Flexible AC Transmission (FACT) devices are normally used to to enhance controllability and increase power transfer capability of the grid. However, in power systems grids, presence of FACT devices such as STATCOM (Static Synchronous Compensator) usually affects the operation of some sub-systems such as distance relays. It is very important that the distance relays do not mal-operate under system fault conditions as this will result in the loss of stability or the security of the system. As STATCOM devices have fast response and their functional characteristics and control system introduce dynamic changes during fault conditions in a transmission line it is important that distance relays perform correctly irrespective of such dynamic changes introduced during fault. This paper presents a complete analysis of distance relay operation in the presence of STATCOM for all types of faults. It has been found from the simulation results that the operation of distance relays prone to either under-reach or over-reach when a STATCOM installed at the mid-point within the protection zone of the relay.
Keywords |
| FACT devices, Distance protection, STATCOM, Power system grid faults, PSCAD. |
INTRODUCTION |
| The measured impedance at the relaying point is the basis of the distance protection operation. There are several
factors affecting the measured impedance at the relaying point as presented in[1, 2]. Some of these factors are related to
the power system parameters prior to the fault instance, which can be categorized into two groups. First group is the
structural conditions, represented by the short circuit levels at the transmission line ends, whereas the second group are
the operational conditions, represented by the line load angle and the voltage magnitude ratio at the line ends. In the
recent years FACTS devices are introduced to the power systems to increase the transmitting capacity of the lines and
provide the optimum utilization of the system capability[3, 4]. However, in the presence of FACTS devices such as
STATCOM in particular, the conventional distance characteristic such as Mho and Quadrilateral are greatly subjected
to mal-operation in the form of over-reaching or under-reaching the fault point[5, 6]. Generally three installation
positions are considered for the STATCOM[6]; at the near end bus, at the mid-point of the line and at the remote end
bus. In the case of ends buses, STATCOM is not present in the fault loop and therefore not affecting the distance relay
operation.When STATCOM is installed at the mid-point, if the fault locates between the relaying point and the midpoint,
the STATCOM is not present in the fault loop, otherwise STATCOM would be included in the fault loop. When
STATCOM is not present in the fault loop for zero fault resistance, the measured impedance is equal to the actual
impedance of the line section between the relaying and fault points. On the other hand, when STATCOM is within the
fault loop, even in the case of zero fault resistance, the measured impedance would be deviated from its actual value.
Therefore, the conventional distance relays are exposed to the mal-operation, in the form of over-reaching or underreaching.
In this case, the effect of STATCOM on the protective zones should be considered accurately[7, 8]. Thus, it is
essential to study effects of FACTS devices on the protective systems, especially the distance protection, which is the
main protective device at EHV (Extra High Voltage) level. |
| This paper presents a detailed analysis of the impact of STATCOM employed in a transmission system on the
performance of distance relay using PSCAD package. At first a briefreview of the effects of a STATCOM connected at
the midpoint of a transmission line on the performance of distance protection relays is presented. Followed by a
detailed simulation study for various types of faults applied on a transmission line protected by distance relay. |
ANALYSIS OF A TRANSMISSION LINE WITH A STATCOM |
| Fig. 1 shows a distance relay protecting a long transmission line with a STATCOM installed at the mid-point.The
STATCOM is controlled to compensate the voltage at the mid-point of the transmission line. Fig. 2 illustrates the V-I
characteristics of a STATCOM. As it can be seen, a STATCOM can provide both capacitive and inductive
compensation and is able to control its output current over the rated maximum capacitive or inductive range
independent of the ac system voltage[9]. This feature is the main advantage of the STATCOM over other FACTS
devices such as Static VAR Compensator (SVC) [6]. SVC can supply only diminishing output current with decreasing
system voltage as determined by its maximum equivalent admittance[10, 11]. This also means that the maximum
capacitive or inductive reactance generated by STATCOM decreases linearly with voltage with constant current. |
 |
 |
| A fault at F1 in front of the STATCOM will not affect the relay performance. The impedance Z calculated by the
traditional distance protection relay using the measured voltage and current, in terms of the reach setting, can be
expressed as in (1). The impedance Z1 is the zone impedance which normally set in terms of the positive sequence
impedance of the transmission line. The rest of the relationship is essential to ensure that the relay operates correctly for
all the earth faults, and compensating for the cross coupling effects with the adjacent lines. |
 |
| where, |
| Z1 is the positive sequence impedance reach setting |
| Ipis the current in the faulted phase |
| Ires = (Ia+Ib+Ic) is the residual current |
| Imutis the residual current in the parallel line |
| Zres = (Z0+Z1)/3 is the residual impedance, which includes the earth impedance. |
| Zmutis the mutual compensating impedance |
| For any fault behind the STATCOM (at point F2) the STATCOM is in the front of the fault. At these conditions,
STATCOM injects a current in quadrature with the line voltage feeding the fault and boosting the voltage at the midpoint,
which is seen as additional impedance by the relay. This impedance can be either inductive or capacitive,
depending on the mode of operation of the STATCOM prior to the fault. In this situation the relationship of (1) does
not apply and therefore the apparent impedance calculated by the distance relay is different from the actual fault
impedance. This scenario leads to possible under-reach or over-reach of the measuring elements of the distance relay.
Hence the relay must be provided with some form of compensation to eliminate the under-reach or the over-reach as
given by, |
 |
SIMULATION STUDY FOR VARIOUS FAULTS |
| A model of a transmission line with a STATCOM installed at the mid-point using PSCAD package is shown in Fig.
3. PSCAD is a graphical user interface, providing a very flexible interface to the electromagnetic transient simulation
software Electomagnetic Transients Program (EMTDC). EMTDC is the library of power system component models
and a procedure which constitutes the simulation software packages are referred to as “PSCAD/EMTDC” and the
combination allows the engineers to set up and run a wide variety of power system simulation[12]. The simulations are
performed for various types of faults ( single-phase to ground, two phase and three-phase faults) applied inside the
protected zone of the distance relay. Based on the developed model, STATCOM is included in the loop of impedance
seen by the distance relay as presented in the following sub-sections. |
 |
| A. Phase to Ground (A-G) Fault |
| In the system shown in Fig. 3, phase-A to ground fault occurs at a distance of 75 kilometers on the right side of
STATCOM. The setting value in terms of the desired voltage for STATCOM is 1.0pu. The apparent impedance trajectories of the system with and without STATCOM together with the distance relay mho characteristic are shown in
Fig. 4. It can be seen that the apparent impedance of the transmission system with STATCOM is greater than that for
the system without STATCOM. The protection zone of the distance relay will thus decrease i.e. it will underreach[13,
14]. Fig. 5 shows the results of simulation of the other units calculation of line-line impedance during A-ground fault
for both conditions (with/ without STATCOM). It is clear that only the impedance of the faulted phases will be
calculated by relay and the trajectories of the apparent impedance of other units do not cross the Mho circle for this
fault condition. |
 |
 |
| When the fault is on the right side of STATCOM, both the apparent resistance and reactance of the system with
STATCOM are larger than that for the system without STATCOM. This can be explained by thethe Ish/Irelay (influence
ratio). Because of the reactive power injection by STATCOM, the voltage at the STATCOM connecting point is higher
compared to the system without STATCOM; in other words, the distance seen by the relay is farther than its real
distance. This increase in the apparent impedance would lead to the under-reaching of distance relay. When the
STATCOM is installed in the middle of the transmission line, and the original distance relay’s reach is set of 80% then,
the reach point Nnewfor the system with STATCOM can be derived from following, |
 (3) |
 (4) |
| where Ish is the shunt current injected by the STATCOM and Irelayis the relaying current.
The setting of STATCOM has a big impact on the apparent impedance[15]. According to different system
conditions, STATCOM may have different setting values for desired voltage, and this setting will also affect the
performance of the distance relay. The next study is conducted for the apparent impedance and during a single phase to
ground fault when the STATCOM settings are 1.2, 1.0 and 0.8 pu respectively.As seen fromFig. 6, 7 and 8, both the
apparent resistance and reactance seen by the distance relay for a single phase to ground fault will increase with the
increase of STATCOM setting reference voltage. This can be explained by the different reactive power injection. When
the setting voltage is high during the fault, to keep the higher desired voltage, STATCOM will inject more reactive
power; in other words, the reactive current injection of STATCOM Ishis high; this will increase the influence ratio and
accordingly increases the apparent impedance seen by the distance relay. |
 |
 |
 |
| It is worth mentioning that for certain conditions, when the system capacity is high and the STATCOM voltage
setting value is low, if a single phase-ground fault occurs outside zone 1, the STATCOM connecting point voltage may
be higher than the setting value, in this case the STATCOM will absorb reactive power in the system, the current Ishwill
become inductive, the influence ratio Ish/Irelaywill become negative rather than positive and the apparent impedance
seen by the distance relay will decrease compared to the system without STATCOM. This may lead to over-reaching of
distance relay, and this is clearly undesirable. |
| B. Phase to Phase (B-C) Fault |
| For a phase to phase fault, the relay voltage input is line-to-line voltage and the current is delta line current. Fig. 9
show the apparent impedance seen by distance relay during a B-C phases fault. The relay voltage is VBCand relay
current is IBC. The fault is located in75km from relay point and the STATCOM setting value is 1.0pu. As can be seen
from Fig. 9, during a phase to phase fault, because of the STATCOM, the apparent reactance increases, but unlike the
single phase to ground fault, the apparent resistance decreases and hence the distance relay can not operate
properly[14]. This is clearly shown in the figure that the apparent impedance trajectory has been moved upward only. |
 |
| C. Three-Phase (ABC) Fault |
| The same system has been simulated for the three-phase faults and the apparent impedance seen by the distance
relay located at the sending-end for the cases with and without STATCOM which is the fault applied inside the
protection zone of the relay. The effect of the STATCOM under fault condition for the cases with the STATCOM
disconnected as well as STATCOM connected are illustrated in Fig.10a and b. It can be seen that in the presence of
STATCOM the apparent impedance trajectory has been shifted outward the Mho circle. |
 |
CONCLUSIONS |
| The impact of STACOM on the distance relay operation for various types of faults has beed presented in this paper.
The analysis is carried out for a STATCOM installed at the mid-point based on simulation study using PSCAD
package. It was found from the simulation results that STATCOM does affect the performance of the distance relay
under fault conditions. It has shown that when STATCOM located at the mid-point of the transmission line, the
conventional distance relays are exposed to the mal-operation, in the form of over-reaching or under-reaching. In this
case, the effect of STATCOM on the protective zones should be considered.It has been found that the apparent
impedance is influenced by the level of reactive power injected by the STATCOM resulting in either under reaching or
over reaching of the distance relay. Since deviation of the measured impedance is not constant, because of the varying
parameters of STATCOM, adaptive methods should be utilized. The distance relay wills under-reach when the
STATCOM supplies the reactive power, and will over-reach when the STATCOM consumes the reactive power. The
setting of STATCOM has a big impact on the apparent impedance. The higher the voltage setting is, the larger the
apparent impedance will be. |
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