In previous chapter we have discussed different power quality problems and the reason in detail. In this chapter we are going to analyse the data which we have obtained from these buildings. The data we are going to analyse is from commercial and residential buildings and it is obtained from the PCC (Point of common coupling). Commercial building includes the office building i.e. Howell building and Tower 'A' which is more an academic building consisting of Labs and different lecture theatres. Similarly the residential building includes a Kilmorey Halls where student are living as university has provided them accommodation. The power quality analyser was installed in these building's PCC for about 5 working days and one week respectively with resolution of 8 minutes.
Before going in to the details and analysing the data we must define that which type of load is attached with these particular buildings. In general when it comes to offices and academics building we have non- linear load which is the main source of power quality issues in the network in terms of generating harmonics. Non-Linear type of loads majorly consists of computers, printers, photocopiers, and other small equipment which most of them are single phase. In commercial building we are analysing will have around 250 or more computer. This huge no. of computer can cause two significant problems as a result of harmonics. First of them is the voltage variations that is generated because of the finite impedance of Lines and second is the overheating of neutral line [ [1] ]. We have to eliminate this problem because this problem will not only disturb the building itself but also those buildings which are located nearby. Now it is a very common practice that we always install neutral lines almost 1.5 times of the ones we are using for the current carrying lines in each phase. Similarly when we are considering the residential building, we have more no. of non- linear loads in form of computers, TVs, Kitchen appliance (Heaters, Boilers, Electric stoves etc.) More over there are more no. of lightening (CFL) as compare to commercial buildings.
As we have mentioned different governing bodies in chapter 2, which have established power quality standard. These standards have limits in which both power supplier and customers should remain to ensure smooth and constant supply of power. We will be analysing and comparing results in accordance with EN-50160 such as measurements of line - phase voltage, line- phase currents, real and reactive power consumption, power factor, flickering and total harmonics distortion.
Commercial Buildings (Tower 'A'-Howell building) at Brunel University:
In this part of the chapter we are going to analyse the results we have obtained from the PCC of Tower A and Howell building. These result will be analyse according to EN-50160.
Frequency:
As we know that frequency of any power network is dependent on the load profile that is attached with a network. Frequency will vary if there is surplus or deficit in generation in respect of demand. If the frequency of any network is changing too much and exceeding the limits defined in the EN-50160 then system may cripple or there will be serious problems in power quality.
Figure : Frequency (Hz)
From above graphical representation, we can observe that frequency is varying because of change in load which is normal. Here we must note that frequency variation is well in range of ±1% (49.5 Hz-50.5 Hz) as described in the EN-50160. The graph also represents that the pattern is almost same throughout the time scale which confirms that there is no serious power quality problems because of this factor.
Current waveform:
In this task we have to analyse the waveform of the current from all three phases and neutral line. This will give us an idea that how and when the maximum current is drawn from the network. Moreover by using the data of unbalance we can check that if the percentage of unbalance is high then we must equally distributed the load in each phase to avoid any power quality issue.
Figure : Line & Neutral Current waveform
Here Figure no. 21 is showing the load current for 3 days and it is clear from the figure that we have a same pattern of load for given time period. In particular we observe that load current and unbalance current increases typically from early morning i.e. 8:00 Am and maintain this profile till late afternoon i.e. 4:00 Pm. As we all know Howell building is office building and the defined time is office timing, so we can expect this pattern of load. Figure 21 also show that the load is not equally distributed because most of the load is single phase. This considers as poor practice that we overload one phase and at the same time other phase is lightly loaded. Due to this we have high current in neutral which can not only increase the losses but possible chances of overheating the equipments. The current in different phases varies from 100A to maximum value of 450A during different time scales.
In figure no. 22, we have particularly taken readings from 26th October as it is considered to be the busiest day due to load current. The maximum current is observed Maximum current at 2:35 Pm in A1 i.e. 472 Amps and similarly minimum current was observed at Midnight i.e. 126Amps in A2. Similarly neutral current was also very high during this day and we observed maximum reading of 111Amps. Reason of high neutral current is unbalance loading of phases and mostly the loads attached with this building is non-linear Load in form of large no. of switch mode power supplies are used in different equipments for example computers, printers, speed drive which are installed mechanical labs lathe machines etc. Because of these machines we have unbalances and voltage distortion. We have also noticed that during night time we are still having considerable neutral current which is because of this non-linear load. It is common observation that most of the staff use to of leaving their computers on sleep mode which still draws current to keep the important parts of computer running so it can restart within no time. Following figure No. 22 gives an overview of the typical day of the Howell building for 24 hours. As we can see that load started to rises from 9:00 am in the morning and by 9:30 am the load reached it maximum value. Usually most of the staff of the university comes into their offices and turn on their PC and other printing devices. At the same time we can see that neutral current started to rise at the same time. We can see that around 2:35pm, we observed the peak load current of 472Amps. Similarly as the office closing time i.e. 5:00 pm, we can see that load current starts to drop but still load current is around 250Amps because most of the PhDs staff keeps on working outside office hours till late night, moreover at night some of the lighting is necessary to maintain enough light so people working during night time don't face any difficulty.
Figure : Load profile on 26th of October
Similarly in Figure no. 23, we can get an idea of unbalance between the 3 phases which shows that load is not equally distributed among them.
Figure : Percentage of Unbalance current between the 3-phases
The current which is introduce by the harmonics in the system in neutral can be more than 100% of phase current and few case studies have shown presence of neutral current between 150%-200% because of high 3rd harmonics current [ [2] ].
In order to avoid the effects of 3rd order harmonics we must put them in groups and regulations should be imposed on their harmonics level with special restriction on negative and 3rd harmonics.
Line Voltage waveform analysis:
Voltage profile is always considered one of the most important areas in power quality analysis. As we have already mentioned above that we are following EN-50160 standard, and it is stated that variation in Low and medium voltages levels must be with in ±10% for 95% of the week. We must also be sure that we are not always providing voltage near to limits which is not good for customer's equipment.
Figure : Typical trend of Line voltage.
Figure no. 24 above is showing the voltage profile for 5 working days of Howell building. This voltage profile is showing that voltage is within permissible range as the minimum voltage of 425.7 Volts was observed on 25th October U2. Similarly the maximum voltage of 445.5 Volts was observed on 27th October on U3.
During our analysis of current waveform, we observed that we usually have high load demand from 9:00 am - 5:00 pm. In figure 25 below we have observed that line voltage keeps on fluctuating between 438 - 445 Volts during high demand condition (circled), but the voltages remained in considerable range ±10% which is defined in EN-50160.
Figure : Line voltage trend on 26th October.
Phase Voltage waveform analysis:
After considering the Line voltages we must also analyse the phase voltage. Following figure no. 26 gives us a brief idea of voltage level for 5 days. As we know that as we are following EN50160, it states that voltage should be in ±10% i.e. 216-264 volts if we assume the nominal voltage of 240 volts. From the figure no. 26 below we can see that voltage is varying from 245V to 257.5 V which is within range of permissible percentage.
Figure : Phase voltage trend over 5 days.
Here we must observe in figure no. 27 which is voltage trend for 26th of October that the voltage is not dropping beyond 245.7 V which is still above than nominal voltage. It is a common understanding in electrical that power delivered to any load is govern by the following equation;
p = v(t) * i(t) = Vmax * Imax * Sin ( t) Sin ( t + θ)
Here it is clear that if we supply excessive voltage to our equipment, it will consume more power which in return will overheat the instrument. This increase the probability that lifespan of different component in the instrument will decrease which in result causing it to mal function or may damage it completely.
Figure : Voltage trend of 26th October
Here in figure no. 28, we have voltage unbalance between the 3 phases. It is clear from the figure that unbalance is not more than 0.3% which is well within range defined in EN-50160 of 2% for 95% of the week in both LV and MV supply voltage.
Figure : Voltage Unbalances between phases.
As we have also discussed in chapter 2, that flicker are also caused by the rapid changes in voltages. They usually occur because of the Intermitted loads, which tends to operate for a very short period of time. These loads can be of any type such as lifts, heavy equipment machines (Installed in Labs of tower A). When these loads operate we suddenly get rapid change in voltage and the same is observed in our data. As figure no. 29 & 30 shows that the magnitude of flickers is less than 0.4% for Pst and less than 0.3% of Plt which is well within range described in EN-50160 & EN-61000-2-2 which is Pst≤1.0 and Plt≤0.8.
Figure : Short term flickers
Figure : Long term Flickers
Real & Reactive Power analysis:
As we have explained in chapter 2 about real and reactive power in detail so here we have analysed the data obtained from this building. We would like to mention that loads attached to the building are not resistive loads which mean that load will have capacitive and inductive component as well for example SMPS( Switch mode power supplies) used for the PC. Due to this we will have all components in of power in our result but we are considering real power consumption. After analysing the data which has span of 5 day we found out that it has moreover similar pattern of power consumption. Power consumption on 26th October, was considered to study the behaviour of power consumption. It was also observed that on this day, the power consumption was on a higher side as compare to other days. Figure no. 31, shows typical power consumption of a working day where we can see that most of the power is consumed during the office timings i.e. 9:30am - 3:00pm. The power consumption in evening and early morning is because of the lighting and the PhD staffs works late in their offices and when they leave the place most of them leave their computers on sleep mode which still consumes power. As we have discussed during current waveform analysis that Phase 1 is slightly more loaded than other 2 phases, so it is visible by the power consumption in Phase 1.
Figure : Power consumption on 26th October.
For same we have also analysed the reactive power consumption is very well balanced throughout the day as shown in the figure no.32. From this we have also noticed that during night time after 10:30pm reactive power is negative which shows that capacitor installed were over compensating for inductive loads.
Figure : Over compensation during night time
Power Factor analysis:
As we have explained in chapter 2 that power factor plays an important role in power generation. If our power factor is not near to unity, then we are wasting some of the generated energy. Following figure no. 33 is giving us a brief idea of power factor in each phase.
Figure : Power factor of a day
In figure 31, we can see that power factor of phase 1 & 3 are much stable as compare to phase 2 which has really poor power factor. There may be several reasons behind this for example; most of non-linear load is attached to this particular phase which is causing harmonics which is affecting the power factor of this phase. To overcome this problem we must divide the load equally or install the power factor correction plant for this particular phase so that we can keep the power factor near to unity.
Total Harmonics Distortion analysis:
As we have already explained about the Harmonic distortion in chapter 2, here we are going to analyse the data obtained from this building. Mostly we observe harmonics up to 40th level but during analysing the result we pay more attention to the first few harmonics i.e. 3rd, 5th, 7th which can put significant effect on the quality of power if not controlled. As we know that Non-Linear load is the source of harmonic current which causes distorted current waveform and when this current passes the system it causes the voltage distortion.
As power system engineers', we know that harmonics in our system can pose serious threat to the quality of power. Some of the common problems that can occur in the system are as follows;
The simultaneous use of capacitive and inductive devices in distribution networks results in parallel or series resonance manifested by very high or very low impedance values respectively. The variations in impedance modify the current and voltage in the distribution network.
The active power transmitted to a load is a function of the fundamental component I1 of the current. When the current drawn by the load contains harmonics, the rms value of the current, Irms, is greater than the fundamental I1.
Derating of the generators by 10% if we have non-linear load up to 30%.
Distortion in supply voltage can disturb the operation of sensitive equipment such as computer, protection relays operating for sensitive equipments and cause distortion in telephone signal.
Size of neutral conductor must be appropriate as current in neutral will approximately equal or greater than the phase current. Similarly the service life of equipment reduces at large such as 32.5% for single phase, 18% for three phase and 5% for transformers.
Data that we obtained from this building contains large no. of Non-linear loads in form of SMPS which are installed in shape of computers, printers and photocopying machines. In this section we are going to analyze both voltage and current harmonics in accordance toEN-50160.
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Figure : Highest Total Harmonic distortion on 29th October.
In figure no. 34 above, graphical representation of the voltage harmonic distortion is given. We can observe from the given data that Total Harmonic distortion was high during the night time. As we know that during October sunset earlier and the lighting off the buildings automatically turns on from 5:00 pm. Since most of the lighting is CFL which is the major contributors toward the harmonics. As we have mentioned in the figure no. 34, the highest occurrence of Harmonics distortion appeared on 29th of October at 12:11Am on all three phases i.e. THDV1= 3.9%, THDV2=3.5%, THDV3=3.7%. Similarly in figure no. 35 we have plotted the odd harmonics up till 9th Harmonic for the time at which the maximum harmonics appeared in the network. According to the EN-50160 the Total Harmonic distortion and Odd Harmonics 3rd, 5th, 7th and 9th all are well within range. We can reduce the harmonic levels to minimal level by changing CFLs with LED lighting which is linear load.
Figure : Highest harmonics level on 29th October @ 12:11 am
Similarly we have also analysed the current harmonic distortion that was taken from the building. As we know that the configuration of the load attached is mostly non-linear load in form of computer, printers and an elevator which are major contributors toward the harmonics. As we have compared our results with EN-50160 and according to IEEE Standards 1159 - 1995, the total Harmonic distortion of current is 20%. As we have highlighted in figure no. 36, the highest THD was observed is A1=19.1%. Although this value is quite high but still under the limit defined by the IEEE 11599 standards.
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Figure : Total Harmonic distortion of current over 24 hours.
In our analysis we have observed that harmonic level was high in different line current at different time so here in figure no.37, we have particularly analysed the components of Harmonics on 29th October at 04:00 Am. Similar to our result in voltage harmonics, here we have focused on odd harmonics. Here in figure no. 37 below, the 3rd and 5th Harmonics in A1 and A2 are high which is around 12% and 14% respectively.
Figure : Highest harmonics level on 29th October @ 04:01 am
Residential Building (Kilmorey Halls) at Brunel University:
Now the data we are going to analyse is from residential building located at Brunel University i.e. Kilmorey Halls. The power quality analyser was installed in these building's PCC for one week. As we know that this is a residential building so it has more computers, CFL, boilers, electric heaters and cooking equipments. During this chapter we are going to analyse the different parameter taken from the building in accordance with EN-50160. All the major parameter which were analysed in the commercial building, will be analysed for this building as well.
Frequency:
Earlier we have explained the relationship between the frequency and load profile. Here in this building we have analysed frequency parameter for whole week and in figure no. 38 we have particularly analysed the frequency data for 30th October. Here in this figure we haven't noticed much deviation from the nominal value of 50 Hz. As we know that according to EN-50160 system frequency should not vary ±1% (49.5 Hz-50.5 Hz) so the graphical representation below, the frequency is in between 49.9 Hz -50.1Hz which is considered very safe margin and no serious power quality problems will be caused by this factor.
Figure : Frequency Variation on 30th October.
Current waveform:
Here we have to analyse the waveform of the current from all three phases and neutral line for residence hall which will give us an idea that when the maximum current is drawn from the network. Similarly we will get an idea of neutral current and unbalances between each phase.
Here in Figure no. 39, graphical representation shows the load current for whole week. It is visible from the figure that the trend is almost similar throughout the week. Since it is a residential building we observe that load current and unbalance current increases typically in the evening i.e. 3 Pm and is profile voltage increase to its maximum level till 7:30 Pm. As we know that Kilmorey halls is a residential building and most of the student are not in their rooms because of the lectures due to this factor we have minimal current during the morning time and high current demand during evening timings. The figure below also shows the current in different phases varying from 40A to maximum value of 145A during different time scales. We can also observe that there is high neutral current which is almost equal to one of the phase current.
Figure : Kilmorey Halls Line currents
In figure no. 40, we have particularly analysed readings from 3rd November as we observe the peak load current. The maximum current is observed at 7:38 Pm in A3=146.2A, similarly minimum current was observed early morning i.e. 40Amps. Similar to phase current, neutral current have the same trend as it was low during the day time and very high during this evening and the maximum reading of 60Amps was observed twice. Earlier we have explained the reason of high neutral current is because of the unbalance between phases.
Figure : Highest activity day i.e. 3rd November.
As we know that this is a residential building and it has electric kitchen equipment so we observe high current of 146.2 Amps during late evening i.e. 7:30Pm because most of the residents prepare their dinner. The highest neutral current of AN=62A was observed at 2:14 Pm where the phase current observed were A1= 61.8A, A2=131.1A, A3=72.1A. This high neutral current will cause overheating in neutral cable. The main reason for this high neutral current is non-linear load. In three-phase circuits, the triplen harmonic current (third, nine, etc.) add instead of cancel. Since it is three time of the fundamental power frequency and equally spaced by 120 electrical degrees with respect to the fundamental frequency, the triplen harmonic currents are in phase with each other, and add in the neutral circuit. This high neutral current in power systems can cause feeders, transformers to overload. [ [3] ]
Line Voltage waveform analysis:
Since we have explained the importance of the Voltage profile in last case study, we have analysed the data obtained from this building in the same way in accordance to EN-50160. Figure no. 39 below is showing the voltage profile for 1 week of Kilmorey Halls. This voltage profile is showing that voltage is within permissible range as the minimum voltage of U2=420.2 Volts was observed on 3rd October at 12:01am. Similarly the maximum voltage of U1=444.1 Volts was observed on 3rd October. As the graphical representation is showing that voltage is in range to the nominal voltage but it is still high. Here in figure no. 41, we have not observed any large voltage disturbance which is causing power quality problem and requires rectification.
Figure : Phase Voltage trend for a week
During our analysis of current waveform, we observed that we usually have high load demand during late evening. In figure 42 below we have observed that line voltage keeps on fluctuating between 420 - 444 Volts. At mid night we observed that voltage dropped to a level of 420.9Volts as it is mentioned by red circled, this could be a reason of under compensation of reactive power as load suddenly increased for instantaneous time but the voltages remained in range ±10% defined by EN-50160.
Figure : Voltage trend for 24 hours
Phase Voltage waveform analysis:
Graphical representation in figure no. 43 below gives us a brief idea of voltage level for 01 week. Since we are following EN50160, it states that voltage variation must be in ±10% of the nominal voltage i.e. 240 volts. As we can observe from the trend of the waveform, mostly voltage remained above the defined nominal voltage which is not suitable for equipment as more voltage means it will consume more power which will only produce heating effect and this will cause degradation of equipment before time.
Figure : Phase Voltages for 1 week
Here in figure no. 44, we have unbalance between the 3 phases but it is clear from the figure that unbalance is not more than 0.3% which is in range defined byEN-50160 for both LV and MV supply voltage.
Figure : 24 hours trend of 2nd/3rd October
While analysing the short and long term flickers, we observed that for a certain time period we have high short term flickers in our system and it is shown in figure no. 45. This short term flicker was observed at 10:19:44Pm where its value was 1.03%. As for this case study we have increased our sampling time to approximately 17 minutes so we assume that the value have remained same until next reading (Although flickers appears for a very short time). The magnitude of flickers is approximately 1.0% which is on the higher end of the limit defined in EN-50160 & EN-61000-2-2 which is Pst≤1.0 for 95% of week.
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Figure : High Short term flicker effect.
Real & Reactive Power analysis:
Kilmorey hall is a residential building and it consist of all types of loads for example SMPS which is used for the PC. Due to this we will have all components of power in our result but we are considering real power consumption. After analysing the data 01 week, power consumption followed a similar pattern throughout the week. Power consumption on 3rd November observed on this day was on a higher side as compare to other days as it can be seen in figure no. 46. We can also observe that the power consumption was after midnight as most of the resident go to sleep as need to attend their classes early in the morning. Spike in power consumption usually observed in late afternoon when most of the resident return to their rooms for lunch break. Power consumption starts to increase in late evening i.e. 5:00Pm and then reach to its peak level around 7:30Pm and later on gradually decrease.
Figure : Power consumption on 26th October.
From graphical representation in figure no. 47, we can analyse that reactive power consumption is very well balanced throughout the day but after midnight reactive power is negative which shows that capacitor installed are over compensating for inductive loads.
Figure : Over compensation during early morning.
Power Factor analysis:
In figure no. 48 below, we can see that power factor in all phases is much stable which means that non-linear load attached to this building is equally distributed. The only point that was observed is that Power factor of third phase is more stable than other two phases but it has no effect on power quality because it is nearly unity.
Figure : Power factor of each phase for 01 week.
Total Harmonics Distortion analysis:
As we have analysed this factor earlier, we are going to analyse the result from the residential building as well. The results obtained from this building are different because it is a residential building where quantity and type of load varies.
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Figure : Highest Total Harmonic distortion on 30th October.
In figure no. 49 above, graphical representation of the voltage harmonic distortion is given. Similar to result earlier we can observe from the given data, Total Harmonic distortion was high during the night time because we have CFL and other non-linear loads. Here in above figure no. 49, the highest occurrence of Harmonics distortion appeared on 30th of October at 12:11Am on all three phases i.e. THDV1= 3.5%, THDV2=3.4%, THDV3=3.4%. Similarly we have plotted the components of harmonics of that particular time in figure no. 50. In this graph we have plotted the odd harmonics. As we know from EN-50160, the Total harmonic distortion in both figures lies well within defined range.
Figure : Highest harmonics level on 30th October @ 04:08 am
As in previous section we have analysed the total Harmonic distortion in current so for this section we are going to do the same for this case study as well. Harmonic distortion observed for this case study was very high on all three phases and the pattern was not symmetrical throughout the time period for which the power quality meter was installed in this building. Here in figure no. 51, the highest THD was observed is A2=28.4%. This level of harmonics in the system is very high in accordance with IEEE 11599 standards.
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Figure : Total Harmonic Distortion of current on 30th & 31st October.
Since we are getting very high level of Total Harmonic distortion in current so in figure no. 52, we have particularly analysed the components of Harmonics on 31st October at 09:43 Am. From this bar chart we get the verification that highest harmonic was observed in A2 5th and 7th component. Since the harmonic distortion is crossing the limit defined by EN-50160 of 20% we must carry out amendments in the network to isolate the harmonics level with the range of 20%.
Figure : Highest harmonics level on 21th October @ 09:43 am
Chapter No. 4 Conclusion and Recommendations:-
