Parallel Operation for 11 kV Ring System with Normally Open Point at Distribution Networks in AADC

Currently, Al Ain Distribution Company (AADC) 11 kV distribution circuits are electrically split by Normal Open Points (NOP)s. NOPs can be closed for load restoration in case of planned or unplanned outages. Also, changing the NOP locations helps in load shifting to achieve network optimization between adjacently located 33/11 kV primary substations. This paper discusses a new operation mode in medium voltage distribution power network of AADC at Al Ain area in the United Arab Emirates by introducing closed loops of parallel 11 kV feeders from different busbars or substations. This operation mode is intended to end the continuous debate of optimal NOP locations in addition to expected reduction in the required time for load restoration in case of failures at distribution (11/0.4 kV) substations. A detailed case study is performed to explore the achieved benefits and enhancements on load balancing, voltage profile, and power factor in medium voltage networks. On the other hand, the impact on load flow, protection scheme, and loading violation are evaluated. Furthermore, the proposed model is in line with the regulations of the Department of Energy (DoE), regulatory body in Abu Dhabi Emirate, and should fully comply with Electricity Distribution Code (EDC) of Security of Supply Standards. Accordingly, while the current used operation mode is still valid, it is found that closed loops arrangement can be only allowed for rings from same 33/11 kV substations feeding from same source with considerable protection limitations. Moreover, it was concluded that closed loops from different 33/11 kV Substations will result in load flow problems.


Introduction
Al Ain Distribution Company (AADC) is a public joint stock company in emirates of Abu Dhabi under TAQA groups companies in United Arab Emirates (UAE).The company's role is to distribute water and electricity in the eastern region of Abu Dhabi Emirate [1].Regarding power network, AADC covers medium and low voltages level starting from 0.4 kV up to 33 kV.As per the business strategy customer satisfaction is always at the forefront of the company's priorities.Therefore, maintaining a secure and safe power supply to client is what AADC is striving for.Moreover, those rings are kept normally open at certain NOPs that act as isolation point between the two sides of a ring.NOPs are defined using rule-based approach and located considering several conditions in addition to the Network Optimization Model such as: -The total load of the feeding (220/33 kV Grid) stations should comply with N-1 criteria.
-The load is preferred to be distributed in a way to increase the number of secured parallel transformers at 33/11 kV primary substations.
-The NOP is preferred to at telemetered station (with remotely switching capabilities from centralized distribution management system) as far as possible, and have easy access by field operation staff (like: near to the roads, not inside private premises, ..etc.) [4].
It's important to highlight the drawbacks of the current operation mode like the difficulty of load distribution between the rings as well as the primary substations.Also, the long-required duration for load restoration and load manoeuvring while performing field operation activities by the field engineers.
The AADC 11 kV network is equipped with different protection schemes that includes protection relays, associated protection wiring, CTs, VTs, auxiliary DC power supply, and tripping devices like Circuit Breakers and fuses.For a simple 11kV ring represented in Fig. 1, any fault that occurs at a distribution transformer (DTR) shall be detected by a simple self-powered overcurrent and earth fault protection device, which sends signal to respective 11 kV circuit breaker (CB) of the DTR to isolate the fault.Furthermore, any fault that occurs at any 11 kV cable will be detected by a DC powered numerical overcurrent and earth fault protection device, which sends signal to the respective CB at the feeder heads X or Y based on the feeding source for the fault isolation.However, it's well known that the failure rate of the cables is much higher where the average failure cable rate in the region during last 5-years is 1.62 failure/100 km/year [5].So, it's more likely to have frequent load disturbance from one side of the ring up to the NOP at any 11 kV fault.So, the protection scheme shall be assessed and updated once the decision of closed loop operation is taken after evaluating the load flow constraints.
Accordingly, this paper discusses a case study of closed loop ring at 11 kV network to overcome the drawbacks of the current operation mode.

Case Study: closed loops at 11 kV networks
As usual practice, AADC performs load management on close/open sequence at 11kV rings by switching the NOP OFF then open another switch on the ring.To perform switching in this sequence, there are three mandatory requirements: matching phases along the whole ring, acceptable short circuit level, and almost equal voltage magnitude and phase angles, to assure normal load flow from a busbar to other in a ring.However, the concept of applying permanent closed loops (11 kV rings without NOPs) is evaluated to achieve certain benefits like minimizing operation cost for planned outages, minimizing interruption time for unplanned outages, and enhancing network performance indices accordingly.Therefore, the following case study is analysed.

Selected Rings
In AADC power network, there are around 2,200 11 kV feeders which are all in ring system which they are not necessarily forming simple rings with two feeders but could be forming multi-rings that could have multiple NOPs.However, each two feeders are still connected in a multi-ring and are still considered a simple ring within a larger ring system.Therefore, to validate the closed loops concept, different 11 kV ring types are selected with different operation configurations: -3 numbers of 11 kV rings from different busbars, from a single or same 33/11 kV substation.(ring1, ring2, and ring3) -3 numbers of 11 kV rings from different 33/11 kV substations feeding from same 220/33 kV grid station.(ring4, ring5, and ring6) -3 numbers of 11 kV rings from different 33/11 kV substations feeding from different 220/33 kV grid stations.(ring7, ring8, and ring9) The daily load profiles of the above-listed rings are presented in the Fig. 2. It's important to highlight that the actual rings' names are not disclosed for confidentiality.The chart in Fig. 3 represents the loading percentages of the rings based on the N-1 firm capacity to comply with DoE regulations [5].In general, the selected rings have different loadings (lightly to fully) loaded for each ring type.Overall, all feeders are complying to security of supply standards and do not exceed the allowable loading limits.This variation in the loading percentages excludes the loading effect on the study analysis.

Load Flow Analysis
AADC currently performing momentary paralleling (closed loop) while performing load management and network planned outages, all selected rings can be set to closed loops for a short time in principle (considered momentary parallel) assuming the selected rings are normally loaded.However, there are not enough studies performed to guarantee normal load flow within the current currying capacity (CCC) of the used conductors under such momentarily switching.Besides, it's important to highlight that there is a history of tripping due to parallel operation that occurred last year -the ring is added to the selected sample as ring9 -due to huge flow with almost 660 A at one feeder head and 470 A at the other end.Accordingly, the high load flow caused tripping on overcurrent protection where all actual load flow measurements are collected through AADC real-time Distribution Management System (DMS) system powered by Spectrum Power 4.4 [4].Typical current ratings of 11 kV cables in the ring varies from 285 to 340 A depending upon the sizes 240-300 sq.mm Copper.Accordingly, it's important to perform load flow analysis simulation to avoid undesired interruptions while performing this case study.
In this paper, load flow analysis is performed while feeding buses' voltages are set to same values at both sides of each ring's feeders by adjusting the onload tap changer (OLTC).Accordingly, simulated load flow analysis is performed using Study Cases and Distribution Network Applications/Distribution Power Flow (DNA/DPF) applications tool equipped with AADC DMS system provided by Spectrum Power 4.4 for all rings.DNA facilitates a comprehensive analysis of power network, enabling the efficient utilization of its assets.These applications, including power flow, voltage-var control, network fault analysis, and load management, are employed in real-time scenarios to assist operators in evaluating the network's status.They support the network operations through optimized controls, as well as in resolving abnormal network conditions like limit violations.The following table 1 represents the load flow results of all sample rings where simulation was performed under average loading condition.For all above listed 11 kV feeders, as the CCC is 300 A of the 240 mm2 copper conductors with 630 A CB rating capacity.Obviously only tings 7, 8, and 9 are violating thermal limits while other rings are having satisfactory results.In addition to its history of high load flow (660 A) during the actual closed loop applied previously, ring9 is also showing the highest load flow currents under simulation (618 A).Accordingly, ring9 is investigated more deeply.The total length of this ring from one end to the other side is almost 3 Km with total 5 distribution substations as illustrated in Fig. 4 below.Each infeed source of the two sides of the ring is a substation with 3 numbers of 33/11 kV parallel transformers with Z% of 15% and 12% respectively.Also, each PRY is equipped with detuned switchable Capacitor Banks that are switched on and assumed to be switched on for the simulation and supplying reactive power at the 11 kV busbar.Such an assumption is made to ignore its impact in this stage of analysis to meet the actual network configuration as the capacitor banks' controller is set for PF only [6].
The following table 2 represents the power flow results of ring9 for actual scenario under closed loop arrangement keeping in mind that the actual results are collected from the tripping analysis report of the fault highlighted earlier in this paper.There is no information about the tap position during the closed loop while the voltages were 11.0 kV and 11.1 kV at both sides.The tap changers are assumed to be at positions 6 and 5 respectively in the two primaries to match simulation results.The PTRs are equipped with 21 total tap position with tap 9 as N-Tap (neutral or mid tap position).In general, it's important to note that negative signs in the above table or the following tables represents reverse flowing of power into the busbar.For the simulation scenarios, the following table 3 represents the base case (open loop) and first scenario titled as Simulation1 where the tap position of the three parallel Power Transformers (PRT)s is 6 and 5 in substations PRY1 and PRY2 respectively.It was found that under this closed loop condition the PTRs at PRY1 are treated as reactive loads consuming reactive power while most of the active load at PRY1 was fed through PTRs at PRY2.The DNA tool can suggest tap positions by enabling the "Use Local Controller" feature under solution options.So, it suggested increasing the tap position at PTR2 to reach to a balance in the load flow between the two PRY substations.After 10 trials, the scenario "Simulation10" was found to be the best arrangement for closed loop arrangement for this ring with minimum load flow.The load flow results are represented in below table 4 showing acceptable values.However, this solution is not practical due to low voltage (around 10.2 kV) resulted at the 11 kV busbars that violates the minimum required voltage at the feeding substations.Also, it is not a recommended arrangement since high voltage will occur at 11 kV busbars of PRY2 when increasing the tap position to position 13 as precondition of balance flow in this case.
Furthermore, more scenarios were simulated by having different combinations between de-paralleling PTRs, switching off capacitor banks, and changing tap positions.Hence, it was found that "Simulation43" resulted in the best closed loop load flow results when all PTRs were parallel in PRY1 at tap 6 and the feeding PTR at PRY2 was independent and set at tap 11.The following table 5 illustrates the results.However, such arrangement would work for this ring only under specific loading and overall network flow.Thus, it's not practical to do more than 4 trials every time we need to have a closed loop arrangement.

Protection Analysis
Similarly, the maximum short circuit currents for simulated 3 phase faults on the closed loops are obtained using DNA/SCC tool of the DMS system.The following table 6 represents the results obtained for the maximum SCC of all sample rings where the listed values are obtained under closed loops arrangement.Obviously, there is no violation to the short circuit withstand and breaking capacities of the equipment and only rings from different substations feeding from different grid stations are showing relatively higher results that might be a concern especially ring7.However, the breaking capacity of the CBs is 25 kA for 3seconds.Hence, it's still acceptable.
In addition, it's important to evaluate the impact of failures under close loop arrangement.The following fig.5 represents a single line diagram (SLD) for two 11kV rings between two different substations feeding from different grid stations.Two fault locations are assumed as illustrated at A and B at the underground cables.So, a tripping at location A on "ring Z" while NOP is closed will result in disturbing power supply for all distribution substations if there are no protection devices installed at the incomer/ and outgoing 11 kV CBs of the distribution substations.On the other hand, if the incomer/outgoing CBs are equipped with protection relays (for example ring T), then a fault at location B will be isolated and no power interruption shall be faced by the whole connected substations.However, such coordination would be considered a real challenge and not feasible, considering discrimination margins required across different protective devices.Alternatively, directional protection could be reinforced which again involves seamless coordination exercise.The third option is that cable differential protection at the two ends of each distribution substation could be introduced for faster isolation of fault without any complex coordination exercise.However, this would not be economically efficient, in view of the huge investment involved, including subsequent operational expenses and expertise for maintaining such a protection system.Consequently, a closed loop configuration will result in enhancing AADC network reliability, only with upgrade of the protection system [7].

Results and Recommendations
Based on the load flow results and the tripping history of momentarily closed loops.We can classify AADC 11 kV Network parallel operation under three main categories keeping in mind that different scenarios were simulated for a single closed loop every time while the impact of having multiple loops is not studied.
The first category, rings from different busbars at the same 33/11 kV substation.Momentarily parallel closed loops are possible without any preconditions if the voltage level at both infeed buses is equal.Permanent loops are not adding additional load balancing benefits as both busbars are in the same station.Accordingly, there is no added value of introducing protection scheme at the distribution substation level.
The second category is the 11 kV ring from different 33/11 kV substations feeding from same 220/33 kV grid station.This configuration still allows momentarily paralleling for outages maneuvering.Furthermore, the permanent closed loop arrangement might help in load balancing while such advantage is already covered by the available DMS control at the distribution substation level.
The last category is a ring from different 33/11 kV substations feeding from different 220/33 kV grid stations.In this configuration no parallel operation shall be allowed without detailed dedicated simulation assessment using the real-time DMS applications.
Overall, it's not recommended to implement the closed loop configuration as it might result in significant risks and considerable operational challenges that might not be a good justification to invest in the protection scheme to meet this proposal.This configuration could be implemented in very limited rings (from the same substation) that require high reliability indexes.

Conclusions
This paper investigated the closed loop arrangement at 11kV rings at AADC for different interconnected networks.It was found that introducing this operation concept will result in additional challenges and has no significant improvement compared to the required upgrade to the existing protection scheme.Mainly, closed loops from different 33/11 kV substations feeding from different 220/33 kV grid stations have significant load flow problems that might trigger the protection scheme and lead to load interruptions.It was confirmed that momentarily loops for 11 kV rings from different busbars at the same 33/11 kV substation or different 33/11 kV substations feeding from same 220/33 kV grid station is possible and can be implemented for load management and planned outages arrangement.

Fig. 2
Fig. 2 Load profiles curves of the sample 11 kV rings from the archived data at AADC records.

Fig. 4
Fig. 4 SLD of Ring9 showing feeding sources of both PRY1 and PRY2.

Fig. 5
Fig. 5 fault examples and impact of closed loops on protection scheme

Table 1 .
Sample Rings Load Flow under Open and Closed Loop arrangement

Table 2 .
Ring9 Load Actual Flow under Closed Loop arrangement

Table 3 .
Ring9 and Feeding PTRs at Primary Substations Simulation1 Scenario for Load Flow under Open and Closed Loop arrangement

Table 4 .
Ring9 and Feeding PTRs at Primary Substations Simulation10 Scenario for Load Flow under Open and Closed Loop arrangement

Table 5 .
Ring9 and Feeding PTRs at Primary Substations Simulation43 Scenario for Load Flow under Open and Closed Loop arrangement

Table 6 .
Maximum Short Circuit Calculations for the Sample Rings Under Closed Loop arrangement