Design, Testing and Application of Phasor Measurement Technology in ESKOM's Transmission Network

by P. Harding, A. Varghese , Alstom Grid, UK, A. Edwards, B.W.D. Berry, ESKOM, South Africa, D.H. Wilson, R. Folkes, Psymetrix, UK


Such a major evolution of the power system will naturally have an impact on the dynamic performance of the system and introduce challenges in managing system stability. Since the South African power system covers an extensive geography with very long transmission lines, stability issues can be particularly challenging. In recent years, there has been a deficit of generation, which has increased the stress on the system. In this context, a wide area monitoring system (WAMS) is a very valuable tool as it can have a positive impact on system planning, transmission optimization, operational security, plant and transmission line commissioning, and control system tuning.

Synchro-phasor based monitoring is being installed as a wide area measurement system (WAMS) in the ESKOM power system, with short term as well as long term objectives. The project will initially cover 15 substations, but will be extended to all critical nodes in the network, with an expected eventual coverage of 52 substations. (See Table 1). The phasor measurement units (PMUs) and phasor data concentrators (PDCs) capture and forward critical system parameters such as bus voltage magnitudes and power angles to a synchronized measurements historian.

The phasor measurements integrate with the existing energy management system (EMS) and give new visualization and stability applications at ESKOM’s National Control Center (NCC).

For instance, the phasor data will allow operators to apply smaller real time margins to transmission limits whilst still guaranteeing safe operation for the dynamic constraints, transient stability and thermal limits. Real time synchrophasor data can show dynamic changes in the system which cannot be seen using existing slower response SCADA systems. Visibility of these changes and their associated oscillations in power flow can enable more flexible connection and optimum operation of less stable renewables such as solar and wind power.

Substation Level Monitoring
The substation-level monitoring system, with provision for up to eight feeders, is shown in Figure 1. One pulse per second timing pulses obtained from GPS receivers are sent via fiber links to synchronize all PMUs in the substation to better than 50ns. This ensures all measurements taken from substations across the country will be accurately synchronized to better than an angle of 0.01 degree. For high reliability, local substation storage using solid state memory in an sPDC (substation phasor data concentrator) gives back up storage of phasor data for up to 7 days (with a full suite of 8 PMUs) in case of temporary loss of communications to the control center. To enhance the overall availability of the synchrophasor data, the substation monitoring system has a dual power supply operating on an automatic chop-over mode.

The PMUs monitoring the three phase system are capable of streaming sequence quantities (positive, negative and zero sequence) of voltages and currents, in addition to the phase vectors. Where necessary, it is also capable of providing phasor values for single-phase inputs, or combinations of three-phase and single phase inputs, for flexibility of application across Eskom’s network. Furthermore, the CT/VT ratio may be configured individually for each of the channels. System frequency and rate of change frequency data is also available from the PMUs to aid wide area applications. Binary information available to the PMU may also be reported via the C37.118 stream where required. This could be used, for example, to report the status of feeder circuit breakers or operation of a protection device in the substation, for integration into a wide area scheme. The synchrophasor reporting rate is settable on the PMU, to suit the application and the communication bandwidth available.

The substation PDC application provides support for up to eight IEEE C37.118 input steams and generates C37.118 output streams combining specified components of the input streams. Up to four independent output streams may be configured, combining phasor data from different input streams. The sPDC has the flexibility of generating output streams at a different reporting rate from the input, by suitably interpolating or discarding phasors from the input streams. Thus Eskom has chosen to stream all the sequence data to the sPDC, but only the positive sequence information is forwarded through to the National Control Center. This optimizes the use of bandwidth, and the 7 day storage capability at the substation level allows Eskom to view more detailed data if required. This data is also retrievable remotely.

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