Protection relays in power systems
In this technical article, protection relays are classified according to the component that is being protected:
4 essential implementations of protection relays in power systems (photo credit: severon.com.au)
- Transmission lines
1. Generator protection
Different protection systems are used to protect generators depending on the type of fault to which they are subjected. One of the most common faults is the sudden loss of large generators, resulting in a large imbalance between the power system and production.
This power mismatch is caused by the loss of synchronism in a certain generator – the unit is said to go out of step. In this case, a phase shift relay can be used to protect the generator in the event of unusual operating conditions, isolating the unit from the rest of the system.
In addition, microprocessor-based protection relays have a built-in function to measure phase angles and calculate the busbar frequency from the measured voltage signal from the VT .
Thus, phase angles and frequency measurements are also available for use in the relay. Figure 1 shows the connection of step protection relays for generator protection.
2. Protection of the transmission line
Transmission lines can be protected by several types of relays. However, the most common practice to protect transmission lines is to equip them with remote protection relays . Distance relays are designed to react to changes in current, voltage, and phase shift between measured current and voltage.
The operating principle is based on the proportionality between the distance to the fault and the impedance seen by the relay . This is done by comparing the apparent impedance of a relay to its preset threshold value.
The characteristics of the distance relays are commonly plotted on the RX Diagram are shown in figure 2a while figure 2b shows the Mho relay which is inherently directional.
For illustration in conjunction with the figure, suppose a fault occurs, the voltage at the relay will be lower or the current will be greater than the values for the steady-state load conditions. Thus, the distance relays activate when the apparent impedance of the relay decreases to any value within the parametric circle.
For this reason, the impedance of the line after the fault can also be used to find the location of the fault.
Like many technical constructs, a backup is used for redundancy . A minimum of two zones are required for primary protection of the remote relays to handle faults at the far end of the protected line section near the adjacent bus.
Such a criterion constitutes a safety factor making it possible to ensure that any operation against faults beyond the end of the line will not be triggered by measurement errors . Multiple protection zones can be built using separate distance measurement units, which allowed for redundancy since both distance units will work for faults occurring in zone 1.
The main difference between the two redundant units is in the time frame. The unit covering Zone 1 would operate instantly, while the unit designated in Zone 2 would have an additional delay between fault signaling and operation. Additionally, by varying the hold and/or operate amounts, the relay control circles can be shifted
In some applications, an additional parameter (Zone 3) is included, which is greater than the Zone 2 parameter. For a fault generated in zone 1, zone 3 is used after a longer delay than that associated with zone 2. Therefore, the delay acts as a temporal tolerance for the fault zone protection schemes.
Delayed operation will start if the tolerance is exceeded.
Therefore, this setting provides some form of backup protection. Figure 3 describes the protection zones of the distance relays.
Typically, Zone 1 is set within a range of 85% to 95% of the positive sequence of protected line impedance . Zone 2 is defined at approximately 50% in the adjacent row, and 25% in the next two rows for zone 3, as described in. The runtime for Zone 1 is instantaneous, while Zone 2 and Zone 3 are labeled T2 and T3, respectively.
Most current microprocessor relays implement multifunctional protection functions. They are considered as a complete protection package in one unit.
In the event of line protection via distance power systems , microprocessor-based protection relays also provide:
- Over-current protection,
- Directional overcurrent protection (for discrimination in the event of multiple parallel lines),
- Under / overvoltage protection,
- Protection in case of circuit breaker failure (in case the circuit breaker does not trip even after receiving the trip command), etc.
Figure 4 shows the connection of a distance relay for line protection.
3. Power protection
Each transformer unit can be protected by a differential relay. The protection principle of this relay is to compare the current inputs to the high and low voltage sides of the transformer.
Under normal conditions or external faults (also taking into account the transformer rotation ratio), the current entering the protected unit would be approximately equal to that leaving it. In other words, there is no current in the relay under ideal conditions, except in the event of a failure of the protected unit.
In addition, microprocessor-based protection relays integrate other protection functions such as thermal overload (which follows the thermal state of the windings) and over / under frequency relays.
These two protection relays work together because the energy losses of the transformer tend to increase with frequency.Therefore , thermal overload relays are also equipped to avoid insulation damage of windings.
Figure 5 shows the connection of a differential relay for transformer protection .
4. Charging protection
Electrical loads are generally sensitive to voltage variations which can cause serious damage to the load when it is high voltage fluctuations occur. In this case, the loads can be protected using over / under voltage protection relays. Figure 6 shows the connection of an over / under voltage relay for load protection power systems
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