LINE TO LINE COMPENSATOR (LLC), A NEW

LINE TO LINE COMPENSATOR (LLC), A NEW GENERATION OF FACTS CONTROLLERS Z. Hooshi, M. Tarafdar Hagh and M. Sabahi Faculty of Electrical & Computer Engineering, University of Tabriz, Tabriz, Iran ABSTRACT In this paper a new power circuit topology for FACTS controllers is introduced. This topology is neither series nor parallel FACTS controller. This controller injects an additive compensating voltage between each of two phases of a three phase power line by using a voltage source inverter (VSI). So, it is named as line to line compensator (LLC). Analytical analysis and simulation results by MATLAB/SIMULINK are presented and a test setup is built to verify the operation of proposed FACTC controller. Index Terms— FACTS, LLC, active power control 1. INTRODUCTION Electric power systems are undergoing some problems due to a increase in power demand. This increment could be satisfied by installing new generation and transmission facilities. This solution is not suitable from economic and environmental points of view. So, electric power utilities would prefer solutions toward full utilization of the capabilities of existing installations. Flexible alternating current transmission system (FACTS) is the most promising technology that provides a viable solution to these requirements [1]. Modern FACTS devices are based on power electronic converters capable of generating or absorbing a controllable amount of reactive power in order to modify and regulate network quantities such as the voltage at the point of common coupling (PCC), current or power flow over a transmission line. Using a power electronic converter with a power source or an energy storage device allows also exchange of active power that results in more advantages [2, 3]. FACTS controllers can be classified in three categories as follows: i) Series FACTS, like SSSC, TSSC, TCSC ii) Shunt FACTS, like STATCOM, SVC iii) Hybrid (or combined) FACTS, like UPFC, IPFC. The new FACTS controller which is introduced in this paper announces a new category, Line to Line Compensator

(LLC). In LLC, three controlled voltages are inserted between each of two phases of a three phase transmission line. This structure can be extended to distribution level, too. These controlled voltages generates by voltage source inverter(s) (VSI) then, a transformer with at least one delta winding in primary and/or secondary side generates compensated line to line voltages. In this paper, a new LLC is introduced, an equivalent series model is presented for it, then, related main calculations are given based upon this model. Some extended features and advantages are introduced through computer software simulation in MATLAB/SIMULINK. Also, some experimental results are presented to verify the analytical and simulation results. 2. POWER CIRCUIT TOPOLOGY AND PRINCIPLE OF OPERATION Figure 1 shows the proposed power circuit topology for LLC. In proposed FACTS controller, the compensated voltages adds to, phase to phase voltage by using VSI. This topology need a delta connection three phase transformer i.e. delta-delta, delta-wye or wye-delta. If there is an installed transformer with at least one delta winding in primary and/or secondary side in power system, LLC can be installed on it. Otherwise, LLC is allocated independent from other transformers location at sending or receiving end or even at the middle of line. In figure 1, three single phase inverters are used in LLC configuration. If the LLC uses active power supplies, like battery, fuel cell and etc, it can exchange active power with the three-phase network.

Fig. 1. Proposed power circuit topology for LLC

     



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Figure 2 shows the effect of line to line voltage insertion in a symmetrical three phase system. When LLC is used, ∧

the system's voltages change from V to V (for example ∧

from Va to Va ).

transformers between two phases. Since the system is considered balance, so calculation of one inserted voltage is enough and other two values can be found through shifting the calculated voltage's phase by ±120 degrees. Figure 3 shows a two machine electrical network with an inductance as a transmission line. This topology is a conventional one to show the ability of controllers in power flow controlling [1]. X V∠δ

V∠0

Fig. 3. Two machine network

The transferred active power through calculated from (2):

X

can be

Fig. 2. Effect of LLC on voltage phasor diagram

In figure 2, Va is the phase to ground voltage of phasea, and Vab is phase to phase voltage, both before using LLC. When the controlled voltage V pq is added to Vab ∧

through LLC, the new line to line voltage becomes Vab . ∧

The Vbc and Vca change in a same manner to Vbc ` and ∧

Vca , respectively. But there, the inserted voltages have -120 and +120 degrees phase shift toward V pq respectively. Changing Vab , Vbc and Vca to new values lead to ∧

change phase voltages. So, Va is the new phase-a to ground voltage which can be calculated from (1). 3 Vˆa = Va + V pq ∠(180 − γ ) 3

(1)

Equation (1) shows that the LLC can operate instead of a Static Series Synchronous Compensator (SSSC) with all of its functions and capabilities. It must be kept in mind that LLC has unique characteristics in comparison with SSSC; some of them can be mentioned as follows: i) Capability of working with two converters. ii) If an open circuit occurs in one of the three injecting transformers or inverters, the compensator can operate, correctly. iii) Possibility of modification if a short circuit occurs in one of the injecting transformers or inverters. iv) No injecting third harmonic current with PWM converter because of delta winding. One of the features of the FACTS controllers is active power control in the transmission lines [4, 5]. Like other FACTS, LLC can control the transmitted active power of a line. In this way, the required voltage must be computed by controller and inserted through inverters and coupling

P=

V2 sin δ X

(2)

Where: P is active power of line. V is sending and receiving end voltages. X is reactance of transmission line. δ is phase difference between sending and receiving end voltages. By adding LLC to this network, the value of transferred ∧

active power changes to new value P as eq. (3). ∧

P=

V2 δ ⎡ δ ⎤ cos( ) ⎢2 sin( ) + K ⎥ X 2 ⎣ 2 ⎦

(3)

Where: K=

V pq

3

V

(4)

So, by controlling the parameter K, the transferred active power could be controlled and set to desired value. Equation (5) shows how percent change in active power relates to magnitude of inserted voltage and transmission angle between sending and receiving ends. For example, to achieve 10% change in transferred active power, inserted voltage must be 1.7% per unit when the sending-receiving end phase angle difference is 10°. According to mentioned equations we have: Pˆ − P K = δ P 2 sin 2

(5)

The following block diagram in figure 4, shows the simplified control block diagram of proposed LLC. The phase of inserted voltage is calculated to guarantee fixed voltage on dc side capacitors [6].

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I

Pref

V

Phase locked loop

Phase computer

∠Vpq

Real Power computer _ +

Gate pattern logic

Magnitude computer

To inverters

Fig. 5. Power circuit topology of simulation circuit

V pq Fig. 4. Control block diagram

3. SIMULATION RESULTS A basic two-machine three-phase system is considered to verify the mentioned capabilities of proposed LLC. Table I shows the parameters of the system and LLC and figure 5 shows the schematic of simulated network [7]. Several functions can be expected to be satisfied through the proposed system. Here, achieving to the desired transferred active power is interested. To show the capability of LLC in comparison with SSSC, both systems are tested in a single phase short circuit fault condition that occurs in phase-c at 0.3sec and continues until 0.8sec. The transferred active power, compensator currents and line currents are shown in figure 6 (a,b), (c,d) and (e,f), respectively . First row shows the results

for SSSC and second row shows same results for LLC. In figure 6(a) the transferred active power reduced during fault period but by using LLC it can be kept constant (figure 6(b). During this fault condition the amplitude of fault currents pass from compensators are compared in figures 6(c,d). Those show LLC encounter lower fault current in comparison with series ones. In all cases, the active power exchange of LLC with utility is zero and since the inserted voltage’s phasor is perpendicular to its current, inverter’s dc capacitor’s voltage is constant. When an inverter is used to produce and insert a voltage to an electrical network, some harmonic voltages are produced, typically. In LLC, because of using a delta winding, the 3 rd harmonic cancels out so, the total harmonic distortion of line current decreases to acceptable value about 4%. It must be noticed that this value is achieved without any harmonic rejection method.

Table I Parameters of the simulated system Parameter Sending End Voltage Receiving End Voltage Phase angle difference Line Voltage Source Inductance Line Inductance Line Resistance Frequency

4. EXPERIMENTAL RESULTS

Value 20 kV phase-phase rms 20 kV phase-phase rms 10 ° 230 kV phase-phase rms 0.001 H 0.5 H 15 Ω 50 Hz

In this section a test system with two machines is considered to show the operation of LLC in controlling the transmitted active power in a transmission line. This system is a prototypic version of power circuit of Fig. 3. Table II shows the parameters of experimental network. Figure 7 shows the experimental circuit.

Fig. 6. Comparison between series compensator (first row) and LLC (second row) in fault condition

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Fig. 8. Voltage and current in sending end (a) without LLC (b) with LLC (CH1:current, CH2:voltage)

Fig. 7. Experimental circuit Table II Parameters of Experimental Circuit Parameter Sending End Voltage Receiving End Voltage Phase angle difference Line Inductance Line Resistance Frequency CRO Probe Rsh

Value 25.47 (V) phase-ground rms 25.47(V) phase-ground rms 30 ° 20 mH 4Ω 50 Hz x10 0.3 Ω

Figure 8(a) and 8(b) shows the voltage and current waveforms in sending end, without and with LLC, respectively. The rms value of inserted voltage with LLC is 12 (V). According to obtained waveforms, transmitted active power per phase without LLC is 70.0 (W). By inserting LLC into network, this value changes to 95.3 (W). As mentioned in section II, the transmitted power can be calculated from (5). Also (6) can be written from (5). ⎡ ⎤ ⎢ V ⎥ pq + 1⎥ × P Pˆ = ⎢ ⎢ 2 3V sin δ ⎥ 2 ⎦ ⎣

(6)

For test system, new active power per phase, with LLC can be calculated as follows: ⎡ ⎤ ⎢ ⎥ 12 + 1⎥ × 70 = 106.7 W Pˆ = ⎢ ⎢ 2 3 × 25.47 × sin 30 ⎥ 2 ⎣ ⎦

(7)

This value is close to measured value that was 95.3 (W).

5. CONCLUSION In this paper, the LLC as a new FACTS controller is introduced. In this structure, an additive voltage is inserted between two phases and in this way, it is become possible to control some parameters of transmission system can be controlled through this voltage insertion. Effect of such a process in active power flow control is shown during paper. Line to Line Compensator (LLC) can be used as a FACTS device or embedded in transformers which leads to new generation of transformers they can expose ancillary services in addition to conventional uses in voltage level adjustment. The mathematical formulas and theoretical claims are verified through simulations in MATLAB software and experiments. 6. REFERENCES [1] N. G. Hingorani and L. Gyugyi, "Understanding FACTS: concepts and technology of flexible AC transmission system", Wiley-IEEE Press, 1999. [2] Paserba, J.J., "How FACTS controllers benefit AC transmission systems", Power Engineering Society General Meeting, 2004. IEEE, 10-10 June 2004, Page(s):1257 - 1262 Vol.2. [3] Arzani, A.; Jazaeri, M.; Alinejad-Beromi, Y., "Available transfer capability enhancement using series FACTS devices in a designed multi-machine power system", Universities Power Engineering Conference, UPEC-2008, 1-4 Sept. 2008, Page(s): 1 - 6. [4] Divan, D.; Johal, H., "Distributed FACTS - A New Concept for Realizing Grid Power Flow Control", Power Electronics Specialists Conference, 2005. PESC '05. IEEE 36th, 16-16 June 2005, Page(s): 8 - 14. [5] Divan, D.M.; Brumsickle, W.E.; Schneider, R.S.; Kranz, B.; Gascoigne, R.W.; Bradshaw, D.T.; Ingram, M.R.; Grant, I.S., "A Distributed Static Series Compensator System for Realizing Active Power Flow Control on Existing Power Lines", Power Delivery, IEEE Transactions on, Volume 22, Issue 1, Jan. 2007, Page(s): 642 - 649. [6] Mhaskar, U.P.; Kulkarni, A.M., "Power oscillation damping using FACTS devices: modal controllability, observability in local signals, and location of transfer function zeros", Power Systems, IEEE Transactions on, Volume 21, Issue 1, Feb. 2006, Page(s): 285 - 294. [7] J. J. Grainger and W. D. Stevenson Jr, "Power System Analysis", McGraw Hill, 1994.

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