On track for partial discharge testing

IN recent years, partial discharge testing (PD) has come to play a significant role in ensuring reliable hydro generator operation. PD tests are sensitive to the majority of stator winding insulation failure mechanisms in hydro generators and thus provide utility generator maintenance engineers with many benefits. These include:

• Warning that the winding has a very high risk of failure, and that the machine should be removed from service especially if the unexpected failure of the machine can have negative consequences in terms of system reliability.

• A warning that immediate maintenance effort or replacement is needed. This avoids an in-service failure, which can damage other components of the generator.

• Warning that deterioration is occurring with sufficient notice that major repairs can be conveniently scheduled with the minimum possible outage, yet restore the winding to nearly new condition.

• If problems are detected, help identify the root cause of any deterioration, since this will guide the user to the most cost-effective repair techniques.

• Identify which generators are in good condition, and thus determine the machines, which can remain in operation without requiring an outage for maintenance.

• Identify which older machines have stator windings in good (or at least stable) condition, avoiding an expensive premature rewinding.

Partial discharge testing involves the measurement of tiny sparks, which occur in the high-voltage insulation of generators. These sparks - or PD pulses - result from high voltage breakdown of air pockets or voids in the insulation, or on the insulation surface. As the generator ages, the number and size of these voids increase and the number and size of the measured PD pulses increase. By tracking the PD activity over several years, it is possible to determine the rate of the electrical insulation degradation and in many cases to predict the particular type of failure mechanism that is occurring. Such information can then be utilised by plant personal to schedule preventative generator maintenance and/or repairs, avoiding catastrophic and costly in-service machine failures.

The PDA test, as it is commonly called, utilises a sophisticated portable instrument to measure the PD on hydro generators while they are in normal operation. A PDA instrument must be capable of measuring PD pulses with risetimes of a few nanoseconds (one billionth of a second) and discriminating between two pulses that may occur less than six nanoseconds apart. Due to the cost, utilities typically have one PDA instrument, which would be shipped between plants, to periodically take PD readings on their machines.

Iris Power Engineering

Founded in 1991 as a commercial venture, Iris Power Engineer-ing was the end-product of research by Ontario Hydro and the Canadian Electrical Association into on-line partial discharge testing of rotating machines. Initially PD testing was very specialised, and required significant effort by experts to collect and interpret the data. Over the past 10 years, PD measurement instruments have been developed which make an on-line PD testing and the resulting data interpretation easier to do, allowing non-specialised plant personnel to reliably perform the test and interpret the data. Today, thousands of hydro generators worldwide are outfitted with sensors needed for on-line PD measurements.

Although portable partial discharge testing has been used for many years, Iris claims to have improved the application and development of automated PD test equipment through its GenGuard, and most recently, Trac lines of products. The most recent product - HydroTrac - developed by Iris with support from epri and the New York Power Authority (NYPA), allows hydro generator users to apply automated, continuous, on-line PD testing to their machines. Utilising state-of-the-art electronics and software, the HydroTrac instrument allows each generator to be outfitted with a cost-effective, permanently installed, PD monitoring system. To achieve this performance in a continuous monitor, HydroTrac employs radical component reduction through the use of programmable logic arrays and surface mount electronics. This development ensured that US-based R&D Magazine selected HydroTrac as one of the top 100 products of the past year.

Hydroelectric industry

The kind of continuous and automated PD monitoring made available by HydroTrac is significant to the hydroelectric industry because there is a trend for many utilities to reduce the number of on-site operations staff. Since power plants are often located in remote sites, an automated PD monitoring system allows measurements to be made, without the time and expense of having to send personnel to the plant. This can greatly reduce overall test costs.

The stator winding PD activity is often affected by rotating machines' operating conditions, including winding temperature, load and voltage. Although these operating condition-dependent results help in interpreting the type of deterioration occurring, they often make it difficult to trend PD data over time. To obtain trendable data, which is important for interpretation, the machine must be tested under the same load, temperature and voltage conditions each time. For critical units which are subject to frequent load cycling, or pumped storage generating units, it is often difficult for the plant operators to recreate exact operating conditions from test to test. An automated PD monitoring system avoids this problem by continuously measuring the generator operating conditions, and then taking the PD measurement only when the desired operating conditions occur.

Collecting PD data at various temperatures and load points can itself be useful in the data interpretation process. At different load points, the winding experiences different forces, thus conditions such as winding looseness can be inferred by comparing full load and no load test results. A continuous PD monitoring system can collect data at specific time intervals, and under selected operating conditions, thus providing multiple points of compatible trend data.

Several expert system software packages are currently under development which continuously monitor all the sensors in a generator to determine if any problems have occurred. This provides plant operators, as well as maintenance personnel, with warnings that a problem may be developing, and also gives advice on corrective action. Since partial discharge activity is an important source of information with regard to stator winding insulation, it is desirable that generators equipped with such expert systems have ready access to PD data. An automated PD monitoring system like HydroTrac can fulfil this function by making PD data available on a continuous basis.

The alternatives

To meet the broadest range of applications, HydroTrac has been designed with a comprehensive list of interface and communications alternatives. The unit itself contains a micro controller, tactile panel, and local LCD display allowing it to operate in a stand-alone fashion, collecting and archiving PD data to local memory and providing external alarm contacts on high PD readings. A local RS232 serial port is available to download the PD history for archiving and trending on an external computer.

Additionally, HydroTrac can be configured with an analog output option, which generates current loop, or DC voltages, proportional to the measured PD levels on the generator. These outputs can then be hardwired to existing Scada, DCS, or PLC inputs, for trending, archiving, and alarming, utilising software and interfaces familiar to the operator on existing plant systems.

In locations with more centralised or automated generator monitoring, the HydroTrac also contains a multi-drop RS485 (fibre or twisted pair) or 10Base-T Ethernet network connection. Utilising these interfaces, multiple HydroTrac units can be remotely controlled via a Windows based software application. Interfacing this application to a plant DCS or Scada will also allow HydroTrac's PD data collection to be triggered based on generator operator conditions.

Versions of HydroTrac to be released in 2002 are already under development, and will provide web embedded capabilities allowing each HydroTrac unit to act as a mini web server. In this configuration, simple control, alarming, and data viewing will be available as web pages using just an internet browser. For clients wanting to access HydroTrac through a company Firewall, HydroTrac will accept emailed commands, returning data or status information via a reply email. The EAlarm feature allows HydroTrac to email PD alarms to any valid email address in the world.

Following the successful verification and field trials of HydroTrac at several NYPA plants, full scale installation and integration of 16 units at the St Lawrence plant has begun. Initially these units will be operated in a stand-alone fashion, with integration of each HydroTrac to the plant systems staged along with unit control system upgrades planned to occur over the next several years. In addition, HydroTrac will play a key role in providing machine condition information to an on-line expert turbine-generator monitoring system under development by Iris and Foster Miller Technologies. This will be prototyped on one unit at NYPA's St Lawrence plant.