Biddle 5kV-DiagnosticTesting 电路图.pdf

RadioFans.CN 收音机爱 好者资料库 MEGGER Insulation Testers1 NOTES Table of Contents INTRODUCTION. 2 WHAT IS INSULATION? . 3 What Causes Insulation to Degrade? . 3 Electrical Stress . 3 Mechanical Stress. 3 Chemical Attack. 3 Thermal Stress . 4 Environmental Contamination . 4 How Can Predictive Maintenance Help Me? . 4 The Benefit of New Technology. 5 HOW INSULATION RESISTANCE IS MEASURED. 6 How an Insulation Resistance Tester Operates. 6 Components of Test Current. 6 Capacitive Charging Current . 6 Absorption or Polarization Current . 6 Surface Leakage Current. 7 Conduction Current. 7 Connecting your Insulation Tester. 8 Selected Typical Connections. 9 Shielded Power Cable . 9 Circuit Breaker/Bushings. 9 Power Transformer . 10 AC Generator. 10 Insulation Resistance Tester Scale. 11 Voltage Characteristics. 12 EVALUATION AND INTERPRETATION OF RESULTS. 13 Interpretation of the Infinity () Reading. 13 DIAGNOSTIC HIGH VOLTAGE INSULATION TESTS. 15 Spot Reading Test. 15 Time vs. Resistance Test . 17 Polarization Index Test . 18 Step Voltage Test . 20 Dielectric Discharge Test. 21 Different Problems/Different Tests. 23 APPENDICES. 24 Potential Sources of Error/Ensuring Quality Test Results . 24 Test Leads. 24 Making Measurements above 100 G . 24 Accuracy Statements . 24 Delivery of Stated Voltage . 24 Interference Rejection. 25 Rules on Testing and Comparing. 25 The Guard Terminal . 26 Effects of Temperature . 28 Effects of Humidity . 31 Ingress Protection . 31 High Potential Testing. 33 Current (nA) Readings vs. Resistance (M) Readings . 33 Burn Capability . 34 Drying out Electrical Equipment . 34 Test Item Discharge. 36 Charging Time for Large Equipment . 36 Motor Driven Insulation Testers. 37 5-kV Insulation Testers Available from AVO International . 38 Distributor: MeterCenter United States: 1-800-230-6008 1-480-659-8351 www.MeterC First Edition: June 2002 2002 AVO International DEDICATION Dedicated to the late T.A. “Ted” Balaska, who pro- vided significant help in structuring this booklet. Ted was an expert in the field of insulation and insu- lation testing and, more important, a good friend. AUTHORS David O. Jones Jeffrey R. Jowett S. Graeme Thomson David S. Danner RadioFans.CN 收音机爱 好者资料库 2A Guide to Diagnostic Insulation Testing Above 1 kV NOTESINTRODUCTION Electrical insulation degrades over a period of time because of various stresses, which are imposed upon it during its normal working life. The insulation has been designed to withstand these stresses for a period of years, which would be re- garded as the working life of that insulation. This often runs into decades. Abnormal stresses can bring about an increase in this natural aging process that can severely shorten the working life of the insulation. For this reason it is good practice to perform regular testing to identify whether increased aging is taking place and, if possible, to identify whether the effects may be reversible or not. The purpose of diagnostic insulation testing is: To identify increased aging. To identify the cause of this aging. To identify, if possible, the most appropriate actions to correct the situation. In its simplest form diagnostic testing takes the form of a “Spot Test.” Most electri- cal maintenance professionals have made spot tests where a voltage is applied to the insulation and a resistance is measured. The diagnosis in this case is limited to “the insulation is good” or “the insulation is bad.” But having made this diagnosis what do we do about it? Its a bit like going to the doctor with a bad cough and the doctor simply telling you, “Youve got a bad cough.” You wouldnt be happy to come away with only that information. You expect the doctor to examine you, carry out a few tests, and tell you why you have a bad cough and what to do about it to cure the cough. In insulation testing, a spot test on its own is the equivalent of the doctor telling you that you are well or you are sick. Its minimal information. This is the sort of test that is typically applied to low-voltage circuits where the cost of a failure is low and equipment can be replaced easily and inexpensively. Since the equipment being tested is low voltage equipment, these tests are typically performed using a 500 or 1000 V test voltage and will be familiar to all electrical maintenance person- nel. However, if the doctor records the results of his examination and compares them with those from previous visits, then a trend might be apparent which could lead to medication being prescribed. Similarly, if insulation resistance readings are re- corded and compared with previously obtained readings, it may be possible to see a trend and to prescribe remedial actions if such are called for. Diagnostic insulation testing at voltages above 1 kV is an area that is less familiar to many electrical maintenance personnel. The purpose of this booklet, therefore, is to: Acquaint the reader with making diagnostic insulation resistance tests. Provide guidelines for evaluating the results of these diagnostic insulation re- sistance tests. Introduce the benefits of multi-voltage testing at higher voltages. A series of appendices are included at the end of the booklet to provide the reader with additional information related to diagnostic insulation testing. RadioFans.CN 收音机爱 好者资料库 MEGGER Insulation Testers3 NOTES This booklet is based on the principles established in the booklet “A Stitch in Time The Complete Guide to Electrical Insulation Testing” first published in 1966 by the James G. Biddle Company. WHAT IS INSULATION? Every electric wire in a facility, whether its in a motor, generator, cable, switch, transformer, or whatever is covered with some form of electrical insulation. While the wire itself is a good conductor (usually made of copper or aluminum) of the electric current that powers electrical equipment, the insulation must resist cur- rent and keep the current in its path along the conductor. Understanding Ohms Law, which is expressed in the following equation, is the key to understanding insulation testing: E = I x R where, E = voltage in volts I = current in amperes R = resistance in ohms For a given resistance, the higher the voltage, the greater the current. Alterna- tively, the lower the resistance of the wire, the more current that flows for the same voltage. No insulation is perfect (has infinite resistance), so some current does flow along the insulation or through it to ground. Such a current may be insignificantly small for most practical purposes but it is the basis of insulation testing equipment. So what is “good” insulation? “Good” means a relatively high resistance to current flow. When used to describe an insulation material, “good” also means “the ability to maintain a high resistance.” Measuring resistance can tell you how “good” the insulation is. What Causes Insulation to Degrade? There are five basic causes for insulation degradation. They interact with each other and cause a gradual spiral of decline in insulation quality. Electrical Stress Insulation is designed for a particular application. Overvoltages and undervoltages cause abnormal stresses within the insulation, which can lead to cracking or delami- nation of the insulation. Mechanical Stress Mechanical damage such as hitting a cable while digging a trench is fairly obvious but mechanical stresses also may occur from running a machine out of balance or frequent stops and starts. The resulting vibration from machine operation may cause defects within the insulation. Chemical Attack While you would expect insulation to be affected by corrosive vapors, dirt and oil can also operate to reduce the effectiveness of insulation. 4A Guide to Diagnostic Insulation Testing Above 1 kV NOTES Thermal Stress Running a piece of machinery in excessively hot or cold conditions will cause over expansion or contraction of the insulation which might result in cracks and failures. However, thermal stresses are also incurred every time a machine is started or stopped. Unless the machinery is designed for intermittent use, every stop and start will adversely affect the aging process of the insulation. Environmental Contamination Environmental contamination covers a multitude of agents ranging from moisture from processes, to humidity on a muggy day, and even to attack by rodents that gnaw their way into the insulation. Insulation begins to degrade as soon as it is put in service. The insulation in any given application will have been designed to provide good service over many years under normal operating conditions. However, abnormal conditions may have a damaging effect which, if left unchecked, will speed up the rate of degradation and will ultimately cause a failure in the insulation. Insulation is deemed to have failed if it fails to adequately prevent electrical current from flowing in undesir- able paths. This includes current flow across the outer or inner surfaces of the insulation (surface leakage current), through the body of the insulation (conduc- tion current) or for a variety of other reasons. For example, pinholes or cracks can develop in the insulation or moisture and foreign matter can penetrate the surface(s). These contaminants readily ionize under the effect of an applied voltage providing a low resistance path for surface leakage current which increases compared with dry uncontaminated surfaces. Cleaning and drying the insulation, however, will easily rectify the situation. Other enemies of insulation may produce deterioration that is not so easily cured. However, once insulation degradation has started, the various initiators tend to assist each other to increase the rate of decline. How Can Predictive Maintenance Help Me? While there are cases where the drop in insulation resistance can be sudden, such as when equipment is flooded, it usually drops gradually