ONLINE DETECTION OF PARTIAL DOWNLOADS IN ELECTRICAL POWER SYSTEMS The present invention is directed towards the detection of partial discharge events in power systems such as cables, motors and transformers, and more particularly towards a method and apparatus for detecting partial discharge events online while the power system is in operation. BACKGROUND OF THE INVENTION Partial discharge events in high voltage power systems, such as high voltage power distribution lines, motors, and transformers, are high frequency discharges that occur in small portions of the system insulation. These discharges may last in the order of ten to fifteen nanoseconds, and generally occur at a peak of the power cycle of c.a. when there is a greater electrical voltage inside the insulation. Partial discharge events generate high frequency electromagnetic pulses that travel through the power systems. High-voltage equipment that is used in electric power systems is conventionally tested off-line for partial discharge activity that could indicate insulation defects and possible insulation faults. These conventional techniques typically involve coupling a capacitor in parallel with the equipment under test and measuring the discharge signals through an external impedance such as a resonant circuit. The resonant circuit expands the pulses of discharge current in the time domain so that the pulses are easier to detect and measure. The amplitude and phase of each partial discharge pulse can be recorded and analyzed relative to the test voltage. Devices of this type are not suitable for the detection and analysis of partial discharge events in power systems while the systems are online. It is therefore a general object of the present invention to provide a method and apparatus for the detection and analysis of partial discharge events in power systems of c.a. which are adapted for online use while the system is in operation, and which can be easily implemented to determine the type and / or location of partial discharges as they occur. BRIEF DESCRIPTION OF THE FIGURES The invention, together with objects, features and additional advantages thereof, will be better understood from the following description, the attached clauses and the accompanying figures in which: Figure 1 is a diagram of Functional blocks of an apparatus for online detection of partial discharge events in AC power systems according to the present preferred embodiment of the invention; Figure 2 is a schematic diagram of the inductive coupler in Figure 1; and Figures 3A, 3B and 3C are graphical illustrations useful in describing the operation of the invention. DETAILED DESCRIPTION OF THE PREFERRED MODALITIES This reference is incorporated to the present in the Detection in Line of Partial Discharges in Cables, ("On-Line Partial Discharge Detect in
Cables, "), I. E. E. Transactions on Dielectrics and
Electrical Insula tion, Vol. 5, No. 2, pp 181-188 (April
1998) by N.H. Ahmed and N.N.E. Srinivas for the purpose of serving as background. Figure 1 illustrates the apparatus 10 according to the present preferred embodiment of the invention for detecting partial discharge events in a partial c.a. 12. For illustrative purposes, it is shown that the power system 12 comprises a cable 14 which connects a load 16 with a power source 18. The apparatus 10 is adapted to detect partial power discharge events in the cable 14 while the system 12 is in line, that is, while power is being delivered by cable 14 from source 18 to load 16. Apparatus 10 is also adapted to detect partial discharge events in other conventional high voltage equipment such as motors, transformers, systems isolated by gas and the like. An inductive coupler 20 is operatively coupled to the cable 14 to detect high frequency electromagnetic pulses in the cable 14 generated by partial discharge events, and to supply such pulses through an adjustable amplifier 22 to an input of a differential amplifier 24. A present preferred embodiment of the coupler is shown in Figure 2 which comprises a coil 26 mounted on a ferromagnetic core 28. The core 28 is of annular construction, having a hinge 30 and opposite ends abutting at 32 to open the core to be able to circulate to the cable 14. The electromagnetic pulses generated by partial discharge events in the cable 14 are high frequency electromagnetic pulses, typically in the range of VHF and UHF. The UHF signals dissipate very rapidly in the power system, so the coupler 20 is preferably adapted to respond to electromagnetic signals in the VHF range, and to exclude signals outside this range, including the electrical power signal in the cable 14 typically at 60 Hz in the USA The differential amplifier 24 has a second input that receives a signal through an adjustable amplifier 34 of an antenna 36. The antenna 36 can be a box or dipole antenna adapted to respond to electromagnetic interference in the surrounding atmosphere within the frequency range of the inductive coupler 20, that is, VHF radio signals. Within the differential amplifier 24, the signals received from the antenna 36 are subtracted from those received from the coupler 20, so that the resulting output from the differential amplifier to a preamplifier 38 indicates the high frequency signals associated with partial discharge events to which have been subtracted from the surrounding electromagnetic interference. The output of the preamplifier 38 is fed through a filter 40 to a spectrum analyzer 42. The spectrum analyzer 42 receives control inputs from a full range control 44 and a zero interval control 46 for the purpose of describes The output of the preamplifier 38 is also fed through a filter 48 to a pulse phase analyzer 50. The pulse phase analyzer 50 also receives a reference voltage 52 indicative of the power signal of c.a. in the cable 14. The spectrum analyzer 42 and the pulse phase analyzer 50 are coupled to a controller 54 to control their operation and provide an automatic analysis of partial discharges. In controller 54 it receives input from operator 56, and is coupled to a visual indicator 58 to display the signal information to the operator. The spectrum analyzer 42 is initially operated in the so-called full-range mode to detect and analyze the input information by amplitude or magnitude as a function of the frequency over the entire frequency range established with the full interval control 44. A typical output of the analyzer 42, under full range control in the frequency domain, is illustrated in Figure 3A. Partial discharge events result in signal peaks at multiple frequencies in the 200 KHz to 200 MHz range. These peaks or lines are indicative of partial discharge activity in the cable. It is a characteristic of the electromagnetic pulses generated by partial discharge events that there is more attenuation at higher frequencies than at lower frequencies while the pulses of the signal travel through the cable. Consequently, the reception of signals predominantly in the lower frequency range, as illustrated in Figure 3A, indicate that the anomaly in the insulation that is causing the partial discharge events is quite far from where the coupler 20 is located. Side, lines or peaks of greater magnitude at the high frequency end of the spectrum would indicate that the anomaly in the insulation is closer to the inductive coupler. Depending on the type of cable in question, the apparatus of the present invention can precisely pinpoint the location of the partial discharge activity up to a range of fifteen meters. The spectrum analyzer 42 is then operated in the so-called zero interval mode to isolate the signal activity in one or more of the peaks illustrated in Figure 3A. For example, Figure 3B illustrates the partial discharge pulse amplitude as a function of time, (i.e., in the time domain) at the 24 MHz frequency illustrated in Figure 3A. The illustration of Figure 3B has a time duration of 50 milliseconds, which corresponds to three cycles of the 60 Hz power signal on the cable 40. It can be seen in Figure 3B that partial discharge events occur alternately in the positive and negative peaks of the power signal. The occurrence of partial discharge events on both peaks, positive and negative, of the power signal indicate that the insulation anomaly in question is close to the center of the insulation between the center conductor and the outer handle or cable shield. If partial discharge events occur only at the positive peaks of the ac signal, this indicates that the insulation anomaly is close to the central conductor, whereas if partial discharge events occur only at negative peaks it indicates that the insulation anomaly is near the shield. Thus, the output of the spectrum analyzer 42 in the full-range or frequency-domain operation mode, and in the zero-interval or time-domain operation mode, indicates the location of the isolation anomaly in a longitudinal and radial way in the cable. The pulse phase analyzer 50 receives the high frequency electromagnetic pulses generated by partial discharge events of the filter 48, and receives a reference voltage 52 indicative of the power signal on the cable 14. The analyzer 50 analyzes the phase angle of the partial discharge signals compared to the reference voltage. Figure 3C illustrates this ratio of pulse counts in pulses per second versus partial discharge magnitude in millivolts versus phase angle. The information provided in the pulse phase analyzer 50 helps determine the type of isolation failure that causes the partial discharge events. The pulse phase analyzer 50 provides: (1) data about the phase angle indicating the angle at which the partial discharge occurs. For example, if the partial discharge occurs at a 90 ° phase angle, this means that the source of the partial discharge is in the air, such as at the cable ends.; (2) if the partial discharge occurs in the positive, negative or both peaks of the c.a. This helps to analyze the type of anomaly, as discussed above; (3) the pulse current indicates the severity of the problem. Thus, a method and apparatus have been presented for the online detection of partial discharge events in power systems of c.a. that distinguish partial discharges from surrounding electromagnetic interference. The spectrum analyzer 42 analyzes the detected signals as a function of frequency. Then one or more frequency lines can be analyzed in zero interval mode. Partial discharge signals occur at peaks of the operational voltage, while noise has no pattern to follow in the zero interval mode. When the partial discharge frequencies are identified, signals at one or more frequencies are analyzed in the time domain mode. The pattern of the phase angle analyzed in the pulse phase analyzer 50 determines whether the partial discharge signal is generated in the equipment being tested or in adjacent equipment. For example, if the partial discharge events are carried out at or near a phase angle of 90 ° with respect to the ac signal, this means that partial discharges occur in the cable under test. If events occur at more than a phase angle of 120 ° or less at a phase angle of 120 °, this means that events happen on adjacent wires. The count and magnitude of pulses in the phase angle analyzer 50 are used to indicate the severity of the problem. The filter system presented allows discrimination between signals associated with partial discharge events and electromagnetic interference in the surrounding atmosphere.