Safety And Efficiency In Shipping The Power Quality (PQ) Connection

Published On: Jul 20, 2020


Like in every other industry that is undergoing a constant modernization, the role of automation, power electronics and data is extremely critical even in the shipping industry. The international trade, although cyclic in nature, has been on a constant rise and so is the growth of shipping industry and the competition within it. Monitoring power efficiency and maintaining certain standards of power quality is critical to the reliability and safety of advances in the systems used on ships. Minimizing risks to safety and increasing the reliability and efficiency of ships is critical for cost-economics in the extremely competitive market. It’s here, that the Power Quality has a deep connection!

INTRODUCTION

The shipping industry is the life blood of the global trade and commerce. The growth rate of the international trade is strongly coupled to the growth of economies worldwide and therefore the industry is often termed as a proxy for global economic growth. As with the global economy, a lot has changed for the shipping industry as well over the last 50 years or so. Tremendous changes are visible at multiple levels – from greater safety regulations for the technicians and engineers, electrification and modernisation of ships to the trend of green shipping – there has been a complete overhaul of the way shipping industry works in every aspect.

Given the harsh and challenging conditions in marine transport, the shipping industry has always been perceived as the one with high risks to safety and life of people working in it. One of the biggest breakthroughs of modern shipping industry has been to change this and make shipping a safer workplace.

Safety on ships and the role of PQ

International regulations and codes were rather scarce at the time of tragedies such as Titanic but the regulations and codes in the shipping today demand systems with guarantee to human safety of the highest order. There are several IEC and IEEE standards concerning permissible levels of electrical power parameters and those determining its quality and measuring methods. Enforcement of Power Quality standards has a key role in reducing the instances of electrical shocks, fires and mishaps induced due to electrical faults such as overheating etc.

Vessel Modernisation and PQ

The electrification of the ships and every system is fast evolving into smart electro-mechanical systems. The heavy-duty electric equipment on ships includes devices for production, transmission, and distribution of electric energy. The issue of electric power quality onboard ships has gained even more importance as most of ship’s thrusters, propellers and other smaller drives are now electric. The most important factor to consider here is that the electrical systems on the ships are isolated systems. Similar systems are installed in aircrafts, oil platforms.

As the earthing system involved is unique and complex in such industries, ensuring the energy efficiency and power quality also come with unique challenges. Power Quality is vital to ensuring reliability of sensitive electronics on the vessel’s systems and the efficiency of heavy duty-equipment.

Staying competitive – the economic rationale for good PQ

With increasing competition, the need for cost-effective operation in shipping industry is on the rise. The shipping companies are finding it increasingly challenging to meet the demands for reduced fuel emissions as per the latest International Maritime Organisation directives and has forced the shift to green solutions for their vessels. IMO 2020 (an environmentally related regulation) is also on the cards, further increasing the uncertainty for the global shipping industry. Every measure that aids in the reduction of a ship’s power consumption contributes to staying competitive. Here too, the Power Quality has emerged as an important parameter, especially, with greater use of power electronic converters.

POWER QUALITY ON SHIPS – UNIQUE NATURE AND CHARACTERISTICS

The isolated nature of electrical systems leads to some differences in the basic characteristics of power quality. The features and maintenance demands of isolated electric establishments are different than on-land systems.

The isolated electrical systems have the characteristics and maintenance challenges of their own and need to consider

  • They have limited power
  • They consist of separate consumers with power commensurable with generators’ power
  • Under dynamic conditions, the deviation of supply voltage and frequency deviation reaches high values, resulting in a distortion
  • They have relatively high short-circuit impedance of generators
  • Electromagnetic disturbances observed in isolated systems are more serious than large connected systems in their normal operation

Here the electrical system is like a pool as compared to an electrical grid that is like an ocean. So, any small disturbance can cause large distortion in functional performance compromising safety and efficiency.

While taking into account the major differences between the isolated and on-land power systems, it is fair to state that, isolated electric systems and maintaining their power quality need special attention. A system that is installed in a mobile object is doing two tasks at once. It is dealing with the electricity that the operation needs, and also decides for the operational control of the mobile object. Power quality in ships is difficult to maintain because of the constant motion.

On isolated power systems, where there is limited energy being generated, it becomes necessary to maintain the phase balance. Imbalance across phases results in increased energy consumption. Improper voltage often leads to reduced equipment life and wastage of energy. The variation in voltage is directly related to the motor performance. Thus, on a moving object like a ship, it is crucial to keep all motor performances up to date. Sudden drop in voltage leads to surges, sags, and transients. Sags are generally caused by the heavy loads on the system. When a design starts operating outside parameters, it leads to surges and results in equipment damage.

Opening and closing of breakers on large loads often causes transients in an electrical system. Since the ship is a resident of numerous heavy-duty devices, controlling transients, surges, and sags leading to poor PQ is a challenging task.

POWER QUALITY IN SHIPS IMPACT ON SAFETY AND ECONOMICS

The electric and sensitive electronics that are meant to monitor and control the power systems are the causes of waveform distortions and also the one’s to be affected by the PQ disturbances. Such disturbances introduce difficulties to safety at many levels in the operation of the ship.

Poor design and installation practices are the biggest reason for EMCs and root cause of PQ disturbances such as voltage level changes and asymmetry.

Compared to the land based industrial sources of power (4-6% impedance) the marine systems are weak power sources (15%-20% impedance). With weaker power sources VFDs breed a stronger harmonics and inter-harmonic distortions and it is not uncommon to observe very high harmonics and voltage distortions onboard ships.

Electromagnetic disturbances

One of the biggest threats to safety and reliability of power systems on the ship is from electromagnetic disturbances. The Electromagnetic Interference (EMI) from radiated and conducted through the ship’s electric networks cause disorder in their operations. Typically, the following are observed as prolonged disturbances:

  • Voltage and frequency variation
  • Voltage asymmetry
  • Harmonic distortions, transients
  • Improper Reactive power distribution between parallel generating sets

Given below are some examples of risks that a typical ship is exposed to from PQ issues in the power distribution system.

Power shut-down due to overload and the induced voltage variation

Overload of a ship’s electric power plant had automatically shut it off leaving the areas with less important services powerless. Also, this event had led to switching on the third generator with same power for parallel operations.

A representation of voltage variations (rms value) during the start-up and running of the thruster driven by 1.3 MW electric motor, when two generators with power 1.75 MVA each operated in parallel.

Artificial overload and real collapse of whole supply system during a ship’s maneuvering

The chart here depicts distribution of active power between generating sets working in parallel as registered during the ship’s manoeuvring. The active power load of generating for the generator set no 2 (PG2) is been depicted as a bold line. The improper distribution of the active and reactive load my cause an artificial overload and lead to complete (and real) collapse of the power supply system by switching the generators off. The impact of the risk stated above during manoeuvring is potentially fatal to the safety of the ship.

Source: Joint research of Marine Electrical Power Engineering Department of Gdynia Maritime University and Electrical and Automation Department of Polish Register of Shipping

Reactive Power and Harmonics concerns

Significant part of electronic devices used on vessels such as control systems, controllers, and navigation devices, are sensitive to power quality disturbances.

The existence of high loads, such as the pods and the thrusters, creates reactive currents and harmonics that cause reactive loads to the electric grid of the vessel. Poor PQ can also lead to operational disturbances of electronic equipment, increase of reactive losses, worsening of insulation or also damage of equipment.

Harmonics from Power Converters (VFDs)

Power converters or VFD’s Variable Frequency Drives used to supply electric motors in order to keep either the motor torque constant or the motor power constant. The VFDs are critical to improving the efficiency of the energy consumption on the ship but these are also a known cause of creating harmonics in the generator systems of the ship.

Impact of Harmonics on the Ship’s systems can be summarized as below:

Generators: Overheating resulting into damage to bearings, winding and sheets packages of generators, often due to a premature thermal ageing of insulation especially on overhang of winding portion.

Electrical power consumers: Other than the impacts such as nuisance tripping, overheating etc. similar to those observed in land-based facilities, the interference with controls such as radio and critical navigation systems and lighting are particularly observed on the ships

Power Distribution network: Cables overheat due to a decrease in the ability to carry rated current (skin effect) and risk of damage due to EMI resonance

THE RISE OF ELECTRIC PROPULSION AND THE NEED FOR IMPROVED PQ

Ship systems like most of the other systems have become more and more electrified. The electrical systems on the ships have now started focusing on reducing the maritime pollution in an optimal way.

The introduction of electric propulsion motors and electronic control systems brought extensive electrification in naval ships. Use of same devices is now extended to commercial ships as well. Thus, these advance power systems are subject to PQ problems as any other electric system. Surges, harmonic distortion, unbalanced operations can show significant adverse consequences and need to be attended accordingly.

As far as the isolated earthing system is concerned, the healthy phases are subject to increased overvoltage. The PQ related issues, thus, are better be merged into the solutions at the occurrence stage. Planning about them at the later stage is always challenging. Many technologies are being installed in order to deal with the PQ issues before the actual generation. These advanced devices help identify the problem and alert the engineers and experts before the event. Hence, they can be prepared well in advance and look for solutions.

NAVIGATING THE SHIPS FOR BETTER PQ

Power factor correction or harmonic corrections are the traditional ways to deal with PQ problems. Use of isolation transformers and inductors are typically tuned for specific harmonics. Use of right size of copper cable with higher insulation grade helps meeting the short-term overheating. Voltage and stability can be ensured by using TVSS, that is, Transient Voltage Surge Suppression that halves the capacity per event.

Some of the generic solutions to addressing PQ issues on ships are installing power quality analysers, continuous PQ monitoring, measurements to maintain the power factor, and more. Specific means to prevent against electromagnetic disturbances and their effects on the operation of the ship can be done by correction of PQ in ship’s power systems and improvement of the electric consumers’ immunity. Continuous PQ monitoring will make designing, construction, classification, and utilization of ship’s electric systems a priority.

Software-defined electricity system has emerged with the development of electrification in shipping industry. A model based computing power electronics system is being installed. It fully synchronises electricity in real time to avoid PQ problems. Software-defined electric systems are equipped with a wide range of advanced features that offer several benefits. Such system removes noise to improve electronics performance.

Also, the digitization of equipment has advanced the pace of dealing with any issues related to PQ. Real-time digital measurement and bi-directional flow control is ensured by such devices.

CONCLUSION

Instances of poor PQ on a large ship poses several risks not only to the operational safety but also has a strong economical dimension. Starting from electromagnetic disturbances to the harmonics induced by VFDs – the ship’s power systems have to battle out several PQ issues.

As stated earlier, PQ issues in isolated electric systems are difficult and more complex to address as there is no fixed system. An isolated power system is continuously on the move and operates on many simultaneous tasks at once. Thus, it is important to know the sources that originate the possible PQ issues on the ship.

When the PQ issues in the ships are accurately known and addressed, the efficiency of its equipment will be high. Thus, in this new era of an increasingly software driven automation, shipping industry will be well served to address the PQ issues to stay safe and competitive. The research of new methods and ways to limit poor power quality are the ways forward for a safe operation of ships now and in the future.

REFERENCES

  1. Electric Power Quality and Ship’s safety – https://www.researchgate.net/publication/237640225_Electric_power_quality_and_ship’s_safety
  2. The Study of Electric Power Systems in a Ship’s system – https://ieeexplore.ieee.org/document/4025818
  3. Improving electric power quality in ships via surge protection devices (SPDs) – https://www.tandfonline.com/doi/full/10.1080/20464177.2015.1095472
  4. Power Quality on Ships Toda and Tomorrow’s challenges – https://www.researchgate.net/publication/286287950_Power_quality_on_ships_Today_and_tomorrow’s_challenges
  5. Power quality analysis for greener shipping by implementing an on-board electric power quality monitoring system – https://doi.org/10.1080/20464177.2019.1658281
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