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Voltage flicker
Voltage dips
Voltage surges and switching disturbances
With regards to load power factor, customers with demand exceeding 100kVA shall ensure that the power factor shall not be less than 0.9 lagging nor shall it go leading unless otherwise agreed to with the relevant Distributor. Should the power factor go beyond these limits, participants shall take corrective action within a reasonable timeframe or as agreed between the parties, to remedy the situation.
In addition, the Distributor shall co-ordinate voltage control, demand control, operating on the Distribution System and security monitoring in order to ensure safe, reliable, and economic operation of the Distribution System. The Distributor shall operate the Distribution System within defined technical standards and equipment operational ratings. Customers shall also assist the Distributors in correcting quality of supply problems caused by the Customer’s equipment connected to the Distribution System.
3.2.1 Assessments and procedures for Grid Code compliance
Through operational liaison, it was observed that most Distributors do not strictly comply with the requirements of the South African Grid codes. To ensure compliance to the System Operating and Network codes within Eskom Distribution, research was done to incorporate specific tasks, procedures and processes into the job specifications and duties of operation engineers. This is summarized in section 3.2.1.1. Section 3.2.1.2 provides a practical application on the concept of improving network performance by minimizing faults and providing effective methods for fault location and protection coordination.
3.2.1.1 The Distribution Network Operation and planning guideline [10]
A portion of this research has been involved in the development of study procedures aimed at providing technical support and guidance to Eskom’s Distribution control centers, for the operational management of distribution networks. Embedded generation has added to the complexity of the distribution system, an environment new to both Operations engineers and Control room operators in Eskom Distribution. Study procedures to operationally manage
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distribution network including networks with embedded generation was developed to ensure that the stability and security of the distribution system is managed effectively [10].
The objective of the procedures is to maintain the power system within operating technical specification during both the normal and abnormal states of the network. Limit compliance, both steady state and dynamic, in terms of voltage, thermal loading, fault level and other grid code specifications are the primary areas of concern to operations engineers. The procedures are defined for daily, occasional and annual assessments. The bulk of the assessments are by means of power system simulation. Since Eskom Distribution makes use of the DIgSILENT Powerfactory simulation tool, the study procedures have been tailored for use in DIgSILENT, including tutorial material.
The daily procedures consists model verification and calibration to suite the specific day, an assessment of system fault levels, the determination of control modes for embedded generation, a contingency sweep of plausible system abnormalities and then dynamic simulations. The steps are iterative so any violation or change required prompts a re-initialization of a basic load flow with a re sweep of assessments. Once the operations engineer is satisfied, changes are implemented and plans are made available for future contingent states.
3.2.1.2 Performance Evaluation of Traction and Utility Network Interface: Fault Location and Protection Coordination [12]
Single phase AC traction systems pose unique challenges to power utilities at interface points of connection on the power delivery network. Traction systems are usually supplied from dedicated utility networks in order to minimize the associated negative effects on conventional three phase loads, particularly in instances where customer equipment may be sensitive to the quality of supply. The dedicated power utility network however, is exposed to faults, short duration thermal overloads, temporary overvoltage, high magnitude transient recovery voltages, load unbalance and harmonics which emanate from the traction system [12].
A method for the determination of fault location and effective discrimination between power utility and traction network faults was developed that is particularly useful in instances where there is a lack of protection coordination or difficulty in achieving coordination between the protective devices between systems [12]. The lack of protection coordination for the networks
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under study resulted in the utility network feeders tripping for traction system faults which lead to unnecessary outages, line patrols and investigations. Fault current “look up charts” were developed by power system simulation that a utility engineer can use to locate faults and take appropriate action. The method involves interrogation of measurement recordings that can be placed on the fault charts for location, thus making it simple to distinguish between utility and traction faults.
The method was further developed into a protection coordination philosophy by translating the fault current charts into the impedance plane. By the manipulation of the impedance reach settings, multiple utility relays can be configured to “see” or “not see” a particular fault. For a traction fault, if only a single utility relay reaches into a fault, a block signal will be instituted as the fault will be identified to be within the traction system. If the traction systems protective devices do not clear this fault in the necessary time, the utility breakers will operate in delayed time.