امروزه ارزيابي كيفيت توان داراي اهميت خاصي در مديريت سيستمهاي الكتريكي است . يكي از جنبه هاي ارزيابي كيفيت توان مطابقت آلودگي هارمونيكي با استاندارد صنعت برق ايران و استانداردهاي بين المللي است . اغلب استانداردها مانند IEC61000-40-30 و IEEE519 نياز به اندازه گيري هارمونيكها تا مرتبه 50 ام دارند .

سنسور كيفيت توان دستگاهي است بسيار ساده و با نصب آسان برويترانسهاي ولتاژ خازني و براي اندازه گيري با پهناي باند وسيع توسط CVT است . با نصب اين دستگاه بروي CVT هاي در حال سرويس ، عملاً خللي در كار CVT يعني نمونه گيري ولتاژ براي رله ها بوجود نمي آيد و بطور همزمان براي مانيتورينگ كيفيت توان مي تواند مورد استفاده قرار گيرد .

اين دستگاه كوچك و ساده با دقت بالايي هارمونيك هاي ورودي به CVT را شناسايي و نمايش مي دهد .مناسب نبودن پاسخ فركانسي CVT ها ناشي از ساختار داخلي آنها باعث شده كه دقت اندازه گيري ولتاژ اوليه تنها در فركانس اصلي قابل قبول باشد لذا اندازه گيري دقيق سطوح ولتاژ اوليه نيازمند پهناي باند بيشتر مي باشد كه از طريق خروجي ثانويه امكان پذير نيست .

با بررسي پلسخ هاي فركانس ترانسفورماتورهاي ولتاژ آشكار مي شود كه نسبت تبديل در فركانسهاي بالا خطاهاي قابل ملاحظه اي دارد كه از حد اقل هاي مقدار استاندارد بسيار بيشتر است . ترانسفورماتورهاي ولتاژ خازني بيشترين كاربرد را در سطح پستهاي انتقال و فوق توزيع دارند همرا با قيمت مناسب و دقت در اندازه گيري خوب . اما از آنجا كه آنها تنها در فركانس هاي نامي تنظيم شده اند لذا به تنهايي قادر به اندازه گيري دقيق هارمونيكي نيستند . فن آوري جديد ABB يعني ساخت دستگاه كوچك و كارا PQ Sensor اين امكان را مي دهد كه به راحتي هارمونيك ها را در سطح پستها اندازه گيري نمائيم .

PQSensor™

The PQSensor™ offers a unique, convenient and costeffective method of faithfully reproducing harmonic

content using Capacitive Voltage Transformers (CVT)without the need for Resistive-Capacitor Dividers (RCD)

− The PQSensor™ offers a practical and economical solutionfor wide bandwidth measurements using CVTs.

− Eliminates the need for special high voltage instrument transformers or wide bandwidth voltage dividers.

− Can be retrofitted to in-service CVTs or installed in new units.

− CVTs can continue to be used in normal ways to feed relays and other standard devices and at the same time to be used for power quality monitoring.

− It can be used to detect CVT internal ferroresonance.

General description

A power quality sensor is available to very accurately measure power quality parameters such as harmonics and flicker over a wide bandwidth from sub synchronous to high frequencies. The PQSensor™ current probes are installed in capacitor voltage transformers (CVTs) at the ground connection points in the secondary terminal box. The PQSensor™ signal-conditioning unit can be mounted on the CVT support structure making the retrofit nstallation simple.

Background

Power quality assessment has become an increasingly important requirement in the management of electric supply systems.

This recognition has led to the introduction of several standards for power quality measurement and monitoring. All standards such as IEEE 519, IEC 61000-4-30 and 61000-4-7 and UK Engineering Recommendation G5/4 require measurements up to the 50th harmonic. Flicker standard IEC 61000- 4-15 require measurement of modulating frequency between 0.5 Hz to 33 Hz.

Options for measuring harmonics

If utilities and users are to monitor ferroresonance oscillations, power quality and other wideband transients in high voltage systems, there is a need for cost effective and accurate means to do so. Sophisticated power quality monitors are now available from various manufacturers. The challenge, however, is to provide inputs to these monitors which accurately eflect the phenomena occurring on the monitored system in a cost effective and safe way. The best performance from a conventional device in terms of a wide bandwidth frequency response is ffered by a resistive-capacitor divider (RCD), which is very expensive, has a very limited output and would not normally be present ina substation environment and does not meet safety requirements for isolation between high voltage and low voltage parts. Most power quality monitors are currently receiving their inputs from wound or inductive type voltage transformers (VTs). The advantage with VTs is that they can also provide inputs into onventional revenue meters and relays and therefore may already be present if harmonic measurement is being considered as an add-on. What is not well understood is that wound VT’s have a limited frequency range. Graph

1 shows the performance of typical wound VT (Reference: CIGRE Working Group 36). It can be seen that the frequency response becomes unacceptable around 500 Hz, well below the frequency limit established in the major standards. The upper limit increases for lower voltage class units but worsens for higher voltage class units.

Capacitor voltage transformers have become the dominant technology for voltage measurement at transmission voltage levels because they provide reliable and accurate performance at reasonable cost. CVTs, because they are essentially tuned to the system frequency, are not in themselves capable of harmonic measurement. Because of the prevalence and reasonable cost of this technology, much effort has been expended to add the functionality of harmonic measurement. There is a solution for employing CVTs for harmonic measurement. A power quality sensor, to be used in conjunction with CVTs, has been developed and patented which overcomes the aforementioned objections. One of the important advantages of the technology, in addition to its cost ffectiveness, is the speed of installation in in-service CVTs