News - Analysis of the problem of linkage between diesel generator sets and energy storage

Here is a detailed English explanation of the four core issues regarding the interconnection of diesel generator sets and energy storage systems. This hybrid energy system (often called a “Diesel + Storage” hybrid microgrid) is an advanced solution for improving efficiency, reducing fuel consumption, and ensuring stable power supply, but its control is highly complex.

Core Issues Overview

100ms Reverse Power Problem: How to prevent energy storage from back-feeding power to the diesel generator, thus protecting it.
Constant Power Output: How to keep the diesel engine running consistently in its high-efficiency zone.
Sudden Disconnection of Energy Storage: How to handle the impact when the energy storage system suddenly drops off the network.
Reactive Power Problem: How to coordinate reactive power sharing between the two sources to ensure voltage stability.

1. The 100ms Reverse Power Problem

Problem Description:
Reverse power occurs when electrical energy flows from the energy storage system (or the load) back towards the diesel generator set. For the diesel engine, this acts like a “motor,” driving the engine. This is extremely dangerous and can lead to:

Mechanical Damage: Abnormal driving of the engine can damage components like the crankshaft and connecting rods.
System Instability: Causes fluctuations in the diesel engine’s speed (frequency) and voltage, potentially leading to shutdown.

The requirement to resolve it within 100ms exists because diesel generators have large mechanical inertia and their speed governing systems respond slowly (typically on the order of seconds). They cannot rely on themselves to quickly suppress this electrical back-flow. The task must be handled by the ultra-fast responding Power Conversion System (PCS) of the energy storage system.

Solution:

Core Principle: ”Diesel leads, storage follows.” In the entire system, the diesel generator set acts as the voltage and frequency reference source (i.e., V/F control mode), analogous to the “grid.” The energy storage system operates in Constant Power (PQ) Control Mode, where its output power is solely determined by commands from a master controller.
Control Logic:
1. Real-time Monitoring: The system master controller (or the storage PCS itself) monitors the output power (P_diesel) and direction of the diesel generator in real-time at a very high speed (e.g., thousands of times per second).
2. Power Setpoint: The power setpoint for the energy storage system (P_set) must satisfy: P_load (total load power) = P_diesel + P_set.
3. Rapid Adjustment: When the load suddenly decreases, causing P_diesel to trend negative, the controller must within a few milliseconds send a command to the storage PCS to immediately reduce its discharge power or switch to absorbing power (charging). This absorbs the excess energy into the batteries, ensuring P_diesel remains positive.
Technical Safeguards:
- High-Speed Communication: High-speed communication protocols (e.g., CAN bus, fast Ethernet) are required between the diesel controller, storage PCS, and system master controller to ensure minimal command delay.
- PCS Rapid Response: Modern storage PCS units have power response times far faster than 100ms, often within 10ms, making them fully capable of meeting this requirement.
- Redundant Protection: Beyond the control link, a reverse power protection relay is usually installed at the diesel generator output as a final hardware barrier. However, its operating time might be a few hundred milliseconds, so it primarily serves as backup protection; the core rapid protection relies on the control system.

2. Constant Power Output

Problem Description:
Diesel engines operate at peak fuel efficiency and lowest emissions within a load range of approximately 60%-80% of their rated power. Low loads cause “wet stacking” and carbon buildup, while high loads drastically increase fuel consumption and reduce lifespan. The goal is to isolate the diesel from load fluctuations, keeping it stable at an efficient setpoint.

Solution:

“Peak Shaving and Valley Filling” Control Strategy:
1. Set Basepoint: The diesel generator set is operated at a constant power output set at its optimal efficiency point (e.g., 70% of rated power).
2. Storage Regulation:
  - When Load Demand > Diesel Setpoint: The deficient power (P_load - P_diesel_set) is supplemented by the energy storage system discharging.
  - When Load Demand < Diesel Setpoint: The excess power (P_diesel_set - P_load) is absorbed by the energy storage system charging.
System Benefits:
- The diesel engine runs consistently at high efficiency, smoothly, extending its life and reducing maintenance costs.
- The energy storage system smooths out drastic load fluctuations, preventing the inefficiency and wear caused by frequent diesel load changes.
- Overall fuel consumption is significantly reduced.

3. Sudden Disconnection of Energy Storage

Problem Description:
The energy storage system might suddenly drop offline due to battery failure, PCS fault, or protection trips. The power previously being handled by the storage (whether generating or consuming) is instantly transferred entirely to the diesel generator set, creating a massive power shock.

Risks:

If the storage was discharging (supporting the load), its disconnection transfers the full load to the diesel, potentially causing overload, frequency (speed) drop, and protective shutdown.
If the storage was charging (absorbing excess power), its disconnection leaves the diesel’s excess power with nowhere to go, potentially causing reverse power and overvoltage, also triggering a shutdown.

Solution:

Diesel Side Spinning Reserve: The diesel generator set must not be sized only for its optimal efficiency point. It must have dynamic spare capacity. For example, if the maximum system load is 1000kW and the diesel runs at 700kW, the diesel’s rated capacity must be greater than 700kW + the largest potential step load (or the storage’s max power), e.g., a 1000kW unit selected, providing a 300kW buffer for a storage failure.
Fast Load Control:
1. System Real-time Monitoring: Continuously monitors the status and power flow of the storage system.
2. Fault Detection: Upon detecting a sudden storage disconnection, the master controller immediately sends a fast load reduction signal to the diesel controller.
3. Diesel Response: The diesel controller acts immediately (e.g., rapidly reducing fuel injection) to try to lower power to match the new load. The spinning reserve capacity buys time for this slower mechanical response.
Last Resort: Load Shedding: If the power shock is too large for the diesel to handle, the most reliable protection is to shed non-critical loads, prioritizing the safety of critical loads and the generator itself. A load-shedding scheme is an essential protection requirement in the system design.

4. Reactive Power Problem

Problem Description:
Reactive power is used to establish magnetic fields and is crucial for maintaining voltage stability in AC systems. Both the diesel generator and the storage PCS need to participate in reactive power regulation.

Diesel Generator: Controls reactive power output and voltage by adjusting its excitation current. Its reactive power capability is limited, and its response is slow.
Storage PCS: Most modern PCS units are four-quadrant, meaning they can independently and rapidly inject or absorb reactive power (provided they do not exceed their apparent power rating kVA).

Challenge: How to coordinate both to ensure system voltage stability without overloading either unit.

Solution:

Control Strategies:
1. Diesel Governs Voltage: The diesel generator set is set to V/F mode, responsible for establishing the system’s voltage and frequency reference. It provides a stable “voltage source.”
2. Storage Participates in Reactive Regulation (Optional):
  - PQ Mode: The storage only handles active power (P), with reactive power (Q) set to zero. The diesel provides all reactive power. This is the simplest method but burdens the diesel.
  - Reactive Power Dispatch Mode: The system master controller sends reactive power commands (Q_set) to the storage PCS based on current voltage conditions. If system voltage is low, command the storage to inject reactive power; if high, command it to absorb reactive power. This relieves the burden on the diesel, allowing it to focus on active power output, while providing finer and faster voltage stabilization.
  - Power Factor (PF) Control Mode: A target power factor (e.g., 0.95) is set, and the storage automatically adjusts its reactive output to maintain a constant overall power factor at the diesel generator’s terminals.
Capacity Consideration: The storage PCS must be sized with sufficient apparent power capacity (kVA). For instance, a 500kW PCS outputting 400kW of active power can provide a maximum of sqrt(500² - 400²) = 300kVAr of reactive power. If reactive power demand is high, a larger PCS is required.

Summary

Successfully achieving a stable interconnection between a diesel generator set and energy storage hinges on hierarchical control:

Hardware Layer: Select a fast-responding storage PCS and a diesel generator controller with high-speed communication interfaces.
Control Layer: Employ a fundamental architecture of “Diesel sets V/F, Storage does PQ.” A high-speed system controller performs real-time power dispatch for active power “peak shaving/valley filling” and reactive power support.
Protection Layer: The system design must include comprehensive protection plans: reverse power protection, overload protection, and load control (even load shedding) strategies to handle the sudden disconnection of storage.

Through the solutions described above, the four key issues you raised can be effectively addressed to build an efficient, stable, and reliable diesel-energy storage hybrid power system.