Summary:Typically, power generation plants utilize a 380V supply system for their low-voltage factory power systems. As societal development continues and the capacity of power generation units increases, the original single-phase distribution method has gradually shifted to motor control centers (MCCs) and low-voltage power centers (PCs). This presents challenges in electrical protection coordination. This article analyzes the coordination of low-voltage factory power protection settings under the existing supply methods, focusing on the study of commonly occurring coordination principles.
Keyword Power Plant; Low-voltage Plant Power; Protection; Setting Coordination
Introduction
When performing relay protection setting calculations, the calculation of the fault current and fault voltage for the protected equipment is fundamental. As long as the fault current and fault voltage are accurately calculated, the subsequent setting process becomes relatively simple. In the current situation, there are corresponding guidelines and procedures for both system protection and generator unit protection. However, during actual operation, there are many different configurations for the relay protection of low-voltage power supply in power plants, with no unified standard. For low-voltage power supply, there are MCC (Motor Control Center) and PC (Power Center) motors, and many switches related to the PC bus, such as incoming line switches and standby incoming line switches, are crucial electrical facilities. Existing technical specifications have clearly stipulated that low-voltage power transformers must be equipped with rapid-interrupt protection, grounding protection, and time-limited overcurrent protection. For large-capacity motors in busbars and their distribution lines, switches with release mechanisms are typically installed, while other switches are usually set to thermal relays or fuses for protection purposes. As for grounding protection, some designs do not include zero-sequence overcurrent protection, while others do. Due to the lack of unified standards, this has greatly facilitated the setting calculation for relay protection in low-voltage systems.
Section II: Analysis of Low-Voltage Power System in Power Plant
In power plants, for large capacity units, the workshop low-voltage transformers are typically configured in a "standby-in-the-dark" setup, meaning two transformers are installed simultaneously—one in use and the other as a backup. Should one transformer be under repair, the standby transformer is activated via a switch.
Low-voltage motors typically have short-circuit protection through thermal relays or fuses, including ground and phase-to-phase short circuits. MCC incoming switch faults or shutdowns are protected by the disconnecting mechanism. The PC incoming switch is also protected by relays or disconnecting mechanisms, as is the case with the sectioned interconnection switches.
For transformers, overcurrent protection, earth protection, and rapid interruption protection are typically installed on the high-voltage side. Neutral point zero-sequence overcurrent protection is usually fitted on the transformer's low-voltage side. Circuit breakers are commonly equipped with earth protection, rapid interruption protection, and time-delayed overcurrent protection.
Section III: Low-voltage Power Plant Utility Protection Setting Coordination Analysis
3.1 Tweak and match according to the principle of low-voltage busbar motor starting current
Industry technical specification standards have explicitly stipulated that transformers used in low-voltage plants must be equipped with current protection to ensure that the switches on both sides of the transformer can be disconnected in the event of a short circuit, thereby protecting the transformer and related electrical components. If the transformer supplies power to two or more sections simultaneously, current protection should be installed on each section.
Due to the fact that low-voltage busbars typically do not have low-voltage protection, when a busbar fault occurs, the power switches connected to the busbar will not operate. At this point, upon voltage restoration, the motor will automatically start. Therefore, when designing busbar incoming overcurrent protection, it is not sufficient to merely consider the motor's starting current. Additionally, taking into account the starting current of the largest capacity motor and the normal operating load current does not align with reality. The correct approach is to comprehensively consider the starting current of all motors.
For the high and low voltage sides of transformers, the factors considered in the design of overcurrent protection are similar; therefore, this article analyzes only the low voltage side situation. Generally, the principle considered when designing overcurrent protection on the low voltage side is to avoid the starting current of low voltage busbars and motors. Although this method is widely used, it actually has certain drawbacks. This is because when using this method, it cannot be well matched with the corresponding thermal relay or capacitor protection characteristics. The resulting problem is that a partial short circuit in the motor can trigger the transformer's overcurrent protection, leading to a trip-out phenomenon. Issues may arise with the slow switching of the transformer's high voltage busbar power supply, so the phenomenon of busbar motors starting in groups should be considered during the design of overcurrent protection. To meet practical needs, the correct approach should be to consider the starting current of all busbar motors in groups, as well as all other load currents, and then check their compatibility with the fuse characteristics.
Generally, the rated current of a fuse is 1.3 to 1.5 times that of the motor's rated current, and its fuse characteristic exhibits an inverse-time feature. Once the current passing through the fuse exceeds 1.2 times the rated current, the fuse can blow within 0.1 seconds.
If the rated current values of the motor and fuse differ significantly, or if the motor cable length is long, the short-circuit current at the cable end may not reach the fuse's threshold and enter the inverse-time characteristic. However, for busbars set by time delay, this has already exceeded the time-delayed overcurrent. In this case, the incoming line overcurrent protection may exhibit a cascading tripping phenomenon. In the low-voltage PC end, if the busbar loses power, it may cause partial unit shutdown or load reduction.
For motors with larger capacities and longer cables, it is necessary to inspect the coordination issue between the busbar incoming current protection and the fuse characteristics when a short-circuit fault occurs at the cable end. If they do not match, measures such as increasing the cable cross-sectional area and replacing the motor fuse with a release switch can be taken. Experience shows that coordination tests should be conducted when the motor rated current does not exceed 100A and the cable length is greater than 150m. For motors with a rated current exceeding 100A, a release switch should be used for protection.
The coordination between different protection delays is as follows: the terminal load is considered instantaneously, while the short-time current for the MCC incoming line, PC联络 switch, PC incoming line switch, and the high voltage side of the transformer are considered at 0.2s, 0.3s, 0.4s, and 0.7s respectively.
3.2 Analysis of Setting Coordination for Zero-Sequence Overcurrent Protection at the Low-Voltage Side Neutral Point of Transformer
Typically, low-voltage transformers are designed with Dyn winding to ground the neutral point of the low-voltage side, aiming to increase the short-circuit current in the event of a ground fault. Consequently, the inter-phase short-circuit protection of the low-voltage system is set as the main protection, while the grounding protection of the branch circuit breaker is used as a backup. The zero-sequence overcurrent protection on the low-voltage side neutral point is divided into two segments: zero-sequence inverse-time and zero-sequence time-delay. If coordination is set based on the time-delay, as low-voltage motors are usually protected by thermal overload relays or fuses, a mismatch issue may arise if the motor cable length is long enough for the short-circuit fault current to not reach the fuse threshold. If coordination is set based on the inverse-time, to match the fuse characteristics when a short-circuit fault occurs at the cable end, the transformer neutral point zero-sequence overcurrent protection must be set to inverse-time characteristics. However, this does not meet the requirement for rapid disconnection, as practical experience shows that if the branch cable length exceeds 100 meters, rapid disconnection cannot be achieved. Therefore, during the design process, the cable length should be kept as short as possible.
It is more suitable to eliminate faults according to a timed export method, but this approach requires a sufficiently large zero-sequence current when a grounding fault occurs. Specific measures include increasing the cross-sectional area of the cables and zero return conductors, or controlling the cable length.
3.3 Analysis of the Coordination of 3.3 Switch Disconnection and Overcurrent Relays, Thermal Relays, and Fuses
Switch disconnectors typically feature a long-time delay operation, a characteristic similar to fuses. Therefore, when matching, one should focus on the operation threshold. If a thermal relay or fuse is set in the final branch circuit, the incoming line switch disconnector can only be adjusted for short-time delay for grounding and overcurrent segments. Only when the cable length is sufficiently long can a瞬时段 be adjusted. For the瞬时段 and short-time delay segments, their adjustment principles are similar to transformer overcurrent protection. The grounding segment usually matches the thermal relay or fuse. If they do not match, matching can be optimized by adjusting the zero-sequence impedance, rather than by increasing the delay measures.Section 4: Selection Guide for ARD Series Motor Protectors
The ARD Intelligent Motor Protector is designed for low-voltage motor circuits with rated voltages up to 660V, integrating protection, measurement, control, communication, and maintenance into one. Its comprehensive protection features ensure the safe operation of motors, featuring logical programmable functions to accommodate various control methods. The product adopts a split structure, consisting of the main body, display unit, and transformer, and is adaptable to various cabinet installations. Optional communication modules are available to meet on-site communication requirements.
4.1 Feature Highlights
■Supports baseband and full-wave power parameter measurements (U, I, P, Q, S, PF, F, EP, EQ), current and current unbalance, positive sequence, negative sequence, and zero sequence components, voltage, three-phase voltage phase angle, residual current.
■ Protection features include overload non-time-limited, overload time-limited, grounding, start-up timeout, leakage, underload, phase loss, stalling, jamming, short circuit, overflow, imbalance (current, voltage), over-power, under-power, over-voltage, under-voltage, phase sequence, temperature, tE time, external fault, start-up count limit, operating time alarm, fault count alarm.
■ 9 programmable DI inputs, defaulting to an internal DC24V power supply, with the option for an external active wet contact.
■ 5 programmable DO outputs for direct start, star-delta start, autotransformer start, and various other start methods; remote control of motor "start/stop" is achievable via communication bus.
Optional Anti-Sag Function: Supports immediate restart after sag and voltage drop restart.
Optional with MODBUS_RTU communication and PROFIBUS DP communication, supporting up to 2 communication interfaces.
Optional 1-channel DC4-20mA analog output interface, compatible with DCS systems, enabling monitoring of field equipment.
■Features records for various events, including fault, start-up, stop, DI position change, and restart.
The display interface features a liquid crystal display and supports switching between Chinese and English.
V. Closing RemarksFor low-voltage overcurrent protection, it's not enough to simply match the setting based on the bus motor's starting current. After considering the self-starting current of all bus motors grouped together and all other load currents, it's also necessary to verify the compatibility with the fuse characteristics. For the transformer's low-voltage side neutral point zero-sequence overcurrent protection, the coordination issue with the thermal overload relays or fuse characteristics needs to be considered.
Reference:
Wang Tingwen. Analysis of Coordination and Setting for Low-Voltage Power Protection in Power Plants
[2] Ankorree Enterprise Microgrid Design and Application Handbook. 2022.05 Edition.
[3] GB14048.4-2020, Part 4-1 of Low-Voltage Switchgear and Control Equipment: Contactors and Motor Starters – Electromechanical Contactors and Motor Starters (including Motor Protectors)
