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DspEdu 2.1 Harmonics When the System is on Grid Supply The Neutral Current - Grid Supply Harmonics When the System is on DG Set Single-phase UPS Loads and System Neutral Current Single-phase input UPS Versus Three-phase input UPS The UPS Units Employed in the System |
POWER QUALITY AUDIT AT AN ELECTRONICS RESEARCH ORGANIZATION - A CASE STUDY
Suresh Kumar. K.S. Department of Electrical Engineering National Institute of Technology Calicut Calicut-673601, Kerala State , India
There is about 43kVA of Single Phase UPS Load in the form of 1kVA, 2kVA, 3kVA and 5kVA Single Phase UPSs used in various laboratories and Computer Centres in the System. All UPS units in the system are of Single-Phase input type and the front end of all the units are of phase controlled thyristor converter type.These UPS units collectively consume about 22kW of power from the mains and contribute to prominent harmonic distortion of line currents in the System. Heavy distortion of currents drawn result in harmonic distortion of Voltages too, and prominently so when the System is on the 100kVA DG Set. 2. Harmonics When the System is on Grid Supply Single Phase UPS units draw a current which is rich in odd harmonics-the predominant ones being third, fifth and seventh. The third and all other triple-n harmonic currents add in the neutral conductor irrespective of distribution of UPS loads among the three phases. If the current drawn from a transformer has more than 15% T.H.D , the transformer losses increase in a significant manner. However, the resultant harmonic distortion in voltages will be marginal due to small short circuit impedance of the transformer. The waveforms shown below in Figure 1 illustrate these points. These waveforms show the phase voltage and line currents at the transformer secondary at the Establishment recorded on 10-3-1999 at around 2:00PM. These waveforms were recorded one at a time using Tektronix THS 710A DSO and hence do not reflect the correct phase relationships between various quantities.
The line currents are heavily distorted due to the single-phase UPS loads. The THD in R Phase current is 14.5%, that of Y Phase is 16.2% and that of B Phase is 28%. The phase voltages are more or less clean and have THDs less than 5%. The R phase current of the 50kVAr capacitor shows a THD of 15.8% and shows a predominant component at 11th harmonic (See Figure 2 below).
A detailed analysis of current waveforms coming from various feeders were carried out and it was found that there is no load that contributes this much of 11th harmonic in the system. Hence this predominant 11th harmonic in capacitor current and transformer current was seen to be the result of partial harmonic resonance between the 50kVAr capacitor at the main bus and the leakage reactance of the 250kVA transformer. Calculations using 4.52% leakage reactance value (found from the nameplate of transformer) reveal that a 50kVAr capacitor can resonate partially at 11th harmonic with this transformer. This partial resonance stresses the capacitor and transformer in addition to the extra stress brought in by the harmonic currents demanded by the non-linear loads. 3. The Neutral Current - Grid Supply The neutral current at the transformer secondary is shown in the waveform below in Figure 3. It shows a fundamental component and a third harmonic component that is more than even the fundamental component. This third harmonic is exclusively the result of single-phase UPS loads. The r.m.s value of this neutral current is 68 Amps and is almost as much as the r.m.s value of phase currents indicating heavy neutral loading.
4. Harmonic Content When the System is on DG Set However, the harmonic distortion picture changes considerably when the system is run on one of the 100kVA DG Sets. Even small degrees of harmonic distortion in generator currents will result in large amount of harmonic distortion in generator voltages through the large synchronous impedance of the generator. Also harmonic currents cause increased losses in stator and field windings of generator. The harmonic distortion in phase currents and phase voltages in the DG Set were measured during the audit on 10-3-1999 and 11-3-1999 and were found to be excessive (See Figure 4 below). Waveform observation confirmed sizeable distortion in currents and voltages. It is well known that a distorted voltage results in loss of capacity of induction motors and overheating of motors. Also distorted voltage has been identified as a major reason for frequent failures of power factor correction capacitors connected across the motors. [Note-These waveforms were recorded one at a time and hence do not reflect proper phase relationships. They can be used only for harmonic analysis and not for power analysis or power factor analysis.]
The generator output voltage shows prominent distortion and ringing due to large harmonic current flow through its winding impedance. The Generator neutral current has a large third harmonic component that is even more than the fundamental amplitude (See Figure 5).
Switching on the 50kVAr capacitor at the main bus was found to result in a relatively clean output voltage waveform from the generator. This happens due to the filtering action of the capacitor. All the harmonic demand of the load goes through the capacitor instead of through the large synchronous reactance of the generator. This will result in a clean voltage waveform at the expense of overheating and possible burnout of the capacitor. Unequal third harmonic currents in the three phases of the generator brought about by unbalance in UPS load among the three phases will result in third harmonic appearing in line to line voltages. Third harmonic content in line voltage will result in circulating currents in delta connected motors which will go undetected by the overload relay. Also, third harmonic in line voltages interferes with AVR of generator and voltage sensing circuits of servo stabilisers etc. 5. Single Phase UPS Load and Neutral Current The most serious effect of single-phase UPS load is on the neutral current. The triple-n harmonic current components of all the UPS loads will add in the neutral and often this results in a neutral current which is between 1 to 2 times the phase current itself in UPS dominated systems. This leads to neutral conductor overheating and fire hazards considering the fact that quite often the neutral conductor has half the size of phase conductor. And, to add to the trouble if the other loads are unbalanced, there will be the 50 Hz unbalanced neutral current along with triplen harmonic component. This is why it is all the more important to balance all the loads to the maximum extent; especially when the system is on DG Set and the total UPS load in the system is about 20% or more of the DG Set capacity. And here there is about 45kVA of distorting UPS load on the 100kVA DG Set. Triplen harmonics and other higher harmonics of current flowing in various neutral conductors and various earth paths in the system will result in significant high frequency voltage drops due to higher path impedance at high frequencies. This results in high frequency noise voltage difference of large magnitude between neutral and system ground at computer centres and this voltage difference has been reported to have caused maloperation/hardware damage in computers at various establishments. Similar analysis of waveforms was carried out for various feeder currents in the system. Based on the waveform study carried out during this audit the following points are brought to the attention of the Establishment. The nameplate rating of a UPS refers to the kVA that the UPS can output. But the contribution of UPS to the connected load must be its input kVA. This value is generally not marked in the specifications. One can assume about 85% efficiency for the UPS inverter. And about 90% for the input converter(These values refer to UPSs manufactured by reputed firms like M/s Numeric Power Systems, M/s Tata Liebert etc.). The input kVA also has a component to supply the charging power to the battery. Most of the UPS units use phase controlled thyristor converters or uncontrolled rectifier followed by controlled d.c side charger to convert input a.c to d.c inside and such converters are well known to have a low displacement power factor and large harmonic components in input current. Thus, the apparent power factor of such UPS units will be about 0.5 or worse under full load conditions. In fact, the power factors measured for the two 5kVA UPS units at the Computer Centre were worse than this. Due to all these factors the input kVA of a UPS under rated load conditions will be about 2.5 times the rated kVA of the UPS i.e. 12.5kVA for a 5kVA UPS.The sizing of UPS wiring and switches and fuses must be done on this basis and not on the UPS rated kVA basis. (Note - These observations are true only for the type of designs described here) This is why though there is only 43kVA of UPS load (obtained by summing up the UPS ratings) in the system, the net contribution of all the UPSs to the system was measured to be 45kVA of distorted power (22kW active power) at the time of audit even though only few UPSs were loaded to more than 50% of their rating. If all the UPS units in the system are loaded fully, there will be about 100kVA of distorted power flow from transformer/generator and there will be extreme overheating in the case of transformer and extreme voltage distortion and system/computer failures in addition to overheating of equipment if the system is run on DG Set. 7. Single Phase Input UPS Versus Three Phase Input UPS Single Phase input UPS units are less efficient at the input converter side compared to a three phase input UPS of same rating. The reduced efficiency is due to increased loss level in the converter transformer and converter power devices. And single phase input UPS draws large amount of third harmonic from line thereby loading the input line equipment to higher levels in addition to creating harmonic related problems in the system. The third harmonic drawn by all such UPS units will add in the neutral and result in overloading of neutral conductor everywhere. Also, harmful ground potential differences can develop due to high frequency components in the neutral. Distributing equal UPS kVAs among the three phases in the form of single phase UPS units does not result in balanced three phase operation since the installed UPS units may not have equal loading at all times. Even if the UPS loading results in balanced three phase loading the third and higher triplen harmonics drawn by them will be additive in neutral. The third harmonic current in the secondary winding of the transformer sets up circulating third harmonic current in the delta connected primary winding of the transformer, thereby greatly increasing the winding losses and causing overheating. Since this is a circulating current it will not be sensed by the primary over current sensing mechanisms. Thus a Single Phase UPS strategy suffers from the following shortcomings. Increased recurring cost due to lowered efficiency. Increased recurring cost from increased losses in wiring, switches cables, equipment etc. due to higher r.m.s current. Increased recurring loss due to neutral losses. Increased recurring cost due to increased harmonic losses everywhere compared to equivalent three-phase input UPS. Loss of life of all distribution equipment including transformer and DG Sets due to disproportionate overheating of windings and core. Increased losses due to unbalance in three-phase system. Possibility of neutral overloading, neutral burning and fire hazards. Possibility of ground potential differences due to high frequency currents in the neutral. Increased Capital Expense in UPS wiringDue to these reasons this Author is of the opinion that the maximum rating that a single-phase input UPS can have should be 2kVA. All UPSs above 2kVA rating should preferably be of three-phase input type. 8. The UPS Units Employed in the System The details of various UPS units and the PC plus Printer Load on them at the Establishment are shown below. a) Room-PA to the Director 1 kVA UPS - one PC , 1 Laser Printer , 1 Fax Machine b) Library 1kVA UPS - 2 PCs and 1 Printer c) Computer and Communications Lab 3kVA UPS - 11 PCs and one Laser Printer d) Software Lab Two 3kVA UPS Units - 23 PCs and 3 Printers e) CAD/CAM Lab 3kVA UPS - 7 PCs and 3 Printers f) Process Control Lab 3kVA UPS - 4 PCs and some equipment in the Lab g) Power Electronics Lab 2kVA UPS - 5 PCs and one Printer h) Microcontroller Lab 3kVA UPS - 7PCs and one Printer i) Room 214 1kVA UPS - 2 PCs and 2 Printers j) General Two 5kVA UPS Units - 24 PCs and 16 Printers k) Computer Centre Two 5kVA UPS Units - 31 PCs and 4
Printers
It was noticed at the time of audit that all the computers and printers in the Software Lab and Computer Centre were in operation.50% utilisation was observed in all other places except in the General Category. The PCs and Printers in this space are distributed among Staff Rooms, Lecture Halls, Library, Office Space etc and may have a distribution factor close to 4.Discussions with the Site Electrical Engineer revealed that all the PCs and Printers except those in the Staff rooms can be ON simultaneously depending on the Schedule of various short-term and long-term courses conducted by the Institution. When that happens all the UPSs put together will draw about 30kW of power from the system at about 60-70kVA and the harmonic distortion and resultant ill effects will be disproportionately more than what was observed during the audit. IEEE Standards and Relevant Practises clearly suggest that all efforts should be made to keep the THD of transformer currents below 15% in the interest of preserving its life. And with a 100kVA DG Set as standby the distorting load content should have been kept below 30kVA,again in the interest of DG set life and energy efficiency. In view of all this, it is indeed surprising to note that an additional highly non-linear load of 4x5kVA single-phase UPS units was allowed on the system (though temporarily). Fortunately two of those 4 units have virtually no load on them and the cables in the Electrical System are overrated. Considering the fact that the Electrical System already had 23kVA of single-phase UPS load on the system at that time the added 20kVA UPS load should preferably have been of three-phase type. 9. Input-side Loading of UPS Units in the System The input loading on various UPSs were monitored during the audit. Important data is abstracted below. 1. It was noted during waveform monitoring at the Power Panel B that current waveform in B phase is highly asymmetric and with low frequency oscillations. The trouble was located at one of the 3kVA UPSs in the Software Lab. The input side converter was found to draw current in only one half cycle of a.c and was quite erratic in its power drawing behaviour. This was pointed out to the Electrical Engineer at site. He informed later that the UPS Manufacturer was contacted and they identified the problem and rectified it. This brings out another potential problem in UPS application. The input side a.c to d.c converter in UPSs are quite fault tolerant and will go on functioning partially even if one or more power device is burnt out or some minor fault in control electronics takes place. This will not result in malfunctioning at UPS output. But the UPS will not be able to take its full load under this condition and UPS will not deliver its specified back up time. But, in a partially loaded UPS, the user may not notice these. Thus, the input side fault will be noticed only when continued operation with a faulted converter leads to further damage and eventual breakdown. Only a waveform study at the input can help in catching the fault earlier. Operation with a faulted converter leads to asymmetric currents in the system that will result in d.c component in a.c voltage in the system. D.C offset in supply voltage is simply detrimental to loads like fans, air conditioner compressor motors, pumps, input transformers of various electronic equipment in the laboratories etc. Such a d.c offset of about 4Volt was observed during the audit at certain points of supply system due to this malfunctioning UPS on the audit day. 2. The second 5kVA UPS kept outside the corridor in the academic space was metered and showed about 13Amps peak current with a highly irregular waveform that is quite unlike the waveforms observed in other similarly rated UPSs. This UPS is suspected to be faulty and it is recommended that manufacturer be requested to trouble shoot and rectify this unit. 3. The 5kVA UPS Units at Student Computer Centre were metered at their a.c input using Tektronix THS 710A Digital Storage Oscilloscope. 11A r.m.s, 30A peak, 6.2kVA, 3.6kW, 0.58p.f ii) UPS 2 17.5A r.m.s, 48A peak, 9.9kVA, 3.5kW, 0.35p.f The PC load on them is balanced and the active power at the input are about the same. But one of them draws 50% more current at a lower power factor. The UPS manufacturer may be asked to look into this and identify the problem. The 5kVA UPS Units (four of them)
seem to employ simple diode rectifier with a capacitor filter as their
a.c to d.c converter. The input current drawn by them show the waveform
typical of such a converter and reveal a crest factor of 3, again typical
of such units. At 5kVA level a full bridge phase controlled thyristor bridge
with d.c side current smoothing choke is almost mandatory in the interest
of lowered currents, reduced harmonic content and improved power factor.
Everything about these UPSs seems to suggest that it is not such a thyristor
converter inside them.
1. The UPS units currently employed are of right rating for the loads currently connected to them. The Establishment is in an evolving stage and considerable additions to the PC and Printer load is likely to take place with expansion of current activities or emergence of new activities. This Audit recommends that no additional load of any kind be put on the existing UPS Units at this Institution. When load addition takes place, new UPS units will have to be procured or the existing UPS units will have to be replaced with new units of higher rating. In any case, all future UPS purchases should insist on three-phase input type UPSs. In view of the fact that all the existing units are of single-phase input type, replacing the existing units with three-phase input type of suitable rating will be the desirable strategy when new loads are added. 2. Detailed technical specifications
covering the input side converter, battery charging process, inverter control
strategy and devices used in the inverter, inverter and converter efficiency,
UPS Shutdown Software etc should be prepared prior to purchase of new UPS.
Verification by detailed testing or test certificates from ERTL , SAMEER,
Dot etc. seem to be the only methods to ensure the efficiency levels of
the UPS units in the market. The market is replete with UPS manufacturers
who claim near 100% efficiency in their products!
3. The two 5kVA UPS Units in the Student Computer Centre account for about half of the power drawn by all the UPSs put together. When the system is on Grid Supply, the harmonic currents cause considerable overheating and loss of life in the transformer. But when it is on the 100kVA DG Set it is still worse and results in impermissible distortions in supply voltage too. Hence this audit strongly recommends that the four 5kVA Single-Phase input UPS units in use should not be allowed to draw power from the a.c mains when the system is running on one of the 100kVA DG Sets. These UPSs must function from the battery when there is no Grid Power available. 4. It is recommended that
only one of the two 25kVAr capacitors be used for power factor correction.
This is to remove the undesirable harmonic resonance at 11th
harmonic that is taking place in the system at present. This move will
not adversely affect the system power factor or the Maximum Demand level.
5. Modern day PCs have the
capability to manage the power consumed by them and to effect power savings.
They do this by the well known methods of hard disk power shut down, video
shut down, clock speed reduction, entering sleep mode etc. Enabling the
Power Management functions and configuring Power Management for maximum
savings can result in considerable reduction in power consumption by a
PC if the PC idles for large amount of time. The Audit observed such idling
of PCs in various labs on 10-3-1999 and 11-3-1999.Hence this audit recommends
that Power Management in all the PCs in the Institution be enabled and
be configured for maximum power savings. The respective lab/dept heads
may be made responsible for implementing Power Management in all the PCs
under their control. This will result in not only energy savings but
also in alleviation of harmonic distortion in system currents. This will
also reduce the load on air conditioning units.
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