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Harmonics in system by UPS:
- UPS or inverter supplies also perform with poor efficiency and inject more harmonic currents in case of unbalances in the system
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Decrease Life cycle of Equipment:
- Unbalanced Voltage increase I2R Losses which increase Temperature. High temperatures, exceeding the rated value of a device, will directly decrease the life cycle of the device and speed up the replacement cycle for the device, and significantly increase the costs of operation and maintenance.
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Relay malfunction
- Unbalanced Voltage flows Negative and Unbalanced Voltage of Voltage or Current.
- The high zero-sequence current in consequence of voltage imbalance may bring about malfunctions of relay operation or make the ground relay less sensitive. That may result in serious safety problems in the system.
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Inaccurate Measurement
- Negative and zero-sequence components of voltages or currents will give rise to inaccurate measurements in many kinds of meters.
- The imprecise measured values might affect the suitability of settings and coordination of relay protection systems and the correctness of decisions by some automated functions of the system.
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Decrease Capacity of transformers, cables and lines
- The capacity of transformers, cables and lines is reduced due to negative sequence components. The operational limit is determined by the RMS rating of the total current, due to ‘useless’ non-direct sequence currents the capacity of equipment is decrease.
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Increase Distribution Losses
- Distribution network losses can vary significantly depending on the load unbalance.
- Unbalance load increase I2R Losses of distribution Lines.
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Increase Energy Bill by increasing Maximum Demand
- Unbalanced Load increase maximum Demand of Electrical supply which is significantly effects on energy bill. By load balancing we can reduce energy bill.
- For Energy Consumption Energy Supply Company does not charge on kVA but on kW for Residential customers. This means that they are charged for the “actual” energy used and not charged for the “total” energy supplied. Thus the power factor and Maximum Demand do not impact residential customers.
- But Commercial, Industrial and H.T Connection charged by its maximum demand . We have to specify the maximum “demand“(in kVA) at the time of connection. During the month if you exceed your maximum “demand” you have to pay penalty (or extra price) for the same. That is the MDI penalty that appears on electricity bills.
- Let’s assume That Two Company has same approved load of 40 KW and runs 30KW for 100 hours.
- Electricity charge = 65 Rs per kWh
- Demand charge = 210Rs per kW
- Example 1: Company A runs a 30 KW loads continuously for 100 hours but It’s Maximum Demand is 50KW
- 30 KW x 100 hours = 3,000 KWh
- Energy Consumption Charge =3000×65=195000Rs
- Demand difference = 50 KW-40KW=10KW
- Demand Charges = 10X210=2100Rs
- Total Bill: 195000+2100=197100Rs
- Example 2: Company A runs a 30 KW loads continuously for 100 hours but It’s Maximum Demand is 40W
- 30 KW x 100 hours = 3,000 KWh
- Energy Consumption Charge =3000×65=195000Rs
- Demand difference = 40 KW-40KW=0KW
- Demand Charges = 0X210=00Rs
- Total Bill: 195000+0=195000Rs
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Failure of Transformer
- Three-phase voltage with high unbalanced may cause the flux inside the transformer core to be asymmetrical.
- This asymmetrical flux will cause extra core loss, raise the winding temperature and may even cause transformer failure in a severe case.
- Ideally any distribution transformer gives best performance at 50% loading and every electrical distribution system is designed for it. But in case of unbalance the loading goes over 50% as the equipments draw more current.
- The efficiency of transformer under different loading conditions
- Full Load- 98.1%
- Half Load- 98.64%
- Unbalanced loads- 96.5%
- For a distribution transformer of 200KVA rating, the eddy currents accounts for 200W but in case of 5% voltage unbalance they can rise up to 720W.
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Bad / Loose connection of neutral wire
- In balance Load condition Bad connection of Neutral wire does not make more impact on distribution System but in unbalance load condition such type of Bad neutral connection make worse impact on distribution.
- The Three Phase power supplies a small a three-floor building. Each floor of this three-floor building is serviced by a single-phase feeder with a different phase. That is the first, second and third floor are serviced by phase R, Y and B. The external lighting load is connected only on R Phase.
- The supply transformer is rated at 150 kVA and connected delta-grounded wye to provide for 430/220 V three-phase four-wire service.
- This Transformer has a loose or Bad Neutral connection with the earth.
- The transformer delivers a load of 35 kVA at 220 V with 0.9 power factor lagging to each floor.
- During the daytime on, most of the Load of the Building are distributed equally over the three floors which is R Phase=30A, Y Phase =32A, B Phase=38A.
- In Daytime The Bad connection of Neutral does not effected the Distribution system due to equal load distribution of the System
- However it is not case in Nighttime. the Load on Y Phase and B Phase are negligible but R Phase Load is high compare to Y and B Phase.
- In R Phase due to High Electrical Load and The fluorescent lamps flash frequently during the Nighttime of external Lighting Load
- In Night time a bad electrical contact of the neutral wire of the supply makes the high contact resistance between the neutral wire and connector .which was about 15 kΩ.
- This extra high impedance caused an unusually high voltage drop in the phase a circuit. In this case, the voltage of phase a dropped from the normal 220V to 182.5V, about 17% based on the nominal voltage. If the contact impedance goes higher than 20 kΩ, it may result in more serious conditions such as extinguishing all lamps.
- This problem can be removed by fixing the bad connection and keeping the contact impedance near to zero.
| Neutral Wire Contact Resistance | Voltage across bad Connection Point | Voltage across Transformer Secondary Side | ||||||||||
| Day Time | Night Time | Day Time | Night Time | |||||||||
| R | Y | B | R | Y | B | R | Y | B | R | Y | B | |
| Proper Connection (0Ω) | 0v | 0v | 0v | 0v | 0v | 0v | 220v | 220v | 220v | 220v | 220v | 220v |
| Bad Connection (15Ω) | 0v | 0v | 0v | 40v | 0v | 0v | 220v | 220v | 220v | 182v | 220v | 220v |
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Neutral wire broken
- The effect of a broken neutral makes voltage imbalance in a Three Phase Four Wire System.
- For a Three Phase Four Wire System, high neutral wire impedance might enlarge a voltage imbalance (Some Phase Voltage increase while some Phase Voltage decreases).
- High Voltage damage the equipment connected and even destroy on other hand low voltage effect operation of equipments.
- The Three Phase star connected lighting loads are fed by a 430 V balanced three-phase voltage source. The fluorescent lamps are all rated at 220 V, 100 W each. The lamps are not equally in R Phase 5 No’s of Bulbs are connected, in Y Phase 3 No’s of Bulbs are connected and on B Phase 3 No’s of Bulbs are Connected. And, the normal impedance of the neutral wire is 1Ω
- In unbalanced three phase load arrangement, high neutral wire impedance will enlarge the voltage across the neutral wire. The voltages of phases B and C at the load terminal raised to 255 V and 235 V, respectively, and gaining 16.15% and 5.77% based on rated voltage. These abnormally high phase voltages might damage the lamps in phase B and C.
- On the other hand, the voltage in phase A was reduced from 220V to 185V. That might cause the lamps to flash.
- If the broken neutral line problem is fixed, then the three phase voltages will go back to normal in near balanced status .however, if the loads are distributed equally to the three phases this problem can also be removed or minimized.
| Conditions | Voltage across the neutral wire | Voltage at the load terminal | ||||
| R | Y | B | R | Y | B | |
| Normal Condition | 1v | 1v | 1v | 220v | 220v | 220v |
| Neutral Broken | 0v | 0v | 0v | 182v | 255v | 235v |
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Unsuitable capacitor bank installation
- For reducing energy loss, utilities always force their customers to maintain the power factor within a limit. Penalty will be applied to the customers if their loads’ power factors run outside the limits.
- Installation of shunt capacitor banks is the most common and cheapest manner to improve the power factor. However, unsuitable installation (single Phase Capacitor instead of Three Phase Capacitor ) may make it worse.
- The supply transformer is rated at 150 kVA, 11kV/430 V, and supplies a three-phase load of 105 kVA with power factor 0.7 lagging.
- A single-phase 20KVAR capacitor bank is connected to B phase to improve system power. The impedance of the shunt capacitor bank is 1.805Ωper phase.
- This kind of single phase Capacitor installation should make the system unbalanced. This unsuitable installation consumes extra real power of 44355 W.
- The extra real power consumption = 1.732x2XV(RB) / 4xXc =(1.732x2x430) / (4×1.805) =44355W
- This case shows that the system balance should be considered when installing a capacitor bank to correct the system power factor for a three-phase power distribution system.
Remedial Action to prevent unbalances Load:
- All the single phase loads should be distributed on the three phase system such that they put equal load on three phases.
- Replacing the disturbing equipments i.e. with unbalanced three phase reactance.
- Reducing the harmonics also reduces the unbalance, which can be done by installing reactive or active filters. These filters reduce the negative phase sequence currents by injecting a compensating current wave.
- In case the disturbing loads cannot be replaced or repaired, connect them with high voltage side this reduces the effects in terms of percentage and even controlled disturbance in low voltage side.
- Motors with unbalanced phase reactance should be replaced and re-winded.
- Distribution of single-phase loads equally to all phases.
- Single-phase regulators have been installed that can be used to correct the unbalance but care must be exercised to ensure that they are controlled carefully not to introduce further unbalance.
- Passive network systems and active power electronic systems such as static var compensators and line conditioners also have been suggested for unbalance correction.
- Load balancing.
- Use of passive networks and static VAR compensators.
- Equipment that is sensitive to voltage unbalance should not be connected to systems which supply single-phase loads.
- Effect of voltage unbalance on ac variable speed drives can be reduced by properly sizing ac side and dc link reactors
- Tight all Neutral Connections of the System.
- Install Proper size of Capacitor Bank to the System.
- Load Scheduling, where the loads in an electrical network are scheduled in a way to turn on and off at precise times to prevent the overloading of any one phase.
- Manual Load Shifting, where an electrician opens a breaker panel and physically removes the loads from one phase and inserts them onto another phase.
- Load Shedding, where the loads in an electrical network are immediately turned off in order to instantly “rebalance” the phases. This is usually done by ranking the loads in a network by how long they can be turned off before it affects operations
August 14, 2018 at 10:16PM by Department of EEE, ADBU: https://ift.tt/2AyIRVT
