EMTP Applications for Power System Protection

Author: Demetrios Tziouvaras, Schweitzer Engineering Laboratories, Inc., USA

Steady-State Applications

Power system operations can cause unbalanced currents and voltages in the network that could impact the operation of protective relays, e.g., during a single-phase trip, unequal gap flashover of series capacitors, or when a three-phase transformer bank consists of different single-phase units. These operating conditions cannot be analyzed using conventional load-flow programs but can be easily studied using steady-state EMTP simulations and demonstrate the benefits of applying EMTP for power system protection applications.

Conventional short-circuit and load-flow programs assume a balanced power system. The assumption of a balanced system is usually adequate under normal operating conditions. In cases where network unbalances exist under normal conditions, conventional programs may not be able to determine the magnitude of normal unbalances. EMTP steady-state solutions are performed in the phase domain, not using sequence components, thereby easily solving networks with nonsymmetrical phase impedances. The EMTP steady-state solution may be used to help quantify the extent of normal unbalances to assist in relay applications.

Open conductors create series unbalances that are not normal conditions, and relays may be expected to protect against them. However, the resulting system unbalances may be significantly less than unbalances resulting from short circuits. In fact, the unbalance currents and voltages are heavily dependent on the magnitude of load currents. EMTP steady-state solutions can readily determine voltages and current measured by relays under such conditions. Cases of interest are: single-phase tripping; 1 phase of a disconnect switch open.

Series capacitor gap flashing during a fault is a special case of a multiple unbalance. Unbalanced gap flashover may occur as a result of a short circuit, or unbalanced bypass may occur as a result of control or bypass equipment problems. EMTP can calculate steady-state unbalance currents and voltages resulting from such unbalanced operation. Knowledge of these quantities can assist in proper protective relay application and settings. Simultaneous or cross-country faults are difficult to analyze with conventional short-circuit programs that use sequence components for analysis but are easily handled by EMTP simulations.

Under some conditions, unbalances are deliberately introduced into a network to address special problems. For example, failure of a large single-phase transformer may necessitate its replacement with another single-phase transformer having different MVA capability and different leakage reactances. EMTP simulations can be used in such a situation to determine the steady-state unbalances resulting from load flowing through the unbalanced transformer impedances. The simulations would reveal the level of system unbalance and the circulating current in the transformer bank tertiary winding. Relay engineers can use the results of the EMTP steady-state simulations to determine the effect on transmission line protection and the required settings for transformer tertiary overload protection.

In addition to unbalanced analysis, steady-state EMTP solutions help to study the effect of non-fundamental frequencies on relays. For instance, the system response at a variety of frequencies generated by a multifrequency injection may be used to test for the presence of system resonances. Such information will help determine the importance of harmonic rejection in relays in particular applications. Power line carrier frequencies propagate in several modes on a multiphase transmission system. Study of these propagation modes is complicated by discontinuities in the transmission circuit, such as transpositions and overhead-tounderground interfaces. At high frequencies, such as those of power line carrier, EMTP steady-state solutions can be used to study the effects of line transpositions and discontinuities on wave propagation.

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