Digital Fault Recorder Deployment at HVDC Converter Stations On line continuous monitoring at HVDC Converter Stations is an important asset in determining overall system performance and an essential diagnostic tool when analyzing faults and unexpected behavior. Monitoring the AC supply, the secondary of the power transformers, the filter banks, the DC output, converter station control signals and the protection scheme provides an invaluable set of data showing the interaction of the DC system with disturbances and transients emanating from the AC supply. Modern Fault Recorders are multi-function devices incorporating power quality measurements to the IEC61000-4-30 standard to class A accuracy. This allows continuous monitoring of the harmonics presented to the AC grid to ensure they remain compliant. Monitoring harmonic levels in the filter banks and secondary side of the power transformer will ensure currents do not exceed rated values. Furthermore it will be possible to incorporate travelling wave fault location into the scheme for monitoring the DC overhead line circuit. This will provide very accurate fault location thereby reducing downtime by dispatching crews to the right spot first time. A Multi-Function Fault Recorder The following functionality is applicable to HVDC stations: High Speed Fault Recording This is the traditional role of a fault recorder whereby analogue signals (both AC and DC) are sampled at a high rate (up to 512 samples / cycle) and a record in the order of 2 or 3 seconds triggered and stored when an abnormal event is detected. The resultant record consisting of pre and post trigger information allows post mortem analyses to be conducted. When combined with digital inputs from relays, circuit breakers and control equipment it is possible to analyse the response of any part of the Station to a transient event to assess performance. Slow Scan or Disturbance Monitoring Also known as Dynamic Disturbance Recording (DDR) this function logs calculated parameters such as system frequency, rms magnitude, phase angle, power (real, reactive and apparent) and sequence components at a selectable rate up to twice per cycle. DDR-C (continuous) stores the parameters in a circular buffer. The typical duration before data is overwritten is 15 days. There is also a triggered variant, DDR-T, where parameters are stored as a permanent record up to 30 minutes long with pre and post trigger segments. Slow scan monitoring is used to determine the response of the Converter Station during periods of instability on the AC grid such as power swing oscillations and frequency and voltage deviations. When digital input status is overlaid on the recordings it is possible to monitor relay and breaker activity as well. Power Quality Monitoring Power quality monitoring providing long term log data of frequency, rms magnitudes, harmonics, voltage dips and swells, unbalance and power must be done by a device
compliant with IEC61000-4-30 Class A accuracy. The results obtained are then validated and can be compared with data from other equivalent devices on the network. Note that harmonic monitoring is particularly important at HVDC sites as the DC conversion process can inject harmonics into the AC grid if the filters are not designed correctly. Note that when measuring harmonics it is important to have a wide bandwidth signal from the instrument transformer. Protection or metering CTs have sufficient bandwidth for accurate current measurements but standard capacitor or inductive type voltage transformers do not. At transmission voltages it is normal to employ capacitor voltage instrument transformers. In such cases it is imperative to add a PQ Sensor device to provide a wide bandwidth output for accurate harmonic measurements. Travelling Wave Fault Location HVDC circuits tend to be long and fault location can be problematical as normal impedance methods cannot be applied for DC. A good alternative solution it to deploy a double ended travelling wave system where the fault induced travelling waves propagate to each end of the line and are detected and accurately time stamped. Distance to fault is calculated from the arrival time at each of the line ends, the total line length and the speed of propagation. Accuracies in the order of 100m are possible. Travelling wave fault location is possible on overhead lines where the travelling wave losses are relatively low. However, the technique is not viable on cable lengths over 50Km due to the much higher losses incurred. Typical Deployment on a +/- 500KV Converter Station The following example is from a project where a submarine cable was used for the HVDC transmission. As such there are no filter banks on the DC side of the Converter as the capacitance of the cable limits harmonics. (DC side filter banks are more common when the HVDC transmission is via overhead lines) The project utilized 2 x bipoles, pole 1 and pole 2 with a common return via the sea. The architecture used at each Converter Station is shown in Fig 1.Data Acquisition Units (DAUs) operate as standalone recording devices. All have Fault Recording and Slow Scan Monitoring as standard with some having a Power Quality capability. None of the devices have Travelling Wave functionality in this case as the losses on the long submarine cable are too great to support the technique. There is one AC transmission line at each Converter Station terminated on busbars. Two Converters are connected to the busbars, Pole 1 and Pole 2. Each Converter consists of one power transformer, filter banks, a series reactor and the AC/DC converter. The power transformers are single phase devices with two secondary windings separated by 30 degrees. In total 6 secondary windings connect to the AC/DC converter. All DAUs are installed in the relevant protection bay to reduce the amount of secondary wiring. All DAUs are connected to the IEC61850 station bus to exchange cross trigger signals, augment the hard wired digital inputs with digital status and messages from GOOSE messages and transfer data to the central analysis station. The central master station has a client server architecture. All data from the DAUs is stored on the central server with the aid of an SQL database. The server is housed in the Converter Station control room and clients can log on locally or remotely from the National Operations Center to view and analyse data and construct reports.
Fig 1 DFR deployment in a +/- 500KV HVDC Converter Station The quantities monitored for each asset type are listed below: DAU Function Incoming AC Line Bay AC Filter and Shunt reactor Bays Transformer Bay DC Pole 500KV and Neutral Common Device Signals Monitored 3 phase currents Residual current 3 phase voltages 3 phase currents to ground Residual current 3 phase voltages Single phase primary and residual currents Current on each secondary winding (12 current channels in total) 3 phase primary voltages Each secondary winding voltage (9 voltage channels in total DC pole current DC neutral current valve side DC neutral current electrode side 1 AC voltage channel to monitor ripple DC pole voltage DC midpoint voltage DC neutral voltage 14 DC voltage channels for control signals 4 DC currents in the common neutral circuit All DC channels had an input range of +/-12V. DC current signals were derived from transducers and DC voltages derived from RCD devices.
PQ Sensors were fitted to the transformer primary voltage CVTs to provide accurate wide bandwidth harmonic monitoring. Harmonic monitoring was also deployed on the AC filter and Shunt Reactor currents for diagnostic purposes. Clock Synchronisation It is possible for the fault recording system to incorporate an internal GPS receiver for accurate time synchronization but in this example an external master clock was available that generated an NTP signal ensuring a time of accuracy of 1 or 2ms for each device. It is very important for all recorded data to be time synchronized so that it is possible to directly compare events between one DAU and another and between each end of the HVDC link. Note that if Travelling Wave fault location is used then clock accuracies better than 1µs is required. End to End Communications Each converter station has an independent monitoring system but there is a communication channel to the converter station at the other end of the link such that data can be downloaded from both sides and compared when necessary. The architecture drawing below shows this. Summary Using a multi-function fault recording monitoring system is an essential tool for on line monitoring of an HVDC Converter Station. Diagnostics are required when a fast acting, complex control system interacting with a dynamic AC grid experiences problems. Some DC
Converter stations have some internal monitoring but it does not substitute for comparing external AC influences with DC response. Adding the Power Quality and Fault Locating functionality adds value to the installation and provides an increased return on investment.