The efficient running of high voltage Alternating Current (AC) networks involves, among other things, the management of reactive power flows. Transmission lines and cables tend to generate reactive power which requires to be compensated for to manage voltage profiles and reduce energy losses on the system. During periods of low loading, the voltage on a long transmission line or cable may increase along the circuit with the potential to fall outside the operational limits and equipment voltage design ratings which could result in equipment failure. One effective way to manage system voltages within desired operational limits is the use of shunt reactors where the system is susceptible to high voltages. The transmission networks in GB have several installations with shunt reactors connected in different configurations including, but not limited to, line connected, busbar connected and auto-transformer tertiary connected to manage system voltages.
Objectives
· An investigation into the ZMP DC current problem in detail (cause, consequences and likelihood).
· An investigation into the capability of circuit breakers to interrupt the prospective fault current
· An investigation into mitigation options and strategies (and developing them)
· Risk analysis of any viable mitigation options and strategies
· Establish the planning, training, operation and maintenance requirements of any viable mitigation options
· Compile reports with results of the study for dissemination and to decide the viability of live trials of the mitigation options
Learnings
Outcomes
A thorough literature review has been carried out and delivered. This review had several findings:
•ZMP is most likely to occur on lines/cables where the compensation level exceeds 50% of the total reactive power;
•ZMP duration can be influenced by system fault level and system impedance’s X/R ratio;
•Five basic “critical” network configurations were identified for testing (to be supplemented by real world examplesfrom SSEN & SPEN);
•ZMP countermeasures can fall into 3 categories:
oPrevention Methods
oMitigation Methods
oHandling Methods
The project then successfully analysed ten standard network configurations, that are susceptible to the Zero Missing Phenomenon, to help deduce suitable mitigation measures. The analysis was also used to determine the severity and effects of the ZMP. The learning from this project was captured in a Technical Guidance Document to be used by Transmission Planners to reduce the number of system studies they have to perform to address this problem. This document will assist GB Transmission operators by providing a structured approach to address the ZMP problem that is becoming more prevalent as the network transitions to low carbon.
The project investigations found that the following factors must be taken into consideration when assessing the risk of the occurrence of ZMP:
1.The background running arrangement of the critical network configuration; as under certain running arrangementssome circuit breakers are at risk of experiencing energisation-related ZMP.
2.The service voltage of the substations/circuits within the network configuration.
3.The switching phase angle on the voltage-to-ground with switching at the instant when the voltage crosses zerocausing the most onerous conditions for energisation-related ZMP. This was assumed always fixed at the mostonerous level.
4.The construction type (air-core or iron-core/oil-filled) of the Shunt Reactor.
5.The rating of the Shunt Reactor.
6.The construction type of the transmission line (OHL/underground cable).
7.The technical characteristics of the transmission line (conductor size/type, circuit length).
8.The compensation level of the reactive power gain of the transmission circuit.
The investigations also highlighted a selection of countermeasures to manage ZMP, such as:
•Circuit-breakers with pre-inserted resistors;
•Point-on-wave controlled switching;
•Energisation in sequence;
•Variable shunt reactors; and
•Sequential switching.
Lessons Learnt
Time and effort were saved creating Transmission Network computer models by collaborating with SPEN’s project “Transient Recovery Voltage” as both projects required network models and both companies use industry-standard components.
Special mention should be made of the successful manufacturer engagement that avoided non-disclosure agreements by posing high-level questions; rather than asking for commercially sensitive computer models.
The overall learning from the project around the ZMP problem and mitigation has been compiled into a Technical Guidance Note to be used by transmission planners. This deliverable can now be used to determine the viability of deploying ZMP mitigation options in the future.
