The Kusile power station is a six unit, 4800 MW supercritical coal-fired power plant that uses a direct dry cooled condenser and pulse jet fabric filter plant with flue gas desulfurisation technology to manage emissions.
Kusile is 30% larger than the average coal-fired power plant in South Africa. When completed in 2020, it will be the largest power plant in Africa, the fourth-largest coal-fired power plant in the world, and the largest such plant in the southern hemisphere. Kusile power station is located in Mpumalanga province, around 100 km from Gauteng and is owned by Eskom which supplies around 90% of South Africa’s power. Kusile will act as a base load plant to address the severe power shortages that South Africa has experienced since 2007. The power will be fed into the national grid and contribute about 12% of South Africa’s generating capacity.
The coal-fired generating process
A coal-fired power plant generates electricity by burning pulverised coal to heat a water-filled boiler, basically a large kettle. The boiling water produces masses of steam, which is fed into a turbine converting steam energy into rotational energy which turns an electrical generator and transforms the rotational energy into electricity. Steam exiting the turbine is converted back into water by means of a cooling system (condenser), and the cycle begins again. Water is pumped back into the boiler by means of large feed-water pumps, again converted into steam, to drive the turbines and generate more electricity.
What is a supercritical power plant?
The term “supercritical” refers to the pressure and temperature conditions of steam produced by the boiler. A critical point in the water vapour cycle is a thermodynamic state in which there is no clear distinction between liquid water and water in a gaseous state. Water reaches this state at a pressure above 22,1 MPa and a temperature of 374°C. In simple terms, at this state no extra heat is needed to convert the water to steam (latent heat).
Most power plant boilers are subcritical. In these boilers, the water exists in two distinct states, namely water and steam. At these pressures, extra heat must be added to convert water into steam (latent heat).
The supercritical thermodynamic cycle has efficiencies of between 40 and 42%, compared to the 36 to 37% efficiency of subcritical power plant cycles, which typically have boiler pressures of around 16 MPa and temperatures of 540°C.
Higher thermodynamic cycle efficiency results in:
Greater boiler efficiency also improves operational flexibility by enhancing temperature control and load change flexibility, reducing start-up times and improving variable pressure operation. Supercritical plants do have much higher water quality requirements.
The Kusile supercritical power plant
The core plant consists of six supercritical boiler turbine units operating with pulse jet fabric filter (PJFF) and flue gas desulfurisation (FGD) systems. To support the core process of power generation, a number of other systems are needed which are known as the balance of plant (BoP). These include:
Responsibilities and scope of supply
ABB is responsible for the control and instrumentation of the entire plant, including all six units and the balance of plant – all of which will be seamlessly integrated and operated from a single control platform.
The company is responsible for the design, manufacture, commissioning and testing of the DCS, the field equipment and the associated cabling infrastructure for the entire Kusile plant. To realise this enormous automation project, the company has drawn on its strong South African engineering resources and combined this with engineering expertise sourced primarily from Italy and Germany. This core team has been supplemented with skills drawn from the company’s European and Indian operational centres to create a truly international project team for the Kusile project.
To automate a plant of this size and complexity requires over 200 000 I/O signals – far more than the 115 000 I/Os that a typical subcritical power plant requires (see Table 1).
When the Kusile power plant is completed, the company will have supplied:
Kusile has several automation firsts
Eskom had several specific requirements for this project, some of which are automation firsts in South Africa.
A single control and protection system for the entire plant
For the Kusile project, Eskom required a unified platform for all control systems across the plant to:
The company supplied not only a single DCS platform, but also provided a protection solution on the same Melody Rack hardware platform. This is the first time that any automation vendor has done this in South Africa.
Benefits of a single operator technology for the entire power plant (unit, electrical and common systems):
Kusile is the first plant with FGD (wet process) in Africa. The plant has to comply with the new plant SO2 minimum emissions standards under the South African National Environmental Management Air Quality Act (NEM: AQA), which stipulates that for coal-fired power stations, daily average SO2 emissions should not exceed 500 mg/Nm3.
It is the first plant the comapny has designed with a centrally managed system covering the entire generating unit and the BoP DCS for:
This was an Eskom requirement for the C&I installation project at Kusile, and the company engineered a solution to meet all of the utility’s requirements.
The benefits of a centrally managed system include:
The company is seamlessly integrating the Alstom/GE turbine control and protection system into its operator interface and automation system to ensure a uniform view of the process (faceplates, alarms and information).
This ensures that Eskom operators have only one operator interface to be trained on, and provides all of the single operator technology advantages listed above.
The plant has three safety systems, which are implemented on a Melody SIL3-rated platform. The plant requires two SIL3-rated protection systems for the water, steam and boiler protection systems. A further protection system is required for the BoP fuel oil system. For the whole plant, the company will provide a total of 13 SIL3-rated safety systems. This is the largest solution of its kind in the world.
Unit control concept
The South African electrical grid has some unique attributes that make it essential for coal-fired power plants to be extremely flexible in their operating ranges, as well as in their ability to support the grid during incidents resulting from the loss of generating units. This is because:
The Eskom specification called for the implementation of the Alstom/GE concept for unit co-ordinator control of the power plant. After discussions, both Eskom and Alstom accepted ABB’s proposal to use ABB’s solution for unit co-ordinator control, Modakond.
As a model-based unit control for co-ordinating the boiler and turbine, Modakond provides significant advantages for Eskom, including:
Project documentation requirements
The basis for completing any engineering activity is the project documentation requirements.
These were clearly defined for each and every major activity, from engineering, installation and commissioning to the maintenance and training stages of the project.
All documentation produced on the project must comply with IEC 61355, VGB B103, VGB R170 C and VGB R 171 standards. Project documentation demands were by far the largest and most comprehensive ever required on a project of this scope.
At the peak of the engineering work, 100 engineers were preparing designs and documentation.
For each unit, the design documentation for the major milestone design freeze (i.e., after Eskom has authorised procurement, manufacture and detailed design to commence) consists of more than 1000 individual documents needing customer approval. For the entire project phase of each unit, the engineering team is required to issue around 4000 documents for various activities.
A number of bus technologies have been applied to interface to the various third-party systems, such as Profibus, IEC 61850, IEC 101/4, Modbus, OPC and Ethernet over radio.
In response to Eskom’s project training requirements, the company has developed 14 Kusile-specific training courses which will be held in 46 sessions with an average participation of eight people per session. By the end of the project, the company will have trained around 368 people.
Training includes three unit operator high-fidelity simulators in service prior to the start-up of the first unit for upfront operator training.
|Automated signalling for Kusile power station|
|Number of hardwired signals per generating unit||24 000|
|Number of hardwired signals (balance of plant)||49 000|
|Number of soft signals||14 000|
|Total I/Os for the total plant||207 000|
RAM and FMECA requirements
RAM (reliability, availability, maintainability)
As part of the technical requirements for the project, the company had to meet very high values for system availability: 99,99% for the DCS portion (equal to downtime of less than 53 minutes per year) and 99,99% for the protection systems (equal to downtime of less than six minutes per year).
To achieve this, the company had to take great care during the design phase to ensure that the systems distribute the redundant mechanical functions across the DCS. The design, together with the inherent redundancy and reliability of the DCS platform, enabled the company to confirm to Eskom via a mathematical reliability study that the system is able to meet the required reliability figures.
FMECA (failure mode, effects and criticality analysis)
As part of the engineering deliverables, the company was required to perform a complete FMECA study on its design. This required analysis of every single input and output of the DCS system and a look at all the possible failures and their impact on plant systems and specific equipment. This was a mammoth task, as it potentially involved analysing nearly 75 000 individual signals for Unit 1, as well as the BoP scope. In order to streamline this process, a way was proposed to reduce the need to analyse all of the I/O based on the way the functions and plant had been distributed in the DCS design. This in turn allowed the company to minimise the overall engineering effort required to produce the FMECA study. The results of the FMECA study did identify certain shortcomings in our allocation of I/O, and we made the necessary corrections prior to commissioning.
The lessons learned from the results of performing the RAM and FMECA studies will be carried forward to future projects.
For this project the company had to manage implementation of all the process alarms provided by various mechanical plant suppliers during the design phase, prior to commissioning.
The work involved identifying all alarms in the system, and then meeting with various process suppliers to review and confirm all the alarms, as well as the expected operator response to them. Eskom documented and signed off on all of this information.
Approximately 50 alarms can be reviewed, discussed and documented per day, and there are 11 000 unique alarms for one unit and the BoP.
A special add-in to the operator interface aspect system was developed for the Kusile project, which enables the operator to easily access documented alarm responses to fulfil the required functionality. On receiving any alarm, an operator can immediately access a detailed description of actions that should be taken to address the problem.
Advanced diagnostics supports the maintenance personnel in checking up the equipment and provides an accurate failure analysis. The application blends in a powerful and intuitive environment the information acquired from vibration analysis, process data, performance calculation and reference data (baseline), provided by a state-of-the-art machine-learning algorithm.
The plant maintenance efficiency is therefore greatly enhanced by reducing the relevant cost and providing an effective predictive maintenance. This greatly increases operator efficiency, because plant disturbances and alarms can be handled immediately and actions taken quickly, increasing overall plant availability.
As part of the project contract, the comapny had to provide comprehensive cyber security solutions to comply with requirements in VGB-R 175 on IT security for power plants. The boundaries of Kusile’s C&I system had to be protected from intrusions via demilitarised zones using redundantly configured unified threat managers (UTM).
Eskom’s Kusile power station will be one of the largest coal-fired power stations in the world when completed. The strict regulations, wet desulfurisation system and demand for full automation makes this project one of the largest the company has undertaken in Africa.
Contact Shivani Chetram, ABB, Tel 010 202-5090, firstname.lastname@example.org
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