Santo Antonio HPS consists of 44 power generators of 82.5 MVA, generating in 13.8 kV and transmitting in 500 kV, by means of three winding elevator transformers, in which two of them are primary windings of 165 MVA (in 13.8 kVA) e one of them is a secondary winding of 330 MVA (in 525 kV). Two power generators of 13.8 kV [1] power each primary winding. The HPS has three powerhouses, each one provided with power lines of 500 kV for the energy transmission to the collector substation built in the city of Porto Velho, with the following disposition:

・ Right Margin―8 generators of 82.5 MVA and two transformers of 330 MVA;

・ Left Margin―24 generators of 82.5 MVA and six transformers of 330 MVA;

・ Riverbed―12 generators of 82.5 MVA and three transformers of 330 MVA.

A collector substation of 500 kV is located at the right margin of the Madeira River, where the transmission lines of Santo Antonio and Jirau HPS are interconnected to the basic SIN network. The interconnection of Santo Antonio HPS is done by three transmission lines of 500 kV; one with duple circuit and two with a simple circuit, in addition to a TL passage with simple circuit between the powerhouse on the right margin and the powerhouse on the riverbed, named:

・ L1―LT 500 kV (right margin)―collector SB (12.5 km);

・ L2―LT 500 kV (riverbed)―collector SB (13.3 km), design objective;

・ L3 and L4―LT 2 × 500 kV (left margin)―collector SB (duple circuit) (14.1 km);

・ L5―LT 500 kV (left margin)―riverbed (0.8 km).

The transmission lines have voltage of 7500 kV in both double circuit and simple circuits, with three aluminum conductors per phase of type CAA 954 MCM (45/7 RAIL) and two lightning rod cables per structure. Figure 1 shows the transmission line from the HPS engine room. The grounding system of each TL has two CAA DOTTEREL lightning rod cables (176.9 MCM, 12/7 formation, 15.42 mm diameter). The foot tower has resistances of 20 Ω. Figure 2 presents

the complete unifilar diagram of Alternating Current (AC) and High Voltage Direct Current (HVDC).

In case of injection of electric current into the earth, either by the occurrence of a fault in the installation or by atmospheric discharges, the currents dispersed through the grounding system causes the raise of electric potential differences between points in the ground surface (surface voltages), which produces touch and step voltages, that poses life risk [2] . Furthermore, there is risk of damage caused by the potential transferred for circuits that are somehow connected to the grounding system and to points distant to the ground surface, or even to other remote grounding systems. The used method consists of injecting a square current at high frequency (25 kHz) and high current (maximum of 20 A), involving the collector substation and the (GG2) engine room with lightning rod cables of the transmission line. For this frequency, the inductive impedance of the ground wire is not significant, which reduces the effect of other adjacent towers to the one being measured [5] [6] .

A device to measure the grounding resistance by selecting the TL-2 as current circuit [2] is used to extract the measurements, including the grounding resistance of the tower supporting foot. The extent of grounding systems, such as grids, balances, and metallic pipes, are measured considering only the nearest passage from the connection point, in a way that the measured value represents the behavior in face of an impulse signal, similar to an atmospheric discharge [4] . In this way, the values that better represent the system capability are obtained in order to conduct the atmospheric discharge to the ground, in a more efficient manner when compared to the values derived from conventional devices with low frequency, even when the ground wire is not connected. The test is performed by circulating a current through the ground diffusion resistance, and through an auxiliary electrode, called current electrode, and measuring the voltage between the resistance grounding and another auxiliary electrode, stuck on the ground in a flat area of the created potential by the flowing current (potential level).

The instrument has a capacitor bank that, by means of the measurement of the resonance frequency, also allows the evaluation of the inductive reactance of the grounding system. The tuning process is automatic and the equipment displays both the value of the equivalent inductance and the capacitance that produces the tuning. The current injected by the earth meter is automatically adjusted to the predetermined value, and the equipment directly indicates the resistance value in its alphanumeric display. Figure 3 shows the practical grounding testing scheme of TL-2. The current electrode, Figure 3(a) (connection point 2), is the phase conductor of the third tower of a TL passage. The test is performed by circulating a current through the ground diffusion resistance, and through an auxiliary electrode stuck on the ground, Figure 3(b) (point 1), from a connection point in the third tower foot, Figure 3(b) (point 2), thus measuring the produced voltage between the grounding resistance and the auxiliary electrode.

In order to avoid the superposition between the regions of influence [7] [8] , the de-energized phases of the TL were used as current electrode, which belongs to the GG2 line passage under testing, thus connecting to the structure and simulating a short-circuit as illustrated in Figure 4.

An experimental practical mounting was developed to evaluate the effectiveness of the grounding system, in which the resistance values were obtained by the design of high current injection and used in a simulation of the protection logic, implemented in the PSCAD/EMTDC software. This simulation derived features of the protection function of the quadrilateral distance of phase and ground for posterior insertion at the simulator Doble F6150 of the fault files COMTRADE in order to perform tests in a commercial relay SEL-421.

The function of a resistance element is to limit the resistive layer for a protection quadrilateral area. Some resistance elements measure the resistance of the added R_F line. The fault resistance values were obtained according to Equation (1), as given by [3] :

R F = I m [ V ¯ ∅ ( 1 ∠ Z 1 A N G ( I ¯ ∅ + k ¯ o ⋅ I ¯ r ) ) ∗ ] I m [ 3 2 ( I ¯ 2 + I ¯ 0 ) ( 1 ∠ Z 1 A N G ( I ¯ ∅ + k ¯ o ⋅ I ¯ r ) ) ∗ ] (1)

where:

・ V ¯ ∅ in the voltage between phase and ground ( ∅ represents phase voltage A, B or C);

・ Z 1 A N G is the relay adjustment, which is equal to the line angle for the positive sequence impedance Z ¯ 1 L ;

・ I ¯ ∅ is the phase current;

・ k ¯ 0 is the compensation factor of zero sequence.

The replicas of line impedances are set according to the reach of faults and adjusted to stop on 10˚ of positive phase sequence. The conditions in the array of protective relay signals are presented below, being:

・ S 1 a , S 1 b , S ′ 1 c the signals of the protection relay for Phase-Ground;

・ Z r a is the replica of the impedance to the stage;

・ U P h − E is short-circuit voltage (effective value);

・ I L is the phase current of short-circuit (effective value);

・ ∅ r is the position of the fault current;

・ I E is the neutral current of short-circuit (effective value);

・ φ U is the phase angle short-circuit voltage;

・ φ L is the phase angle of the short-circuit current;

・ φ E is the phase angle of the neutral current of short circuit;

・ R E / R L e X E / X L are the parameters set in the relay for residual compensation.

The effect of fault on load (phase A) is calculated according to Equation (2):

S ′ 1 a = I R E S ⋅ r ( I a + k ⋅ I R E S ) = Z ′ r a = | Z ′ r a | < ∅ r a S ′ 1 a = | Z ′ r a | ∠ ∅ r + ∅ r a (2)

The effect of fault on load to the (stage B and C) is calculated according to Equation (3):

S ′ 1 b = | Z ′ r b | ∠ ∅ r + ∅ r b S ′ 1 c = | Z ′ r c | ∠ ∅ r + ∅ r c (3)

For performance of distance protection simultaneous measurements of impedanceare performed with 6 loops (L₁-E, L₂-E, L₃-E, L₁-L₂, L₂-L₃, L₃-L₁). Ground loops are evaluated when there is a recognition of fault to earth. Phase-phase loops are evaluated when the current and both phases exceeds this same set value, according to Equation (4).

I L 1 ⋅ Z _ L − I L 2 ⋅ Z _ L = U _ L 1 − E − U _ L 2 − E (4)

where U _ e I _ are complex values measured and Z _ = R _ + j X _ is the impedance of the line.

The impedance of the line is calculated according to Equation (5):

Z _ L = U L 1 − E − U L 2 − E I L 1 − I L 2 (5)

Generalizing the impedance calculation between phases (A and B) we obtain the Equation (6):

Z L 1 − L 2 = U _ L 1 − U _ L 2 I _ L 1 − I _ L 2 (6)

According to [3] the faulty stage is not always in phase with the total current of fault, I_F, thus, is not very suitable as reference signal or polarization. The negative sequence current is a more efficient option. Equation (7) displays the measured residual current in each of the busbar presented in Table 1. For systems where ∠ Z 0 S = ∠ Z 0 L = ∠ Z 0 R (homogeneous systems), the phase angle of I_R is equal to that of I_F, regardless of the charging condition and fault resistance.

I R = ( 1 − m ) ⋅ Z 0 L + Z _ 0 R Z 0 S + Z 0 L + Z _ 0 R (7)

According to [3] , the major advantage of this fault resistance element is that its measurement is not significantly affected by the loading conditions. This allows the resistive limit to be set to a value greater than the apparent minimum charge impedance.

Relay SEL-421 uses MHO properties in the distant protection of phase and ground. Zones 1 (Z1P) and 2 (Z2P) are fixed in the “forward” direction, remaining zone 4 (Z4P) is adjusted forward, and zone 3 (Z3P) is adjusted as reverse, which results into three forward zones and one reverse zone, with high-speed distance elements. Those high-speed elements use voltage and current phasors derived from a medium cycle fast filter to provide opening times of sub-cycle order. The adjustments are automatically associated to the extent of the elements zone. Figure 5 shows the adjustments performed by the graphic editor of the AcSELerator QuickSet® software.

The range of MHO distance element of phase Z1P was set as 80% of the impedance of the positive sequence of the secure line. The distance element of phase Z2P is part of the implemented protection assisted by communication― POTT2 (Permissive Overreaching Transfer Trip), with directionality in the forward direction. The adopted range was of 200% of the impedance of positive sequence of the secure line. For the distance element of phase Z3P, with POTT2 scheme, the directionality was set in the reverse direction, allowing it to have a higher range when compared to Z2P. Thus, a range of 200% of the impedance of positive sequence of the secure line was adopted, and phase Z4P was adjusted as

conventional zone 2 protection (Z2P), that is, 120% of the impedance of positive sequence of the secure line. Figure 6 presents the adjustments for the logic diagram of distance protection.

The first step of the practical scheme is to simulate diverse fault conditions considering the resistance values of grounding (R_T) measured in field and fault resistances (R_F) collected calculated with PSCAD/EMTDC software. Those measurements are presented in Table 2.

The extraction of fault files is done in the second step in COMTRADE format (Common Format for Transient Data Exchange for power systems). In the third step, these files are provided to the simulator (Doble F6150) for the files in COMTRADE format for characterization of certain situations tests on the protection logic. The fourth step is characterized by the evaluation of the relay operation behavior regarding the simulated situations. Figure 7 shows the testing layout set in the laboratory.

An automation logic was implemented for the automatic voltage setting at all phases if this voltage exceeds 500 kV. If it is the case of a permanent condition of high voltage on the 500 kV level, an opening command is sent to the circuit breaker. Figure 8 shows the logic diagram of the automation setting on the phase voltage. A complete scheme of the TL was modeled by using the software PSCAD/EMTDC to simulate diverse fault conditions, as demonstrated in Figure 9.

The circuit breakers B1 e B2 are monitored by function 50BF (Break Failure) and acted independently, as presented in Figure 10, for the logic diagram implemented in relay SEL-421, and as presented in Figure 11, for the modeled logic diagram in the software PSCAD/EMTDC [9] [10] .

Functions BFPU1 and BFPU2 in the SEL-421 relay are implemented and tested in a time based manner, in which it must be continually present to result in the trip for multiple faults of the fault protection logic of circuit breakers 1 and 2, with maximum operating time of ten cycles for B1 and B2. The detection logic of the high-speed open pole was set for a pickup current below the minimum charge current, providing sensitivity without harming the speed of the dropout time. This condition is met, even in scenarios where there is a delay that drops the current in the CT secondary to zero, due to the magnetic flow, the detection the of the circuit breaker opening is performed at high speed [11] [12] and [13] .