Microcomputer Bus Protection and Its Setting Calculation

With the rapid development of computer technology today, microcomputer relay protection devices have the advantages of advanced principles, high reliability, simple operation, and convenient maintenance and management. The line protection in the relay protection devices of power grids above 220KV in Guangdong Province has basically been microcomputerized, and component protection is also transitioning to full microcomputerization. At present, there are more than 170 sets of 220KV busbar protection in substations and power plants in Guangdong Province, of which there are more than 90 sets of various types of microcomputer busbar protection.

The role of busbar protection
If a busbar fault is not equipped with a dedicated busbar protection, it needs to rely on the protection of adjacent components as a backup, which will extend the fault removal time, and often expand the scope of power outage, and even cause a systematic large-scale power outage. Because busbar protection involves more switches and the consequences of false operation are particularly serious, it is required to have higher safety than other protections. In the "Technical Regulations for Relay Protection and Safety Automatic Devices", it is stipulated that the installation of high-voltage power grid busbar protection should follow the following principles:
For 220~500KV busbars, busbar protection that can quickly and selectively remove faults should be installed. For a half-circuit breaker wiring, two sets of busbar protection should be installed for each busbar group.

Principle of busbar differential protection
The operating principle of busbar differential protection is based on Kirchhoff's current law. Considering the busbar as a node, the sum of the current flowing into the busbar during normal operation and external fault is zero, while the total short-circuit current is obtained during internal short circuit. Assuming that the transformation ratio of the current transformers of each outgoing line on the busbar is the same, and the same polarity ends of the secondary side are connected together, the current in the relay is zero during normal and external short circuit according to Figure 1.
In fact, due to the error of the current transformer, an unbalanced current appears in the relay during external short circuit, and the starting current of the differential protection must avoid the largest unbalanced current to ensure selectivity.

Several problems to be solved by the three microcomputer busbar differential
1 The problem of saturation of the fault current transformer outside the zone
In the case of external short circuit, the fault line current in the outgoing line of the busbar is the sum of the currents of all non-fault lines. As shown in Figure 1, the current of the fault line is very large, its current transformer is saturated, and the secondary current is very small. At this time, the unbalanced current of the differential protection is very large. The differential protection should not lose selectivity in this case.

Since the saturated CT has the following two characteristics: a. No matter how large the primary current is, the CT cannot be saturated at the same time when the system fails. From the time the fault occurs to the time the CT is saturated to 1/4 cycle, the CT can correctly transmit the primary current. b. After the CT enters saturation, the secondary current waveform is distorted and defective, but near the zero point of the primary current, there is still a linear transmission area on the secondary side of the saturated CT.
1) The anti-CT saturation scheme used by the WMZ-41 busbar protection device is called the synchronous identification method, which is to determine whether the "fault start" and "differential current over-limit" occur synchronously.
If they occur synchronously, it is considered that the "differential current over-limit" is caused by a fault in the busbar area. At this time, the differential protection quickly determines that it is a fault in the area before the CT is saturated within 5ms, sends a differential output signal and memorizes it to ensure that each circuit breaker trips reliably.
If the "fault start" occurs first and the "differential current over-limit" occurs later, it is considered that the "differential current over-limit" is caused by a fault outside the busbar area and the CT is saturated. At this time, the bus differential protection will be locked for one cycle, and in the next cycle it will be determined whether the fault outside the zone develops into an internal fault. If so, a differential exit command will be issued; otherwise, it will continue to determine whether there is a fault development in each subsequent cycle until the fault outside the zone disappears.
2) The WMH-800 bus protection device also uses the characteristic that the differential protection action time lags behind the fault occurrence time when the CT is saturated to deal with this problem. That is, first determine the time of the fault occurrence. If the differential protection does not move at this time, it is determined to be an external bus fault, and the differential protection is locked for one cycle. Then the waveform recognition method is used to open the differential protection so that the differential protection can operate when the bus zone turns from an external fault to an internal fault.
3) The CT saturation detection element of the BP-2B bus protection device is called an adaptive full-wave transient monitor. This monitor determines that the fault component differential current ΔId element and the fault component and current ΔIr element have different action timings when the fault occurs in the zone and the CT is saturated outside the zone, and uses the characteristics of differential current waveform distortion when the CT is saturated and the existence of a linear transition zone in each cycle to detect the time when saturation occurs.
4) RCS-915 busbar protection sets two CT saturation detection elements according to the characteristics of CT saturation waveform.
CT saturation detection element 1 adopts adaptive impedance weighted anti-saturation method, that is, the voltage power frequency variation starting element is adaptively opened with weighted algorithm. When a fault occurs in the busbar area, the power frequency variation differential element ΔBLCD and the power frequency variation impedance element ΔZ and the power frequency variation voltage element ΔU basically act at the same time, while when a fault occurs outside the busbar area, since the fault starting CT has not yet entered saturation, the action of the ΔBLCD element and the ΔZ element lags behind the ΔU element. Using the characteristics of the relative timing relationship of the action of the ΔBLCD element, the ΔZ element and the ΔU element, the adaptive impedance weighted criterion for anti-CT saturation is obtained. This criterion makes full use of the characteristics that the time difference flow of CT saturation when the fault occurs outside the area is different from the time difference flow of the fault inside the area, and can quickly cut off the fault inside the area and the fault from outside the area to inside the area when it is anti-CT saturation.
CT saturation detection element 2 is composed of harmonic braking principle. The characteristics of differential current waveform distortion when CT is saturated and the existence of linear transition zone in each cycle are utilized to detect whether CT is saturated according to the waveform characteristics of harmonic components in differential current. In the case of conversion fault after CT saturation outside the fault zone, the bus fault can be quickly cut off.
2 Problems with changes in bus operation mode
When the bus is connected as a double bus, when a short circuit occurs on one bus, only the faulty bus should be selectively cut off to allow the healthy bus to continue to operate. Therefore, the bus protection is required to adapt to any connection mode of the bus. Since the secondary side of the current transformer is not allowed to be open-circuited and cannot be switched accordingly with the primary lead-out line, it is difficult to ensure the selectivity of the double bus differential protection under any bus connection mode.
The solution to this problem is to install differential protection on each bus. As shown in Figure 2,

The large differential element can only distinguish whether the busbar is short-circuited internally or externally, while the small differential element is used to select the faulty busbar.
Traditional analog protection relies on the secondary circuit wiring to complete the current addition. Since the connection mode of the lead-out wires on the busbar often changes, the secondary side of the current transformer connected to the differential protection must change accordingly to ensure the correct operation of the differential protection. In order to avoid switching, a method of fixing the busbar connection mode is adopted. That is, only when the busbar is operated in the prescribed connection mode can a faulty busbar be selectively cut off, otherwise both busbars will be directly cut off without differential protection. When there are many lead-out wires on the double busbar, the busbar often cannot be operated in the prescribed connection mode, so the differential protection rarely works.
The development of microcomputer protection has solved this problem. By inputting the auxiliary contact position of the disconnector into the microcomputer busbar protection, the software realizes the switching of the current transformer extraction current.
3 The problem of current transformer ratio consistency
Analog protection requires the installation of an intermediate converter for adjustment, while microcomputer protection compensates through software inside the device, which greatly simplifies the solution of this problem.

4. Comparison of several commonly used microcomputer bus differential protection principles
1. WMH-800 and WMZ-41
The current differential criterion without ratio braking (1) is used as the start-up exit condition; the current differential criterion with ratio braking (2) is used as the differential exit condition.

If the above two basic criteria are met at the same time, the differential protection will be activated. The braking characteristics of the differential protection are shown in Figure 3.

2. BP-2 type
Its differential element consists of a phase-separated compound ratio differential criterion and a phase-separated sudden change compound ratio differential criterion.
1) The action expression of the compound ratio differential criterion is

Figure 4 shows the action characteristics of the compound ratio differential element. Compared with the traditional ratio braking criterion, the compound ratio differential criterion has a strong braking characteristic when there is a fault outside the busbar area, and no braking when there is a fault inside the busbar area, so it can clearly distinguish between faults outside the area and faults inside the area.

2) The fault component compound ratio differential criterion uses the following digital algorithm to extract the fault component:

where i(k) is the current current sampling value; i(kN) is the sampling value one cycle ago. Within one cycle after the fault occurs, its output can more accurately reflect the fault component including various harmonic components.


Due to the transient characteristics of the current fault component, the fault component compound ratio differential criterion is only put into use in the first cycle after the current sudden start, and is blocked by the compound ratio differential criterion using a low braking coefficient.
3. RCS-915 type
The conventional ratio differential element composed of differential current and the power frequency variation current constitute the power frequency variation ratio differential element, which is combined with the conventional ratio differential element with a low braking coefficient to form a fast differential protection.
The advantages of the power frequency variation ratio differential element are that the device is not affected by the load current and is less affected by the transition resistance; the action is sensitive. Its action criterion is:

Setting calculation of five bus differential protection
The main workload of calculating bus differential protection lies in the calculation of the following values. After summarizing and inducing, the calculation method is as follows:
1 The ratio differential threshold of the ratio differential element is calculated according to the minimum total short-circuit current (phase current) of various types of short circuits in the bus under various operating modes including maintenance mode, with sufficient sensitivity, sensitivity ≥4, and avoid the maximum load current of the bus outgoing line as much as possible.
The ratio differential threshold must be set to avoid the maximum load current of the bus outgoing line to prevent the bus differential protection from malfunctioning when the CT is disconnected.
2 Low voltage blocking element
Differential elements based on current criterion can be matched with voltage blocking elements to improve the overall reliability of protection. Composite voltage blocking includes bus line voltage (phase-to-phase voltage), bus triple zero-sequence voltage, and bus negative-sequence voltage. Its action expression is:

If any of the above three criteria is met, the voltage blocking element of the bus section will operate.
Uset is set according to the bus symmetrical fault with sufficient sensitivity, sensitivity ≥1.5. And it should not operate at the lowest operating voltage of the bus, but can return reliably after the fault is removed. Generally, it is 65% to 70% Ue.
​​U0set is set with sufficient sensitivity according to the bus asymmetric fault, and the sensitivity is ≥4. And it should avoid the zero-sequence component of the maximum unbalanced voltage when the bus is in normal operation. Generally, it is 6 to 10V.
U2set is set with sufficient sensitivity according to the bus asymmetric fault, and the sensitivity is ≥4. And it should avoid the negative-sequence component of the maximum unbalanced voltage when the bus is in normal operation. Generally, it is 4 to 8V.
In 1995, Guangdong 220KV Meilin Station once had a problem because the #2 knife pillar of the Shuimei line knocked down the disconnector and broke the arc, causing the knife switch on the A-phase line to be grounded. The differential relay of the bus differential protection of Meilin Station was activated, and the bus differential protection action signal dropped, but the bus differential protection did not trip at the outlet. The bus differential protection only has negative-sequence voltage and low-voltage lockout, and no zero-sequence voltage lockout. After the accident, according to the waveform analysis and simulated fault calculation, the fault point was far away from the Meilin 220KV busbar, and the negative sequence voltage and low voltage did not reach the set value, so the bus differential protection was not opened in time. In this fault, the bus differential protection of Meilin Station did not act in time to isolate the fault point. About 3 seconds later, the bus side blade caused the II busbar to ground fault, the busbar voltage decreased, and the bus differential protection outlet tripped. This has caused the accident to expand. Therefore, any criterion in the low voltage locking element is necessary.

Seven Conclusions
At present, various types of microcomputer bus protection are used in the Guangdong Power Grid, and the operation is good. Since the probability of bus failure is small (according to incomplete statistics, each busbar fails once every 17 years), it is necessary to accumulate experience and analyze and count its operation status in a longer period of operation and use to better master the performance of various types of microcomputer bus protection.