Hello, learners. Welcome to this module on Bus Bars Protection, Load Shedding, and Frequency Relaying. Let's start with the topic on bus protection schemes. At the end of this topic, you should be able to state the requirements of bus bar protection, list the various types of bus configurations present in the power system network, classify various bus protection schemes. Let's begin with bus bars. Bus bars are conductors to which multiple circuits are connected. The material utilized should have a low resistance, higher softening temperature, minimal cost, and good mechanical qualities. Strain insulators are used to support the structure. The flexible bus is elevated above the various substation equipment. Let's see the protection of bus bar. The bus bar protection includes the bus and the apparatus such as circuit breakers, isolating switches, instrument transformers, etc. Bus bars are used to connect incoming and outgoing links or circuits in generating stations and substations. If a fault occurs on a bus bar, unless some type of quick-acting automated protection is provided to isolate the faulty bus bar, there will be substantial damage and supply interruption. There are various types of bus configurations present in the power system network as shown in the screen. Click on each one to learn more. Radial bus configuration. One primary bus is used in the radial bus configuration. Circuit breakers, motor controlled or manually operated disconnect switches are used to connect transmission lines, transformers, and shunt capacitor banks to the main bus. Radial bus substations are most cost-effective, need less land, and are easier to expand and require less complex protective relay than other designs. When a bus or a breaker fails, the substation in a radial bus layout scheme has to be taken out of operation, which reduces reliability and maintenance flexibility. When reliability is not an issue, the radial bus topology is typically employed in distribution voltage substations up to 161 kilovolts. Partitioning or splitting of a radial bus is one way to improve the performance of a bus. Sectionalizing or bus tie circuit breaker link two radial buses in this design. Sectionalized radial bus substations have the following advantages over radial buses. They take up less space, are more reliable, have more operational flexibility, and are easier to expand. Higher expenses, increased operational complexity, and increased protective relaying application complexity are all disadvantages as compared to the radial bus. The sectionalized radial bus layout is commonly used in distribution voltage substations up to 161 kilovolts and in locations where system dependability isn't a priority. A single breaker double bus configuration has been added to the sectionalized radial bus. In this design, a tie circuit breaker connects two main buses. It is used in transmission and distribution voltage levels. Bus bar maintenance without affecting circuit. Fault on a bus disconnects only the circuits being connected to that bus. The primary and transfer bus are two further radial bus variations. This design employs both main and transfer buses. Circuit breakers and transfer switches are used to link all circuits to the main bus and the transfer bus. The circuit is protected when a transfer bus circuit breaker is servicing a circuit's linked circuit breaker. Characteristics of this bus are operational flexibility is improved. During a bus fault, all breakers must be tripped, transfer bus for breaker maintenance. Double bus, single breaker with transfer bus. Each circuit has one circuit breaker which is connected to the main bus through disconnect switches on both sides. To balance the load, isolate key circuits or locate sources on either bus and connect all circuits to one bus in the event of a secondary bus outage. This design permits circuits to be attached to either the main bus or the secondary bus. A circuit's transition from one bus to another is not automatic and must be done manually. This bus has the following characteristics. It has a high-level of operational flexibility. Bus will be rerouted for breaker maintenance. Double bus, double breaker. In this system, each circuit has a double breaker and disconnected to both bus bars: bus bar 1 and bus bar 2. Despite its high cost, such a plan provides the best circuit breaker maintenance facilities. The load can be easily switched to the other circuit breaker when one of the circuit breakers is opened for maintenance or regular inspections. Characteristics of this bus are operational flexibility is high. The bus segment is protected by line protection between two CTs. The power to the circuit is not disrupted by a bus fault. In reality, the ring bus configuration is made up of a succession of radial buses that have been sectionalized and connected together to form a ring. Each bus is referred to as a position. Only one slot is taken out of operation in the event of a circuit or a bus failure. The circuit breakers supplying power to the defective site are activated. Two positions will be removed from service if a breaker fails to work, owing to a line or bus problem. Without causing a power outage, any circuit breaker can be pulled out of operation for maintenance. A multiple substation version of the ring bus concept is the one-half breaker design. To isolate a damaged line or transformer, two breakers similar to the ring bus must be tripped. One of the half breaker configuration's breakers is normally found on the transmission lines at their end. Substations A, B, C, D, and E form an extended ring bus in this diagram. Two major buses make up the breaker and one-half configuration. There are three circuit breakers in each bay between the major buses. Between each pair of circuit breakers, a circuit is interrupted. Each circuit has its own circuit breaker and shares one with the next circuit in this configuration, giving each circuit one and a half breakers. Characteristics of this bus are used on higher voltage levels, more operating flexibility, requires more breakers, middle bus sections covered by line or other equipment protection. Bus bar faults are generally single line to ground faults and phase-to-phase faults. The main causes for a fault in a bus bar are the failure of a support insulator causes an earth fault. There is a flashover over the support insulator during overvoltages. Due to a heavily contaminated insulator, there is a flashover. Failure of connected equipment to the bus bar. Various environmental conditions. Bus protection schemes are classified into five types. Click on each one to learn more. Blocking scheme. A blocking scheme can provide basic protection for distribution bus bars. Protection devices in the outgoing base will send out blocking signals to the approaching base protection device, which is normally the transformer bay or feeder bays. Whenever one of the feeder bays or outgoing ports detects an error or pick up signal, it must send a blocking signal to the protection of the incoming port. The entering bay is prevented from tripping by the signal. Activation of the block signal may only last a few milliseconds. On all incoming circuits and outbound feeders, overcurrent OC relays are placed. The fault current on the feeders are detected by the feeder OCs. Unless any of the feeder OC relays are blocked, the incoming circuits OC is configured to trip the bus bar. To a wide race situations, a short coordination timer is typically required. All of the basic OC functions can be combined into one or a few microprocessor based multi-functional release. This provides for not just a decrease in wiring, but also a reduction in coordination time and a faster operation of the scheme. To provide rapid peer-to-peer communication, modern relays use protocol like the UCA with the GOOSE mechanism. This eliminates the need for wires and allows the blocking signals to be transmitted via the Internet. GOOSE, generic object-oriented substation events, is an IEC 61850 compliant control architecture that allows event data to be sent across whole electrical substation networks quickly and reliably. This strategy ensures that the same event message is received by multiple physical devices when used with multicast or broadcast services. In this figure, Fault 1. When a fault occurs in a feeder, the protective device closest to the problem transmits a trip signal to the incoming bay, as well as a block signal. Fault 2, when a problem develops on a bus bar, the incoming base protection mechanism issues a quick trip with the I greater than, greater than stage after 50 milliseconds. No block signal is issued since none of the feeder bays pick up. Overcurrent differential scheme. Externally, a differential current is formed by combining all of the circuit currents and then supply to an overcurrent relay. The CT ratio should ideally be the same. Matching CTs are required if this isn't the case. As a result, the load on the primary CTs may grow, worsening the saturation problem. Percent differential scheme. Furthermore, percent differential relays can apply a percent or restricted characteristic to the differential signal. The restraining signal options are maximum, average, and some bus currents. Applying voltage across the differential junction points activates high impedance protection. It is imperative that CTs have a low leakage rate, completely distributed windings or toroidal coils. Due to a decreased relay input impedance, the voltage does not rise above a certain level during external faults, even if some of the CT channels are excessively saturated. Linear couplers scheme. A linear couplers or air-core mutual reactor output voltage is proportional to the derivative of the input current. Because linear couplers use air cores, they do not saturate. The primary current and secondary voltage have a linear relationship. In a series loop, the coupler secondaries for each bus are linked to the relay. When the currents entering and exiting the bus are equal, the net induced voltage in the relay loop is zero. When there is a failure on the bus, the net induced voltage, which is proportional to the fault current, operates the relay.
