75 CHAPTER 5 CONCEPT OF PD SIGNAL AND PRPD PATTERN 5.1 INTRODUCTION Partial Discharge (PD) detection is an important tool for monitoring insulation conditions in high voltage (HV) devices in power systems. As the insulation of high voltage equipment ages, it may breakdown due to mechanical, thermal and electrical stress, resulting in the catastrophic failure of the equipment. Therefore it is important to have a system that is capable of warning users of potential insulation problems so that they may be repaired during a scheduled shutdown. In order to design a system to detect and locate this phenomenon within an HV system, it is important to understand why PDs occur and what methods are currently employed to detect and locate PDs. This chapter discusses the basic concepts of partial discharge signal and phase resolved partial discharge pattern. 5.2 OCCURRENCE OF PD Partial discharge can be described as an electrical pulse or discharge in a gas-filled void or on a dielectric surface of a solid or liquid insulation system (Cavallini et al 2003; Cavallini et al 2005). This pulse or discharge only partially bridges the gap between phase insulation to ground, or phase to phase insulation. These discharges might occur in any void between the copper conductor and the grounded motor frame reference. The voids may be located between the copper conductor and insulation wall, or
76 internal to the insulation itself, between the outer insulation wall and the grounded frame, or along the surface of the insulation. The pulses occur at high frequencies; therefore they attenuate quickly as they pass to ground. The discharges are effectively small arcs occurring within the insulation system, therefore deteriorating the insulation, and can result in eventual complete insulation failure. The other area of partial discharge, which can eventually result, is insulation tracking. This usually occurs on the insulation surface. These discharges can bridge the potential gradient between the applied voltage and ground by cracks or contaminated paths on the insulation surface. 5.3 MECHANISM OF PD INITIATION A partial discharge in an insulator occurs when the electric field in a localized area changes in such a way that a localized arcing is created. This localized arcing manifests itself as an electrical pulse that is measurable. Voids are defined as gaps in a more dense dielectric material, such as gas bubbles in oil, or cracks and fissures in the paper insulation. The void region has a lower dielectric constant than the surrounding material, creating a capacitance. A partial discharge can then occur when the electric field difference across the void exceeds minimum breakdown field strength. However, reaching this minimum field strength does not guarantee an immediate partial discharge. In order for a PD to occur, two criteria must be met. First, the electric field difference across the void must be higher than the breakdown value, which is determined by the field s ability to accelerate an electron to the point that if it impacts another molecule, more electrons are knocked loose than are absorbed. Second, there must be a free electron present within a specific volume, whose size is proportional to the voltage across the void, to accelerate within the field. If these conditions are met, then
77 the buildup of electrons in motion grows exponentially and a streamer, or electron channel, is created and current can flow across the void and return the voltage across the void to zero. However, the presence and location of a free electron is a random process dominated by the presence of ambient radiation knocking electrons loose from surrounding materials. Even the largest source of ambient radiation in most systems, cosmic rays, creates very few free electrons. In summary, in order for a PD to occur, a free electron must be present within a voltage dependent volume while the electric field strength is high enough to cause a cascading flow of electrons from the movement of a single free accelerated electron. This need for free electrons makes the PD phenomenon very unpredictable and a PD can occur within minutes or within hours of reaching the breakdown field strength within the void. The resulting discharge manifests itself as an observable electrical, acoustic, and sometimes optical signal. It should be noted at this time that the exact mechanisms and resulting signal properties of a partial discharge are not completely understood, though there are loose guidelines that device designers can use in building detection systems. 5.4 MODELING OF INSULATION SYSTEM USING R-C CIRCUIT A simplified model of an insulation system can be represented by a capacitance and resistance in parallel as shown in Figure 5.1. This is the concept employed in the use of power factor testing of insulation systems. The leakage current is split between the resistive and capacitive paths. The power factor is the cosine of the phase angle between the total leakage current and the resistive component of leakage current.
78 Figure 5.1 R-C Equivalent circuit model of electrical insulation The above model is also used for attenuator circuits in electronics. Signal attenuation results in reducing the amplitude of the electrical signal. This underlies the problem with partial discharge detection. The insulation medium, which is being exposed to the partial discharges, acts to attenuate the signal, therefore weakening this damaging signal which we are trying to identify at our sensor locations. In addition, the attenuated partial discharge signal can be masked by sources of electrical noise. 5.5 PHASE RESOLVED PARTIAL DISCHARGE PATTERN The illustration of the partial discharge activity relative to the 360 degrees of an AC cycle, which is commonly known as Phase Resolved Partial Discharge (PRPD) Pattern, allows for identifying the prominent root cause of partial discharges, therefore appropriate corrective actions can be implemented ((Cavallini et al 2003; Cavallini et al 2005; Contin et al 2000; Contin et al 2002; Contin et al 2006). A typical PRPD pattern of a insulator is shown in Figure 5.2. The first concept to review is the characteristic trait that partial discharges occur only during the first and third quarter of each cycle. This is the initial rising positive signal, and the initial rising negative signal.
79 Figure 5.2 Typical PRPD pattern of insulation system Effectively, during the initial rising positive signal, all of the capacitive components are being charged until the partial discharge inception voltage is reached across each specific void, and partial discharges commence. When the positive wave cycle begins to decrease the positive voltage across each void is reduced, since some capacitive charge remains. Some level of charge must exist since the voltage across a capacitor cannot be changed instantaneously. During the first quarter cycle we are creating a positive charge and the resultant partial discharges. During the third quarter cycle, this positive charge is effectively reversed, resulting in a positive charge in the reverse direction, and the resultant partial discharges. The second concept to review is that partial discharges are measured as voltage pulses; therefore, during the positive waveform cycle, a discharge, or a partial short-circuit, results in a negative, downward oriented pulse. This is referred to as a partial discharge with a negative polarity, and occurs during the first quarter-cycle of increasing positive voltage applied to
80 the void. During the third quarter-cycle, a partial short-circuit results in a positive, upward oriented pulse. This is referred to as a partial discharge with a positive polarity and occurs during the third quarter-cycle of the increasing negative voltage applied. These partial discharges, which are measured as a high frequency change in the power signal in milli volts to a few volts, cannot be observed with a standard scope. The two measurements illustrated on the two dimensional graph are partial discharge peak magnitude, usually represented in milli volts and pulse repetition rate, represented by the number of partial discharge pulses during one cycle of an AC waveform. The partial discharge magnitude is related to the extent of damaging discharges occurring, therefore related to the amount of damage being inflected into the insulation. The pulse repetition rate indicates the quantity of discharges occurring, at the various maximum magnitude levels. Both play a role in determining the condition of the insulation under test. Whereas seldom possible with on-line motors, the maximum magnitude level should be calibrated to reflect the actual charge, measured in pico-coulombs. The benefits of such calibration are offset by the relative comparison of similar motors, and more importantly by trending of the partial discharge activity over time. On-line partial discharge testing allows for such trending and analysis of the electrical equipment.
81 5.6 TYPES OF PD PATTERN (i) (ii) (iii) Figure 5.3 PRPD pattern (i) Noise (ii) Surface discharge (iii) Corona discharge
82 During PD measurements, it is possible to get surface partial discharges or corona partial discharges. Sometimes, if the measurement is not carried out properly, it may lead to measurement of noise signals. Therefore proper usage of filters are always recommended to avoid noise signals during partial discharge measurements. Figure 5.3 shows a typical noise pattern, surface discharge and corona discharge pattern respectively. 5.7 PD SIGNAL CHARACTERISTICS Figure 5.4 shows a typical partial discharge signal measured during experimental studies. In the waveform, T1 and T2 represents front time and tail time of PD signal respectively (Cavallini et al 2003; Cavallini et al 2005). The various significant parameters generally used to differentiate PD pulses from different sources are, Figure 5.4 Typical shape of PD signal 1) Rise time of PD pulse 2) Tail time of PD pulse
83 3) Peak value of PD pulse 4) Frequency of PD pulse 5) Phase angle of pulse 5.8 PD DETECTION METHODS Over time, the insulation begins to breakdown due to mechanical, thermal and electrical stress. Because partial discharges are both symptomatic of insulation breakdown and a mechanism for further insulation damage, PD detection is used to evaluate the condition of and diagnose problems with the insulation. Over the past forty years, several methods have been developed to detect PDs within high voltage systems. These can be grouped into four categories, based on the PD manifestation that they measure: chemical, electrical, acoustic and optical detection. Mostly electrical detection is preferred in high voltage systems. 5.9 ELECTRICAL PD DETECTION Electrical detection focuses on capturing the electrical pulse created by the current streamer in the void. These pulses last on the order of single nanoseconds and have measurable frequency components in excess of 1 MHz. The pulse shape, its relative phase location within the AC cycle of the high voltage insulation, and the signal intensity all lead to information about the type of PD fault and the severity of the insulation damage. Electrical measurements are grouped into two categories, direct probing and RF emission testing. The direct probing method requires that capacitive couplers or inductive couplers be connected to the terminals of the equipment. The second group, RF emission testing, is conducted by using antennas in the area of the transformer. Both methods require a time domain recording device, such as a data storage oscilloscope, to capture the PD signal. The PD is then
84 identified using several digital processing methods. These processing methods make online electrical PD detection very attractive because it makes realtime monitoring of HV systems possible. Electrical detection has its own limitations. The primary limitation of electrical testing is its susceptibility to noise. The high voltage system environment contains high levels of electrical noise, both narrowband and broadband. In some cases, it is extremely difficult to distinguish between noise and a PD because of the short PD pulse width. This problem leads to false detection in online electrical PD systems. Another problem with electrical detection is that the received pulse characteristics are highly dependent on the geometry of the high voltage system. Different components within the high voltage system can distort the pulse shape needed to characterize the type of PD fault and can again result in erroneous detection. Although electrical detection has several problems, these systems are widely used in power plants around the world and provide equipment managers with valuable information about the condition of the insulation system. 5.10 CONCLUSION This chapter analysed the basic concepts of partial discharge signal and phase resolved partial discharge pattern in detail. In addition, mechanism of PD initiation and PD signal characteristics are also studied. Method of PD detections are also studied in details.