Evaluation of Primary Frequency Response (PFR) of Generating Plants

Article by : Chandan Kumar Raj Protim Kundu Saibal Ghosh

It is important to evaluate primary frequency response to ensure it is available in adequate and generators are able to deliver the same whenever there is a large frequency excursion due to load/generation loss. There are various data sources through which it can be evaluated and is discussed below along with their merits.

1. During real-time operation: Status of Governor (FGMO/RGMO) and quantum of Frequency Influence through SCADA system at Control centre (ISOs)
2. After frequency event: SCADA data at control center (4-15 seconds update rate), Generator DCS data (0.5-1 second update rate) and PMU data (25-30 Samples per second)

Real-time data: The real-time governor status and generation quantum can provide details of quantum of Primary response reserve available in operation.

Control Centre's (ISO) SCADA data: For evaluation of PFR provided by generating plants, SCADA data can be used at control centres (ISOs) which updates at every 4-10 seconds interval. However, SCADA data is skewed in nature as it is not time synchronized. It is expected that PFR of generating units starts to get deployed within 4-5 seconds and gets fully deployed in the next 30-45 seconds. Thus, SCADA data may not be able to provide adequate performance details. Figure 1 shows one example of SCADA data-based evaluation of primary frequency response of generating units. It can be observed that whether response was fast enough could not be completely accessed. This can be improved with changes in scan rate, reporting rate of RTUs and SCADA data processing rate and data historian capabilities.

Generating Plant Distributed Control Systems (DCS) data: During any frequency response event, generators can share unit-wise generation and frequency/speed data. As these data are captured at generating unit terminal, approximation for unit auxiliary consumption and loss are not required to be taken care of during evaluation. However, the analysis of the performance shown by any generating unit depends on the resolution of data shared by the plant (generally 0.5 to 1 seconds and higher time level accuracy as measured and stored locally at plant). It can be observed in the below figure that though the resolution of unit distributed control system (DCS) data is better than the SCADA data, however, the initial perturbation of generating unit power output is not captured in generating unit DCS data. Further any oscillatory or spike kind of response also will not be captured in DCS data.?

Synchrophasor data at Control Centre (ISO): To overcome the inherent resolution and time synchronization related issues of SCADA data, PMU data can be utilized for frequency response analysis. The resolution of PMU data is very high and is captured at a rate of 25-100 frames per second with time synchronization. However, PMUs may not be available at all generating unit terminals. To overcome this limitation, PMU data from all outgoing feeders from the generating stations can also be used to estimate total power output from the generating station. This will provide the entire power plant’s PFR during any frequency event. The same Generating plant total incoming power flow can be measured with PMUs installed on its evacuation lines. The plant auxiliary and other power consumption and transmission lines’ power loss are much less compared to its total power generation. Further, auxiliary power consumption and power loss do not vary with frequency. So, these can be ignored and thus power flow summation of outgoing lines measured at remote ends provides a better overview of the total power generation from the power plant. The PFR evaluation for the same event using PMU data is shown in below figure. It is clear from these figures that steady-state change in frequency and power generation are captured in SCADA, as well as PMU data however transient variation, has been captured with PMU measurement.

In addition, while evaluating primary frequency response, observing the transient behavior of generating unit, particularly at the time of sudden frequency change, is extremely important. During this time, generating unit’s’ electric power output may induce some low frequency oscillation which cannot be captured by SCADA data or unit DCS data. These low frequency oscillations may be detrimental when the system is in a stressed condition. To monitor the performance of generating unit, it is recommended to install PMU devices at generating unit terminal also.?These measurement can help in accessing plant response during any event, testing and other activities.

Reference :

[1]?IEEE Task Force Report. 2017. “Measurement, Monitoring, and Reliability Issues Related to Primary Governing Frequency Response,” Technical Report PESR-24, October.

[2]??S. K. Soonee, S C SAXENA, “Frequency Response Characteristics of an interconnected power system – A case study of regional grid in India”, 6th International R & D Conference on Sustainable Development of Water and Energy Resources – Needs and Challenges - 13-16 February 2007, Lucknow, Uttar Pradesh, India

https://posoco.in/papers/Frequency%20Response%20Characteristics%20of%20an%20interconnected%20power%20system%20-%20A%20case%20study%20of%20regional%20grids%20in%20India_2007.pdf

[3]?Power System Operation Corporation Limited, “Analysis of Grid Events using Synchro-phasor Technology” - December 2019

[4] Vivek Pandey, Srinivas Chitturi, “Primary frequency response in an RE-rich grid-Operational experience of the Western Regional Grid”, National conference on Renewable Energy Technologies and its Integration with Grid 2018, Vadodara, India

https://www.researchgate.net/publication/329921475_Primary_frequency_response_in_an_RE-rich_grid-Operational_experience_of_the_Western_Regional_Grid

[5] Chandan Kumar, Raj Protim Kundu, Saibal Ghosh, Shabri Pramanik, Chandan Mallick, Saurav Kumar Sahay, Surajit Banerjee, “Improvement in performance of primary frequency response of generating units in Indian power system”, 2022 CIGRE Canada Conference & Expo,?Calgary, Alberta, Oct.31 – Nov. 3, 2022

https://www.researchgate.net/publication/365776717_Improvement_in_performance_of_primary_frequency_response_of_generating_units_in_Indian_power_system