Our old focus was selling electricity and water. Our mission now is " to provide quality energyand water services that our customers value." We have developed a performance model tomeasure this value. This enables us to match our services to customers requirements and also toprioritize our expenditures on the same basis. The model aligns strategic plans to business plans toaction plan, right through to plan implementation and measures. It is a dynamic model to ensureevery dollar we spend is giving the customer the most value for their money.
Customer demands for lower prices, improved and more flexible services are driving changes atScarborough PUC. We are using advances in technology, a flexible, skilled and value-drivenworkforce and partnerships with the private sector and other public sector organizations to betterserve our customers.
Scarborough PUC is committed to ongoing improvement of its delivery systems and maintainingpower quality to meet customers' needs. System enhancement projects are ranked based on"customer impact costs." The cost to a customer of not having electrical supply or having powerquality problems far exceeds the price charged for a kilowatt-hour. Our comprehensivemaintenance program is aimed at the prevention of customer outages. We use diagnostic testing,such as our infrared scanning program, to identify pending component failures and takeappropriate action.
Like other utilities, we have been facing increased challenges in our social and economicresponsibilities as a result of significant changes in technology, customer demands, and their loadrequirements over the last 30 years. Many types of equipment can no longer tolerate blips, bumps,and momentary outages that always occur in systems. Solid state power electronic technology hasintroduced higher non-linear loads which inject non-sinusoidal waveforms into the distributionsystem. This has caused power quality problems including harmonic pollution, lower powerfactor, and unbalanced currents in the power system.
The harmonic pollution that introduces distorted waveforms to the system may cause undesirableeffects such as instability of converter control; increased system losses; an increase in motorlosses; over heating in transformers, switchgear, and capacitors; and misoperation of protectiverelays, fuses, and metering devices.
Another major power quality problem is the transient voltages which are a result of lightningstrikes, fault-clearing switching operations, switching lines, and capacitor bank switchings. Of these transient phenomena, the overvoltages caused by capacitor bank switching has been ofmost interest to distribution utilities and their customers.
Capacitor banks are used for power factor improvement, elimination of voltage drops on longfeeders, and control of reactive power. The capacitors at the transformer stations are switched onin the morning as the peak load increases to reduce the on-line generation and regulate systemvoltage. At night, the peak load is significantly decreased and therefore the capacitors can beswitched off the line since there is no requirement.
Voltage transients due to the switching operations of the capacitors at the transformer stationsresult in amplified overvoltages which propagate to the downstream distribution feeders causingvariable speed drives to experience frequent trippings. The interaction of the overvoltage, causedby the switched capacitor, with variable speed drives is easily confirmed through correlation of thetime logs for the switching operation and the interruption incidents at the customer's site. Theseovervoltages are compounded by the distorted harmonic waveforms generated by the customer'snon-linear loads and amplified, causing low voltage power electronic devices to fail or the variablespeed drives to trip out.
These interruptions to the normal operation of motor drives and the consequent inconvenience tocustomers have been of significant concern for electric utilities. As a customer-focused utility,Scarborough PUC realizes that our customers expect and deserve continuous supply of qualityproducts and programs. Thus, joint initiatives are undertaken with the customers to investigateand identify the power quality problems to obtain the best solution.
Switching of Capacitor Banks
Figure 1 is a typical single line diagram showing a distribution feeder supplying a low voltagecustomer-owned transformer which feeds a variable speed drive controlling an induction motor.The configuration also shows a switched capacitor bank at the source bus, on the secondary sideof the transformer station for power factor correction and reactive power/bus voltage control.
The variable speed drive consists of either a six-pulse or twelve-pulse converter which rectifiesthe input AC voltage to a DC voltage, makes it available to a DC link, and allows an inverter tochop the DC signal and change it to a variable frequency AC signal used to control the inductionmotors. Those variable speed drives which are of the voltage source inverter (VSI) type use alarge capacitor in the DC link between the rectifier and inverter to smooth the DC voltage andprovide a relatively constant DC voltage. Figure 2 shows the typical AC voltage and currentwaveforms of a VSI drive controlling an induction motor when no switched capacitor is in serviceat the transformer station. Note that both waveforms are distorted due to the non-linearcharacteristic of the drive systems.
Figure 3 illustrates the waveform for the voltage at the main service entrance, where the capacitorat the transformer station has been energized causing overvoltage in the supply voltagepropagated to the main service entrance. Before the capacitor switching, the supply voltage wasat peak of 23.38 kV L-N (nominal peak voltage of 22.53 kV L-N). As a result of the capacitorswitching, the medium voltage supplying the customer transformer reaches a peak voltage of32.9 kV L-N which is 41 per cent higher than its steady state value.
The overvoltage at the primary of the customer-owned transformer is reflected to its secondaryside causing overvoltage distortion of the AC input voltage to the motor drive during the transientperiod. Figure 4 shows the 600V of the nominal input voltage to the motor drive reaches to avalue of 707 V from its steady state 580 V peak as a result of switching operation of the capacitorat the transformer station.
It has been reported that the switched capacitors have resulted in nuisance tripping of the motordrives due to the capacitor switching operation and its interaction with customer's power factorimprovement capacitors and the drive's DC link capacitors. The drives are mainly tripped due tothe excitation of the L-C circuit formed by the capacitors at the customer's side and the inductorsin the system.
The L-C circuit is formed as a result of either of the following two different situations:
Either of the above L-C circuits can be excited by the switching operation of the capacitor bank(s)located at the transformer station. When a capacitor is energized, a transient oscillation occursbetween the capacitor and the system inductance. The switching performed on the capacitor bankcan result in magnified transient voltages on the customer's bus causing a high surge current toflow through the DC link capacitor.
As a result of this situation, a high magnitude DC overvoltage is resulted in the motor drive. Ifthis overvoltage (which is of a transient nature) is not controlled, electronic devices in the drivesuch as capacitors, thryristors, diodes, and other power electronic devices which are sensitive tothe overvoltages, will fail and severe damage to the equipment will occur. In fact, the high energygenerated by these transients may exceed the energy duty of the MOVs which are used to protectthe electronic components at the motor loads.
To protect the drive and its components, the DC bus voltage is monitored and the drive is trippedif the DC voltage exceeds a preset value such as 1.2 times its rated value. For the drives rated inrange of 800 to 840 V, the trip level is typically set at 950 V.
Magnification of the Harmonic Components
Solid-state electronic devices have had many applications in industrial facilities for variable speedAC and DC motor drives and motor controllers. These devices are known to inject non-sinusoidalcurrent waveforms into the distribution system causing damage to different electrical equipment .
The AC/DC converters are the main source of harmonics resulting in distorted voltage andcurrent waveforms in the distribution system. These waveforms consist of harmonic componentson both the AC and DC sides of the converters and are generated as a result of non-linear loads.For a six or twelve pulse converter, the order of the characteristic harmonic currents is given inTable 1 as the amplitude of the harmonics decreases with increasing harmonic order.
An examination of the harmonic spectrum of the AC input current to the six pulse convertershows that the fifth, seventh, eleventh, and seventeenth harmonics are the most pronouncedharmonic contents of the waveform. However, due to the imperfections in the operation of thedrive systems, e.g. lack of AC side symmetry and un-equal firing of valves, the non-characteristicharmonics also exist. Thus, the current and voltage waveforms contain all harmonic components.
The problems of distorted currents and voltages can be significantly magnified if excitation of theresonance condition occurs as a result of the addition of the power factor correction capacitors atthe low voltage customer bus. The high impedance resonance occurs at a frequency determinedby the inductance of the source system and the capacitance.
A time-domain simulation using Electromagnetic Transient Program (EMTP) was performed topredict the harmonic spectrum of the distorted voltage waveform of Figure 3, as recorded at themain service centre. The measured harmonic contents of that voltage waveform as shown inFigure 5 show that the results of the simulation are completely correlated with those of themeasurement and the characteristic and non-characteristic harmonics which were predicted by theEMTP simulation are verified by the measurement (see Appendix A).
If this parallel resonance frequency coincides at or close to one of the characteristic ornon-characteristic harmonic currents generated by the variable AC drive, then the distortedharmonic waveform of the voltage is magnified. If the electrical system has natural frequencies inthe vicinity of generated harmonics, overvoltage and/or overcurrent can be experienced in thesystem. In this case, there is a possibility for continuous excitation of system harmonics that aregenerated from the non-linear loads. This situation magnifies voltages and propagates them to theDC side of the motor drives causing them to trip. Figure 6 provides an overview of the impacts ofthe switched capacitor operation on the distribution system for different possible scenarios.
Controlling the Overvoltage
One of the common methods for limiting the transient overvoltage of the DC side of the drive,during the capacitor switching, is to place a reactor between the convertor and invertor of thedrive. This addition of the DC link will decrease the peak overvoltage and prevents multiplenuisance tripping.
Another approach is to place the isolation transformers or reactors on the AC side in series withthe convertor AC input terminals. Study results indicate that the addition of the reactor on the ACside is a more effective way for reducing the peak overvoltage on the DC side as compared to itsplacement in the drive's DC link. Figure 7 depicts that if a 10% reactor is instal-led in front of avariable speed drive, the overvoltage is clipped and the problem is mitigated. Without this reactor,the level of the overvoltage will be unacceptable resulting in nuisance drive tripping. The size ofthe reactor should be optimized; otherwise the reactor will be ineffective or may be damaged if itis undersized, or it can magnify the harmonic distortion if it is oversized.
Parallel type zero threshold surge suppressors which are capacitor based and respond to theovervoltages may be preferred to the series reactors since they do not dissipate extra energy anddo not require down time for installation or repair .
One of the effective ways of decreasing the impact of the overvoltages at the low voltage systemis to prevent the excitation of the characteristic or non-characteristic harmonics and solve theharmonic problems. This can be done by converting the power factor correction capacitor banksat the customer's site to the harmonic filters. In this regard, there are two approaches which offersolution to harmonic filters. The first approach is geared to the selection of capacitor bank sizes tokeep the resonance harmonic orders away from characteristic/non-characteristic harmonics. Thesecond approach is to remove the characteristic/non-characteristic harmonics of interest from thesystem by utilizing tuned filters .
Harmonic currents can be controlled from flowing through the power system by diverting themthrough a low impedance shunt path. The shunt path can be designed as either a single-tuned orhigh-pass filter to minimize the voltage distortion caused by the harmonic contents. The powerfactor correction capacitors can be converted to the filter banks by adding series reactors which incombination is detuned to the series harmonic contents.
Conclusions
Switched capacitor operation at the transformer stations can cause overvoltage transientphenomenon on the customer's system causing undesirable trippings of variable speed drives.These trippings depend not only on switching operations, but also are functions of the capacitorsizes at the motor drive and the customer's plant that are used for power factor correction. Thelevel of trippings can be significantly increased if the interaction of the transformer stationcapacitor switching operation excites one of the harmonics which are generated by the customer'snon-linear motor loads. The interruptions can be minimized by utilizing series reactors at the driveand converting the power factor correction capacitors to harmonic filters.
D. Babaie-Azadi, M. Stoddart, K. Allen and J. Bailey are with Scarborough Public UtilitiesCommission. M.R. Iravani is with the University of Toronto.
