Relay Coordination Review for 2,743 MW Integrated Power & Water Plant, Saudi Arabia
Power Projects was engaged to perform a detailed relay coordination review for the Marafiq Independent Water and Power Plant (IWPP) in Saudi Arabia, assessing whether existing protection settings satisfied a critical 1.1-second plant/grid coordination requirement.
Project Overview
Power Projects was engaged to perform a detailed relay coordination review for the Marafiq Independent Water and Power Plant (IWPP) in Saudi Arabia.
The facility has a net generation capacity of 2,743 MW and a desalinated water production capacity of 800,000 m³/day. Its electrical system consists of four power blocks connected to the National Grid through eight Step-Up Transformers and four 380 kV transmission lines.
The plant electrical infrastructure includes:
- 4 Steam Turbine Generators rated at 363 MVA and 478 MVA
- 12 Gas Turbine Generators rated at 234 MVA
- 8 Step-Up Generating Transformers
- 12 Unit Auxiliary Transformers
- 380 kV double-busbar GIS
- 6 bus sections
- 4 bus couplers
The study was performed with IEEE 242-2001 as the reference standard.
A key project requirement was that, for faults occurring on the external National Grid SA transmission system, the Step-Up Transformer and unit protection systems at the plant should not operate before 1.1 seconds.
Power Projects reviewed the existing protection settings to determine whether this coordination requirement was being achieved.
Protection Coordination Challenge
The plant operates in parallel with a 380 kV transmission network, making coordination between plant-side protection and the external grid protection an important part of the review.
For an external transmission fault, the applicable National Grid protection is expected to clear the fault. Plant-side backup protection must provide the required coordination time so that an external fault does not unnecessarily result in the tripping of a Step-Up Transformer or generating unit.
For this project, the critical requirement was a minimum operating time of 1.1 seconds for applicable plant-side protection during external transmission faults.
The review therefore examined whether a fault on the National Grid side could cause any existing plant protection element to operate within the 1.1-second coordination window.
The assessment considered fault direction, fault-current contribution, protection zone, relay characteristic and operating time together with the existing relay settings.
Protection Systems Reviewed
The review covered protection associated with the interface between the generating plant and the 380 kV transmission system. The major protection systems reviewed included:
- Transmission line circuit breaker protection
- Bus-tie and bus-section protection
- 380 kV GIS cable feeder protection
- Step-Up Transformer protection
- 380 kV busbar protection
- Unit Auxiliary Transformer protection
- Steam Turbine Generator protection
- Gas Turbine Generator protection
Protection functions including 50, 51, 51G, 67, 67G, 87T, 87UAT, 87REF, 87B, 87G, 27, 59GB and breaker failure protection were assessed based on their application and response to the external transmission faults considered in the study.
Three 380 kV Fault Scenarios
The relay coordination review considered three representative transmission fault scenarios rather than a single fault location.
Fault at the 380 kV Jubail Bus
A fault was considered directly at the Jubail 380 kV bus, where four transmission lines are interconnected. This represented a severe external network disturbance and was used to assess whether plant-side protection could respond before the required coordination time.
Fault at Transmission Line End, National Grid Side
A fault was applied at the remote transmission line end towards the National Grid. This scenario assessed how the plant protection responded to a grid-side line fault with contributions from both the Jubail system and National Grid.
Fault at Transmission Line End, Plant Side
The third scenario considered a line-end fault towards the Marafiq plant, with the four power blocks contributing to the fault and the identified BK-2 breaker maintained in the open condition.
For each scenario, fault-current contribution and the corresponding response of individual protection functions were reviewed.
Engineering Review Approach
The review was performed using the existing plant protection documentation, including:
- 380 kV GIS single-line diagrams
- Generator nameplate information
- Protective relay trip matrix
- Electrical interlocking schemes
- Existing relay setting calculations
- Electrical design criteria
- Step-Up Transformer technical specifications
- UAT technical data
- Generator protection information
Each protection element was assessed against the calculated fault contribution for the three external fault scenarios.
The engineering review checked whether each element would pick up, whether it was directional or zone-restricted, and how long it would take to operate.
This approach separated protection functions that were naturally secure for external faults from non-zonal backup functions that could potentially cause unwanted plant-side operation.
Protection Performance Assessment
The review confirmed that the main zone-based protection functions behaved correctly. The following protection functions were restricted to their defined protection zones:
- Transformer Differential Protection 87T
- Unit Auxiliary Transformer Differential Protection 87UAT
- Restricted Earth Fault 64R/87REF
- Busbar Differential Protection 87B
- Generator Differential Protection 87G
These functions were therefore not expected to operate for faults occurring outside their respective zones on the National Grid transmission system.
The 380 kV cable directional protections, including 67 and 67G, were also configured based on fault direction and did not create the primary coordination concern for faults towards the grid. The critical finding came from the non-zonal backup protection.
Critical Protection Coordination Issue
Several Step-Up Transformer overcurrent and ground overcurrent protection elements were found to operate within or close to the required 1.1-second coordination period under certain transmission fault scenarios.
For example, the review identified Step-Up Transformer protection functions where:
Phase Time Overcurrent 51 had definite-time operation as low as 0.3 seconds under an applicable scenario.
Ground Time Overcurrent 51G had definite-time operation of approximately 0.5 seconds.
Certain 51G functions could therefore respond well before the required 1.1-second plant/grid coordination boundary.
This created insufficient time discrimination between the plant-side Step-Up Transformer backup protection and the National Grid SA transmission protection.
The study therefore established that the existing protection coordination did not fully satisfy the client's 1.1-second requirement.
Engineering Recommendation
Power Projects recommended reviewing and modifying the applicable 51/51G settings associated with the Step-Up Transformer protection systems.
The objective was not simply to increase relay operating time. Any revised setting needed to achieve two requirements:
- Protect the transformer and plant equipment for genuine internal faults
- Provide sufficient grading time for National Grid SA protection to clear external transmission faults without unnecessary plant-side tripping
The review therefore recommended optimisation of the time-current characteristics of the affected 51/51G elements and other applicable time-delayed protection functions.
Project Outcome
The study identified a protection coordination issue that may not have been evident from reviewing the differential protection schemes alone. The primary zone protections were correctly configured, while the concern existed in the backup protection layer.
By analysing three different 380 kV fault locations and checking the response of individual protection functions, Power Projects demonstrated that selected non-zonal Step-Up Transformer protection elements could operate before the required National Grid coordination period had elapsed.
The study provided a clear engineering basis for reviewing these settings while retaining dependable protection for the generating transformers and plant equipment.
For a 2,743 MW generating facility connected through four 380 kV transmission lines, protection coordination at the plant-grid interface forms an important part of the protection assessment during external transmission faults.
Project Details
- ProjectMarafiq Independent Water and Power Plant
- LocationSaudi Arabia
- StudyReview of Relay Coordination
- Net Generation Capacity2,743 MW
- Desalinated Water Production Capacity800,000 m³/day
- Grid Voltage380 kV
- Reference StandardIEEE 242-2001
- Power Blocks4
- Steam Turbine Generators4 × 363 MVA and 478 MVA
- Gas Turbine Generators12 × 234 MVA
- Step-Up Generating Transformers8
- Unit Auxiliary Transformers12
- 380 kV GISDouble busbar
- Bus Sections6
- Bus Couplers4
- 380 kV Transmission Lines4
- Coordination Requirement1.1 seconds
- Fault Scenarios Reviewed3
- Identified Breaker ConditionBK-2 maintained open
- Phase Time Overcurrent Example51 – as low as 0.3 seconds
- Ground Time Overcurrent Example51G – approximately 0.5 seconds
- Protection Functions Assessed50, 51, 51G, 67, 67G, 87T, 87UAT, 87REF, 87B, 87G, 27, 59GB, breaker failure
About Power Projects
Established in 2006, Power Projects provides power system consulting services for utilities, transmission projects, renewable energy developments and industrial electrical networks.
The company undertakes protection, relay coordination, grid compliance, EMT, dynamic stability, power quality and electrical network studies using engineering platforms including DIgSILENT PowerFactory, PSS®E, PSCAD and ETAP.
