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Ferro Resonance - Introduction,Classification and Characterstics

Introduction
The term "Ferro-resonance ", which appeared in the literature for the first time in 1920, refers to all oscillating phenomena occurring in an electric circuit which must contain at least:

• a non-linear inductance (ferromagnetic and
  saturable),
• a capacitor,
• a voltage source (generally sinusoidal),
• low losses.

Power networks are made up of a large number of saturable inductances (power transformers, voltage measurement inductive transformers (VT), shunt reactors), as well as capacitors cables, long lines, capacitor voltage transformers, series or shunt capacitor banks,voltage grading capacitors in circuit-breakers,metalclad substations). They thus present scenarios under which ferroresonance can occur.
The main feature of this phenomenon is that more than one stable steady state response is possible for the same set of the network parameters. Transients, lightning overvoltages,energizing or deenergizing transformers or loads, occurrence or removal of faults, live works, etc...may initiate ferroresonance. The response can suddenly jump from one normal steady state response (sinusoidal at the same frequency as the source) to an another ferroresonant steady state response characterised by high overvoltages and harmonic levels which can lead to serious damage to the equipment.
A practical example of such behaviour (surprising for the uninitiated) is the deenergization of a voltage transformer by the opening of a circuit-breaker. As the transformer is still fed through grading capacitors accross the circuit-breaker, this may lead either to zero voltage at the transformer terminals or to permanent highly distorted voltage of an amplitude well over normal voltage.
To prevent the consequences of ferroresonance (untimely tripping of protection devices,destruction of equipment such as power transformers or voltage transformers, production losses,...), it is necessary to:

• understand the phenomenon,
• predict it,
• identify it and
• avoid or eliminate it.

Little is known about this complex phenomenon as it is rare and cannot be analysed or predicted by the computation methods (based on linear approximation) normally used by electrical
engineers. This lack of knowledge means that it is readily considered responsible for a number of unexplained destructions or malfunctionings of equipment.
A distinction drawn between resonance and ferroresonance will highlight the specific and some times disconcerting characteristics of ferroresonance.
Practical examples of electrical power system configurations at risk from ferroresonance are used to identify and emphasise the variety of potentially dangerous configurations.Well-informed system designers avoid putting themselves in such risky situations.
Ferro Resonance
The main differences between a ferroresonant circuit and a linear resonant circuit are for a given ω :

• its resonance possibility in a wide range of
  values of C,
• the frequency of the voltage and current waves
  which may be different from that of the sinusoidal
  voltage source,
• the existence of several stable steady state
  responses for a given configuration and values

Classification of ferroresonant modes
Experience of waveforms appearing on power systems, experiments conducted on reduced system models, together with numerical simulations, enable classification of ferroresonance states into four different types.
This classification corresponds to the steady state condition, i.e. once the transient state is over, as it is difficult for a ferroresonant circuit to distinguish the normal transient state from ferroresonant transient states. However, this in no way implies that transient ferroresonance phenomena do not present a risk for electrical equipment. Dangerous transient overvoltages can occur several system periods after an event (for example following energizing of an unloaded transformer) and persist for several power system cycles.
The four different ferroresonance types are:

• fundamental mode,
• subharmonic mode,
• quasi-periodic mode,
• chaotic mode.

The type of ferroresonance can be identified:

• either by the spectrum of the current and voltage signals,
• or by a stroboscopic image obtained by measuring current i and voltage v at a given point of the system and by plotting in plane v, i the instantaneous values at instants separated
  by a system period.

The characteristics of each type of ferroresonance are defined below.
Fundamental modeVoltages and currents are periodic with a period T equal to the system period, and can contain a varying rate of harmonics. The signal spectrum is a discontinuous spectrum made up of the fundamental f0 of the power system and of its harmonics (2f0, 3f0 ...). The stroboscopic image is reduced to a point far removed from the point representing the normal state.

Subharmonic mode
The signals are periodic with a period nT which is a multiple of the source period. This state is
known as subharmonic n or harmonic 1/n.Subharmonic ferroresonant states are normally of odd order. The spectrum presents a fundamental equal to f0/n (where f0 is the source frequency and n is an integer) and its harmonics (frequency f0 is thus part of the spectrum).A stroboscopic plotted line reveals n points.

Quasi-periodic mode
This mode (also called pseudo-periodic) is not periodic. The spectrum is a discontinuous spectrum whose frequencies are expressed in the form: nf1+mf2 (where n and m are integers
and f1/f2 an irrational real number). The stroboscopic image shows a closed curve.


Chaotic mode
The corresponding spectrum is continuous, i.e. it is not cancelled for any frequency. The stroboscopic image is made up of completely separate points occupying an area in plane v, i known as the strange attractor.

Since it is not possible to discuss the different case studies here iam giving the links related to different cases of ferroresonance

Ferroresonace - Link1 Link2
Examples of ferroresonance in a high voltage power system - click here
Modeling Ferroresonance Phenomena in an Underground Distribution System - click here
Examples of Ferroresonance in Distribution sysems - click here

MicomPXXX(PDF)

Hipot test اختبار الهايبوت المقصود منه

ماذا يعني اختبار “Hipot” ؟

اختبار الاستمرارية هو اختبار مألوف للعديد من الناس , فاختبار الاستمرارية يستخدم لتدقّيق الارتباطات و التوصيلات الجيدة Good Connections حيث يكون التأكد في الاختبار من سريان التيار الكهربائي من نقطة إلى نقطة أخرى.

أما اختبار هيبوت Hipot فهو ليس مألوفا للعديد من الناس , فكلمة “هيبوت Hipot ” هي مختصر يعني جهد عالي (فولطية عالية) , فاختبار هيبوت هو لغرض التدقّق من العزل الجيد , فيعمل اختبار هيبوت للتأكد بأن لا تيار سيمر من نقطة إلى أخرى , فهو بشكل من الأشكال اختبار نظير لاختبار الاستمرارية.

في حالة بسيطة في اختبار هيبوت يأخذ موصلين معزولين ويطبق عليهما فولطية عالية جدا ويراقب سريان التيار بعناية , و في الحالة المثالية سوف لن يسري تيار كثير , فإذا مر تيار أكثر من اللازم فهذا يعني أن النقطتين أو الموصلين لم تعزل بصورة حسنة و بذلك يفشلا في الاختبار.

لماذا نختبر بفولطية عالية ؟ Why high voltage test?

نستعمل اختبار الهيبوت لتأكيد أن عندنا عزل جيدة بين أجزاء الدائرة , فالعزل الجيد يساعد في امتلاك الضمان والأمان النوعي في الدوائر الكهربائية.

فاختبارات الهيبوت تساعد في إيجاد الحزوز في العزل أو العزل المسحوق ، أو سلك ضالّ محصور أو ضفائر الحماية أو موصل ملوث أو عزل متآكل أو هناك مشكلة في البعد بين طرفية وهذا ما يجدث عادة في الكابلات و من ثم تسبب للأداة بالفشل.

ما هي أنواع الاختبارات بالفولطية العالية؟ What kinds of high voltage tests are there?

بصورة مختصرة جدا هناك ثلاثة اختبارات عامة بالفولطية العالية وهي.

· اختبار انكسار العازل Dielectric Breakdown Test

· اختبار تحمل مقاومة العازل Dielectric Withstanding Test

· اختبار مقاومة العزل Insulation Resistance Test

كيف تؤثر اختبارات الهيبوت على النوعية؟ How dose “hipot” tests affect quality?

كلّ هذه الاختبارات هي الأدوات التي يمكن أن تستعمل لفهم بشكل أفضل كيف سيؤدي كابل كهربائي مثلا عمله و كذلك لمراقبة أيّ تغييرات في أداءه مستقبلا.

فاختبار انهيار و انكسار العازل Dielectric breakdown testing يستعمل في مراحل تصميم و تأهيل المنتجات.

في الكثير من المواصفات يتطلب إجراء اختبار تحمل مقاومة العازل Dielectric Withstanding Test كما هو في الكابلات على سبيل المثال ويجرى الاختبار عادة في حوالي 75 % من فولطية التوقّف المثالية Typical breakdown voltage , وهو يعمل كشبكة أمان.

إنّ هذا الاختبار حسّاس إلى الأقواس الكهربائية Arcs أو الهالة Corona لذا نجد مشاكل في المباعدة بين أطراف الكابلات في صناديق الربط في خلايا المفاتيج الكهربائية لقواطع الدورة في أغلب الأحيان.

إنّ اختبار مقاومة العزل Insulation Resistance test فهو اختبار نموذجي يعمل على كلّ سلك يراد اختباره , فهو يعمل عادة في جهد 300 – 500 فولت دي سي (DC) مع مقاومة 100 – 500 ميكاأوم , وهذا الاختبار حسّاس جدا إلى التلوّث خلال عملية التجميع , كجريان اللحيم و الزيوت و الرطوبة.

الآن جاء دور السؤال الذي أثير في موضوع صيانة قواطع الدورة للفولطية المتوسّطة Maintenance Of Medium Voltage Circuit Breakers التحقق من الفراغ خلال عمليتي التدقيق و الصيانة وهل نفحص قنينة الفراغ ونختبرها كعمل نموذجي وكما لو كانت هي في المصنع ، أو هل يجيب علينا أجراء ذلك الاختبار عند أجراء فحص القبول؟

يذكر في أحد المواصفات أن أجري الاختبار الحقلي فيجب أن يكون معمول بموجب توصية و تعليمات المنتج.

فأن هذا الفحص يكون قد أجري في المصنع فأنّ اختبار الحقل يكون متروكا لأنه ليس من الإجراءات القياسية , لأن المواد لا تشحن قبل أن تكون مفحوصة.

هناك من يعترض على ذلك بحجة أن تكون المواد قد تعرضت للتلف خلال الشحن , فكيف سيحدد الضرر من الشحن أن لم يجرى الفحص الموقعي , وهذا جدل بدون طائل المعدات عادة تكون تحت بند الضمان و الشركات المنتجة تكون حريصة سمعتها فأن حصل شيء لا سمح الله فسيقوم المنتج بتقديم البديل والحل المناسب للعميل , لذا فأن اغلب المستهلكين قد استغنوا عن فحص سلامة قنينة الفراغ , علما بأن فحص اختبار الـ DC من سلامة قنينة الفراغ هو عديم الفائدة , لذا تؤكد NETA بتوصيتها على عمله بقول “إذا” كان ذلك قابل للتطبيق و بعد “موافقة المنتج و الالتزام الصارم ببياناته المنشورة “.

SUBTRANSMISSION UNDERGROUND CABLES (اختبارات الكابلات

 PROCEDURE

6a  IR Test
The Insulation resistance (IR) test measures the dc resistance of the insulation of the cable
installation using an Insulation Resistance Tester. It involves measuring both the ph-ph & ph-earth insulation resistances. Because of cable capacitance, the IRT shall be applied until a stable reading is obtained.
IR tests shall be performed after laying and prior to jointing each section of cable.

Acceptable IR values are: 

Screen to Earth test will record an IR after one (1) minute. This test shall be performed first to assist in the reconnection process of the screen to earth connection.
Phase to Screen and Phase to Phase testing requires the measurement of IR at 1kV after one (1) minute duration.
Note : IR tests shall be applied for a period of 1 minute for cables less than 100m and 3 minutes for cables greater than 100m in length.
6b  Conductor Resistance Test
This is carried out to determine the effectiveness of the conductor joints and terminations by
measuring the dc loop resistance. Results are compared with the manufacturer’s conductor
resistance usually expressed in ohms/km in the cable specification.
6c  Phase Identification 

Phase identification should be checked by the use either of the phasing resistors or a continuity check of each individual core wherever possible after all jointing work has been completed. Alternatively, the current injection phasing method may be used. Phase identification checks must be carried out by jointing staff to ensure cables have been
connected correctly in line with the system phasing diagrams. Phase identification relates to both conductors and sheaths.
6d  Cross Bonding Test 

Cross bonding tests on the cable sheaths shall be performed to verify the integrity of the cross bonding system. This test ensures circulating sheath currents generated by induced voltages at full load will not adversely affect the cable rating.
This test involves injection of a 3 phase currentinto the cores of the cable, and measuring of
voltages and currents induced into the sheaths at each cross bonding point along the complete cross bonding section. An injection current of greater than 50A shouldbe used. Sheath currents and voltages are to be measured at each cross bonding point, earth point and termination. The cross bonding connections shall be rearranged to prove incorrect connection, and checked again after correct restoration.
The procedure shall be repeated for each complete cross bonding section of the cable run.
Current and voltage measurements are to be scaled up to the rated load current. Voltage values at isolated (not solidly earthed) cross bonding pointsshall be less than the rated load current scaled voltages of the installation design.
6e  Sequence Impedance Measurements
These tests are carried out for protection settings, earth potential rise and fault analysis. The tests are circuit dependent. The cable measurements shall include DC resistance, positive, negative and zero sequence impedances, and shall be expressed at a reference temperature of 20deg C. The measured values shall be compared with the calculated theoretical values or those provided by the cable manufacturer. 

6f  Serving Tests
A high voltage DC test between the metallic sheath to earth is performed to test the integrity of the outer sheath. An IR test is performed to and after the high voltage test to assess the insulation integrity of the cable.
The test voltage level and test period for serving tests on subtransmission cables are :
  33kV & 66kV – 10kV/1min.
  132kV – 15kV/1min.
6g  Sheath or Screen Resistance Test
The dc resistance of the metallic sheath and connections is measured and compared with the manufacturer’s sheath resistanceusually expressed in ohms/km in the cable specification.
6h  Sheath Voltage Limiters (SVL) SVL’s are connected to the cable sheaths to limit the transient voltage rises to avoid puncturing the cable servings under fault conditions.
The units are tested to ensure their compliance with their original characteristics. The test shall be carried out in accordance with the manufacturer’s recommendations.
6i  HV Tests
The requirement is to complete the prescribed tests as specified. All cables must be fully discharged for a time duration equal to the test time upon completion of each testby means of discharge function on test equipment or an independent earth. HV tests shall be carried out after laying & bedding has been completed. The following table shows the test voltages and times required for cable system voltage.