Published On: Mar 06, 2015

Power Quality is one of the major issues in electricity distribution segment today. It is an issue, which is becoming increasingly important for electricity consumers at all levels of usage. PQ related issues are most discerning because of the extensive and increasing use of sensitive power electronic equipment. Power quality issues originate from utility as well as from end-customer; hence the solutions/mitigation should be adopted by both of them. Poor quality of power supply causes disruption to an end-customer’s process and equipment, leading to a loss of revenues. Conversely, a customer’s usage also affects the supply quality back into the electricity network. It is therefore important that to improve power quality; a systematic approach is adopted engaging all stakeholders.

Power Quality intervention techniques can be conventional, contemporary or hybrid (combination of first two) depending upon each stakeholders and their requirement. Some of the conventional intervention techniques include System design, Network Planning, Voltage Regulator, On-line Voltage stabilizer, K-rated transformer, Zig-Zag Transformer, Isolation transformer, PFC panel, Passive Filter etc. These techniques can be implemented at optimum cost, if properly planned during designing phase to ensure good PQ environment. While there are conventional techniques/methods in place for PQ mitigation, this blog attempts to focus on some of the key innovative technologies for PQ intervention and their benefits accrued to the concerned stakeholders.

INNOVATIVE INTERVENTIONS FOR PQ MITIGATION

Considering today’s scenario with usage of high power electronic devices, there is an urgent and immediate need of innovative/latest techniques to mitigate PQ.

Before we move ahead to understand the innovative PQ techniques, let us briefly see some of the key causes and effects due to poor PQ:

Table 1. PQ Phenomenon, its causes and effects

From the above table, it is quite evident that there is a need of addressing PQ issues using innovative techniques. Leading among innovative techniques is Flexible AC Transmission Systems (FACTS) based devices that are used to shield sensitive load against the most significant PQ problems. FACTS devices are also used by utilities to improve power transmission capability, voltage control, enhances voltage stability and improves power system stability.

Modern day solutions like Dynamic Voltage Restorer (DVR), Distributed Static Compensator (D-Statcom), Unified Power Quality Compensator (UPQC), Automatic Power Factor Controller Panel (APFC) panel, Active Filter and others are very effective in addressing PQ issues, those are vexed and dynamic. Although these solutions come with benefits, however there costs are also higher than contemporary solutions.

Key devices explained below are used by utilities and high-end industries both to mitigate power quality issues:

• Dynamic Voltage Restorer (DVR): has become popular as a cost effective solution for the protection of sensitive loads from voltage sags and swells and improves the voltage stability. It is a series of connected compensator designed to maintain a constant RMS voltage levels against the voltage disturbances. The main function of a DVR is to protect sensitive equipment likes Variable Frequency Drives (VFDs), Induction motors, etc. from voltage sags/swells coming from the grid. For low voltage applications, single-phase DVR topology is used and for medium voltage applications (i.e. up to 33kV), three-phase DVR topology is considered.
• Distributed Static Compensator (D-Statcom): is a FACTS device, which is used for balancing source current, power factor correction, harmonic mitigation and has the capacity to maintain bus voltage sags at the required level by supplying or receiving of reactive power in the distribution system. A D-STATCOM is a converter based flexible AC transmission controller used at the distribution level. At the transmission level, STATCOM handles fundamental reactive power and provides voltage sag control, while a D-STATCOM is employed at the distribution level or at the customer end for dynamic compensation. D-Statcom is normally used by transmission and distribution utilities to compensate the reactive power, which is one of the main cause of increasing distribution system losses and various power quality problems.
• Unified Power Quality Compensator (UPQC): mitigates voltage flicker/imbalance, reactive power, negative sequence current and harmonics. UPQC is one of the most effective devices for mitigating these issues. It consists of combined series and shunt active power filters (APFs) for simultaneous compensation of voltage, current disturbances and reactive power. They are applicable to power distribution systems, being connected at the point of common coupling (PCC) of loads that generate harmonic currents. UPQC has the ultimate capability of improving the power quality at the installation point in the distribution system. This system can replace the UPS, which is effective for the long duration of voltage interruption, because the long duration of voltage interruption is very rare in the present power system.
• Automatic Power Factor Controller Panel (APFC): Most of the electrical loads are inductive in nature resulting in severely lagging power factor. The most practical and economical solution to improve the power factor (PF) is to provide reactive compensation by installing power capacitors of suitable rating at strategic load end locations. But this has to be operated manually and may result in the over voltages, switching surges, damage of the electrical equipment, saturation of transformers, etc. PF correction is more important in electrical distribution systems. In dynamic system for accomplishing the same, low voltage (LV) capacitors are being extensively used both as Fixed Capacitor banks and as Automatic Power Factor Correction (APFC) Panels. Application of APFC Panels are becoming more attractive due to their techno-commercial advantages like:
  • Ability to maintain PF at the required high value (close to unity) and hence avoid the penalties imposed by electrical supply companies due to low PF
  • Avoid over compensation during low load conditions
  • Improved efficiency of the system due to reduction in losses
• Static VAR Compensators (SVC): is an excellent device, which uses a combination of capacitors and reactors to regulate the voltage and it prevents from fluctuating it. A SVC is typically made up of coupling transformer, thyristor valves, reactors, capacitance (often tuned for harmonic filtering). The main advantages of SVCs over mechanical switched compensation are their near-instantaneous response to change in the system voltage. For this reason they are often operated at close to their zero-point in order to maximize the reactive power correction. They are in general cheaper, faster, and more reliable than dynamic compensation schemes such as synchronous compensators (condensers). Due to its high speed switching and simple control, it is widely used by utilities and industries worldwide.

Above devices are mostly used by high-end industries and distribution utilities. While many end users may be unable to perceive or realize the impact of PQ on their systems and energy bills, however with deployment of innovative solutions through precise application engineering in PQ affected areas can help them realize these benefits.

For residential/small commercial premises, proper engineering and design at initial level will help reduce PQ issues significantly. Proper selection of equipment with right design and specification holds high relevance in PQ mitigation. Further, usage of Transient Voltage Surge Suppressors (TVSS) is the most basic, simplest and least expensive power protection device that protect against high transient level voltages. More details can be read from our previous blog: Common Mass perception around PQ. Affordable common PQ initiatives.

OTHER WAYS OF PQ MITIGATION

While we discussed above technological advancement for PQ intervention, it can also be managed by policies drafted by National Standard Bodies like Bureau of Indian Standards (BIS). It is the central body that establishes technical committee on Power Quality and develops standards on power quality and its related issues. Much of the PQ issues can be avoided/reduced, if planned properly at the design and construction stage based on the principle set of regulations under ‘National Electrical Code of India’. Electrical systems play an important role and it is of utmost priority to give attention to the Power Quality as well as electrical safety during any electrical installation. The onus lies upon utilities and industries to adopt the code in their business model for right implementation of PQ techniques.

Original Equipment Manufacturer (OEM) initiatives (like Computer Business Equipment Manufacturers Association (CBEMA) curve adopted by computer manufacturers themselves) are some of the forward ways of mitigating PQ issues. CBEMA Curve is one of the most frequently employed power acceptability curve. The CBEMA curve was originally derived to describe the tolerance of mainframe computer business equipment to the magnitude and duration of voltage variations on the power system. In addition, it is also designed to point out ways in which system reliability could be provided for electronic equipment. Eventually, it became a standard design target for sensitive equipment to be applied on the power system and a common format for reporting power quality variation data. The CBEMA curve was adapted from IEEE Standard 446 (Recommended Practice for Emergency and Standby Power Systems for Industrial and Commercial Applications – Orange Book), which is typically used in the analysis of power quality monitoring results.

A CASE STUDY INDICATING ECONOMIC BENEFITS OF USING INNOVATIVE PQ TECHNOLOGIES

CASE STUDY: Mitigation of Harmonic and Reduction in Operating Cost for a Cement Plant

Problem Statement: The facility is one of the major cement plant of India with the state of the art technology. It has two manufacturing lines and each line has 8 cooler fans, 4 ESP transformers and raw mill classifiers. The facility receives power from state electricity board at 33kV level. In order to maintain quality of the product and optimize energy cost, all the cooler fans in the system were installed with variable frequency drive. But post installation of VFD’s the facility started experiencing problems like,

• Nuisance tripping of circuit breakers
• Failure of motor winding due to high temperature
• Increased temperature of transformer, though loaded at 27-30% level, windings

Solutions and benefits: A power quality study was carried out to investigate the cause of above problems and it was noticed that both the current and voltage harmonics level were up to 32.5% and 7.4% respectively. On the suggestion of power consultant, the facility management decided to install active harmonic filters at all places where VFD’s are installed. Post installation study was carried out to ascertain the benefits gained by installation of PQ mitigation equipment. The team noted that along with mitigation of harmonics, the energy losses have also been reduced which are associated with harmonics. This has resulted in a huge difference in utility’s electric bills. The table below shows top numbers:

Conclusion: The facility gained following additional benefits in addition to annual savings:

• Reduction in losses at motors, distribution system and transformers giving direct energy savings
• Avoided any nuisance tripping of circuit breaker
• Reduction in Failure rate of sensitive electronics, lesser production Downtime & lesser production Waste
• Improved efficiency of transformer as load losses is reduced

(For detailed case study, refer bullet #5 in References section below)

CONCLUSION

The deterioration of power quality levels; proliferation of more sensitive loads, and the increase in processes based on power electronics have in recent years been attracting growing concerns on power quality issues. These disturbances and outages have substantial economic losses.
The availability of different power electronic and mechanical based mitigation devices as well as customized solutions provide a range of equipment that satisfies user requirements both technically and economically. Hence, a holistic approach in selection of PQ devices with right design and specifications is a key to effective PQ mitigation.

REFERENCES

1. Innovative System Solutions for Power Quality Enhancement – K.Chan, A.Kara ABB High Voltage Technologies Ltd.
2. Mitigation of Power Quality Problems Using FACTS Devices – Rajeev Kumar Chauhan & J.P. Pandey, International Journal of Electronic and Electrical Engineering.
3. Study of Different Methods for Enhancing Power Quality Problems – Priyanka Chhabra, International Journal of Current Engineering and Technology, June 2013
4. Various Control Techniques for Power Quality Improvement Using D-statcom – R. Geetha, M. Aishwariya Devi, International Journal of Power Control Signal and Computation (IJPCSC), April –June 2012
5. Case study – Mitigation of Harmonic and Reduction in Operating Cost for a Cement Plant