TRANSFER CAPAbility evaluation in deregulation environment

Sarika Khushalani, S. A. Khaparde Senior Member IEEE, S. A. Soman

Department Of Electrical Engineering,

IIT Bombay, Powai, Mumbai-400076

Abstract:

Available transfer capacity (ATC) calculation is a vital issue in the power system operation under deregulated environment and can be a major hurdle to trade of electricity if not properly implemented. This paper presents the calculation of ATC through two different methods viz. Optimal Power Flow and Power Transfer Distribution Factor (PTDF) calculations. The function of ATC comprises of allowing or disallowing bilateral transmission transaction based on effect of transaction on transmission. In deregulated environment with open market conditions Optimal Power Flow (OPF) has revived interest with different objectives. These calculations help the Independent System Operator (ISO), to manage the congestion problem over transmission lines and ensure security and reliability. The two methods have been applied to sample system and confirm that each evaluates power system security but differs in manner, that is, whether requested transaction can cause congestion or not and the other (OPF) is a corrective technique which suggests new generation schedule to prevent congestion.

  1. INTRODUCTION

A condition where demand for power transmission exceeds system's capability is called transmission congestion. Congestion can be controlled by planning and design i.e. by increasing network’s capacity to meet the need. Observing the behavior of competitive market place and transmission grid, system planners can identify congestion problems through optimized engineering studies. It can also be controlled by Regional or Zonal congestion transmission charges i.e. it costs more to move power across zone boundaries. Those needing local access pay local fee but someone wanting to move power from one zone to the other pays interzonal rates [1]. Congestion is one of the most challenging aspects in a multibuyer competitive wholesale system [2]. This is a simple concept but difficult to implement since it must be done in a dynamic, real time manner to be most effective. Without careful attention towards congestion management and economics of energy market, market inefficiencies can take away the savings deregulation promises to society.

In the recent bid to open up access to electric power transmission networks in order to foster generation competition and customer choice, the ATC information must be made available on a publicly accessible Open Access Same Time Information System (OASIS). ATC is thus defined as a measure of transfer capability or available room in the physical transmission network, for transfers of power for further commercial activity, over and above already committed uses. Accurate identification of this capability provides vital information for both planning and operation of bulk power market [3]. ATC information can help Independent System Operator (ISO) to determine the validity of bidding results in an open access deregulated electricity market when timely ATC information is very important. It can also help the power market participants to place bids strategically when congestion happens.

The present computations [4,5] are usually oversimplified or time consuming. Without a fast computation algorithm the center computer of the ISO would not calculate at a faster speed as the market would appreciate. The DC model proposed in [4] has many assumptions, which leads to erroneous results. Continuation Power Flow suggested in [6] gives accurate results but involves many iterations hence time consuming. However, fast computations [7,8] with AC model termed as Linear Methods can improve the accuracy and realism of transfer capability computations. Here the ATC is calculated by sensitivity factors [4,7,9,10]. [4] gives good insight on the essence of calculating linear sensitivities, however, it is based on DC model. There are not many fast ATC calculation algorithms available today. The ATC determined here is without the contingency consideration, however, linear factors corresponding to its consideration can be included [4].

ATC can also be defined as the Total Transfer Capability (TTC) minus the base case flow. TTC is the largest flow in the selected interface for which there are no thermal overloads, voltage limit violations, voltage collapse and/or any other system security problems such as transient stability. This concept is utilized in another way to deal with the Congestion problem that is through the Optimal Power Flow (OPF) formulation. It is formulated as a minimum cost problem in this report and it is seen how effective it is in removing congestion. The limitations on power system performance that is considered here is the transmission line flow limits, voltage magnitudes and voltage collapse. All these limits can be handled in an AC Load flow power system model. Limits due to transient stability or oscillations are not addressed; these limits have to be crudely approximated by flow limits.

  1. Power Transfer Distribution Factor

For ATC determination the MW flows must be allocated to each line or group of lines in proportion to the MWs being transmitted by each transaction. This is accomplished through the use of the linear power transfer distribution factor (PTDF). From the power flow point of view, a transaction is a specific amount of power that is injected into the system at one bus by a generator and removed at another bus by a load. The coefficient of linear relationship between the amount of a transaction and flow on a line is called the PTDF. PTDF is also called sensitivity because it relates the amount of one change, transaction amount, to another change, line power flow. Line power flows are simply function of the voltages and angles at its terminal buses. So PTDF is function of these voltage and angle sensitivities.

Consider a n-node system with nodes 1… g as PV nodes (generator buses) and g+1… n as the PQ nodes (load buses). Bus 1 is taken as slack bus. A transaction is defined by a 4 tuple (t, i, j, Pt) where t is the transaction number, i and j are the source and sink nodes and Pt is the MWs transacted. The change in flow for an arbitrary line l-m can be evaluated by sensitivity analysis as follows.

(1)

From the converged base case Load Flow solution we have

(2)

where J is load flow Jacobian. For a MW power transaction number t,

(3)

where (k=1,…..n ,k  i, j). Substituting (3) in R.H.S. of (2) and then revised (2) in (1) we have

dt is the PTDF. The effect of multiple transactions (not necessarily disjoint) in the flow of line l-m can be obtained by superposition i.e. for tk transactions we have

3. ATC Calculation using PTDF

ATC is determined by recognizing new flow on line from node l to node m, due to a transaction from node i to node j .The new flow on the line is sum of original flow and the change.

(9)

where is the PTDF for line lm due to transaction ij, is the base case flow on the line, and Pij is the magnitude of the proposed transfer. If the limit on line lm, the maximum power that can be transferred without overloading line lm, is then

(10)

is the maximum allowable transaction from node i to node j constrained by line from node l to node m. ATC is the minimum of the maximum allowable transactions over all lines.

(11)

Using the above eqn, any proposed transaction for a specific hour may be checked by calculating ATC. If it is greater than the amount of the proposed transaction the transaction is allowed. If not the transaction must be rejected or limited to the ATC. The use of ATC was first envisioned as a completely decentralized approach shown in [4]. So when an ISO posts the ATC from node l to node m

1) it means that the entire network is capable of carrying the posted ATC MW for a transaction whose source is at node i and whose destination is node j.

2) it does not mean that the ATC is the capacity of the interface connecting node i and node j.

  1. Optimal Power Flow (OPF)

Use of OPF is becoming more important in deregulated power industry to deploy the resources optimally. In the past time, researchers focused on how to formulate some practical constraints, environment concerns etc. and how to solve OPF problem efficiently. The objective functions can be varied so as to maximize TTC or the maximization of active power flow over tie lines [5,3]. Under deregulation all generators are required to submit supply bids to a system operator reflecting their respective marginal costs. Assuming system is lossless the system operator decides on dispatch with given supply bids by solving OPF for the entire system. However, here the OPF is treated as a static generation cost optimization problem. The problem was solved in MATLAB using constr function of the Optimization toolbox, which uses a Sequential Quadratic Programming technique for optimization.

  1. Results

ATC calculation is accomplished via two methods the linear method in subpart 1 and through OPF in subpart 2.

  1. ATC Calculation by linear method

Here two sample systems are considered viz. a 6 bus and a 39 bus system. The data for 6 bus sample system is given in tables 1 and 2.

Table 1 Line data for 6 bus system

From

Bus

/ To
Bus / Resistance
[p.u.] / Reactance
[p.u.] / Line Charging
[p.u.] / Line Limit
[MVA]
1 / 2 / 0.04 / 0.08 / 0.02 / 100
1 / 5 / 0.04 / 0.08 / 0.02 / 100
2 / 4 / 0.04 / 0.08 / 0.02 / 100
3 / 5 / 0.04 / 0.08 / 0.02 / 100
3 / 6 / 0.04 / 0.08 / 0.02 / 100
4 / 5 / 0.04 / 0.08 / 0.02 / 50
4 / 6 / 0.04 / 0.08 / 0.02 / 100

Table 2 Bus data for 6 bus system

Bus
Number / Load
[MW] / Load
[MVAR] / Generation
[MW]
1 / 70 / 10 / 18
2 / 45 / 10 / 100
3 / 50 / 10 / 86
4 / 30 / 10 / 130
5 / 35 / 10 / 0.0
6 / 100 / 10 / 0.0

Table 3 Maximum allowable transaction over all lines

/ Line Numbers
119.155 / 1-2
216.650 / 1-5
174.182 / 2-4
745.279 / 3-5
1019.906 / 3-6
31.133 / 4-5
230.301 / 4-6

Sample System-I

For the 6 bus test system the flow on line 4-5 is 36MW in the base case. The ATC calculated for a transaction between bus 2 to bus 5 is 31.133 MW (table 3) due to 4-5 line limitation. Note especially that the capacity of the line from bus 4 to bus 5 is 5036=14, which is much smaller than the ATC. Simply put, the ATC measures the capability of the entire network to carry this transaction. One implication of this is that transactions that neither originate nor terminate in a zone can affect the ATC to and from that zone. Hence, a transaction between 2-6 of 45MW was established and effect on 3-4 transaction ATC was noted and was observed that ATC for the transaction 3-4 prior to transaction 2-6 was 144 and afterwards reduced to 109.413. The results were checked with PowerWorld [11] software and PTDF’s were found to be matching. However, PowerWorld calculates ATC only of those lines that are connected to generator bus as found in educational version of the software. In this work, ATC can be minimum of line other than that connected with generator buses.

Sample System-II

Also the program was run for 39 bus New England system, the data of which was taken from [12] and it was found that for a transfer from bus 3 (generation 650)-bus 2 (load 1104) base case flow on line 20-3 is 650 (limit of 20-3 is 900). Minimum ATC corresponds to line 20-3 which is 250. It was checked by injections that after increasing transfer i.e., on bus 3 (650+250) and on bus 2 (1104+250), the line flow on 20-3 comes 900. Thus the result of ATC is verified.

  1. ATC calculation by OPF

The three bus system [13] shown in tables 4, 5, 6 is used to illustrate and examine ATC. Here Area one consists only of the generator and load at bus 1. Area two consists of the generator and load at bus 3 plus load at bus2 and the line from bus2 to bus3 and tie lines are 1-2 and 1-3. Area one has expensive generation, while area two has cheaper generation (table 6). For the indicated loads and no area transfers the cost of operation for the total system is 26233 monetary units (mu)/hr. For this study, the loads at all buses remain fixed at the values shown. Generation in each area is sufficient to supply the area loads. The base case has zero scheduled megawatt transfer between the two areas, however, 73 MW of area two’s generation flows through area one.

Consider the situation where area one wants to purchase power at considerable savings from area two. Similarly, area two wants to sell power to area one. We first examine the available transfer capability of this system for transfer from area two to area one. An OPF solution is used to find the maximum power which can be transferred from area two to area one. The voltage constraint for power quality used in this example is 0.96 Vi  1.04. A global OPF solution which minimizes the total cost of providing the total load will attempt to increase area-two generation and decrease area-one generation. This power transfer will change the voltage at bus 2. When the transfer is such that the voltage reaches its lower limit (0.96), the OPF solution will terminate at that constraint. The calculations are for the case in which a nominal 15 MVAR capacitor bank is installed at bus 2 for reactive power support. The available transfer capability for these constraints is 234 MW. The cost of operation for the total system is 22014 monetary units/hr. Thus this maximum power transfer has resulted in a reduction of total cost by 4218.6 monetary units/hr. Additional transfers are not possible without violation of the voltage constraint at bus 2.

Table 4 Line data for three-bus system

From

Bus

/ To
Bus / Resistance
[p.u.] / Reactance
[p.u.] / Line Charging
[p.u.] / Line Limit
[MVA]
1 / 2 / 0.02 / 0.08 / 0.0 / 500
1 / 3 / 0.03 / 0.12 / 0.0 / 500
2 / 3 / 0.02 / 0.06 / 0.0 / 600

Table 5 Bus data for three-bus system

Bus
Number / Load
[MW] / Load
[MVAR] / Generation
[MW]
1 / 500 / 100 / 504
2 / 300 / 100 / 0.0
3 / 100 / 30 / 411

Table 6 Generator Cost Data

Bus / A(mu/hr) / B(mu/MWhr) / C(mu/MW2hr)
1 / 100 / 25 / 0.025
3 / 150 / 13 / 0.01

6. Conclusions

OPF and ATC programs were implemented as tools for solving Transmission Management problems. A novel fast computational method to determine the simultaneous power available transfer capability in a power system was implemented. It was seen that ATC calculation by fast approach has the drawback that transactions that neither originate nor terminate in a zone can affect the ATC of that zone. ATC values change if base case changes i.e. loads or generations etc. However ATC schedules generation prior to operation and if the values calculated are posted on a website the buyers or sellers will be aware if such a transaction is possible. OPF type formulation tries to minimize cost and is different due to its consideration of voltage limit violations as well. Thus every system should have OPF type of post congestion management technique to accommodate real time variations in power system operation.

References

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[13] J.D.Weber, “Implementation Of A Newton-Based Optimal Power Flow Into a Power System Simulation Environment,” M.S. thesis, Department of Electrical Engineering, University of Illinois at Urbana-Champaign, 1997.

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