The “Volt / var with var priority” function is considered as one of the functions of the Smart Distribution Grid [1]. The primary objective of this function is to support voltage quality and/or var requirements by providing more reactive power from the distributed energy resources (DER) by reduction of the real power of the DERs.
The available reactive power from an inverter depends on its rated power factor, actual kW, and voltage at the terminals of the inverter. For each fixed value of the DER real power, the available reactive power of the DER is presented in Table 1 for the voltage equal 1pu, and in Table 2 for the voltage equal 0.95pu. The rated real power is assumed 100%, and the reactive power is presented in the same scale. As seen in the tables, even with PF=1, there is available reactive power, when the real power is below 100%. The choice of the rated PF should be based on a benefit/cost study of the distribution system with inverters during system planning and/or during the process of the interconnection studies.
Table 1. Var capabilities of inverter (without kW reduction). Rated kW = 100%, DER Volt=1
Table 2 presents the case when the voltage at the DER terminals is 0.95 pu. As seen in this table, the available kvars are less than in Table 1.
The same reduction of kW, but from different initial kW of the DER, does not provide the same additionally available kvars, as seen in Table 3.
Table 2. Var capabilities of inverter (without kW reduction). Rated kW = 100%, DER Volt=.95
Table 3 presents the additional kvars that become available after the kW are reduced by 10%, For instance, when the PF=0.90 and the voltage is 0.95pu, reduction of the kWs by 10% from the initial 100% provide 21.4 % of additional kvars. Reduction of the kWs by 10% from the initial 90% of kWs provides 13.7 % of additional kvars. As seen in the table, the lower is the initial DER kW, the smaller are the additionally available kvars.
Table 3. Additional kvars available due to reduction of DER kW by 10% from different initial DER kWs, %. DER Volt =0.95pu
If the additional reactive power is needed to raise the voltages in some voltage-critical points, the impacts of both the reduction of the DER real power and the increase of the DER reactive power should be compared. The following conditions should be met:
Δkvar(DERk) x Reactance(k-l)+ ∆kW(DERk) x Resistance(k-l) > 0 (1)
Volti,min ≤Volti ≤ Volti,max (2)
Ampj ≤ Ampj,max (3)
(Operating kW Reserve) ≥ (Operating kW Reserve)min , (4)
Where
k – is the k-th DER involved in the kw-kvar exchange
k-l - is the electrical path either between the l-th critical point and source of supply, or between the k-th DER and source of supply, depending on the mutual allocation of the critical point and DER
i – is the i-th node in the subject network
j – is the j-th element in the subject network
min – is the minimum limit
max – is the maximum limit
As follows from (1)
∆kvark / ∆kWk > resistance(k-l) / reactance(k-l) (5)
Table 4 presents the ∆kvar / ∆kW ratios for different initial DER kWs. As seen in the table, if the resistance to reactance ratio is 1, all cases in Table 4 represented in italics would not provide voltage increase in the sought points due to the additional available kvars. If the resistance to reactance ratio were 1/2, no cases in Table 4 represented in bold numbers would provide voltage increase, and if the resistance to reactance ratio were 1/3, no cases in Table 4 represented in red would provide voltage increase.
Table 4. Ratios of the increase of DER kvars over reduction of kW from the initial kWs. DER Volt =0.95pu.
There is another function of the Smart Distribution Grid – the Volt-Watt function [1]. The main objective of this function is reducing the overvoltage by reducing the kW of some DERs. When the DERs have the capability of absorbing kvars, this function is similar to the Volt/var with var priority function. In this case, reduction of DER’s kW releases additional available absorbing kvars. In many cases, the additional absorbing kvars have a greater effect on voltage reduction, than the reduction of kW.
When the kWs and kvars of DERs are changed, the power flow in both the distribution and transmission networks is also changed. This means that the losses, nodal voltages, loading of elements, operating reserves, and other dependent parameters are changed. We can say that there are “cost” and “benefits” of such changes. It is a challenge to adequately determine the cost/benefit relationship between the reduced kWs and additional kvars in the near real time. To make such an assessment, an optimization procedure based on comparative power flow simulations should be used.
Consider an example of the application of the “Volt/var with var priority” function. Figure 1 presents the example diagram. The transmission equivalent represents a 115kV network. The distribution is a 12kV network. The sample operating conditions are such that the secondary voltages in nodes 1210 and 1209 are below the lower voltage limits. The voltage cannot be improved by changing the voltage at the bus 1201. Hence, the Volt / var with var priority function is considered. Different cases of DER kW reduction have been analyzed. The paths between the voltage critical points and the main source of supplies do not include the DERs in nodes 1202, 1203, and 1204. That is why these DERs were not considered in the analysis. A number of other combinations of reductions of DER kW for additional kvars were considered (see Table 5). The effects of these combinations on the voltages in the critical nodes are presented in Table 6.
Figure 1. Diagram of the example circuits
Table 5. Different cases of reduction of the DER’s kWs (%) for additional kvars
As seen in Table 5, Case 1 represents the initial conditions, when the voltage in node 1210 is 92%, and in node 1209 it is 94.1% (see Table 6). Cases 2 through 6 represent different degrees of DER kW reduction in the most critical point – 1210. The reduction ranges from 10% through 100%. Even with 100% of reduction of DER kW in the secondaries of the critical node, the voltage does not reach the standard level (95%). Combinations 7 through 11 also do not provide the desired results. Combination 12, which involves all DERs located fully or partially on the paths between the critical points and the source of supply, brings the voltages in the critical points into the standard range.
Let us consider the “cost” of such improvement of the voltage quality. As seen in Table 6, the real power losses in the distribution network increased by 47%, and the losses on the transmission network increased by 4%.
Table 6. Effects of kW reduction for additional kvars
Changes of other parameters are presented in Table 7. As seen in the table, the kW flowing into the distribution system from the transmission system increased by 51%, while the kvars were flowing into the transmission system from the distribution network. The natural real load in distribution increased by 2.7% and the reactive load increased by 8.1% due to the increase in the weighted average secondary voltages by 2.7%. The total DER’s kW reduction was 29%, while the DER’s kvars increased four times. The loading of the transmission line feeding the subject substation increased by 3%.
Table 7. Additional factors affected by the reduction of DER’s kW for more kvars
As follows from the above analyses, the many entities of different ownerships paid a significant “cost” to improve the voltages in two distribution nodes.
The sample distribution circuit (for cases 1-12) was an overhead distribution system with the R/X ratio of the involved primary circuits around 0.55. If the circuit were with a higher ratio (e.g., underground), the effect would be much smaller, and the cost would be much higher. On the other hand, if the circuit were of a smaller R/X ratio, then the effect would be greater, and the cost would be significantly smaller. Such an example is presented in case 13. In this example, the R/X ratio is around 0.3.
As seen in case 13, the kW flowing into the distribution system from the transmission system increased only by 16%, while the kvars were still flowing from the transmission system in the distribution network. The natural real load in distribution increased by 1.3% and the reactive load increased by 3.4% due to the increase in the weighted average secondary voltages by 1.3%. The total DER’s kW reduction was 9%, while the DER’s kvars increased about two times. The loading of the transmission line feeding the subject substation increased by 1%.
Consider another use of the Volt/var with var priority function. In this case, all DER owners autonomously run the Conservation Voltage Reduction (CVR) mode of Volt/var control. In order to reduce the voltage, they reduced the injection of vars in the circuit. Because of this, the voltages in the entire circuit were reduced, and in one node, it was reduced below the standard voltage limits. The DER in this voltage-critical point could not produce enough, if any, reactive power to support the needed voltage at its bus. In order to provide the customers in this node with quality voltage, the distribution
system operator (DSO) or the DMS needs to increase the voltage at the bus of the main feeding substation, increasing the voltages in the entire distribution network fed from this bus. In such a case, reduction of the kW injection by the DER in the voltage-critical point to release its reactive power may help in supporting the voltage in this point and, at the same time, avoid raising the substation voltage.
Consider an example. The sample circuit is the same as in the previous example with the R/X ratio about 0.55.
The results of the analysis are presented in Table 8 (cases 14 and 15).
Table 8. Results of the analysis of the CVR case.
As seen in the table, the combined actions by the DSO and the DER owner in node 1210 (case 15) result in 138 kW reduction of total customer loads, in 91 kW reduction of distribution losses, and in 89 kW reduction in transmission losses. These reductions benefit all customers. However, the customer in node 1210 needs to take additional 300 kW from the grid to compensate for the reduction of its DER injection. This additional cost to the customer 1210 can be considered as payment for the improved voltage quality, or can be fully or partially compensated from the benefits of other customers. This matter is beyond the scope of this paper.
Case 16 in Table 8 presents a case when the Volt/var function is coordinated by the DSO/DMS. In this case, the objective of the function is still CVR, but the DSO requests providing maximum available kvars from the DERs. The additional supply of vars from the DERs allows the DSO to reduce the substation bus voltage by additional 3%. The difference between this case and the case of autonomous Volt/var controls is presented in the last column of Table 8. As seen in the column, the distribution losses are reduced by 252 kW, the transmission losses are reduced by 301 kW, and the total customer load is increase by 55 kW. The overall conservation of kWs is 498 kW. Whether this case is beneficial for the customers depends on the ratio of the cost of a consumed kW over the cost of a lost kW. In this example with about ten-fold difference between the loss reduction and load increase (553/55), it is most likely that the case is beneficial for all customers.
Conclusions
1. The efficiency of the Volt / var with var priority function in distribution with high penetration of DER depends on a number of factors, such as:
the dominant R/X ratio of the circuits,
the mutual allocation of the voltage-critical points and the DERs,
the sizes of the DERs,
the rated Power Factors of the DERs,
the initial loading of the DERs,
the availability of the DERs to participate in the Volt / var with var priority function.
2. To take into account all the above factors and find the optimal solution, a comparative power flow analysis should be performed, in other words, an optimization procedure should be applied.
3. The cost of mitigating voltage violations in distribution by using the Volt / var with var priority function in distribution circuits with the dominant R/X ratios above 0.5 may be very high. If such conditions may happen often, an upgrade of the circuits may be a more efficient solution and should be considered in the planning stage.
4. Coordination of the Volt/var control function by the DSO/DMS instead of autonomous controls by the DER owners, following the same objective, may provide more benefits to all customers.
References and further reading
John Berdner, Advanced Inverters and Grid Support. Available: http://www.clean-coalition.org/site/wp-content/uploads/2014/05/Grid-support-With-Advanced-Inverters.pdf
Nokhum Markushevich and Alesandr Berman, “New Aspects of IVVO in Active Distribution Networks”, Presented at IEEE PES 2012 T and D
Nokhum Markushevich, ‘What will the Microgrids and EPS Talk about?’ Part 1 and 2. Available: http://www.energycentral.com/gridtandd/gridoperations/articles/2858 and http://www.energycentral.com/gridtandd/gridoperations/articles/2864
Nokhum Markushevich and Edward Chan, Integrated Voltage, Var Control and Demand Response in Distribution Systems, IEEE, March 2009, Seattle
N. Markushevich and A. Berman, Distribution Automation and Demand Response, DistribuTech2008, Tampa, Fl, January, 2008; North American Policies and Technologies, Electricity, Transmission & Distribution, 2008, Volume 20, No. 8 ; 2009, Volume 21, No. 1
http://www.electricity-today.com/download/issue8_2008.pdf; http://www.electricity-today.com/download/issue1_2009.pdf
Nokhum Markushevich, Applications of Advanced Distribution Automation in the Smart Grid Environment, T&D Online Magazine, January-February 2010 issue. Available: http://www.myvirtualpaper.com/doc/Electric-Energy/january-february-2010/2010012802/#22
Development of Data and Information Exchange Model for Distributed Energy Resources, EPRI, Palo Alto, CA: 2010. 1020832. Available: http://www.epri.com/abstracts/Pages/ProductAbstract.aspx?ProductId=000000000001020832&Mode=download
Coordination of Volt/var control in Connected Mode under Normal Operating Conditions. Available: http://smartgrid.epri.com/Repository/Repository.aspx/
Update aggregated at PCC real and reactive load-to-voltage dependencies. Available: http://smartgrid.epri.com/Repository/Repository.aspx/
Updates of capability curves of the microgrid’s DERs. Available: http://smartgrid.epri.com/Repository/Repository.aspx/
Understanding Coordinated Voltage and Var Control in Distribution Systems: Is Power Factor = 1 Always a Good Thing? Available: http://www.energycentral.com/gridtandd/gridoperations/articles/1553/Unde...