1 Ancillary Service for Transmission Systems By Tap Stagger Operation in Distribution Networks Linwei Chen, Haiyu Li, Steve Cox, and Kieran Bailey Presented by Dr Haiyu Li haiyu.li@manchester.ac.uk Senior Lecturer in Power System Automation & Communication Systems The University of Manchester, UK
2 Outline Motivations VAr Supports to Transmission Systems by Tap stagger Operation in Distribution Networks Voltage Problem Case Studies in Transmission System Economic Studies Dynamic Response Studies Conclusions
3 Motivations The growing integration of low carbon technologies, e.g. wind power and electric vehicles, into the grid has increased the difficulties in balancing generation and demand. Voltage issues in transmission systems have become more dynamic and may occur in different time periods or in different regions. The UK transmission grid currently has experienced high voltage situations under low demand conditions [1]. [1] National Grid, "Electricity Ten Year Statement 2014," pp. 269 271, 2014. [Online]. Available: http://www2.nationalgrid.com/uk/industry information/future of Energy/Electricity ten year statement/
4 Motivations The main reasons for UK high voltage issues: Development of underground cables in transmission & distribution networks. Decommissioning of coal generators in specific areas, resulting in a lack of generator voltage control. Reduction in reactive power demand during periods of minimum load. Negative reactive power demands have been observed at the National Grid transmission system. Possible contributing factors: changes in line loading patterns due to embedded generation, energy efficiency measures, e.g. a switch to energy efficient lighting.
5 Motivations In 2013, The UK Office of Gas and Electricity Markets (Ofgem) approved the 9 million Customer Load Active System Services (CLASS) project to investigate new techniques for a smarter integration between transmission and distribution networks. CLASS was led by Electricity North West (ENW), with collaborations from experienced partners (http://www.enwl.co.uk/class).
6 Motivations CLASS has carried out site trials to investigate: Demand reduction/boost for the purpose of frequency response; Demand reduction at time of system peak through the voltage reduction of primary substations; Reactive power supports through the tap staggering operation of primary substation transformers; and The impacts on transformer health and the quality of supply to customers. Academic outcomes from the University of Manchester: Voltage/demand relationship matrices with 24 h / 4 seasons; Demand response capabilities; Reactive power absorption capabilities; and Healthy analysis for Transformer and tap changer operation.
7 VAr Supports by Tap Stagger Operation of Parallel Transformers in Distribution Networks To apply the tap stagger operation to primary substation transformers in distribution networks....
8 Methodology of VAr Supports using Tap Stagger Tap stagger is used to introduce inductive loading onto the system by circulating reactive power. Source V p V p (Primary voltage) -k taps T 1 T 2 I c +k taps I L + I c I L -I c V s V s (Secondary voltage) Load
9 Methodology of VAr Supports using Tap Stagger The difference in tap positions should only be allowed within a small range to prevent the transformers from overloading. The transformer heath studies based on CLASS site trials have confirmed that the impact of tap stagger is minimal. % per unit % per unit per unit TAP % :the tap position increment per tap on the pair of parallel transformers, k: an integer representing the number of tap steps different from the initial position TAP.
10 Methodology of VAr Supports using Tap Stagger With small staggered taps (e.g. k = 1 or 2), the secondary voltage V S will remain almost constant, leaving the downstream network voltages and demands unaffected. % per unit
11 Methodology of VAr Supports using Tap Stagger The potential reactive power absorption capability for the entire GB distribution network based on the CLASS research [2] : Load flow studies were carried out for primary substation transformers with up to 6staggered taps (i.e. 3 taps up for one and 3 taps down for the other). The amount of VAr absorption changes slightly with load demands, due to the change of line reactive power losses. An optimal control scheme for the tap stagger to minimise the power losses as well as the number of OLTC switching operations was proposed in [3]. [2] Electricity North West, Reactive power absorption capability assessment final report CLASS research," Sep. 2015. [Online]. Available: http://www.enwl.co.uk/class/knowledge learning/closedown report/class research. [3] L. Chen, and H. Li, Optimised reactive power supports using transformer tap stagger in distribution networks, IEEE Transactions on Smart Grid, accepted in Mar. 2016, In Press.
12 Voltage Case Studies in Transmission System To analyse the economic and dynamic impacts of the tap staggering technique on the transmission system. Bus 6: occasionally (358 hours in a year), Bus 10: frequently (2000 hours in a year).
13 Voltage Case Studies in Transmission System The tap staggering technique has been applied to mitigate high voltages. The tap staggering impacts have been simulated by equivalently increasing the load consumption at the transmission bus. The load increment was determined using the VAr absorption capability matrix:,,,,, where the matrix element Q i,t indicates the aggregated VAr absorption measured at the GSPs by tap staggering i pairs of primary transformers at the time point t. The matrix Q was determined based on the VAr capability study from the previous work [4]. [4] L. Chen, H. Li, V. Turnham, and S. Brooke, Distribution network supports for reactive power management in transmission systems, IEEE PES Innovative Smart Grid Technologies (ISGT) European, Istanbul, Oct. 2014.
14 Economic Studies Cost analyses of applying the tap staggering technique when high voltage problems arise occasionally or frequently. Results have been compared to the use of shunt reactors. The annual cost for the TSO to apply tap stagger: where h denotes the total hours of using tap stagger in a year (i.e. utilisation hour). The first term C gen (h) represents the cost to compensate the generation changes. The second term represents the payment for the VAr support providers, i.e. calculated based on the unit price of reactive energy C VAr (in $/MVArh).
15 Economic Studies The annual cost for the TSO to apply a shunt reactor: where: C gen (h) is the cost for generation changes, which can be negative as the reactor can reduce system losses. I is the annual equivalent investment cost for a shunt reactor.
16 Economic Studies Case 1: Cost analysis for bus 6 with 358 hours (occasional V problems) Solution 1: Installing an additional shunt reactor of 6 MVAr at bus 6, Solution 2: Applying tap stagger at bus 6. 358
17 Economic Studies Case 2: Cost analysis for bus 10 with 2000 hours (frequent V problems) Solution 3: Installing shunt reactors of 6 MVAr at buses 6 and 10, Solution 4: Applying tap stagger at buses 6 and 10. Bus 10 requires 35 MVAr to eliminate all high voltage issues. In practice, distribution networks may not be able to provide such large VAr absorption through the use of tap stagger.
18 Economic Studies Case 3: Combined methods of reactors and tap stagger: Solution method Utilisation hour of tap stagger (hrs) Bus 6 Bus 10 Annual cost ($/year) Cost reduction (%) Solution 5 (reactors only) Solution 6 (stagger at bus 6 with 4 MVAr & reactor at bus 10 with 28 MVAr) Solution 7 (stagger at bus 6 with 4 MVAr & reactor with 24 MVAr + stagger with 4 MVAr at bus 10) 0 0 145.8k 0 358 0 138.1k 5% 358 189 124.5k 15% The potential cost reduction for the UK VAr compensation could be $8 million. An average of 7.5 $/MVArh cost reduction was estimated based on Case 3. The annual reactive energy absorption required by the UK transmission system was 22 10 6 MVArh in 2014. The utilisation factor of tap stagger is assumed to be 5%.
19 Dynamic Studies Dynamic studies aim to analyse the transmission system voltage responses to the successive tap changer switching in the downstream distribution networks. Modified the transmission network model by connecting five pairs of primary substation transformers to bus 6.
20 Dynamic Studies Compare the tap stagger results with the reactor switching: Switched on Switched off Staggering activated Staggering deactivated Settling time (s) Overshoot (%) Settling time (s) Overshoot (%) Settling time * (s) Overshoot * (%) Settling time * (s) Overshoot * (%) Bus 6 0.99 0.020 0.74 0.046 Bus 2 0.97 0.016 0.81 0.015 Bus 10 0.94 0.011 0.66 0.019 Bus 6 0.093 0.010 0.076 0.024 Bus 2 0.084 0.008 0.052 0.009 Bus 10 0.070 0.007 0.047 0.018
21 Conclusions For buses with only occasional high voltage issues, the results indicate that the tap staggering technique is more cost effective than the use of reactors. For more frequent voltage violations, a combined method of applying tap stagger and reactors together can be used to reduce VAr compensation costs. In the transient studies, the tap staggering method has been adopted to reduce the settling time and overshoots of the transient voltages. The research outcomes may contribute to establishing a new reactive power ancillary service, which allows distribution networks to use their existing parallel transformers to absorb temporary reactive power surplus.
22 Thanks Questions & Answers