Power quality (PQ) issues are present in every electric system. PQ problems range largely from supply interruptions and harmonic distortions to transients and imbalances in voltage and current, flickers, voltage sags and swells, frequency excursions and problems associated with reactive power. For instance, although reactive power is essential for the functioning of any AC power system, it needs to be managed in order to reduce demand, improve voltage stability margins and reduce transmission and distribution network losses.
Harmonic distortion is a common PQ problem that power distribution systems need to tackle. The problem of harmonics arises due to the use of non-linear load or solid-state component. Traditional distribution equipment such as rotating machines and overloaded transformers produce harmonics. Further, harmonics of a varying magnitude create additional stress on the networks, potentially leading to equipment malfunction and making installations run less efficiently.
Technology solutions such as static var compensators (SVCs), static synchronous compensators (statcoms), unified power quality conditioners and harmonics filters can thus be deployed to overcome PQ issues. Meanwhile, energy storage solutions such as batteries can be used to overcome the intermittent nature of renewable power and provide backup power in areas with poor PQ. A look at some of the key technologies and solutions for PQ improvement levels…
SVC technology is important for reactive power control. SVCs are devices that can quickly and reliably control line voltages. They increase the power transmission capability of transmission lines and improve the transient stability and load factor of the system, reduce losses and mitigate active power oscillations.
An SVC consists of a number of fixed or switched branches, of which at least one branch includes thyristors, and the combination of branches can vary a lot depending on requirements. The most commonly used SVC schemes include thyristor-controlled reactor, thyristor-switched capacitor, self-reactor, thyristor-controlled reactor-fixed capacitor and the thyristor-switched capacitor–thyristor-controlled reactor. The main advantage of using thyristor-switched capacitor branch is to reduce losses by reducing the filter size.
There are two main situations where SVCs are used – when connected to the power system for regulating the transmission voltage and when connected close to the large industrial loads for PQ improvement.
A statcom is a voltage regulating device. It provides variable reactive power as per grid requirements and enables dynamic voltage control. This is particularly useful in cases of intermittent influx of power from variable sources of energy such as wind and solar, which impact the quality and reliability of the grid system. Hence, even if the system voltage drops significantly in case of variable sources of energy supply, the maximum reactive power output is not affected. This property of a statcom enables continued adoption of renewables, as it reacts according to the voltage source converter principles and a unique pulse width modulation, and has the ability to switch within milliseconds.
For dynamic compensation, statcom technology is preferred over SVCs, in view of its faster response, requirement of less space, and above all, its state-of-the-art technology. The response time of a statcom is shorter than that of an SVC, mainly due to the fast switching times provided by the insulated gate bipolar transistors (IGBTs) of the voltage source converter. A statcom also provides better reactive power support at low AC voltages than an SVC, since the reactive power from a statcom decreases linearly with the AC voltage. Statcoms may be combined with mechanically switched reactors and capacitors controlled by a statcom controller. A statcom would be primarily for dynamic compensation while mechanically switched reactors/capacitors would be for reactive compensation under a steady state.
Energy storage systems
Energy storage systems improve PQ and protect downstream loads against short-duration disturbances in the grid by offering accurate and rapid response. In other words, energy storage systems allow the grid to draw power from the storage system in case of high demand and store power during low-demand periods, thus bridging the gap by providing power as per the load requirement. These solutions enable quick response to the varying grid requirements, thus maintaining grid stability and high quality power supply. An example is the battery energy storage system, which helps power smoothing in generation systems in which power flow variations can occur. It can store the excess energy generated from the renewable energy source and can reuse this energy in times of high demand.
One of the key hurdles in the large-scale adoption of energy storage technologies has been the high cost associated with it. However, given the current deployment levels and industry research and development efforts under way globally, most experts estimate that energy storage costs will reduce at a faster pace than expected.
Harmonic filters help in maintaining the harmonic distortion below permissible limits by injecting controlled current to remove harmonic current from the source side of electrical systems, in order to correct the poor displacement power factor. They are divided into three broad categories – passive (require constant loading condition), active (provide harmonic mitigation under any load conditions) and hybrid (utilise properties of both active and passive filters). The performance of passive harmonic filters depends on the source of impedance, which is hard to determine and varies with system changes. However, they are able to improve the power factor and reduce high frequency harmonics of a large size. Active harmonic filters allow the control of output current and provide stable operations against AC source impedance variations. They can respond quickly, irrespective of the order and magnitude of harmonics. However, compared to passive filters, active filters tend to be more expensive as their initial and running costs are usually higher.
Further, for low-order and uncharacteristic harmonics, other harmonic mitigation techniques are used. For instance, phase multiplication is effective in reducing low-order harmonics as long as there is a balanced load. Harmonic injections can remedy uncharacteristic harmonics, but because system impedance is not a part of the design criteria, these may give rise to low-order harmonics. Harmonic mitigation techniques with pulse width modification are capable of obtaining harmonic reductions in very minor frequency deviations.
Constant voltage transformers (CVTs) provide a barrier to the spikes and electrical noise disturbances, while also working in a reverse mode to prevent any such elements from disturbing the main load at the grid. They are capable of correcting main voltage sags and surges by keeping the iron core of the transformer’s secondary section saturated, thereby generating a constant voltage output. These transformers have a unique capability to store energy for about half of the cycle due to their specific design, which when combined with an inverter and a static transfer switch, can provide uninterrupted power transfer to an alternative source. In case of a fault or overload, this feature of a CVT enables it to maintain power supply to the grid, thus preventing a total loss. CVTs are able to drown any considerable input voltage variations and provide nominal output voltage regulation. CVTs ensure that any voltage sag is rectified immediately, though they are not effective during instantaneous voltage interruptions or extremely deep voltage sags.
Besides these, a number of other devices and technologies can also be deployed by utilities to improve PQ such as power conditioning devices, uninterruptible power supply (UPS), lightning surge arrestors, low voltage capacitor banks, transient voltage suppressors, etc. In areas that suffer from frequent power outages, distributed generation (DG) systems can also be used to improve power reliability. It is also known as on-site generation and refers to the use of small-scale power generation technologies located close to the load being served. Diesel generators, gas turbines, solar photovoltaic systems, fuel cells and wind turbines are some types of DG technologies. Although DG systems have a number of benefits to offer, they can also reduce harmonics in the system. Besides these solutions, end users are also looking at remote monitoring software of PQ correction equipment.
Moving ahead, the magnitude and complexity of equipment and consumers connected to the grid will grow and PQ disturbances are expected to increase proportionally. Utilities will thus need to provide energy that is clean, reliable and always on. The increase in the influx of variable power into the grid by renewable sources of energy will also lead to greater unpredictability in grid operations, resulting in an imbalance in the production and consumption of power in the system. By using next-generation power quality correction and monitoring technologies, utilities will be able to meet end users’ expectations more effectively and minimise equipment downtime and malfunctions.