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Harmonic Mitigation in VFD Used for Seawater Cooling System Pumps Onboard Ship.

Mr. Sanjay Dabadgaonkar, Fellow (IEI), Sr. Associate Prof. (Electrical & Electronics),

Tolani Maritime Institute, Induri, Pune - 410507. (M.S.) E-Mail: sanjayd@tmi.tolani.

Dr. Asim Kumar Sen, Fellow (IEI), Principal, Yadavrao Tasgaonkar College of Engineering & Management (YTCEM), Bhivpuri Road Railway Station, Chandhai, Tal: Karjat, Dist: Raigad (M.S.) E-Mail: asim_sen@linuxmail.org

Abstract

In the maritime industry, engine room cooling sea water pumps used on tanker vessels are generally oversized (32oC to 38oC) during design stage and constantly operate at maximum flow. The process, however, requires maximum capacities only for a very short time period during the operating cycle, resulting in more power consumption and constant energy loss. This will increase fuel consumption and give rise to emission of GHG from international shipping. This paper deals with effective use of Variable Frequency Drives that can achieve reduced flow by adjusting the speed of pumps according to process requirements in order to optimize the energy efficiency of the vessel & to reduce GHG emission. However, the operation of VFD introduces harmonic distortion of voltage and current, which can cause serious problems. This may result in overheating and destruction of power factor correction capacitors as it has caused catastrophic failure of capacitor in aft harmonic filter room on passenger cruise ship RMS Queen Mary-II. So, the shipboard system is also designed to comply with marine harmonic standards i.e. as per IEEE-519 standard the Total Harmonic Distortion must be below 5%.

Keywords

Harmonic Mitigation, Power Quality, Universal Harmonic Filter, Variable Frequency Drive, Energy Efficiency, Energy Conservation, GHG Emission.

INTRODUCTION

With the increasing use of ac and dc electric drives in marine vessel applications, such as electric propulsion, Pump control and offshore oil-drilling operations, poor power quality due to harmonic distortion has become a critical safety issue. Electric propulsion offers significant benefits, such as

greater redundancy, lower emissions, improved maneuverability, and reclaimed deck space for cargo and other important uses. Oil drilling platforms could not function without variable speed control of their motors. However, by drawing current in a nonlinear or non-sinusoidal manner, the VSDs can introduce excessive levels of both current and voltage harmonics. The nonlinear

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loading on marine vessels can now reach 80% of the onboard electrical generating capacity and results in total harmonic voltage distortion (THDv) in excess of 20% (Fig.1).

Fig. 1 Voltage harmonic spectrum of offshore oil production platform at 600V. THDv/phase was 24.1% HARMONIC DISTORTION

All marine classification bodies are extremely concerned about harmonic voltage distortion and the possible consequences should some critical item of equipment malfunction or fail. Often viewed as a potential safety of life at sea (SOLAS) issue, classification bodies have imposed strict limitations on the magnitude of harmonic voltage distortion permitted on vessels classed under their rules. A bigger issue with harmonic currents is that they create voltage distortion as they flow through the power distribution system. To meet voltage-distortion limits, current harmonics must be reduced. Therefore, by establishing limits for voltage distortion, the marine standards indirectly limit current distortion as well. If the combination of harmonic currents and system impedance is high enough, system voltage can become severely distorted. In marine applications, the combination of heavy nonlinear loading and relatively high-source impedance of the supply generators results in excessive levels of voltage distortion.

The shipping industry is facing ever greater challenges, especially regarding energy savings and reducing greenhouse gas emission. Therefore, the improvement in

energy efficiency has become most important factor for sustainable development in shipping industry. Cost of energy is one of the major component not only necessities the installation of energy efficient technology but also inculcate energy efficient practices and methods to be used for pollution free eco-friendly green shipping. As a result, we expect to see an improvement in the fuel consumption on ships by the use of proposed variable frequency drive for electric propulsion and for pumping in seawater cooling system on-board ship.

It is generally acknowledged that more than 90 percent of international trade is carried out by sea. The amount of goods actually loaded aboard ships has reached up to 10 billion tons. The advances in technology made shipping an increasingly efficient and swift method of transport. Pumping systems with motors on-board ship accounts for 35% of energy use. A recent study concluded that the average pumping efficiency on-board is less than 40%. Therefore there is much more scope for improving energy efficiency in pumping systems on-board ship. So, there is a large potential for retrofitting existing ships with new fuel-reducing technologies.

Fig.2 Active Filter and associated waveforms Because of traditional methods of flow controlling, the current fleet is missing an opportunity for high fuel savings and environmental rewards. In this research study

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we have developed a Simulink model of Variable Frequency Drive (VFD's) technology for large capacity seawater cooling system and validated the results by using experimental test-rig. Harmonic analysis was also carried out with and without harmonic mitigation and compliance report with IEEE 519 std. was generated, so as to validate improvement in power quality of the proposed system on-board ship. VFD's not only provides improved control performance but also offers significant energy savings. The benefits of variable speed pumps are now widely recognized e.g. energy efficiency, decreased pump maintenance, ease of automatic control. SEAWATER COOLING In a Sea Water Cooling System, Cooling water pumps used on-board ships are especially dimensioned to have their rated capacity at a water temperature of 32°C to 38°C. When operating in cooler seas, this means that unnecessary high volumes of cooling water are pumped through the systems, resulting in high cost of energy, burning of more fuel i.e. emission of GHG and wear of the mechanical equipment. In passing through different climatic zones with changing temperature conditions, a Marine frequency converter controlled cooling system will ensure constant and very stable storage room temperature. In case of the main cooling freshwater system, the heat is taken away from heat sources such as the main engine jacket, main engine air coolers, main engine lubricating oil etc. and this heat is transferred to the seawater in the central freshwater coolers and the hot seawater is sent overboard. Thus the sea acts as the heat sink. The seawater temperature however is not constant and it changes as the ship moves through various waters and climatic zones. The maximum seawater temperatures are experienced usually in the tropical zones and in the Persian Gulf. Therefore as the ship keeps moving, the cooling system will experience maximum seawater temperature, which is for

considerably a short period of time, in other words, the maximum flow rates in the cooling water system will be for short spans. As the seawater temperature decrease, the flow rate in the cooling water system needs to be reduced. Therefore by installing proposed green technology for controlling flow rate of centrifugal pump, we can able to regulate the total energy consumption according to actual cooling demand and thereby saves fuel consumption required GREEN SHIPPING

There are various means to reduce burning of fuel and saving of energy on-board ship. In this research study our focus is on improving energy efficiency using speed reduction technique. According to Affinity laws the power consumption is directly proportional to speed cube. Therefore slight reduction in speed we can able to save cube root of the power. So, the speed reduction gives tremendous amount of energy saving on-board.

On-board ship, there are a large number of pumps serving multiple purposes and the majority of these pumps are of centrifugal type. These pumps are the variable torque type non-linear loads. In this research study we have selected Sea Water Cooling system used in engine room on-board ship.

Fig. 3 Affinity Law for variable torque loads Pump applications increasingly use AC variable speed drives. However, their operation introduces harmonic distortion in voltage and current, which can cause serious problems. We are going to investigate effects of harmonic distortion and the recommendations that have been put in place to minimize the problems,

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and the passive and active harmonic mitigation solutions currently available for ship board systems. These harmonic currents cause havoc in power systems, especially with generators. These are ‘weak’ sources where impedances can be as high as 15–18% compared to ‘stiff’ sources (4–6%) more common in utility applications. The ‘weaker’ the source, the higher is the harmonic voltage distortion, for a given harmonic current distortion. As a non-linear load, a VFD draws current in a non-sinusoidal manner, rich in harmonic components. These harmonics flow through the power system where they can distort the supply voltage, overload electrical distribution equipment (such as transformers) and resonate with power factor correction capacitors among other issues. EFFECT OF HARMONICS

Typical effects of harmonic distortion include 1. Overheating and sustained damage to bearings, laminations and winding insulation on generators, fires in distribution transformers, causing early life failure. 2. Overheating and destruction of power factor correction capacitors. (Catastrophic failure of capacitor in aft harmonic filter room on passenger cruise ship RMS Queen Mary 2) 3. Overheating of the stator and rotor of fixed-speed electric motors; risk of bearing collapse due to hot rotors. This is especially problematic on explosion-proof motors with increased risk of explosion; in any case if the voltage distortion is over the prescribed limit stated on the certification, the motor is no longer certified, losing any third party assurance as to its safety. 4. Spurious tripping of electrical circuit breakers. Interference with electrical, electronic and control system equipment, including computers, radio communications, measuring devices, lighting, etc.5. Overheating of cables and additional risk of failure due to resonance. The other problems that may result from harmonic distortion include:

• Overheating of rotating equipment, such as electric motors and generators • High voltages and circulating currents caused by harmonic resonance • Equipment malfunctions due to excessive voltage distortion • Increased internal losses in connected equipment resulting in component failure and shortened lifespan • False operation of protection equipment • Lower system power factor preventing effective utilization of system capacity • Voltage regulator problems on generators AFE Vs AUHF

In the marine and offshore sectors, voltage distortion can be extremely severe. As per classification society that they have evidence of up 24% voltage distortion on an offshore installation. This is obviously why the marine classification societies are setting strict limits, usually 5%, on the permissible voltage distortion: to force harmonic mitigation to be installed. The shipboard system is also designed to comply with marine harmonic standards i.e. as per IEEE-519 standard the Total Harmonic Distortion must be below 5%. Active Front End (AFE) Drive are better over standard 6-Pulse VFDs equipped with diode bridge rectifiers such as, reduced line current harmonics, improved power factor and inherent regenerative capabilities. But fact is that current harmonics are much higher when measured above the 50th and that very serious problems can result from the introduction of these higher frequency harmonics. Also, they will downplay a substantial loss in efficiency due to the increased losses in the input IGBTs. The reality is: 1. AFE’s are not the best solution for a low harmonic VFD. 2. A properly designed Wide Spectrum Passive Filter, such as the Lineator AUHF, can outperform AFE especially when harmonics up to the 100th are taken into consideration.

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3. AFE’s generate high frequency harmonics which can have more serious consequences than low frequency harmonics. As a passive device, Lineator AUHF cannot introduce high frequency harmonics and will, in fact, help reduce them when they are present. 4. If there is a mixture of 6-Pulse and AFE Drives on the same switchboard, the ripple in voltage from the AFE Drive can raise the DC bus voltage in the 6-Pulse VFDs creating overvoltage conditions. 5. Although an active solution, AFE’s still require input passive filters (LCL and EMI/RFI filters) to control switching frequency harmonics and to attenuate ripple in the mains side voltage and current. 6. LCL and EMI/RFI filters are more likely to resonate with the power system at rectifier harmonic frequencies (ie. 5th, 7th, 11th, etc.) than the Lineator AUHF. Also under lightly loaded conditions, the reactive power of the LCL capacitors can cause over-excitation of generators. 7. AFE’s generate significant levels of ground leakage current which can cause inadvertent ground fault trips and failure of sensitive equipment. 8. AFE losses are significantly higher and efficiencies much lower than a 6-Pulse VFD with Lineator AUHF. Fig. 1a shows a typical AFE drive topology. The problems associated with the operation of AFE rectifiers are related to the converter design characteristics, switching frequency and interaction with the power system. You will note that ahead of the input bridge is a passive LCL filter. The function of this filter is to reduce the switching frequency harmonics introduced by the IGBTs. HARMONIC MITIGATION All AFE manufacturers, however, include LCL filters that are only minimally effective because a more effective filter would be much more expensive and physically larger. In fact, this passive filter would very likely be larger and more expensive than the Lineator AUHF that

provides equivalent current harmonic mitigation on a simple 6-Pulse VFD (Fig 1b).

Fig. 4 AFE Drive topology with LCL Filter Both topologies have an input passive filter but the Lineator/6-Pulse VFD topology is much simpler, more reliable and less expensive.

Fig. 5 6-Pulse topology with Lineator AUHF AFE Drive technology is not the best solution for a low harmonic variable frequency drive despite claims by their manufacturers. It is true that they reduce the low frequency harmonics introduced by a VFD, but they do so with very significant negative consequences. These include (i) introduction of high levels of high frequency harmonics, (ii) an input passive LCL filter that performs poorly and can resonate with the power system, (iii) higher levels of common-mode ground leakage current, (iv) much higher losses, (v) increased complexity which reduces reliability and (vi) significantly higher costs. A much better solution is the combination of a Lineator AUHF Wide Spectrum Filter with a simple 6-Pulse VFD.. Key advantages are no introduction of high frequency harmonics, compatibility with the power system (including resistance to resonance and low capacitive

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reactance for generators), higher efficiencies, improved reliability and both lower installed and operating costs. Table 1. Efficiency comparison of AFE Vs AUHF

The LINEATOR AUHF will perform as advertised to reduce harmonic distortion caused by AC Variable Speed Drives and other non-linear loads equipped with 3-phase, 6-pulse, and Diode Bridge rectifiers. A properly selected and installed LINEATOR™ will: (i) Reduce Current Total Harmonic Distortion (ITHD), measured at the LINEATOR™ input terminals at full load, to:

Fig. 6 LINEATOR AUHF

(1.) < 8% when background voltage distortion is < 5% and voltage imbalance is < 3% (2.) < 5% when short circuit ratio (Isc/IL), as defined by IEEE Std 519, is < 20 and when background voltage distortion is < 0.5% and voltage imbalance is < 1% (ii) Reduce Current Total Demand Distortion (ITDD), measured at the LINEATOR input terminals over its entire operating range, to levels defined in Item 1 above. (iii) Minimize the contribution to Voltage Harmonic Distortion of all VSD’s equipped with the LINEATOR™ to < 5% total and < 3% for individual harmonics, as defined by IEEE Std 519-1992. (iv) NOT become overloaded by other upstream harmonic sources. (v) NOT resonate with other power system components. (vi) NOT have compatibility problems with engine generator sets properly sized for the load. Computer simulations, using MIRUS’ SOLV software, showed that with the LINEATORS installed, the THD(V) on the ships reduced to well within the required limits of <5% THD(V).

CONCLUSION As the marine industry has moved toward electric propulsion, Pump control and other uses for VSDs, harmonics have become a major power-quality concern. This has led classification bodies to demand that voltage distortion remain below acceptable levels. A new technologies being adopted to mitigate the effects of harmonics have been reviewed, and analysis was carried out By using MIRUS SOLV software. It describes effect of harmonics and improvement in performance of VFD and output power quality by harmonic mitigation using Advanced Ultra Harmonic Filter. The Results of Harmonic Mitigation analysis using MIRUS’ SOLV6 software is shown below, 1. The VFD system is without Advanced Ultra Harmonic Filter and without voltage source distortion and line voltage imbalance. FAIL compliance report says that system generates high harmonics around 17.7 %. 2. By using Advanced Ultra Harmonic Filter with VFD reduces harmonics below 5%. Thus PASS compliance report mentions harmonic contents as 3.8 % which is sufficiently below 5% as per IEEE-519 standards. 3. The energy consumption by both scenarios with and without harmonic mitigation is also compared at the end. 4 Thus the VFD used for ship board systems e.g. Electric propulsion, Seawater cooling, Centrifugal pumps control, Oil platforms etc with the use of proposed harmonic mitigation technique will be electrically safe. 5. The energy conservation, reduction in fuel consumption and reduction in GHG emission is also possible by the use of proposed VFD with AUHF which further leads to concept of eco-friendly green shipping.

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