Given the country’s complex power grid, ensuring system security and stability is a formidable task. The key emerging technologies in this regard are flexible alternating current transmission systems (FACTS) and high voltage direct current (HVDC). These can be deployed to reduce transmission losses by optimising power flow; minimise cascading disturbances; prevent blackouts; support grid integration of intermittent renewable energy resources; and increase the transmission capacity and system stability.
The applications of FACTS can be divided into two categories – steady-state and dynamic. The former includes 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, reduction in the impact of primary disturbance, voltage stability enhancement, and elimination of subsynchronous resonance (SSR).
Based on how the controllers are connected, they can be divided into four basic groups – series controllers, shunt controllers, series-shunt controllers and series-series controllers. While series FACTS devices boost stability, shunt FACTS devices provide reactive power compensation.
FACTS devices can also be classified into two generations. First-generation devices, which have been in commercial use since the 1970s, 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. The key first-generation controllers are static VAR compensators, thyristor-controlled series capacitors, and thyristor-controlled phase-shifting transformers.
Second-generation devices have been in use since the mid-1980s. They employ voltage source converters (VSCs) with features such as gate turn-offs (GTOs) and insulated gate bipolar transistors (IGBTs). VSC-based FACTS controllers employ self-commutated direct current (DC) to alternating current (AC) converters using GTOs and IGBTs that can internally generate capacitive and inductive reactive power for transmission line compensation without the use of capacitors or reactor banks. The important VSC-based FACTS controllers are shunt-connected static synchronous compensators (STATCOMs), series-connected static synchronous series compensators (SSSCs), combined series-series interline power flow controllers (IPFCs), and combined shunt-series unified power flow controllers (UPFCs).
STATCOM: This is a shunt-connected reactive power compensation device that is capable of generating and/or absorbing reactive power. STATCOMs offer dynamic voltage control, power oscillation damping and system stabilisation, which enhance the capacity and quality of power. They are compact devices that have high response speeds and minimal environmental impact.
SSSC: This is a series-connected synchronous voltage source that can vary the effective impedance of a transmission line by injecting voltage with an appropriate phase angle in relation to the line current. It has the capability of exchanging both real and reactive power with the transmission system. SSSCs are useful for controlling the power flow and suppressing SSR.
IPFC: This is ideally a combination of two or more SSSCs that are coupled through a common DC link to facilitate the bidirectional flow of real power between the AC terminals of SSSCs. It provides independent reactive compensation for adjusting real power flow in each line and maintains the desired distribution of reactive power flow among the lines.
UPFC: A combination of a STATCOM and an SSSC, a UPFC is one of the most versatile FACTS devices. A UPFC device comprises two VSCs coupled through a common DC link. One VSC is connected with the transmission line in shunt through a coupling transformer while the second is inserted in series with the transmission line through an interface transformer. The DC voltage for both the converters is provided by a common capacitor bank. 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.
Transmission utilities across the world are increasingly deploying HVDC technology for bulk power transfer. HVDC technology enables power transmission over long distances in a more efficient and economical manner than alternating current (AC). It provides the necessary features to ensure system stability and assist in the prevention of cascaded disturbances. Considering these aspects, HVDC is expected to play an important role in the development of smart grids with better controllability of power flow.
Power Grid Corporation of India Limited (Powergrid) is currently executing the world’s longest multi-terminal HVDC transmission line. The first pole of the ±800 kV Biswanath Chariali-Agra ultra HVDC link was commissioned in September 2015. It is also the first ±800 kV HVDC line in India. With a length of 1,750 km and a power transfer capacity of 6,000 MW, the bipole line extends from Biswanath Chariali in Assam to Agra in Uttar Pradesh via Alipurduar in West Bengal. The line is part of the Northeast-North/West interconnector project.
The feasibility of HVDC technologies can be partly attributed to the successful development of converters comprising semiconductors with high dielectric strengths, for rectification (conversion from AC to DC) at the beginning of the transmission route and inversion (conversion from DC to AC) at the end.
The HVDC system can be divided into two categories – the conventional line-commutated converters (LCCs) HVDC system and the VSC HVDC system.
The LCC HVDC system uses semi-conductors, which can withstand voltage in either polarity and can be turned on by control action while for turn-off and commutation, they rely on the external circuit. They require strong AC systems and large amounts of reactive power. These systems have good overload capability and lower station losses and are highly reliable. Another advantage of LCC HVDC systems is that their DC side fault clearance capability is inherent in the converter.
In VSC-based HVDC systems, active and reactive power is controlled independently and the control of AC voltage is almost 20 times faster than conventional HVDC. These systems operate in all four quadrants of their capability curve and can be used as STATCOMs.
Moreover, there is no need for fast telecommunication between two stations. A standard transformer design can also be used without special requirements to withstand the DC voltage or harmonic currents in the symmetrical monopole configuration.
However, both LCC HVDC and VSC HVDC have downsides. For instance, LCC HVDC does not have black-start capability and faces commutation failure issues. In addition, its converter transformers are exposed to DC. The reversal of power in this system requires polarity reversal of the DC voltage, which takes a considerable amount of time. VSC HVDC, on the other hand, has lower power capability, weak overload capability, high station losses, high cost and low reliability.
In the Indian context, FACTS and HVDC will play an important role in the years to come. They will help in the efficient utilisation of the existing transmission system, improve dynamic performance in various contingencies, prevent the disintegration of grids, and support greater integration of renewable energy.
Based on inputs from a presentation by Oommen Chandy, Executive Director, Engineering, Powergrid