The Eighteen Skills an Excellent Inverter Should Have

1. Introduction

The inverter is connected to the photovoltaic system at one end and to the power grid at the other end. It is the bridge connecting the photovoltaic power station and the power grid, and plays the most critical role in improving the grid-friendliness of photovoltaic power.

However, the photovoltaic system in front of the inverter is very unfriendly to the power grid.

1 Characteristics of photovoltaic power generation grid-connected system

1. Intermittent, random, and periodic

2. Grid connection through power electronic equipment, no rotating parts, no inertia

3. Co-development of centralized and distributed systems

2. The inverter as a bridge needs to achieve the following goals

This enables the photovoltaic system to be better integrated into the multi-energy interconnection system, as shown in the figure below.

(The above picture comes from: Cinda Securities)

To achieve the above goals, what excellent performance should an excellent grid-friendly inverter have?

Lower grid current harmonics

Better fault ride-through capability

More flexible and faster scheduling response capabilities

2. Functions that inverters should have in large-scale photovoltaic power stations

1Low voltage and zero voltage ride-through function

Low voltage ride-through 2s, zero voltage ride-through 150ms

2. Active and reactive power dynamic adjustment function

Active and reactive power dynamic adjustment function (Pf, Qt, Q(p), Q(u))

3High voltage ride-through function

1.2pu ≤UT≤1.3pu, should run continuously for at least 0.5s

4 Reactive power dynamic response

Current harmonic reactive dynamic response is less than 30ms

5Stable fault ride-through capability

The fact that a single device or a single generating unit passes the low voltage ride-through does not mean that the entire power station can meet the requirements.

The more devices there are, the more difficult it is to coordinate with each other, and the greater the system risk.

6. The resonance phenomenon after multiple low-harmonic inverters are connected in parallel, especially in weak grid areas, increases the risk

7 Intelligent scheduling interface to meet AGC and AVC scheduling requirements

8. Fully exploit and utilize SVG functions to meet the grid's 30ms fast reactive power dispatch response requirements and have the ability to replace SVG

The inverter power factor is continuously adjustable between -0.9 and +0.9. When outputting 500KW of active power, it can also generate 242kVar of reactive power.

Reactive power from inductive 200kVar→0, response time 23.9ms

Reactive power tolerance 221kVar→0, response time 26.6ms

The inverter can generate reactive power according to the QU curve based on the grid voltage.

"Automatic voltage-reactive power control mode" and "dispatch mode" are optional

2. Functions that inverters should have in distributed photovoltaic projects

1. A reactive power control solution should be available, and no additional SVG equipment is required. 1) Performance without a "reactive power control solution"

The power factor of the property boundary point decreases after the distributed photovoltaic system is connected to the grid

Typical primary wiring diagram

Active power fluctuations are more obvious at the property boundary point.
The monthly power factor is 0.509 when the system has no reactive power control.

2) Performance when having "reactive power control solution"

Typical primary wiring diagram

After the system added automatic power factor control, the monthly power factor was 0.972

2Automatic active and reactive power control to support the power grid

Photovoltaic poverty alleviation projects and rural power grid environments are very poor, which brings challenges to grid connection. The power grid in rural areas is relatively weak. After photovoltaic grid connection, abnormal situations such as three-phase imbalance and severe power grid fluctuations may occur.

At the end of the power grid, the power grid voltage fluctuates violently.

The three-phase power grid voltage is unbalanced. The national standard GB/T 15543 requires a limit of 2%.

Automatic active power control (P(f))

and automatic reactive power control (Q(u)). Multiple modes are available.

3. Relay protection configuration

After a line fault occurs, the relay protection and reclosing action will be affected by the photovoltaic power generation system. The impact on the three-stage current protection based on the circuit breaker is the most significant. This will lead to reduced sensitivity of the line protection and failures such as refusal to operate and false operation;

There is confusion about how to configure relay protection when distributed power is connected to the distribution network. It is necessary to comprehensively consider the setting of inverters, access systems, and relay protection in the original distribution network to balance safety and economic indicators.

4. Improve power quality function

Power quality

5Island protection function

6. Response scheduling function

7One-stop solution to distributed grid connection problems

3. The ultimate function to improve grid friendliness: energy storage

Virtualize the energy storage inverter to be a synchronous generator to further improve system performance

"Peak shaving and valley filling", reduce power generation restrictions

, suppress fluctuations, and improve grid-connected characteristics