth electrode station and construct electrode stations, which require a large amount of steel rods and coke. With this project, the construction of a new transmission line between the earth electrode station and the respective HVDC terminal station is also avoided. The project has two bipoles with a combined capacity of 3,000 MW. Each pole is rated for 1,500 MW power with 1.1 p.u. continuous overload and 1.2 p.u. overload for two hours. Both bipoles share the same HVDC line and DMR conductor.

Role of FACTS devices

The applications of FACTS devices can be divided into two categories – steady-state and dynamic applications. Steady-state applications include voltage control, increased thermal loading, post-contingency voltage control, loop flow control, reduction in short-circuit levels and power flow control. The dynamic applications of FACTS controllers include transient stability improvement, oscillation damping (dynamic stability), dynamic voltage control during system contingencies and reduction in primary disturbances, voltage stability enhancement and elimination of sub-synchronous resonance (SSR).

These devices can also be divided into four categories based on how they are connected – series controllers, shunt controllers, series-shunt controllers and series-series controllers. Series FACTS devices boost stability, while shunt FACTS devices provide reactive power compensation. In addition, FACTS devices can be classified into two generations. First-generation devices have been in commercial use since the 1970s and employ conventional thyristor-switched capacitors and reactors with only current turn-on features. They employ capacitor and reactor banks with fast solid-state switches in traditional shunt or series circuit arrangements. First-generation controllers are static VAR compensators (SVCs), thyristor-controlled series capacitors (TCSCs), and thyristor-controlled phase-shifting transformers.

Second-generation devices have been in use since the mid-1980s and employ VSCs with features like gate turn-offs (GTOs) and IGBTs. VSC-based FACTS controllers employ self-commutated DC to AC converters with GTOs and IGBTs, which can internally generate capacitive and inductive reactive power for transmission line compensation without having to use capacitors or reactor banks. The main VSC-based FACTS controllers are STATCOMs, shunt-connected; static synchronous series compensators (SSSCs), series connected; interline power flow controllers (IPFCs), combined series-series; and unified power flow controllers (UPFCs), combined shunt-series.

Fixed series capacitors (FSCs) are the simplest source of series compensation. They can increase the transfer capability and reduce the transmission angle. They increase system stability by limiting load dependent voltage drops.

TCSCs have several advantages over FSCs. Their key benefits are that they can be used for load load-flow control and power oscillation damping.

Meanwhile, SSSCs can vary the effective impedance of a transmission line by injecting voltage with an appropriate phase angle in relation to the line current. IPFCs, on the other hand, are ideally a combination of two or more SSSCs that are connected through a common DC link to facilitate the bidirectional flow of real power between the AC terminals of SSSCs.

STATCOMs offer dynamic voltage control, power oscillation damping and system stabilisation, which enhance the capacity and quality of power. These are compact devices that have high response speeds and minimal environmental impact.

A combination of a STATCOM and an SSSC, UPFCs are one of the most versatile FACTS devices. A UPFC device comprises two VSCs coupled through a common DC link. The UPFC is able to simultaneously or independently control the transmission line voltage, impedance and angle. It can control both real and reactive power flows in a transmission line.

Further, SVCs stabilise voltages and control dynamic reactive power. The typical response time of an SVC ranges from 30-40 milliseconds, which is shorter than that of STATCOM.

Conclusion

The biggest challenge that the transmission sector is facing today is RoW, which can be overcome by high intensity transmission corridors. Power Grid Corporation of India Limited has seamlessly upgraded the ± 500 kV Talcher–Kolar HVDC terminal from 2,000 MW to 2,500 MW without changing any equipment. This has been achieved by enhancing the cooling of transformers and smoothing of reactors at negligible costs.

Going forward, FACTS and HVDC will play increasingly important roles in Indian transmission networks as they can increase the efficiency of the transmission system, improve dynamic performance under various contingencies and help prevent grid disintegration.

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