The power sector in India has undergone a sea change. The integration of regional grids into a single national grid in addition to simultaneously providing near-universal electricity access has helped the sector achieve surplus power. The next milestone is to ensure quality and reliability of power supply on a 24×7 basis while limiting outages and minimising volatility in load and frequency. Retaining power quality while facilitating the seamless integration of increasing variable renewable energy, distributed generation and complex loads into the grid will prove to be a challenging task.
Modern-day power quality devices provide information that benchmark the overall system performance, assist in preventive maintenance, monitor trends and conditions, assess network performance and sensitivity to process equipment, and improve energy rates. A network of power quality monitors can be installed on supply systems, and their raw measurement data can be aggregated to correlate and help identify sources of disturbances. Power quality monitors can also be a part of embedded equipment design for tighter integration and control. A unique electrical signature of a machine can be captured to understand the overall health. Conclusions from data analysis and diagnostics can provide reliable input to design next-generation protection algorithms and products to improve power quality.
Voltage and reactive power control technology options
Power quality technologies will be especially useful given the rising share of renewable energy-based generation, distributed grid systems, etc. as higher renewable energy generation leads to a corresponding increase in non-linear loads. There are devices, such as distribution static synchronous compensators (STATCOM), static VAR compensators (SVCs), multivoltage source inverters (MVSIs), battery energy storage systems (BESSs), superconducting voltage energy systems and thyristor switched capacitors, which can be deployed in T&D networks as they become increasingly bidirectional and distributed.
STATCOMs are based on power electronic device technology and are used in order to supply and adjust to rapid changes in active power as well as reactive power of utility grids. In other words, STATCOMs help in achieving corrections in power factor as well as balancing of load and filtering of harmonics. STATCOMs combine voltage source converters in parallel with capacitors, which act as DC energy source link tied to the transmission line. Similarly, SVCs are used to decrease disturbances caused by changes in voltage fluctuations in the normal operation of transmission lines and industrial distribution systems. These compensators balance the disturbances caused by line faults and non-linear components such as thyristor controls and rapidly fluctuating reactive loads. STATCOMs and SVCs perform similar roles in moderating current; however, STATCOMs respond faster, use less space given that they are modular and deliver high quality performance even during low voltage conditions. Several studies recommend utilising distribution static compensators that are equipped with BESSs and interfaced to distribution network with solar photovoltaic (PV) energy integration to improve power quality.
SVC technology is being deployed in several substations as it enhances the capacity, security and flexibility of power transmission systems. It is a solid-state reactive power compensation device based on high power thyristor (semiconductor) technology. Installing an SVC at one or more suitable points in the network can increase the transfer capability and reduce losses while maintaining a smooth voltage profile under different network conditions. For instance, two SVCs with 140 MVAr inductive capacity each were installed at the Kanpur substation on the Rihand-Delhi HVDC line. These SVCs helped avert a cascade tripping across the network when the grid voltage dipped by 10 per cent on a faulty line.
The increasing share of distributed renewables alongside hybrid renewable energy installations such as solar-wind hybrid necessitate power converter options for high-power applications such as MVSIs. These inverters typically synthesise the staircase voltage wave from several levels of DC capacitor voltages. MVSIs are able to convert from power voltage as well as convert the electricity generated from AC/DC or DC/AC (in case of solar). Moreover, in the case of rooftop solar, these MVSIs ensure that the PV arrays are operated at maximum power point while also aiding at maintaining a sinusoidal current into the grid. Additionally, due to the modular structure of MVSIs, the hardware implementation is rather simple and the maintenance operation is easier than of alternative multilevel converters.
The BESS stores electricity in batteries for discharging it as per requirement. The ability of a BESS to store and discharge electricity makes it suitable for application in regulating voltage and load volatility. It is especially useful in managing voltage sags and voltage swells. In comparison to STATCOMs, BESSs can play a major role in making a difference to quality as they can control frequency, actively smooth power output in renewable energy plants, improve the damping of electromechanical power oscillation, and provide voltage and power quality support. In addition, BESSs can reduce network congestion and enhance power transfer capability within interconnected power systems.
Superconducting voltage energy systems are a branch of emerging technology that is still at a nascent stage. Superconducting magnetic energy storage (SMES) systems store energy in the magnetic field created by the flow of direct current in a superconducting coil which has been cryogenically cooled to a temperature below its superconducting critical temperature. The stored energy can be released back into the network by discharging the coil. The power conditioning system uses an inverter/rectifier to transform AC power to DC or convert DC back to AC power. SMES systems make it extremely suitable for deployment for role of voltage and load stabilisation specially to prevent voltage swells, as it can provide high power output for a brief span of time.
Apart from variations in supply, there can be volatility and non-linear load consumption by end-consumers resulting in harmonic distortions. Such distortion leads to increased AT&C losses and reduced efficiency, stresses capacitors leading to their breakdown, errors in metering equipment, etc., hence, many end-users, especially industrial units, need to install active or passive harmonic filters to improve power factor and reduce impedance. In passive harmonic filters, the harmonics are reduced as they are absorbed by the filters from the downstream load. It relieves harmonic stress on load end equipment and solves its failure and tripping problem. Therefore, it facilitates the end-user, generally industrial units, to achieve a full load run with reduced energy consumption.
Similarly, active harmonic filters are parallel filters (which means the current doesn’t go through the filter) that are used to reduce, or mitigate, harmonics to tolerable levels. Active filters are significantly more expensive than passive filters and take up more space as they segregate and filter volatile harmonics before current reaches the consumption unit. Active harmonic filters are more suited for deployment in the substations whereas passive harmonic filters are better suited for deployment in industrial units and commercial establishments.
The way forward
The increase in the share of renewables and decentralised energy generation, coupled with the central government’s focus on improving power quality, will drive investment in power stabilising technologies such as harmonic filters, thyristors and SVCs. Emerging technologies such as BESSs, hybrid harmonic filters and super capacitors will also elevate the degree of power supply to newer levels. Moreover, these investments will have supplementary benefits such as fewer outages, reduced technical losses, longer asset life and increased efficiency.