Monitoring and Recording Current and Fault Parameters in a DC-AC Conversion Station

At any power station, keeping up with maintenance is crucial for safety but also an incredibly complex and challenging task. When faults occur, the technician in charge quickly needs to pinpoint the causes. Where it happened, and why, acting fast to fix it. If the staff does not immediately address a problem at the plant or in the line, it could develop into something even more complex, causing prolonged downtime. 

Personnel onsite can’t visually keep track of the status of each component. That’s why power plants use computerized systems to track their equipment's real-time status and record any faults. The understanding of why faults occur and how often increases efficiency and safety.

The newest and most modern power stations have state-of-the-art control units and monitoring systems, but what about older ones?

Converter stations

DC to AC conversion power stations, also called inverter or converter stations, are crucial components in electricity transmission systems, especially in systems incorporating renewable energy sources such as solar or wind power.

In such systems, electricity generated from renewable sources is often direct current (DC). For efficient long-distance transmission and integration into the grid, this DC electricity needs to be converted into alternating current (AC), the standard for most electrical grids worldwide.

These stations house equipment converting current from DC to AC. These inverters or converters typically use semiconductor devices such as insulated gate bipolar transistors (IGBTs) or silicon-controlled rectifiers (SCRs) to switch the DC input into a synchronized AC output.

The main functions of a DC-AC conversion power station include:

• Inversion: Converting DC power from renewable sources (such as solar panels or wind turbines) into AC power suitable for transmission and distribution in the grid.

• Synchronization: Ensuring that the output AC power matches the frequency, phase, and voltage levels the grid requires to maintain stability and compatibility with existing infrastructure.

• Grid Interconnection: Facilitating the connection of renewable energy sources to the electrical grid, allowing the distribution of the generated power to consumers.

• Control and Monitoring: Managing the operation of the inverters to regulate voltage, frequency, and power output, monitoring performance, and diagnosing faults or abnormalities.

DC-AC conversion power stations are strategically located within electrical transmission networks to facilitate the integration of renewable energy and the efficient distribution of electricity. By harnessing renewable resources and reducing reliance on fossil fuels, the stations play a vital role in communities’ transition to a more sustainable and decentralized energy system.

The issue

In a 1960 380kV HVDC power station, the engineers needed to replace a fault recorder designed and made by a small Italian company in the early 90s. Due to its age, the old one generated reliability problems. It would run only on Windows 95, which gave the engineers difficulties in maintaining custom hardware and software. They needed something more modern and maintainable with spare parts available, capable of running software for today’s computers, and with upgradable software.

However, the old system, considering the time of creation, was fairly cutting-edge:

• 500kHz of the acquisition rate,

• user-friendly analysis software,

• custom views for faults coming from the AC and DC lines, and

• Excellent database to research faults by name.

The system included a data acquisition system for 160 digital signals and 32 analog channels, all with custom-made boards.

Analysis

We did a preliminary site inspection to collect information and to understand the issues.

The DAQ recorder was in the single rack on the right - see Figures 2 and 3 - and would receive data from the field. A multi-rack on the left collected these data. Digital channels would go through an isolator board before going to the DAQ device.

A PC with custom software served for recording and post-analysis.  

The cable from the digital isolation board would go on the single rack, connecting to a PC with Altera boards and a real-time Linux OS. For the time, the early 90s, this was amazing!

We realized that we needed custom connection boards to test the signals from the field without being too intrusive. To make things a bit more complicated, the Altera FPGA would also control a relay set. Since the signals should bypass the original system, we needed a new custom board to control the relays for the digital and analog channels.

Initial test of the original system

To complicate things even further - the original Altera system had been out of function for some time. We now needed to collect all the information about signals, their functions, and amplitude by reading old documentation stored in metallic cabinets.

Returning to our Parma office, our engineers prepared a few boards to allow preliminary tests on the original system to collect data and signals. After spending a few weeks preparing what was needed, we went again to the client, ready for testing.

We used:

• an isolated Dewesoft SIRIUS modular device (model: SIRIUSi-4xHV-4xLV) 

• custom connection boards and 

• a power supply for the relay boards - see Figure 4.

After some cabling, we started collecting and recording data from the field.

It is possible to recognize the voltage of the three AC phases and the functionality of some thyristors. A thyristor is a four-layer semiconductor device consisting of alternating P-type and N-type materials (PNPN) for high-power applications. It usually has three electrodes: 

• An anode

• A cathode

• A gate - also known as a control electrode. 

Solution architecture and setup

For this system, we chose a mixed Dewesoft family configuration as we needed 160 digital inputs and 32 analog outputs but with output impedance up to 10MOhm,

To monitor the structure, we had from the customer some constraints:

• The system must run 24/7 without interruption, 

• the data acquisition and post-analysis software should run separately, 

• 10Mohm input impedance is required for the analog channels to avoid perturbation on the power station monitoring system,

• GPS for time sync,

• the possibility of exporting in Comtrade,

• customized views for different errors,

• alarms should integrate with the internal SCADA and

• the Dewesoft DXD file names must contain the error codes.

To meet their specification, we offered a mixed Dewesoft solution.

• 2 x SIRIUSie-HD-16xLV Ethercat data acquisition systems with 10 MOhm output impedance.

• 1 x IOLITE R8 chassis loaded with 5x IOLITEi-32xDI digital input boards.

• 1 x industrial PC server to run DewesoftX data acquisition software

• 1 x desktop PC to collect data from the server and 

The solution was the fully configurable and customizable IOLITE R8 chassis with five IOLITEi-32xDI boards for the digital inputs and the SIRIUSie-HD-16xLV for the analog inputs - see Figure 8.

Customization

One of the client’s requests was for our solution to manage alerts and integrate these with their internal SCADA monitoring system.

We had to divide the alerts into groups according to the nature of the fault. Dewesoft recorded files needed to follow the same procedure, with the alert name included in their names.

Our developer at LEANE wrote a customized plugin called LIAlarmManager. The plugin handles such actions, linking Dewesoft with the SCADA and renaming all Dewesoft recorded files according to the error occurring - see Figure 11.

Conclusion

We installed our proposed Dewesoft system more than a year ago, and it’s keeping track of faults on the Powerline 24 hours a day. With Dewesoft’s easy-to-use software and a tailored DewesoftX setup, the technicians at the conversion station can now immediately pinpoint power line problems and quickly act to fix the fault.

When something happens, a triggered recording mechanism within Dewesoft records and saves data on disk. Via the multi-display Dewesoft tab feature, each fault has its distinct display showing only the channels related to the issue. Such a quick event overview lets the operator clearly understand what happened and why - a substantial advantage for the customer!

Lastly, because Dewesoft is compatible with commonly applied programming tools, we could integrate the data with the customer’s SCADA system with a custom plugin.