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MODELING AND ANALYSIS OF CUSTOM POWER PARK BY PSCAD/EMTDC

PROGRAM

Özel Amaçlı Güç Parkının PSCAD/EMTDC Programında Modellenmesi ve Analiz Edilmesi

Yusuf Alper KAPLAN Mehmet TÜMAY Elektrik Elektronik Mühendisliği Elektrik Elektronik Mühendisliği ABSTRACT

The advances in power electronics based devices to improve power quality

have been increased the interest on Custom Power Park (CPP). CPP concept

means the integration of multiple CP Devices within the Industrial/Commercial

Park, which offers customers high quality of power at the distribution system

voltage level. CPP is one of the most useful solutions to prevent voltage

sags/swells and harmonics from distribution lines.

In this thesis, CPP has been modeled by using PSCAD/EMTDC program. CPP is composed of Static Transfer Switch (STS) and Dynamic Voltage Restorer (DVR). The operation principles of STS and DVR have been explained in detail and then they have been modeled with PSCAD/EMTDC program. Simulation results have been comprehensively investigated. Different types of faults are applied for STS and DVR in CPP and the response of the system for these disturbances are examined. Keywords: Power Quality, Custom Power Park, Dynamic Voltage Restorer,Static Transfer Switch

ÖZET

Güç kalitesini iyileştirmek için kulanılan güç elektroniği tabanlı donanımlardaki gelişmeler, Özel Amaçlı Güç Parkına olan ilgiyi arttırmıştır. Özel Amaçlı Güç Parkı, endüstriel ve ticari park dahilinde çoklu özel amaçlı güç aygıtlarının bütünlüğü anlamına gelip, dağıtım sistemindeki gerilim seviyesinde müşterilere kaliteli güç sunar. Dağıtım hatlarında oluşan gerilim düşümü/yükselimi ve harmonikleri önlemek gerekmektedir ve bunun için Özel Amaçlı Güç Parkı uygulamaları en uygun çözümlerden biridir.

Bu tezde Özel Amaçlı Güç Parkı PSCAD/EMTDC programı kullanılarak modellenmiştir. Özel Amaçlı Güç Parkı, Statik Transfer Anahtarı (STA) ve Dinamik Gerilim İyileştirici (DGİ) kullanılarak oluşturulmuştur. STA ve DGİ’ nin çalışma prensipleri detaylı olarak anlatıldıktan sonra PSCAD programında modellenmiş ve simulasyon sonuçları incelenmiştir. Özel Amaçlı Güç Parkının STA ve DGİ

* Yüksek Lisans Tezi

-MSc Thesis

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kısımlarında değişik hatalar uygulanmıştır ve bu hatalara sistemin tepkisi incelenmiştir. Anahtar Kelimeler: Güç Kalitesi, Özel Amaçlı Güç Parkı, Dinamik Gerilim İyileştirici, Statik Transfer Anahtarı Introduction

Custom power is the employment of power electronic or static controllers in medium voltage distribution systems for the purpose of supplying a level of reliability and/or power quality that is needed by electric power customers sensitive to power quality variations. In other words custom power is intended to protect the customers from interruptions and voltage reductions originating in the utility system as well as those transfered to customers from other customers via the utility system and even internal disturbances. Custom power devices, or controllers, include static switches, DVRs, injection transformers, energy storage modules that have the ability to perform current interruption and voltage regulation functions in a distribution system to improve reliability and/or power quality (Daniel and Sannino, 2003).

Electric power is a form of energy we have come to depend on, so much so, that in many automated product line businesses can not tolerate its loss for even a few tens of milliseconds. With the ever-increasing role of electricity in improving the quality of life, productivity of manufacturing and service industries, and efficient energy use, power electronics will play significant part (Narain G, 1998).

Custom Power is a concept based on the use of power electronic controllers in the distribution system to supply value-added, reliable, high quality power to its customers. For many customers this is a preferred alternative to the customer improvising utility power by their own means, mostly in a band aid manner with numerous uninterruptable power supplies, as is done now. Many utilities are moving in the direction of value-added Custom Power service to their large customers. Simulation study of a custom power park (CPP) is presented. It is assumed that the park contains unbalanced and nonlinear loads in addition to a sensitive load. Two different types of compensators are used seperately to protect the sensitive load terminal voltage. Additional issues such as the load transfer through a static transfer switch, detection of sag/fault etc. are also discussed. The concepts are validated through PSCAD/EMTDC simulation studies on a sample distribution system. Power Quality Problems Power quality can be defined as having a bus voltage that closely resembles a sinusoidal waveform of required magnitude. Increasing number of sensitive devices to variations, the requirement for reducing losses and the

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behaviors of interconnected networks are some reasons which increase the importance of power quality concept. In the industry, power quality polluting loads are increasing day by day. The main reasons for concern with power quality (PQ) are as following (Dong, 2004):

End user devices become more sensitive to PQ due to many microprocessor based controls. Complexity of industrial processes; the re-startup is very costly. Large computer systems in many businesses facilities. Power electronics equipment used for enhancing system stability, operation and efficiency. They are major source of bad PQ and are vulnerable to bad PQ as well. Deregulation of the power industry. Complex interconnection of systems, which results in more severe consequences if any one component fails. Continuous development of high performance equipment; Such equipment is more susceptible to power disturbances. Voltage sag is a fundamental frequency decrease in the supply voltage for

a short duration. Voltage swell is defined as the increase of fundamental frequency voltage

for a short duration. An interruption occurs when the supply voltage (or load current) decreases

to less than 0.1 per unit for a period of time not exceeding 1 minute. Figure 1 shows the common voltage disturbances.

Custom Power Park Concept The layout of the custom power park is shown in Fig.2 The power to the

park is supplied either by source vs1 through the preferred feeder or through the source vs2 through the alternate feeder. Of these, during normal operations, vs1 supplies power through the preferred or primary feeder (PF). During a fault or long duration voltage sag in the preferred feeder, the STS rapidly transfers the supply to vs2 through the alternate feeder (AF).

Figure1 Voltage Disturbances (Ghosh, 2005)

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Figure 2 Layout of Custom Power Park (Ghosh, 2005) The loads in the park are divided into three categories. Load L–1 is unbalanced, load L–2 is a rectifier load, which draws harmonic current, and load L-3 is both balanced and harmonic free. Load L–3 is sensitive or critical, and needs continuous uninterrupted supply. To facilitate this, a power electronic-converter-based device is connected at the CPP bus. The DVR can inject a voltage in series to cancel the unbalance and harmonics created in the CPP bus so that load L–3 gets balanced supply. Therefore all three loads get balanced voltage irrespective of harmonics and distortion in L–1 and L–2. As a consequence, the current drawn through the feeder is balanced and sinusoidal. The capacitor Cf is provided to bypass the harmonic currents generated by the nonlinear load L-2. The custom power park is also equipped with a thyristor based STS, which can transfer the CPP load from the PF to the AF in case of a voltage sag or fault in the PF. In case of a catastrophic failure in which both feeders are lost, the diesel generator (DG) set is turned on for auxilary supply. In the interim period when the feeders are off and the DG is not on, the solid state converter based device (DVR ) regulates the voltage of the critical load L–3 (Ghosh, 2005). Static Transfer Switch The STS is used in uninterruptible power supply systems and in distribution networks to provide connection to alternate sources of ac power for critical loads when the main sources fail.

The STS can be used very effectively to protect sensitive loads against voltage sags, swells and other electrical disturbances. The STS ensures continuous high-quality power supply to sensitive loads by transferring, within a time scale of milliseconds, the load from a faulted bus to a healthy one.

To ensure continuity of electrical power supply with sensitive process controls, a critical load is normally supplied from two independent sources, one being the primary selection. Tradiationally, the sources are often connected to mechanical switches incorporating controls that can recognize loss of the main

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power source and then automatically transfer to the alternate source, thus maintainig a highly reliable source of power.

However, as processes and process controls have become increasingly sensitive not only to loss of the power source but also to fluctuations in the voltage supplied (i.e. voltage sags and swells), mechanical transfer switches can not transfer quickly enough to eliminate customer interruptions when such interruptions occur.

The Static Transfer Switch (STS) essentially consists of a pair of back-to-back thyristor switches. It takes the place of the mechanical transfer switch and enables a seamless transfer of energy from the main source to the alternate source in order to avoid service interruption. As a result, this arrangement can provide reliable power to the customer (Mokhtari, Dewan and Iravani, 2002).

Dynamic Voltage Restorer

The dynamic voltage restorer (DVR) is a power quality device that has gained an increasing role in protecting industries against disturbances such as voltage sags and swells related to remote system faults. Power quality has become an increasingly important topic in the performance of many industrial applications. One of the major issues in improving power quality in distribution networks is the mitigation of voltage sags. The effect of voltage sag can be very expensive for the customer because it may lead to production downtime and equipment demage. Voltage sag can be mitigated by voltage and power injections into the distribution system using power electronics based devices. Different approaches have been proposed to limit the cost causes by voltage sag. One approach to address the voltage sag problem is the use of dynamic voltage restorer (DVR) which is a custom power device employing gate turn off thyristors. During a sag disturbance, the DVR uses a power electronic inverter to inject voltage in series with the source. The amplitude and the phase angle of the injected voltages are variable thereby allowing control of the real and reactive power exchange between the device and the distribution system. Short time energy storage is used to provide the supplemental energy through the inverter to keep the load voltage at acceptable levels. The voltage sensitive equipment in industrial sector has made industrial processes more vulnerable to supply voltage deviations. Such voltage deviations in the form of voltage sag, swell or temporary outage cause severe process disruptions resulting in millions of dollars of loss of revenue. Therefore power supply authorities as well as customers have been desperately looking for a cost effective solution currently to ride through momentary power supply disturbances. As such, the preposition of a novel custom power device called dynamic voltage restorer (DVR) for compensating voltage disturbances in distribution systems has generated a great deal of interest recently. Apart from the DVR, some researchers have proposed several other devices to mitigate momentary disturbances.

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Modeling Of CPP By Using PSCAD/EMTDC Figure 3 shows the PSCAD modeling of Custom Power Park. The power to the park is supplied either by source vs1 through the preferred feeder or through the source Vs2 through the alternate feeder, the STS rapidly transfers the supply to Vs2 through the alternate feeder. The DVR can inject a voltage in series to cancel the unbalance and harmonics created in CPP. The custom power park also equipped with a thyristor based STS, which can transfer the CPP load from the preferred feeder to the alternate feeder. System Quantities Values System frequency 50 Hz Source Vs1 12 kV (L-L), phase angle 0º Source Vs2 12 kV (L-L), phase angle 0º

Preferred feeder (PF) impedance 0.05 + j0.0048

Alternate feeder (AF) impedance 0.05 + j0.0048 PSCAD/EMTDC Simulation Tool PSCAD/EMTDC is an industry standard simulation tool for studying the transient behaviour of electrical networks. Its graphical user interface enables all aspects of the simulation to be conducted within a single integrated environment including circuit assembly, run-time control, analysis of results, and reporting. Its comprehensive library of models supports most ac and dc of power plant component and controls, in such a way that, custom power systems can be modeled with speed and precision

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Figure 3 CPP Model in PSCAD

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Simulation Results of STS Part

Figure 4 and Figure 5 shows the addition of line currents of preferred source and alternate source which were measured from the high voltage side of the transformer.Figure 5 represents preferred source currents and figure 6 represents alternate source currents. It is seen that load is feed from only one of the sources, there isn’t any source paralleling and there is not any cross currents. The critical load is supplied from the alternate feeder at the fault time. Fig 4 shows the Valt signal at the fault time. Vprf is “off ” and Valt signal is “on” at the fault time. After fault they return their initial values

Figure 4 Valt Signal at the fault time Fig 5 shows the preferred phase currents at the fault time. It can be seen from Fig. 4 and Fig. 5 that even though the interruption occurs at 0.2 s and the transfer signal is generated at the fault time.

Main : Graphs

0.00 0.10 0.20 0.30 0.40 0.50 ...

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Figure 5 Preferred Phase Currents Figure 6 shows the alternate phase currents at the fault time. It is clearly seen that load is supplied from alternate feeder at the fault time. After the fault load is supplied from the preferred feeder.

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Main : Graphs

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Figure 6 Alternate Phase Currents Figure 7 shows the load phase currents it can be seen that the fault has no effect on the load phase currents.

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Figure 7 Load Currents Simulation Results of DVR Part Figure 8 shows the load voltage. There is a fault at 0.16 sec and it is 0.05 sec duration. It is seem that at the fault duration the load voltage is undisturbed. Simulations have been carried out to evaluate the performance of the DVR in a distribution system under various operating conditions. From the simulation results, the designed DVR responded well in compensating voltage sags. The harmonics generated in the DVR can be significantly reduced by connecting a pasive filter to the system.

-8.0

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Figure 9 illustrates the phase load voltage after the DVR. The result shows that fault has no effect on the CPP load voltage at the fault time. At the fault time DVR injects a voltage to the system.

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Figure 9 Phase Voltage After DVR CONCLUSIONS This thesis has presented electromagnetic transient models of a custom power park. The park is composed by STS and DVR. The highly developed graphic facilities available in PSCAD/EMTDC were used to conduct all aspects of model implementation and to carry out extensive simulation studies.

Custom power park offers to the customer; no interruptions, low harmonic voltages, and acceptance of fluctuating and non-linear loads without effection terminal voltage.

In this thesis custom power park concept has been studied. Advantages of custom power devices have been pointed out. CPP has been modeled by using PSCAD/EMTDC program. CPP is composed of Static Transfer Switch (STS) and Dynamic Voltage Restorer (DVR). The operation principles of STS and DVR have been explained in detail and then they have been modeled with PSCAD/EMTDC program. Simulation results have been comprehensively investigated. Different types of faults are applied for STS and DVR in CPP and the response of the system for these disturbances are examined. The methodolgy of the integration of the custom power devices to the park is also discussed.

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A.GOSH, “Performance Study of Two Different Compensating Devices in a Custom Power Park”, IEEE Proc. Gener. Transm. Distrib. Volume 152, no:4, July 2005

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NARAIN G. HINGORANI, “Overview of Custom Power Applications” Summer Meeting Panel Session on Application of Custom power devices for Enhanced Power Quality. IEEE/PES, 1998