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Relay Protection and Substation Automation of Modern Power Systems (Cheboksary, September 9-13, 2007)

S7-5: Relay protection reliability

Load encroachment resistance settings calculation features for digital distance protection K. BRINKIS, D. DROZDS AS Latvenergo Latvia [email protected]

KEYWORDS Distance protection setting calculation, load area, healthy phase impedance, single-phase fault conditions, parallel transmission line zero sequence current, overreaching zone, load resistance vector.

1. INTRODUCTION Digital distance protection principal schemes and algorithms allow to realize the coordination function for the load resistance vectors with the help of load encroachment in the distance protection characteristic’s resistance area. However existed methodical directions for the load encroachment setting calculation do not take into account all the possible (worst) conditions for the 330 kV transmission network. Taking up the distance protection with the different functional and algorithmic features concerning the load area setting calculation we can use the well-known methods [1,2].

Fig. 1. Polygonal tripping zones for the distance protection of ABB (Fig.1a) and Siemens (Fig.1b)

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The load resistance vectors area can be represented as the distance protection tripping zone encroachment. This load area settings calculation until now had been made with the determination of the maximal angle of the load vectors (ϕmax) and the minimal load resistance value (RLoad). Z Load =

U min , 3 ⋅ I max

R Load = Z Load ⋅ cos ϕ Load .

where Umin – minimal voltage value; Imax – maximal load current value; ϕLoad – maximal load vectors angle value. However, the load vectors adaptation only for the symmetrical working conditions of the transmission line (TL) is not enough from autorecloser function usage point of view for the single phase faults, which can liquidate this kind of nonsymmetry with the minimal deviations in the transmission network without all three phases disconnection (70 % of the single phase faults have no steady state conditions). 2. LOAD ENCROACHMENT RESISTANCE SETTINGS CALCULATION DURING THE SINGLE PHASE AUTORECLOSER CYCLE Calculating the boundary settings values for the load area it is necessary to take into account the conditions from p.1 of the load resistance vectors, but it is more important to have a good look at the healthy phases vectors for the single-phase open conditions from only one TL end, and of course for the single-phase autoreclosing from both TL ends. 2.1. Single-phase open conditions only from one end of the TL As the result of the single-phase short-circuit with the single-phase cascade circuit breaker open only from one TL end, if there are used the autorecloser function and the power flow have the high value we can examine that the healthy phase vector value to fall into the distance protection overreaching zone, which can be calculated reasoning from the sensitivity conditions for the worst network mode of operation, e.g., for the Fig. 2a conditions before the short-circuit total disconnection from both TL ends if the single-phase fault with the fault resistance about 14 Ω is on the point 1 near the substation B busbar, and the impedance measuring take place on the substation B.

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Fig. 2. Single-phase fault for phase «А» with the cascade circuit breaker open only from one TL end with the maximal power flow value (fig. 2a) and the resistance vectors arrangement in the complex plane for the healthy phases ZB and ZC measured from substation B side (fig. 2b)

It is well known, that the single-phase autoreclosing function application can help to keep the normal network working conditions for about 70 % of all single-phase faults (statistical data for the Baltic 330 kV network). Exactly why it is not expedient to allow the three-phase open after the single-phase cascade circuit breaker autoreclosing because of the healthy phases superfluous reclosing from the overreaching distance protection zones. From the Fig. 2b can see the healthy phase «B» impedance disposition measured from sunstation B busbar when the short-circuit place move from substation A to substation B side at the single-phase open conditions only from substation B busbar side.

Fig. 3. Estimated diagram for the positive sequence impedances (where x, y, m – calculated impedance factors relatively for the single unit length ZL)

We must take into account that the healthy phases impedances depends on: active power flow value and direction, fault resistance value on the place of short-circuit, equivalent impedances of the TL ends and the TL own impedance (for the research it is taken the 330 kV TL single unit length ZL about 50 km, relatively for the equivalent impedances of the TL ends). 2.2. Single-phase open conditions for all TL ends (autoreclosing) Short-circuit modeling along all TL length have not the critical condition, when TL length is long (> 3ZL, i.e. more then 150 km). In that case the calculation model is:

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Fig. 4. Calculation model of operation for the single-phase open conditions from all 330 kV TL ends (single-phase autoreclosing)

In the Fig. 4 there is the variant of the single-phase open conditions – faulty phase open from both TL ends – non-symmetry cross mode replaces by longitudinal. It is rational to measure the healthy phases impedances when the active power flow direction is coincide with the place of measuring (i.e., if the active power flow from substation B busbar to substation A direction it is rational to measure the impedances from substation B busbar). As well it is necessary to mention about the teleaccelerated (overreaching) distance protection zones checking up for the maximal allowable active power flow values at the same single-phase open conditions. 3. LOAD VECTORS RESISTANCE LIMIT VALUES CALCULATION FEATURES FOR THE PARALLEL TRANSMISSION LINES

Fig. 5. Parallel TL schematic sketch in the equivalent values.

It is significant to point that the calculating conditions of the load encroachment limits for the teleaccelerated distance protection zone can be the single-phase open conditions only from one TL end just on one TL (Fig. 6), as in this case on the healthy phases impedances affects extra zero sequence current from the load at the single-phase open conditions. As the result on these parallel TLs flows the zero sequence ring current.

Fig. 6. Single-phase short-circuit with the cascade circuit breaker open from one parallel TL end with the maximal active power flow on its.

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For the case of the parallel TLs, when the single-phase fault is only on the Line 1 near the substation B, and the faulty phase open from only Line 1 end, and the active power flow from substation B to substation A direction it is necessary to measure the healthy phase impedances from substation A busbar just for control the overreaching zone setting to prevent the false trip of the distance protection relays on the substation A of Line 2. Fig. 7. Healthy phases impedances in the complex plane for overreaching distance protection zone.

In the Fig. 7 there are the healthy phases impedances vectors for 330 kV TL Line 1 and Line 2, when the modeled short circuit place is moving from substation B to substation A direction. These checking of the healthy phases impedances vectors of Line 1 is to carry up the distance protection overreaching tripping zones from substation A side not to trip in case of short circuit. Fig. 8. Zero sequence current vectors for both two parallel transmission lines (3I0)

Zero sequence current vectors in the Fig. 8 represents for both TLs - Line 1 with the short circuit in phase A and Line 2, when the short circuit place is moving along the Line 1 phase «A», and the active power flow for both lines is about 1000 MW. Also we can see that the zero sequence current value and the place in the complex plane on Line 1 and Line 2 is changing because of the zero sequence current of the open phase conditions on Line 1. This 3I0 current for Line 2 has the opposite sign from Line 1. The overreaching distance protection tripping zone for Line 2 from substation B doesn’t trip because the short circuit place is out of forward direction of the tripping zone for substation A side. But for the Line 1 this overreaching zone for substation B side (for healthy phases «B» and «C») can trip before the faulty phase A will trip from substation A side. This untripping can be realized with the help 5


of load area resistance encroachment, not to minimize the reactance setting of these overreaching zones. It is necessary to take into account the open phase conditions for one of the parallel line to calculate the settings for the load area encroachment, because these limitations in some cases can be conclusive, when we have to prevent the false trip of the distance protection overreaching zones. 4. CONCLUSIONS 1. Calculating the load area resistance encroachment values for the distance protection overreaching zones it is necessary to take into account the open phase conditions from only one side of the TL (single-phase autoreclosing), as well as for the parallel transmission lines cases. 2. It is necessary to take into account the zero sequence ring current, which value depends on the load current, and it can significantly decrease the impedance values of the healthy phases. 5. REFERENCES [1] ABB Line distance protection terminal application manual. [2] Siemens SIPROTEC Distance protection manual.

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