Transformer Reverse Power Protection — Distributed PV & CHP, Directional Detection & 32R Relay

Introduction

When distributed generation (DG) sources — rooftop photovoltaic arrays, combined heat and power (CHP) units, or wind turbines — are connected downstream of a distribution transformer, the power flow can reverse direction. Under light load and high DG output, the transformer experiences reverse power flow: energy is exported back through the transformer to the upstream grid. While net metering makes this desirable from a revenue perspective, it presents serious protection challenges. The transformer must be protected against motoring (for generator step-up transformers), islanding, and undetected faults. This article covers the complete reverse power protection scheme for DG-interconnected transformers.

1. Understanding Reverse Power Flow

1.1 Normal vs. Reverse Power

Condition	Power Direction	Transformer Mode
Load > DG output	Grid → Load (normal)	Step-down
DG output > Load	DG → Grid (reverse)	Step-up
Grid loss + DG	DG → Island	Uncontrolled voltage/frequency

1.2 When Reverse Power Becomes a Problem

Transformer motoring: In generator step-up transformers (GSU), if the generator trips and the HV breaker remains closed, the transformer draws magnetizing current from the grid and acts as a motor — overheating within minutes if the generator breaker doesn't open.
Islanding: The DG remains energized on a section of the grid that is disconnected from the utility — creating a hazard for line workers and causing voltage/frequency excursions.
Protection blinding: Fault current from the DG may not be detected by upstream overcurrent relays set for grid-only fault levels.

2. ANSI 32R — Directional Reverse Power Relay

2.1 Operating Principle

The 32R relay measures the phase angle between voltage and current to determine power flow direction:

P = V × I × cos(φ)

cos(φ) > 0 → positive power → normal (grid to load)
cos(φ) < 0 → negative power → reverse (load to grid)

2.2 Pickup Setting

The pickup is typically set at:

P_pickup = 0.5% to 3% of transformer rated power, with a time delay of 0.5–5 seconds

Rationale:

Transformer magnetizing power (1–3% of rated) can appear as reverse power during low-load conditions when DG is absent. Set pickup above the no-load losses.
Time delay prevents nuisance tripping during transient reverse flows (e.g., motor regeneration).

2.3 Typical Settings

Application	Pickup (% of S_r)	Time Delay (s)	Notes
GSU transformer anti-motoring	0.5–1.0%	2–5 s	Small pickup to detect even low-level motoring
Distribution TX with DG	1–3%	0.5–2.0 s	Higher pickup to avoid nuisance on load swings
Intertie transformer	1–5%	1–5 s	Depends on contractual power exchange limits

3. Generator Step-Up Transformer Anti-Motoring

3.1 The Problem

When a synchronous generator trips (prime mover failure, emergency stop), the generator breaker opens. If the HV grid-side breaker remains closed, the transformer remains energized from the HV side and draws:

Magnetizing current: ~1–3% of rated, reactive
Core losses: ~0.2% of rated, active

While the current magnitude is small (~1–3% of rated), the transformer is driving no load. The active power drawn from the grid represents pure loss. More critically, if the generator breaker fails to open and the prime mover stops, the generator becomes a synchronous motor, driving the turbine. Steam turbines can overheat within 30–90 seconds when operated without steam flow (windage heating).

3.2 Protection Logic

IF (P_reverse > P_pickup) AND (Generator CB is OPEN) AND (t > t_delay):
    → Trip HV breaker

Interlock with generator breaker status is critical — without it, the relay could trip during normal generator synchronization when brief reverse power transients are expected.

4. Distribution Transformer with Embedded DG

4.1 Protection Coordination Challenge

Consider a 1600 kVA, 10/0.4 kV transformer with 800 kWp of rooftop PV:

Scenario	Load (kW)	PV Output (kW)	Net Power at TX	Power Direction
Peak day, full production	400	720	320 kW export (reverse)	← Grid
Cloudy day	600	200	400 kW import (normal)	Grid →
Night	300	0	300 kW import (normal)	Grid →
Grid outage, PV on	350	500	150 kW to island	Island

4.2 Protection Functions Required

ANSI Code	Function	Purpose
32R	Directional reverse power	Detect sustained reverse flow (optional, utility requirement)
27	Undervoltage	Detect grid loss (voltage collapse)
59	Overvoltage	Prevent island overvoltage from PV inverters
81U/O	Under/over frequency	Detect island frequency excursions
67	Directional overcurrent	Discriminate grid faults from DG contribution
25	Synch check	Prevent out-of-sync reclosure onto island

4.3 Anti-Islanding vs. Reverse Power

Reverse power (32R) ≠ Anti-islanding: A 32R relay detects reverse power flow magnitude and direction but cannot detect islanding when local load matches DG output (zero power exchange with the grid). Anti-islanding requires rate of change of frequency (ROCOF, df/dt, ANSI 81R) and vector shift detection, which are typically built into the DG inverter's grid protection relay.

5. VT and CT Configuration for Directional Measurement

5.1 Connection

The 32R relay requires both voltage and current inputs with correct polarity:

CT polarity: P1 → Line, P2 → Transformer (conventional)
VT connection: Phase-to-phase (V_AB) or phase-to-neutral, depending on relay

Wiring rule: The CT secondary current must be in-phase with the VT secondary voltage for normal power flow (grid to load). If wired backwards, the relay sees reverse power as forward and will not trip when required.

5.2 Phase Rotation and Angle Compensation

For a three-phase relay using V_AB and I_A:

Normal power factor = 0.85 lag → I_A lags V_AN by arccos(0.85) = 31.8°
V_AB leads V_AN by 30°
Therefore: I_A lags V_AB by 31.8° + 30° = 61.8°
The relay must be set with a characteristic angle of 0° (wattmetric) or compensated for the VT connection

5.3 Test Before Commissioning

Inject secondary test signals:

Apply rated voltage, inject rated current at 0° phase shift → verify "forward" indication
Shift current to 180° (reverse) → verify "reverse" indication
Gradually increase reverse current until pickup → verify trip after delay

6. Integration with SCADA and Utility Requirements

6.1 Utility Interconnection Requirements

Most utilities require transformer reverse power protection at the point of common coupling (PCC):

IEEE 1547-2018: Mandates anti-islanding detection but allows reverse power flow if the interconnection agreement permits export.
DNO (UK) G59/G99: Requires a G59 relay with ROCOF and vector shift, plus directional overcurrent.
VDE-AR-N 4105 (Germany): Specifies voltage, frequency, and anti-islanding protection settings for LV-connected DG.

6.2 SCADA Monitoring Points

Parameter	Alarm	Trip
Reverse power (kW)	Exceeds 50% of pickup	Exceeds pickup + time
Reverse reactive power (kVAr)	—	Utility-specified limit
Frequency	49.5 / 50.5 Hz	47.5 / 51.5 Hz
Voltage	±10%	+15% / −20%
ROCOF	0.2 Hz/s	0.5 Hz/s

FAQ

Q: What is the difference between reverse power (32R) and directional overcurrent (67)?

The 32R relay responds to active power magnitude and direction (wattmetric measurement). The 67 relay responds to current magnitude and direction regardless of power factor. 32R is insensitive to reactive power flow — a purely capacitive or inductive reverse flow will not cause a trip. 67 trips on any current exceeding the pickup in the reverse direction, regardless of whether it represents active power export or capacitive charging current.

Q: My factory has a 500 kW CHP unit. Do I need reverse power protection on the 10 kV utility transformer?

If your CHP unit is sized smaller than the minimum site load (all CHP output is consumed on-site), reverse power protection may not be strictly required for operational reasons. However, the utility interconnection agreement almost certainly requires it as a condition of parallel operation. Even if export never occurs in practice, the protection must be installed and functional to prevent inadvertent export during abnormal conditions.

Q: How do I coordinate reverse power protection with automatic power factor correction (APFC) capacitor banks?

APFC capacitor banks inject leading reactive power that can affect power factor measurements but should not cause active power reversal. However, if the 32R relay uses a wide-angle setting or has poor harmonic rejection, capacitor switching transients may cause nuisance indications. Ensure the relay has adequate filtering (DFT-based measurement) and set a sufficient time delay (≥2 seconds) to ride through capacitor switching events.

Q: Can a 32R relay protect against backfeed from a UPS system?

No — a standard UPS operates only in discharge mode during grid outages and does not export power to the grid. A grid-tied bidirectional UPS or energy storage system (ESS) capable of export must be treated as distributed generation and requires full interconnection protection including reverse power and anti-islanding. Standard transformer 32R settings (1–3% of rated) will detect the ESS export but may not provide the fast anti-islanding response required by IEEE 1547.

Q: What happens if I set the 32R pickup too low?

A pickup below the transformer's no-load losses (~0.2–0.5% of rated for modern transformers) will cause nuisance tripping whenever the DG output closely matches the load and small measurement errors produce an apparent reverse power reading. CT and VT accuracy at 1–5% of rated current is poor (Class 0.5 extends to 1% of rated), making sub-1% measurements unreliable. The pickup should always exceed the combined measurement uncertainty and transformer no-load losses.

Q: Is reverse power protection required for purely solar-only installations where export to the grid is never intended?

If the PV inverter is certified to IEEE 1547 (UL 1741 SA) and includes built-in anti-islanding protection, reverse power protection at the transformer level may not be required if the interconnection agreement specifies a zero-export profile. However, the inverter's zero-export control must be proven reliable, and many utilities still require a separate 32R relay at the PCC as a defense-in-depth measure. A reverse power relay with a 5-minute averaging window can discriminate between transient export (acceptable) and sustained export (violation).

References & Standards

Document	Title	Relevance
IEEE 1547-2018	Standard for Interconnection of Distributed Energy Resources	DG interconnection requirements
IEC 60255-12	Directional relays and power relays	32R relay specification and testing
IEEE C37.2	Standard electrical power system device function numbers	ANSI 32/32R definitions
UL 1741 SA	Inverters for use with distributed energy resources	Inverter anti-islanding certification
VDE-AR-N 4105	Generators connected to LV distribution network	German LV DG requirements
G59/3 / G99	UK DNO requirements for generation connection	UK-specific DG protection

*Du Fu, ZY POWER Production Engineer — Power knows its direction; your protection should too.*